History UsefulNotes / Planets

The moon systems of ice giants are a trickier matter to pin down. While it's known that gas giants can have moons rivalling or even surpassing true planets in volume, it is not known if this is also common for ice giants because our only two concrete examples consist of one with relatively small moons, and one whose moon system is in the eternal aftermath of the capture of a dwarf planet. There is currently no means of concluding what a typical ice giant moon system looks like.

What's different about dwarf planets is that they aren't massive enough to dominate their orbits, so they are found in belts of various space debris: asteroid belts, Kuiper belts and scattered discs. They are also commonly found in orbital resonances with a true planet. In very rare cases, a dwarf planet may be gravitationally captured into direct orbit around a true planet and converted into a moon.

A long-standing staple in space opera, a double planet is a system of two planets acting as each other's moons. They will orbit a common centre of mass which, unlike a typical planet-moon pairing such as Earth and its moon[[note]]The barycenter of the Earth-Moon system lies in a shifting spot in the Earth's mantle.[[/note]], lies in the empty space between the two objects. The gravitational interactions caused by two large bodies being in such close proximity will result in a two way tidal lock; no matter what, each planet will always show its partner the same face, like a pair of dancers holding hands and spinning in a circle, and thus will never rise or set in each other's skies. This configuration is possible and even experienced directly by our scientists: the dwarf planets Pluto and Charon are in this configuration.

Of the five pulsar planets we know, only the three around the [[https://en.wikipedia.org/wiki/PSR_B1257%2B12 PSR B1257+12]][[note]]Appropriately named [[{{OurLichesAreDifferent}} Lich]] (an undead thing that can control other undead things), while the planets are [[{{OurZombiesAreDifferent}} Draugr]], [[{{Poltergeist}} Poltergeist]], and [[{{Myth/ClassicalMythology}} Phobetor]] respectively.[[/note]] pulsar are believed to be examples of these elusive second gen planets,[[note]]The other two are pretty much confirmed to be a captured planet and a Chthonian planet/Stripped star.[[/note]] and are likely some of the most inhospitable places we know. Not only are they constantly sterilized by the extreme radiation that pulsars natively radiate, but the beams that gives them their name would be concentrated enough at such a short distance that they would carve deep grooves into the planet's surfaces. Ironically, despite their rarity and how unlikely their very existence was thought to be, pulsar planets were the very first extrasolar planets to be detected.

Another type of hot gas giant are those that are in wide, Jupiter-like orbits, but are orbiting a luminous star such as a red giant[[note]]An old, evolved, more or less Sun-like star[[/note]]. Because of that are as strongly irradiated as epistellar Jupiters, but at the same time have some of the properties cold gas giants (are expected to) have such as fast rotation (thus no tidal lock (see below)) as well as a retinue of large moons as explained in the next section. This is the fate Jupiter and Saturn -at least- will experience when our Sun goes red giant, five billion years from now.

A Megaearth as [[https://en.wikipedia.org/wiki/BD+20_594b BD+20 594b]], more than twice as largue as Earth and sixteen times more massive, or [[http://en.wikipedia.org/wiki/Kepler-10c Kepler-10c]] was thought to be (seventeen times[[note]][[ScienceMarchesOn newer observations]] suggests is [[https://arxiv.org/abs/1707.06192 half as massive]], thus with much less rock and metal and much more volatile elements as water[[/note]] more massive than our planet and twice as large) is basically a superearth up to eleven, with heavier mass--thus with more gravity--and larger. Going even further we've [[http://en.wikipedia.org/wiki/HD_149026_b HD 149026 b]], that has a mass similar to that of Saturn, but it's two thirds as large, suggesting a rocky-icy core of 60 Earth mass or even more below a (still) massive hydrogen atmosphere. If you don't have enough with this, some theoretical research suggests ''really'' massive solid planets, up to several thousand times the mass of the Earth, could form in the protoplanetary disks of metal-rich massive stars whose strong UV evaporation and stellar winds would strip the lightest elements, leaving just the heaviest ones.

Even if they seem to be abundant[[note]]Unless it's an observational bias caused by our present technology, that is[[/note]], no superearths have been found in our Solar System. [[https://en.wikipedia.org/wiki/Grand_tack_hypothesis#Lost_super-Earths it has recently been proposed]] the early Solar System ''could'' have had some of them, but Jupiter's interactions and movement within the protoplanetary disk caused such a huge mess that, among other things, hypothetical superearths that could have formed in the innermost Solar System would have [[ColonyDrop crashed among themselves]] or [[HurlItIntoTheSun fallen into the Sun, along with the debris of those collisions]].

Gas giants vary in size. The smallest ones, like UsefulNotes/{{Uranus}} and UsefulNotes/{{Neptune}}, have solid icy cores of significant size in comparison to their whole volume. Their hydrogen is the most impure, with the largest amounts of helium, methane, ammonia and other gases that often dye them in funny colors (Uranus is sky-blue, Neptune is darker blue). For this reason, they are commonly referred to as ice giants. Those smaller than Uranus and Neptune are sometimes called gas dwarfs. Regardless of which type a gaseous planet falls into, they often have intense storms that are visible from space.

The chemical composition of a planet depends partly on it's location in the solar system (lighter elements will naturally coagulate further out, hence why outer planets are almost always gas giants) as well as the composition of the protoplanetary disc that spawned the planet, in earth's case being mostly a mixture of iron, oxygen, silicon and magnesium.

Another type of Chthonian planet takes this UpToEleven. Instead of a gas giant merely being stripped from migrating to close to their sun, these where stars in binary orbits with a neutron star (such as [[https://en.wikipedia.org/wiki/PSR_J1719%E2%88%921438_b PSR J1719−1438 b]] or a white dwarf ([[https://en.wikipedia.org/wiki/EF_Eridani EF Eridani B]] being an example) that has been there for so long that pretty much all of their lighter materials has been removed by their partners gravity, leaving behind a planet sized stellar remnant consisting mainly of oxygen and crystalline carbon[[note]]aka diamond, another name for these types of stellar remnants are ''Diamond Planets''[[/note]] and some other metals depending on what stage of life the star was in prior to being stripped.

A Megaearth as [[https://en.wikipedia.org/wiki/BD+20_594b BD+20 594b]], more than twice as largue as Earth and sixteen times more massive, or [[http://en.wikipedia.org/wiki/Kepler-10c Kepler-10c]] was thought to be (seventeen times[[note]][[ScienceMarchesOn newer observations]] suggests is [[https://arxiv.org/abs/1707.06192 half as massive]], thus with much less rock and metal and much more volatile elements as water[[/note]] more massive than our planet and twice as large) is basically a superearth UpToEleven, with heavier mass--thus with more gravity--and larger. Going even further we've [[http://en.wikipedia.org/wiki/HD_149026_b HD 149026 b]], that has a mass similar to that of Saturn, but it's two thirds as large, suggesting a rocky-icy core of 60 Earth mass or even more below a (still) massive hydrogen atmosphere. If you don't have enough with this, some theoretical research suggests ''really'' massive solid planets, up to several thousand times the mass of the Earth, could form in the protoplanetary disks of metal-rich massive stars whose strong UV evaporation and stellar winds would strip the lightest elements, leaving just the heaviest ones.

If the primary star emits enough radiation [[UsefulNotes/{{stars}} (it's a G or bluer, or a severely flaring M)]], iceballs may be covered with tholin, a substance not unlike frozen brown gunge. It is formed when various ices (water, methane, ammonia) react under irradiation and contains various naturally-occurring organic polymers. In our Solar system, tholin can be found on Titan, Ganymede, Callisto, all moons of Uranus, Triton and most far fringe dwarf planets. Speculations have been made that tholin could be a very useful resource to space explorers if we could bio-engineer micro-organisms (bacteria and algae) capable of processing this gunge into food and fertilizer.

AlienSky is perhaps the most important visual cue of another planet. However, it's also a common way of making an [[ArtisticLicenseAstronomy astronomy goof]]. This section contains tips on what cool alien skies can other planets have.

Of the five pulsar planets we know, only the three around the [[https://en.wikipedia.org/wiki/PSR_B1257%2B12 PSR B1257+12]][[note]]Appropriately named [[{{OurLichesAreDifferent}} Lich]] (an undead thing that can control other undead things), while the planets are [[{{OurZombiesAreDifferent}} Draugr]], [[{{Poltergeist}} Poltergeist]], and [[{{Myth/ClassicalMythology}} Phobetor]] respectively.[[/note]] pulsar are believed to be examples of these elusive second gen planets,[[note]]The other two are pretty much confirmed to be a captured planet and a Chthonian planet/Stripped star.[[/note]] and are likely some of the most inhospitable places we know. Not only are they constantly sterilized by the extreme radiation that pulsars natively radiate, but the beams that gives them they're name would be concentrated enough at such a short distance that they would carve deep groves into the planets. Ironically, despite their rarity and how unlikely their very existence was thought to be, pulsar planets were the very first extrasolar planets to be detected.

As ridiculous as it might sound, scientist have proposed theories for how this type of planets can potentially host life. Among the hotchpotch of different types of radiation that pulsar radiate are visible and infrared rays, meaning that they do have a 'habitability' zone in the biggest possible stretch of the word. Now if the planet in question also have a thick enough atmosphere (as in, ''millions'' of time thicker than earth with Mariana trench levels of pressure), it could also be enough to shield it from it's host's more dangerous outputs of X rays and Gamma rays. While said radiation would still Ionize the atmosphere layer by layer, it would take it a billion years or so (and possible even more if the planet also has a deep ocean it has to go through) before the atmosphere is thin enough that the rays reach the surface in deadly amounts, which is plenty of time for at least basic life to possible form.

Pulsar planets are planets orbiting the husks of dead large stars. Exactly how they ended up there isn't really understood,[[note]]Pulsars and other neutron stars are created when stars go nova, a process that should have cleared whatever solar system they had previously.[[/note]] but its believed to be trough some sort of Second round of planetary creation that pulsars might go trough extremely rarely. (unless they where just captured from another star that is)

Of the five pulsar planets we know, only the three around the [[https://en.wikipedia.org/wiki/PSR_B1257%2B12 PSR B1257+12]] pulsar are believed to be examples of these elusive second gen planets,[[note]]The other two are pretty much confirmed to be a captured planet and a Chthonian planet/Stripped star.[[/note]] and are likely some of the most inhospitable places we know. Not only are they constantly sterilized by the extreme radiation that pulsars natively radiate, but the beams that gives them they're name would be concentrated enough at such a short distance that they would carve deep groves into the planets. Ironically, despite their rarity and how unlikely their very existence was thought to be, pulsar planets were the very first extrasolar planets to be detected.

Another type of hot gas giant are those that are in wide, Jupiter-like orbits, but are orbiting a luminous star such as a red giant[[note]]An old, evolved, more or less Sun-like star[[/note]]. Because of that are as strongly irradiated as epistellar Jupiters, but at the same time have some of the properties cold gas giants (are expected to) have such as fast rotation (thus no tidal lock (see below)) as well as a retinue of large moons as explained in the next section. This is the fate Jupiter and Saturn -at least- will experiment when our Sun goes red giant, five billion years from now.

Inner-system dwarf planets are found in asteroid belts and are essentially small rockballs. Our system has one: Ceres. Other stars may have asteroid belts in their outer systems that contain icy dwarf planets. But the most common places to find dwarf planets are Kuiper belts: orbiting discs of primordial icy debris that never coalesced into true planets, found in the dark and cold outer fringes of star systems. Our Solar system has a Kuiper belt, and several have been confirmed around other stars. Kuiper belt dwarf planets are very cold and covered by frozen nitrogen and methane that could be their atmospheres if they were a little warmer. In our system, Pluto is a typical example, along with Eris, Haumea, Makemake and the most extreme of them, Sedna, which ranges out to a hundred [=AUs=] from the Sun at the farthest point of its orbit.

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This UsefulNotes page deals with planets, moons, dwarf planets, or any similar big and round objects hanging out in space, both solar and extrasolar, the types of them, their properties and how to avoid obvious mistakes when making ones up.

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[[folder:Recipe: How to build a planet]]

!!Ingredients, or what are planets made of?

As far as scientists know, most planets are made of 4, maybe 5 main types of materials. Minor components may be mixed in, but planets will usually be made of layers or mixtures of:

* Metals. Mostly iron and metals that dissolve well in Iron. Found in the cores of rocky planets and moons including Earth, and many meteorites/asteroids/other small bodies are mostly Iron. This is the most dense material.
* Rocks: Rocks have a variety of chemistries, with silicate based compounds most common, often ionic as the most common. This is usually the second most dense material.
* Volatiles/Ices: Lighter compounds including Water, Ammonia, Carbon dioxide, monoxide, hydrogen sulfide, and a number of what are normally gases or liquids on Earth. Less dense than rock.
* Gases: Hydrogen and Helium mostly, also some minor low boiling components like Neon.
* Carbonaceous materials: Carbon planets are a hypothetical types of planet formed when carbon outnumbers oxygen in planet formation. If this happens, lots of carbon based materials: sludges, char, tars, and such will be found throughout the planet.

Why these materials? Why not, say, a planet made of solid gold, or a uranium core with an aluminum shell? Or a purely dry ice planet? The reasons are a combination of elemental abundances, chemistry, and phase separation.\\
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In the universe, the vast majority of all elements are hydrogen and helium: remaining lighter elements are formed by fusion in usually older stars. As described in UsefulNotes/Stars, stars through most of their lives fuse hydrogen into helium and stop there, only when the star runs out of hydrogen in the center do they start to create heavier elements. Going down the periodic table, Lithium Beryllium, and Boron are often consumed in nuclear reactions, so as stars start to fuse helium at the end of their lives, carbon, oxygen, and some nitrogen become the next elements created, Many stars stop there, only heavier stars continue to fuse further, creating sodium, magnesium, aluminum, silicon, etc. As the elements get heavier, they get less common, and generally even numbered ones are more common. At Iron, something unusual happens: Iron is the most stable element, with the lowest energy per nucleon. Fusing iron absorbs energy, as does breaking it down. As a result, Iron is the stopping point for stars, and it is much more common than its atomic number would suggest.\\
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Heavier elements are created in supernovas, or in neutron star collisions, which are quite rare.\\
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The result of all this together: Hydrogen and helium are the most common, to a large excess, next more common are Oxygen, Carbon, Nitrogen, with the next row of Sodium, Magnesium, etc. following, and Iron also somewhat common, with other elements being rare. Now, chemistry takes over.\\
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Hydrogen is available in a large excess, as is helium, and helium doesn't react with anything anyway. Oxygen, Nitrogen, and Carbon can react with hydrogen and somewhat each other: the resulting chemicals freeze at roughly similar temperatures (compared to anything else common) and often dissolve in each other, creating the volatiles/ices. Joining them are some compounds of sulfur, phosphorus, and a few others. Silicon and many lighter metals are very energetic when binding to oxygen, forming rocks. Iron usually travels with rocks in space, but when combined into a planet, excess Iron can separate out, forming its own material. If Carbon is more common than Oxygen, the excess carbon can form its own wide array of compounds, as you can see in crude oil, tar, or overcooked food char and grunge when cooking: this is what may make carbon planets.\\
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Finally, phase separation takes place. Each component condenses/freezes at different temparatures, and has a different density. Gases are light and condense at extremely low temperatures. Volatiles are denser and condense at higher temperatures. Rocks are denser and condense hotter still, and Iron is the most dense, though it melts at lower temperatures than rocks. Like also mixes/dissolves with like: Anything in gas form can mix, Volatiles can bind together, rock chemistry is somewhat flexible. Minor elements tend to bind to whatever major component they mix with best: Neon, for example, is low boiling and unreactive like hydrogen and helium, Uranium is very reactive in a similar way to light metals and mostly ends up in Rock, metal sulfides and some metals mix well with Iron. On Earth and likely other rocky planets, the [[https://en.wikipedia.org/wiki/Goldschmidt_classification goldschmidt]] classification gives a rough guide of which elements tend to go where. As a result, planet forming materials separate into the 4-5 types of materials described above. Within a planet, these materials are often separated in different layers, lighter towards the surface, heavier inside.\\


!!Cooking Directions: How planets form

To the best of our knowledge, planets form along with stars. Their formation begins with gas and dust clouds in space. If these clouds get dense enough, or something disturbs them (supernova shocks are common guesses), their gravity is strong enough to pull them inward. As it collapses, the cloud does two things: heat up, and spins. The heat comes from gravity: As the cloud is pulled together, the gravitational energy of material falling in must go somewhere: friction and compression mean a lot of it ends up as heat. The spin could come from outside gravitational or friction forces, or from random motion: if its motion adds up to any kind of rotation at all, it will cause the cloud to spin up as it is pulled in (see the common analogy of an ice skater/dancer drawing arms in).\\
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Most material in such a collapsing cloud will go to the central star, or a couple of central stars, but the fact that the cloud is spinning means some will orbit instead. This orbiting material forms what is called an accretion disk: centrifugal effects mean that the gas and dust is pulled outward at the equator of spinning, and flattened. The center is hotter: infalling material has fallen farther, releasing more gravitational energy, and is nearer the hot still-forming star in the middle. Gas fills the space in the disk, but solid material forms dust particles, and these particles are what will form comets, asteroids, planets, and any other bodies.\\
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Smaller dust grains clump together, mostly from friction or static electricity, forming somewhat larger, boulder sized particles. Scientists are still not sure how these larger particles stick together, collisions would be expected to destroy them faster than they form, but obviously something must stick together to form planets. When the material gets large enough, gravity gets strong enough to hold the bodies together. Some of these bodies will stop growing and form comets, asteroids, or other small bodies. Others keep growing, becoming so called planetesimals. As bodies grow larger, their gravity effects the orbits of anything further and further out, these destablizations lead to further collisions, throwing some material and bodies into the central star, other material and bodies into distant orbits, or ejections from the system. The end result of this process: large bodies that dominate their orbital regions, or smaller bodies in belts or clusters. The larger bodies are planets, the smaller ones dwarf planets, or asteroids, comets, etc.\\
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The material a planet is made of depends on where it is formed. Nearer the star, only silicates can solidify, and more rocky/metallic planets form. Further out, volatiles can more easily freeze, and become a larger mix within a planet. If enough material accumulates, a planet can pull in hydrogen and helium. Once formed, collisions and impacts can further change composition: volatiles may be brought in from further out regions, impacts can remove outer layers, and other changes occur.\\
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Moons can form in a number of ways. Some may be captured: impacts, or gas around a forming planet slow them down enough to be caught in orbit. Tidal interactions in a double pair of objects may split the pair, also capturing a moon. Large gas giants form their own accretion disks when forming, and moons can form in a similar way as planets.\\


!!Placement and Presentation: Location, Orbit, and Rotation

Planets, moons, and similar bodies revolve around a star or other larger body, and rotate as well. They don't necessarily orbit where they form, a big discovery in the past few decades is that planetary orbits can move quite a bit over time. Planet can fling smaller bodies in or out, or can interact with gas in their accretion disks, or each other, to change orbits and move closer or farther from a star. In our own solar system, it's thought that Neptune, Saturn, and Uranus have moved outwards from where they were formed. Small bodies thrown outwards make up the kuiper belt and scattered dick, Probably Oort cloud also. In other systems, gas giants are thought to move in, forming hot Jupiters. It is possible for some planets to get thrown out of a system, becoming rogue planets that float through interstellar space near no star.\\
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More orbits of anything are not exactly circular, elliptical is more common. Hugely elliptical orbits are found with some small bodies such as comets in our system, their outer points are thousands, millions, or even more time further out than their inner points. Most larger bodies, including planets, are more circular: a more elliptical orbit gives a great chance to collide with something, and big planets will attract each other to make such a collision more likely.\\
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Large orbiting bodies strongly effect smaller bodies nearby: part of the definition of a planet is something that clears (dominates would be a better term) its orbital neighborhood. Most obviously, larger bodies can have moons. The volume things can orbit in is called a hill sphere, a region where the smaller body's gravity dominates the effects of the larger body it is circling around. This sphere gets larger when the planet/orbiting body gets more massive, the star/central body less massive, and the further away from the star/central body. So, in our own system, big gas giants further away from the sun can have more moons than smaller planets further in.\\
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Big bodies also disturb smaller bodies further away. At so called lagrange points (stable points for a small body near two larger ones.), most planets in our system have so called "trojan asteroids". Jupiter shapes the asteroid belt, keeping it from clumping and keeping it in a band, Neptune plays a similar role with the kuiper belt, and possibly further out. Some moons do the same with planetary rings. However, two large bodies in the same orbit is a no-no, such systems are unstable, and most likely lead to a collision of some sort, possibly a change in orbits or ejection for one of the bodies relatively quickly.\\
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An important feature of orbits are so called resonances: where one body and anonther body complet whole numbers of orbits in the same amount of time. (for example, a 2:1 resonance means one body orbits once for each 2 orbits of the other). Resonances can do interesting things: transferring energy between bodies, making orbits more elliptical, avoiding collisions, are among the examples.\\
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Planetary rotation is not well understood. All (or 99.999...% of) bodies rotate, but the rotation speeds of many are not understood, apart from tidal locking (see next paragraph and the one after.). Most likely, bodies rotate in the same general direction as the whole system rotates when they formed, and being big, planets and moons don't change direction easily. However, gravity from other planets, or impacts, or changing mass distribution within a planet can change their rotation speed and axis.\\
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If two bodies are close enough, tides become an important effect. The cause: the slightly different gravy between near, far sides, and the section around the middle of the body, cause it to distort and become egg shaped with two bulges on the near and far sides, and the circle in the middle squeezed in. Earth tides are caused this way by the noon: the bulges are high tides, the squeezed circle creates low tides.\\
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Tides can cause tidal locking of moons, and planets too close to a star: as the moon rotates, a tidal bulge on it is moved away from the central body. the central body pulls back in the tidal bulge: the effect is to slow or speed the rotation of the smaller body until the tidal bulge is always facing the central body, which happens when the planet/moon rotates as fast as it orbits. they can also change the orbits of bodies in a similar way: if a body orbits faster than the tidal bulge in whatever it is orbiting, that tidal bulge pulls back, lowering the orbit and causing the central body to rotate more in the direction of that orbit. The opposite happens if a body orbits more slowly than a tidal bulge. Bodies in low orbits as a result will crash into whatever they are orbiting as a result, bodies in higher orbits may spiral away.\\
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Tides do a lot more interesting things, but that's found in the next big section.\\


!!Portion Size: How big is a planet?

Obviously, the more material a planet has, the more massive and, usually, larger it is. The exact size and mass depend on the type of material, the same mass of more dense material obviously forms a smaller planet (by volume) than one made of less dense material.\\
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Planetary density also depends on the size of the planet: larger planets compress their insides more, making them dense. This gets extreme at about Jupiter size: compression works so well that planets at about this volume stay at about a similar size as more material gets added. (They can get larger if heated, however, the heat expands their atmospheres.) Material inside can be squeezed and just get smaller, or can even chemically change under high pressure.\\
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A body's surface gravity is proportional to its mass, and inversely proportional to the radius squared. As a result, gravity increases with mass, usually, but not as quickly as the actual mass does. Planets made of less dense materials can have weaker gravity than more dense but smaller planets: examples in our own system include Uranus and Earth (Uranus is slightly smaller), and Mercury and Mars (Almost the same gravity despite mercury being about 1/3 as massive).\\
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Defining a body's radius, size, or zero altitude isn't as obvious as you might think. Rotating planets bulge at their equators, cause by centrifugal effects (The equator rotates further out than the poles, so centrifugal effects pull it further out), planetary radius's will be a bit higher near equators as a result. A solid body has mountains, valleys, highlands, and other higher and lower terrain, so some arbitrary altitude must be picked as the radius, based in some criteria (Average radius if a body is well mapped maybe the go to option). A Gas Giant may be either no surface, or a surface so deep that you'd lose a huge fraction of the planet by defining it there. For our own solar system, the radius where atmospheric pressure equals earth pressure is used. Atmospheres don't have an upper edge, they instead smoothly get thinner and change into whatever is in surrounding space, so are not any help. Planets with large oceans have the easiest measurement, simply measure an average sea level. (This is how Earth altitudes are done.)\\

[[/folder]]

[[folder: Volcanoes, Weather, Life, and other features of planets]]

!!LethalLavaLand or Geologically Dead? Volcanoes, Quakes, and Tectonics

To have geological activity, a planetary body must have a couple of things: an internal heat source, and to be made of the right materials. Obviously, a planet must have a solid surface to have geology: internal heat escaping in a liquid or gaseous planet creates ocean currents or weather instead. The material is important in another way: material that softens and melts at lower temperatures means less heat is needed to create geology.\\
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Where does internal heat come from? Scientists know of three major sources today. A heat source every body has is heat from formation. The heat released from a planet sized mass of material pulling itself together is enormous: to get an idea, think of a reentering spacecraft or asteroid impacts, the heat released when going from space to settling on a planet could vaporize or destroy a spacecraft if not properly protected. Larger impacts create craters, and even melt material. When a planet forms, a good amount of this type of heat gets contained within.\\
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Closely related is heat from differentiation: if a planet is warm enough, heavier material can sink through lighter material, releasing gravitational energy that further heats the body. Related chemical and physical changes can occur through the lifetime of a planet, material can freeze or melt, dissolve or come out of solution, and the resulting material can sink or rise, often resulting in further energy release. The freezing of Earth's core, for example, releases some energy.\\
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Radioactivity is the second well known source of heat. Radioactive elements tend to concentrate in rocks, so within a system, the more rocks a body has, the more radioactive heat it generates. The amount of radioactive material likely changes from system to system, depending on the history of where it formed, and how many heavy elements were around when it formed.\\
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Most complex is tidal heating, requiring three things: a body with large tides, an elliptical orbit, and material that can deform but resists deformation (hot rock, not to cold ice, and most liquids all work). If a body orbits elliptically, the size of the tide will change as it obits, and the tidal bulge will move back and forth a bit as well (the body rotates at a constant speed, but orbits at a slightly changing speed, pulling the tidal bulge a bit ahead of or behind the central body as it moves). If made of the right material ,with some plastic or fluid properties, the resistance to this motion creates heat. To last enough time, some outside force must keep the orbit elliptical, otherwise this resistance would make the orbit circular.\\
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All else equal, larger and younger bodies should be more active. Larger bodies have something like the SquareCubeLaw occurring: Radioactive heating is proportional to mass (if made of the same substances), accretionary heating is proportional to the square of mass (more stuff pulled into the planet * more gravity pulling each piece in), tidal heating increases with radius to the fifth power (a few effects that add together). The amount of surface area to escape from, however, increases as radius squared, or a bit less than mass to the 2/3 power. Younger bodies haven't cooled as much from accretion, and their radioactive elements haven't decayed as much, generating more radiation.\\
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If this heat can melt material close to the surface, volcanoes form. If the material has lots of gas, or is under great pressure in another way, explosive eruptions or plumes can occur, throwing material great distances. with less gas,liquid can flow out more smoothly, creating lava flows, channels, tunnels, or other such formations. Material building up over time can build up mountains. Some eruptions might come at spreading areas or cracks, creating fissures or ridges.\\
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Internal heat also can create tectonic activity: if material inside convects, the movement distorts the surface. Rising plumes can create highlands, downward movement can create depressions, sideways movement can compress differnt regions to form mountains, or extend regions to form valleys or plains. This type of movement is a major source of quakes, as surface material is pushed and pulled, it can build up stress: the release of stress with sudden movement is a quake.\\

!![[Literature/JourneyToTheCenterOfTheEarth Journey To The Center Of The Planet]] Planetary interiors

Before we begin: the Earth's mantle (and that of most rocky planets, most likely) is solid, not magma. Pop culture often confuses this, for understandable reasons: it is very hot down there, magma does come from there, and lots of descriptions of flow and convection sure sounds liquid like. What is actually down there is a solid, but plastic material. Like a lot of materials, rock melts at higher temperatures under higher pressures; mantle rock would melt if brought to the surface, but is under enough pressure to stay solid. the rock is plastic, technically: it would feel solid to us if we could feel it, but over time and with enough force can creep (this is the actual materials science term), allowing convection. A similar material is tar, the [[https://en.wikipedia.org/wiki/Pitch_drop_experiment Pitch Drop experiment]] shows how a seemingly solid material can look liquid like over long enough periods. A good analogy is fudge: when cold, it feels mostly solid, but put something heavy enough and let it sit for long enough, and it will deform. [[Film/TheCore The Cor]]...uh, [[FanonDiscontinuity a movie that was not made in 2003 and does not exist]] had this as one of its many, many, many scientific issues, the liquid mantle also made some other phenomena even less realistic (not that they were that realistic anyway.)\\
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While rocky planets may have magma layers, it's likely most are mostly made of solid rock. The reason is that liquids flow easily, and transport heat very, very quickly: unless a huge amount of heat is produced, or the planet is recently formed, or close enough to a star to melt on the surface, a molten interior will very quickly lose that heat until much of it freezes and the resulting solid rock insulates the interior.\\
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How does anyone know what the interiors of planets are like? We can't drill directly and see: Even on Earth we've only gotten a few miles underground at most, let alone other bodies. Scientists have a few ways. Just knowing the size and mass of a planet tells you roughly what materials it is made of. Further details are provided by the shape of a body and the details of its rotation. Spinning bodies are slightly larger at the equator than at the poles due to centrifugal effects, the size of this bulge, and the change in gravity it causes, tell how material is distributed in a body. Tidal bulges can provide similar information, as can changes in rotation rate as a body interacts with other bodies. Finer scale gravity measurements can also give details of convection or temperature distribution, smaller than expected gravity over and area shows lighter, probably hotter, material is underneath it, the opposite for higher than expected gravity. A magnetic field shows that something electrically conductive is inside the body, tectonics shows heat exists, among other features.\\
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Sound waves or seismic waves can travel through a body and provide information. On Earth, seismic waves from earthquakes have been the main way to see what the interior is like: Different types of waves speed up or slow down as they pass through different material, or even stopped. Seeing how long waves from an earthquake (or [[BreadEggsMilkSquick nuclear test]]) take to reach different regions tells something about the material in between, with more powerful computers and enough earthquakes and seismometers, [[https://en.wikipedia.org/wiki/Seismic_tomography relatively detailed maps]] can be created. Sound waves can be used for gaseous bodies, the sun's interior was figured out in this way.\\
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If you can access the surface of a body, material from the inside is sometimes available. On Earth, mantle material has been brought to the surface in a few places through volcanoes or tectonics. Foreign material trapped in diamonds, which are formed in Earth's mantle, has provided some details of history and chemistry. On gaseous or liquid bodies, currents may bring deeper material to the surface.\\
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Finally, scientists may use physics/chemistry knowledge to find out what materials do inside planets. they may extrapolate from known material properties, or use quantum mechanics/atomic/molecular level calculations to see what materials do at the temperatures and pressures inside planets. Using special equipment, they can study materials directly: a common method these days is the diamond anvil cell, which uses diamonds to concentrate force to create extremely high pressures, than use laser heating if needed to heat the material. Diamonds are clear, allowing examination of the results. New types of minerals were found this way, with the right properties to explain seismic studies. Scientists can also copy from planet to planet: planets with similar compositions and sizes probably have similar interiors.\\
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Still, scientists might know what an interior is mostly like, but many details will still be unknown. Even on well studied Earth, scientists have only recently discovered evidence of high pressure ice in parts of the mantle. The existence of diamonds and jade (formed in the mantle) would be hard to spot from space. Scientists guessed that mantle convection existed, but the discovery of [[https://en.wikipedia.org/wiki/Large_low-shear-velocity_provinces two giant piles of....something]] was a surprise. If you are writing a story, the insides of planets gives much room for speculation. In Jupiter, the core was recently (from when this page was written) found to be mixed with metallic hydrogen above it, not a completely separate layer or nonexistent as had been predicted.\\
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So, what are planetary interiors like? Whatever a body is made of, the weight of mile/tens/hundreds of miles of stuff creates enormous pressures, often changing material properties. many bodies are also hot inside. Almost always the further in you go, the hotter it gets. (for a simple reason, things cool from the outside.) Only planets still forming might break this, since the inner material accreted as a smaller body with less gravity, it may be cooler than shallower material. Big enough bodies will have different layers of material. Some layers form as the same material changes under different pressures and temperatures. Others form from different chemicals: if a bodies forms from different material at different times, or gets warm and soft enough for heavier material to sink, and the body is not mixed afterward, that body will have layers of heavier material towards the center and lighter material towards the surface.\\
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Iron layers may be liquid or solid depending on the temperature and pressure it is at. Rocky layers may be liquid or solid as well. The minerals also may change deeper into a planet, the same mix of elements can form denser minerals as pressure increases, and change properties with temperature changes. Water can form liquid water or several types of ice. Higher pressure ices can stay solid at boiling temperatures on earth. Put enough pressure on water, and it breaks down, separating the hydrogen and oxygen and forming ionic or superionic water. Hydrogen when compressed becomes liquid, than metallic. Carbon under higher pressures forms diamonds, carbon reacting with metals or rocky elements forms carbides, either liquid or solid.\\
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Smaller components also do unusual things. At some pressures and temperatures, hydrogen and helium are thought to separate, the helium forms a separate layer. Volatile chemicals react in unusual ways, hydrocarbons may separate and form carbon, creating diamonds. Rocks may be able to hold water or other volatiles, creating resevoirs of such materials. Gaseous or water rich planets may have blocks of solid materials. Minor components are where the speculation can run wild; scientists can make some predictions, but data from gravity or surface features reveals little.\\
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If the interior of a planet is hot enough (how hot depends on the materials), it will convect. Convection can mix the interior of a planet, bringing material to the surface, and/or bringing surface material deeper where the hotter/high pressure conditions cause it to react in unexpected ways. On solid bodies, it creates tectonics, and brings heat to the surface to create volcanoes. In liquids and gases, it creates weather, and (probably) ocean currents.\\
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!!Something In The Air: Atmospheres

Can we breathe on a random planet? [[AllPlanetsAreEarthlike Are all planets Earthlike]]? The answer is almost certainly no, for a smple reason: Oxygen is an extremely reactive and energetic gas. Good for powering an energetic warm blooded body, bad for lasting a long time in an atmosphere. Carbon, hydrogen, [[BreadEggsBreadedEggs hydrocarbons]], most other organic chemicals, ammonia, carbon monoxide, sulfur, sulfides, silicon, free metals including iron, and many more chemicals will react within extremely short timeframes with oxygen if available. Even rock can react: some rocks are not as oxidized as they could be, and with lots of rock making up a planet, there is plenty to react with wisps of oxygen in the air. The only way to maintain an oxygen atmosphere is to continuously replenish it, and we know of only three cases: Earth, where oxygen is produced by photosynthesis in life, and Europa and Ganymede, with atmospheres in name only (detectable by instruments, still pretty much a vacuum.) where wisps of oxygen are produced by ultraviolet rays and radiation from the occasional water molecule sputtered off the surface by more radiation.\\
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Even with oxygen, the air may not be breathable. the air may be too thick or thin (too much oxygen is poisonous over time), or have other toxic gases.\\
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So what is in an atmosphere, and where can I get one? Many bodies are surrounded by gas, but to hold a decent atmosphere for a good length of time, a body must be large enough and cold enough. Gases are held in place by gravity, but can escape if molecules are given enough energy to reach escape velocity, or are blown off the planet in some other way. Larger planets have higher escape velocities (even if surface gravity isn't as high, the total energy needed to leave a large body's influence is higher than for a small body), meaning gas can be more easily trapped. Hotter atmospheres mean individual molecules have more energy, and are more likely to reach escape velocity. Atmospheres can also be exposed to other energy sources, such as radiation or impacts, that under the right conditions can cause atmospheric escape. In a sense, no atmosphere is really permanently bound: a few molecules here and there can always escape by random chance, but there is still a vast difference between a long lasting, thick, tightly bound collection of gas and a barely there, almost a vacuum, escapes as fast as it is produced gas envelope that some bodies have.\\
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The ease of escape also effects what kind of gases could make up an atmosphere. Colder and/or larger planets can hold hydrogen and helium. Warmer/smaller than this and these gases escape, but volatiles and gases made from them (nitrogen, methane, water, carbon dioxide, etc.) can be held. Vaporized rock and iron could conceivably exist if the body gets hot enough. This brings us to the other end of the scale: if too cold, an atmosphere will freeze out, although wisps of gas will always exist in equilibrium with the solid surface, and this gas will be bound to the body. Some outer solar system bodies have atmospheres like this, where nitrogen, methane, and/or other components freeze into the surface when farther away from the sun, and evaporate more when closer.\\
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Atmospheres can be formed in a few ways. Some, called primary atmospheres,form along with the planet. Gas giants in our solar system are like this. Earth, Venus, and mars have atmospheres formed mostly from outgassing: chemicals in the interior are released as gases and create one. Impacts can also create atmospheres, either bringing material themselves, or causing material in the body to be released.\\
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What gases are expected? If the body can hold helium and/or hydrogen, these gases will probably make up most of its atmosphere, being so common. Among the volatiles, nitrogen, carbon dioxide, methane, possibly water if hot enough are quite common, other gases are often found in trace amounts. The exact mix of gases will change from body to body, but chemical reactions will likely create these gases. Nitrogen mostly comes from ammonia, stellar radiation tends to break ammonia down into nitrogen, and nitrogen is itself quite stable, doesn't react easily, and doesn't react energetically with much. Carbon dioxide is similar, it is a relatively stable molecule. Methane and water can be broken down similar to ammonia, but are also quite common and often replenished.Other gases are usually more reactive, and therefore found in trace quantities, as they react very quickly after being formed. Iron or silicate vapors are not found within our system, nothing is anywhere close to hot enough except perhaps deep inside gas giants where we can't see anyway, so their properties are speculative if they exist.\\
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If some gases can condense, clouds can form from small particles of liquid or solid. All solid and liquid substances have a so called "vapor pressure": let a system go to equilibrium, and there will be some pressure of evaporated material in equlibrium with the solid or liquid stuff, freezing or condensing as fast as evaporation happens. If the concentration of a gas is higher than the vapor pressure, and has any kind of small particle or disturbance to condense on to, it will form a cloud. Since vapor pressure almost always decreases at lower temperatures, this most often happens when air cools. Clouds of water, ammonia, and methane have been seen directly, and ammonium sulfide, iron, and rocks are likely on some bodies.\\
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Closely related are organic hazes: These are created when organic chemicals react in an atmosphere, usually driven by photochemistry from starlight or ultraviolet rays, forming chemicals that are condensed at the temperatures they form at, creating a fog. On Earth, this process makes smog. Less damaging to respiratory systems, outer planets and some moons also seem to have hazes. Similar photochemical reactions likely help create Vanus's sulfuric acid clouds.\\
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And finally, we get to temperature. If you ever see someone arguing that global warming is exaggerated or doesn't exist, give them a good slap. The basic physics works not just for Earth, but also for any planetary body. The main challenge to models is working out the effects, Earth is a complicated planet with lots of systems effected (ice, life, etc.), but there is no question that some warming will occur, and the general predictions of most models are happening as predicted.\\
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The temperature of a planet roughly depends on the amount of heat coming in, and the amount going out. Planetary bodies gain heat from their stars light, and some from internal heating: Starlight is by far the most important for planets near a star, internal heating gets more important as you move further away. The amount of light coming in depends on how close a planet is to a star, how bright the star is, and how reflective the body is: bodies close to luminous stars with non-reflecting materials will absorb lots of light, bodies far from dim stars made of reflective materials will be very cold. Reflection can come from clouds and hazes, or from surface features if a planet has them. Particularly reflective are Venus's sulfuric acid clouds, and snow and clean ice (this is one of several reasons melting ice caps are an issue, losing them means more light gets absorbed).\\
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If a planet's temperature of stable, it is losing heat as quickly as it gains it. Space being the vacuum it is, heat loss happens by thermal radiation: the body gives off energy in electromagnetic radiation, the exact mix depends on temperature. Thermal radiation generates the light from stars (see [[UsefulNotes/Stars]]), at most planetary temperatures, infrared emission is the main form emitted. Greenhouse gases work their magic here: They absorb some radiation, than emit the energy in all directions, sending some back down to the planet. Being high in the atmosphere, they are also often colder, and emit less radiation overall. As a result, the planet emits less radiation overall, and heats up until a balance is restored. Due to the workings of molecules, most gases made of more than one element absorb infrared radiation but do nothing to visible light, meaning they retain heat on a planet and don't reflect anything, heating a planet without cooling it.\\
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Different gases absorb different types of infrared, and a single gas has diminishing returns in how much infrared it can absorb. Absorption is also effected by interactions with other gases nearby. the result is that a mix of gases absorbs more than the same amount of a single gas, and traces in an atmosphere can have a seemingly disproportionate effect on temperature. Without any greenhouse effect at all, Earth would be freezing, but slightly too much of one, and lots of issues happen.\\


!!A Dark And Stormy Night: Weather and Atmospheric Structure.

If you've ever been outside (not a sure thing for a TV Troper, obviously) or heard a weather report, you'll know that atmospheres do things. Wind blows, rain falls, lighting strikes, clouds form and disappear. Look at other worlds and you'll see even more exotic weather: storms lasting decades or centuries, bands of wind, turbulence.\\
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Ultimately, weather is driven by heat. Temperature differences of various sorts: lower atmosphere to upper, equator to poles, night side to day side are the main possibilities, are what ultimately drive weather. Our good friend thermal convection shows up here, if you read this entire useful notes all the way through, you'll notice convection is [[{{Understatement}} somewhat important]] in planetary science.\\
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Also important is rotation: this creates centrifugal effects and something called the coriolis effect. Centrifugal effects are the familiar "spinning objects tend to pull outwards". The coriolis effect occurs as air moves between different latitudes. While all of a planetary body rotates at the same rate of spinning, the actual speed the planet moves depends on the latitude: near an equator, the surface moves the fastest, it has farther to go in the same time period. he poles are the slowest, staying in the same location as the planet spins. Air moving towards the equator keeps the same velocity, but appears to by moving slower compared to something rotating with the planet. The opposite happens as air moves poleward. Combine centrifugal and coriolis effects, and moving winds tend to rotate, atmospheric scientists model this by pretending an extra force exists, defecting air as it moves.\\
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Temperature differences between lower and higher in the atmosphere cause the familiar bouyant convection you know about: hot air is less dense than colder air, so tends to float upwards. This rising air draws in surrounding air, which creates wind at whatever level it is rising from. Horizontal temperature differences also create winds: because air is less dense, less hotter air is above a particular point than equivalent colder air. The change in air pressure between different heights is equal to the weight of air between the two heights. A column of hotter air, as a result, has slower pressure changes than a column of cooler air, this means that at some point in the atmosphere, a pressure difference will exist between air of different temperatures. This pressure difference will drive winds. Actual examples include day to night winds, and winds between the usually hotter equator and the colder poles.\\
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Once winds are created, the Coriolis and centrifugal effects cause them to deflect. Air drawn in to the convective system described above will rotate as it is drawn in, and leaving will also rotate. Winds moving towards the poles will deflect towards the equator, until the pressure difference caused by different temperatures balances the coriolis effect. Winds can also shear against each other, creating turbulence and more rotation. These, and other coriolis effects, cause lots of rotating vortexes in an atmosphere: if allowed to spin ,these vortexes can transfer energy into each other, sometimes merging and becoming larger, sometimes driving further winds.\\
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Heat sources can include both internal heating and heating from a central star. The strength of the winds does '''not''' simply depend on the amount of energy available: in our own system, Earth probably gets the most energy per atmosphere, but has relatively weak winds. Having a solid surface obviously slows winds through friction, gas giants have longer lasting and stronger winds than solid bodies in our system. It also may be that more heat creates turbulence that disrupts winds and slows them down. Thicker atmospheres also experience more friction, and are harder to get up to speed as well.\\
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Rising and falling air doesn't stay at the same temperature. Air pressure in an area is equal to the weight of air above it: as height increases, less air is above that height, and the air pressure is lower. Rising air as a result is moving from higher to lower pressure: the pressure decreases, the pocket of air pushes the surrounding air outwards, doing work. The energy to do this comes from their air's heat, as a result the temperature of the rising air drops. The opposite happens with falling air, which heats up. The expected temperature change with height is called a lapse rate: in an active atmosphere, with lots of rising and sinking air, the temperature at different heights will follow the lapse rate, cooler up high, warmer down below, the change occurring roughly at a constant rate with height.\\
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If their air has something that can condense, it forms clouds when cool: rising air cooling down is a common location for clouds to form. Condensation or freezing has another effect: it releases heat. As a result, rising air with condensation doesn't cool as quickly as dry rising air. one resulting effect: if the cloud remains behind and the air packet is forced back down in some way, it gets compressed and heated to a higher temperature than a dry air packet following the same path. This has been seen on Earth and jupiter, it creates so called chinook winds on Earth. If the air is allowed to rise, and has enough condensable material, it's upward speed will increase, generating an extremely tall cloud with strong winds. In other words, a storm.\\
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Which brings us to lighting. Lightning is effectively very powerful static electricity, and is probably generated the same way, by two types of particles rubbing each other in an atmosphere, becoming charged, and being separated. the different charges build up an electric field, is something lets electricity flow, it discharges to create the lightning. On Earth, water ice and liquid water in thunderstorms cause this. On other planets, we're not sure of the source, though it has been detected. It's probably safe to say that a planet with lots of clouds and active weather has some sort of lightning.\\
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Most weather happens in the lower atmospheres of planets. As height increases, the air thins, and the air gets hit with radiation and incoming sources of energy. At some point, this energy is enough to heat the air up significantly, and temperature decreases less, or stops decreasing with height. Hotter air above colder air does not allow convection: rising air becomes heavier than surrounding air, and sinks, higher air that sinks becomes much warmer and lighter than surrounding air, and is forced up again. At around this height in the atmosphere, most weather stops, winds still exist but activity is otherwise slower and calmer. The structure of the atmosphere at this point depends on the amount of radiation absorbed and the resulting temperatures.\\
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Atmospheres don't really have a ceiling, they instead smoothly get thinner, and change to resemble whatever is in surrounding space. At some point, an atmosphere gets thin enough that winds cannot mix it, an the chemistry begins to change significantly with height. As the air gets thinner and more radiation comes up, most atmospheres start to heat with altitude above a certain point. Radiation also ionizes some gas, and their air is thin enough that recombination doesn't occur quickly. This air can conduct electricity, and is called an ionosphere, and a number of interesting things happen here.\\

!!AlienSky: What does the sky look like from a planet?

As the trope describes, and alien sky is a great way to show you aren't on Earth. What would the sky from other bodies actually look like?\\
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If a planet is covered in clouds or haze, probably pretty boring. You'd just see the haze, of whatever color. Possibly it would be lit if enough starlight got through, or dark is facing away, light may also be diffused throughout the clouds, reducing the day/night contrast. Rain of some sort may fall, or lightning if a planet had some.\\
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Clearer skies make life more interesting. Clouds on other planets, if they exist, are probably variable like they are on earth, with lumpy, flats, filaments, and other familiar shapes. Planets with multiple types of clouds might have different colors in sky sky, see Jupiter's colored bands and turbulence and imagine from lower down. Faster winds if they exist may visibly distort or move clouds, and turbulence would create interesting sights also.\\
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The day or night sky in a clear atmosphere, or a nonexistent one, would allow views of astronomical objects. Different types of central stars would appear to be different colors, see [[UsefulNotes/Stars]] for more. Obviously, stars would appear larger and brighter the close a body was. Binary stars, depending on the size of their orbits, would appear to be separate bodies, more or less close to each other. A rogue planet would obviously have no nearby stars, the sky would always appear like a night sky.\\
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Other nearby bodies would appear in the sky much like Earth's moon does. A gas giant would appear large in its moon's sky, though depending on orbital distance might not seem that large. Rings, unless looking from the equator, would fill a large chunk of their sky if visible, or would block out stars if not.\\
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The stars seen from a sky would change from system to system. Planets in the space between galaxies would have little visible, just faraway galaxies. Planets in a galaxy would see it in the sky much like the milky way is visible from Earth (in rural areas without too much light.) Planets in star clusters would have a very bright night sky, possibly with gas visible.\\


!!Ring Around The Rosie: Planetary Rings

When you think of Rings, you probably think of Saturn's big bright disk. However, all gas giants in our system have rings, Jupiter, Neptune, and /Uranus's are smaller and made of darker/dustier materials. All rings are made from dispersed small bits of material: dust, ice particles, carbonaceous material. The material can come from a number of sources: a destroyed body, impacts, material already floating from outside the system, eruptions, anything that produces dusty material. The particles as they orbit clump together and get knocked apart by impacts.\\
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Rings exist because of tidal forces. Normally, a ring of material around a body would come together into a body, this is how planets and moons originally formed. However, within a certain distance of a body, tidal forces are stronger than the gravitational force holding such a body together. These tidal forces could destroy a body that gets too close (It's thought that Saturn's rings may have formed this way), and also prevent a body from accumulating around a planet. This distance is larger the higher gravity from the central body, and also changes with material (A denser material resists tidal forces more, it is smaller for the same mass, and this means lower tidal forces and higher surface gravity to keep the orbiting body together.)\\
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Once formed, rings can be organized and energized by resonances with other orbiting bodies. Moon can act something similar to Jupiter with our system's asteroid belt, interacting with the dust particles, keeping them together, and adding energy to the system. Planetary rings will orbit near their equator, or near the plane of whatever system they are in. A planet's equatorial bulge pull ring material towards it, material also tends to align with a system's plain through gravity.\\
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Ultimately, rings are expected to decay over time. some material may evaporate, or get changed through radiation. Rings also create drag on each other: ring material orbiting slightly further in moves faster, further out moves slower, and these materials drag on each other, slowing the inner material and causing its orbit to drop, while further out material is pulled faster and its orbit moves further out. These effects cause rings to spread out, eventually, material will collide with something, merge with a central planet, and overall become too faint or dispersed to be a structure anymore. This process will happen faster in heavier ring systems, lighter in fainter/dispersed ones.\\


!!Ahoy, Mate! Oceans, Rivers, and other surface liquid

Of the common phases of matter, liquids exist at the narrowest range of conditions. Below a certain pressure or temperature, only solids and gases can exist, lower pressure/higher temperature leads to gases, higher pressure/lower temperature to solids. Above this temperature and pressure, liquids can exist in a range of temperatures between the boiling and freezing points, both of which change as pressure changes. However, above a certain temperature and pressure, liquids and gases merge into something called a supercritical fluid, which does not act completely like a liquid.\\
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The smaller window of conditions means surface liquid isn't likely as common as atmospheres or fully solid surfaces. To have such a liquid, a body must have an atmosphere with a high enough pressure, have a temperature not too high or low, and have enough material that can be liquid at the body's conditions. How much material exists determines what liquid features are found. Small amounts give river, creeks, and such. Larger amounts give deeper and deeper oceans. If the liquid makes up a large fraction of a planet's mass, the ocean is itself a major layer of the body as a whole.\\
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Most likely, this liquid will be a volatile of some kind: their liquid ranges are the most likely to exist on the surfaces of planets. Our own solar system has water oceans and rivers (on Earth), and hydrocarbon rivers and lakes (on titan), these are likely common materials elsewhere: water is very common in the universe, and carbon compounds are also common, with a variety that can liquify at very different conditions. More exotic materials like ammonia can also be liquid at the right conditions, but is easily destroyed by photochemical reactions, so would need some protection or replenishment to last long. Magma oceans are a lot less common, but certainly exist somewhere: forming rocky planets will have a magma ocean if large enough, thanks to their heats of formation/impacts raining down. How long this ocean lasts is still not known for sure, ranges of hundreds or thousands to millions of years are estimated by scientists. Planets very close to hot stars would have magma oceans also, but amount of orbits where this an occur is very small. Iron and hydrogen/helium oceans are far less likely: iron can melt, but is so heavy it will sink to the middle of a planet unless it is all metal. Hydrogen/helium boil at extremely low temperatures, and if they condense out, no other materials exist to maintain an atmosphere to create needed pressures for liquids.\\
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What do surface liquids do? If rivers exist on a solid surface, they erode the surrounding terrain, creating channels and valleys. On earth, rivers tend to start as creeks and small flows, merging into larger structures as they flow to low lying terrain, and this shape likely occurs on other planets as well. Liquids can also chemically interacts with surface solids: preferentially dissolving some materials, possibly chemically reacting with others, and creating new types of material as a result.\\
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As bodies of liquid get larger and deeper, they are able to carry large amounts of heat. Liquids will not flow as fast as gases, but being much denser they can carry a large amount of heat in a smaller space. On Earth, this creates weird weather changes (El Nino is the most famous), and oceans help transfer heat from the equator to the poles. Our good friend convection will create currents in an ocean, driven by temperature differences on the surface or by internal heat, other currents may be created by wind or tides. Large bodies of water also chemically influence and are influenced by their atmospheres and deeper processes. Absorption and release of gases buffers an atmosphere, removing gases that dissolve easily, and dissolving deeper materials or volcanic gases effect the oceans and change materials further down.\\


!!Dust to Dust: Soil, Erosion, Glaciers, and other solid stuff

Planets made of solid material are not necessarily completely solid on their surfaces: instead, powders/dust are common. Some solid material is due to weathering, the material being broken up in various ways. Impacts from space break down material on any planet. Wind and surface liquid help as well on planets with these hings. Temperature changes may also break things down through thermal expansion and contraction.\\
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An unusual form of erosion is found on some icy moons: darker areas absorb light and heat up, while lighter areas reflect light and stay cool. ice evaporates from the darker areas and deposits in the lighter areas, leaving other (often darker) chemicals behind, weakening the structure and further darkening it to continue he process.\\
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(In technical terms, weathering is the breaking down of material, while erosion is the carrying away of material. Just in case you are talking to the scientists in question.)\\
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Surface dust or soil may not be the same material as the bulk of the planet. Chemical reactions may produce new materials that coat the surface, atmospheric aerosols are a common way to do this. Material can freeze, producing equivalents of frost or snow if conditions are right. Life on Earth helps create soil, and may do so on other worlds with life as well.\\
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Dust on the surface may simply build up, creating a layer on top of other material. If an atmosphere exists, wind will transport the dust: This can create dune or other structures if wind is constant enough, sometimes dust storms form with strong enough intermittent winds and light enough dust. Dust can participate in weathering: wind, glaciers, liquids, or anything else carrying it causes the dust to hit material on the surface, sometimes breaking it apart, or to scrape against nearby material. Given long enough, built up dust can form new rocks: pressure and/or heat may bind the particles together.\\
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A more dramatic form of movement of called mass wasting: in common terms, and avalanche or landslide. This can happen any time solid material is undermined or builds up in too big a pile: if the structure is weakened in some way, material will slide down to some sort of lower point. Mass wasting, given enough time, can erode large structures, flattening the terrain out. The evaporation of ice on icy moons has eroded craters in this way, for example, as the leftover debris cannot support itself as well.\\
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And finally, there are glaciers and ice caps. On Earth, glaciers are water ice on a rocky planet, but other worlds can have their own version of a lower melting/evaporating material forming large structures. Local ice can build up in colder areas: Poles, shadowed craters, mountains are common sites in our system. If the ice is on a higher place, it can creep (This is the actual term from materials science, the material deforms in the direction of force. This is the same process causing convection in solid planetary interiors), flowing over time. Glaciers can make *fantastic* eroders and weatherers: being solid, they can hold dust and larger blocks of material much better than liquids or atmospheres, and scrape away almost anything underneath them. Ices, if more or less reflective than other surface material, can influence weather, forming cold or warm spots which can create winds, even cooling or warming an entire planet.\\


!!Giant Rocks from Space: Impacts

Sometimes, a planet encounters something else in space. Sometimes, these two things get quite close together. Sometimes, thy get close enough to meet, and given speeds in space and the force of gravity, these meetings are very, very rough. How rough depends on the speed of impact, the size of the objects, and any gravity between them that pulls them together faster.\\
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Impacts from space are extremely energetic: to get an idea, think of a reentering spacecraft, most must be shielded to prevent them burning up. The energy comes from a combination of Earth's gravity and the speed of the orbit (A spacecraft coming back from another world would have even more energy). Gravity may be smaller or larger on a planet (but still pretty strong), while speeds in space are almost certainly greater (remember, most spacecraft are orbiting Earth and moving with it as a result, a body moving in a star system is most likely not moving with a planet, and that difference in direction and/or speed creates a larger speed difference, adding energy to the collision) If you are reading through this entire useful notes, the energy of impact is how accretion energy/heat of formation is added to a planet.\\
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Impacts can range from little dust grains though planet sized bodies. Small enough objects will burn up in a planet's atmosphere if it has one, or if moving slowly enough, may more gently become some of the dust that probably already exists. Smaller impactors that reach the surface contribute to weathering, hitting the ground and helping to break it up.\\
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Make an object large and energetic enough, and it can form an impact crater. Craters are not simply formed by the impactor punching downward, it is instead an explosion. the fast moving impactor quickly compresses anything in its way, creating a shockwave. The compressed material than explodes outward, throwing stuff out of the area surrounding the impact. Hit with enough energy, and some material vaporizes or melts. If thrown hard enough, material may leave the planet: as likely, it falls back to the surface, possibly creating rays leading away from a crater, possibly coating the surface far away with ejected material, effecting the entire planet in a large enough impact. Shock waves from the impact will travel through the planet, possibly creating quakes as they pass, and possibly disrupting the opposite side of the crater as the shocks converge.\\
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Hitting liquid or atmosphere obviously doesn't leave a crater, but is energetic all the same. Breaking up in an atmosphere is effectively like an air exploded bomb: impacts of Jupiter create visible fireballs and smoke clouds afterwards, and Earth has had its own explosions in the [[https://en.wikipedia.org/wiki/Tunguska_event Tunguska Event]] and [[https://en.wikipedia.org/wiki/Chelyabinsk_meteor Chelyabinsk meteor]] (Meteors love Russia I guess. A large land area does mean it is more likely to get hit) Hitting a liquid will vaporize and throw liquid over the surface, and create enormous waves from the impact site.\\
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A general rule for craters: the more craters a planet has within a system, the less other stuff (volcanoes, weather, glaciers, etc.) it has. Craters form at a rate that does not depend on anything else happening on a planet (apart from, say, atmospheres burning up smaller impactors), and build up relatively slowly, giving time for other processes to wear them down/fill them in/smooth them out. A dead, airless body shouldby a ball of craters, a highly active planet should have almost none.\\
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Two planet sized bodies hitting each other is a [[ApocalypseHow Planetary level Apocalypse event]]. Such srtong shockwaves and so much heat are released thatanything on a surface or atmosphere (If the planets have one) are destroyed, material will melt and/or vaporize all over. Large amounts of material will be ejected, but often not as much as you'd think: gravity is strong enough that such collisions likely lead to mergers rather than destruction. It's thought that collisions like this helped form our moon, broke up Jupiter's core, and changed the rotation rate/angle of Venus and Uranus. Merging bodies obvious can have different compositions than the original bodies did, both by mixing and by the impact evaporating more volatile material and throwing it into space, or throwing material closer to the outside of the bodies into space.\\
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Back to a smaller scale, craters can have different forms depending on how strong the impact was, how strong the material it is made from is, and other material properties. Lower energy impacts form simple bowl craters, more energetic impacts can form peaks in the center and/or rings within the bowl, called complex craters. the process is similar to a water drop falling in water and producing ripples and a peak, which got frozen in place, though the exact mechanism is a bit different. Craters in stronger material will of course last longer, craters in weaker material will tend to slump over time, leaving flat circular features if they slump far enough. Even if eroded or slumped, traces of the impact still exist underground, shocked minerals and other circular patterns will show that an impact happened.\\

!![[Franchise/PiratesOfTheCaribbean A Compass that Doesn't Point North]] Magnetism

Wnat to trip some people up? Question: Where is Earth's magnetic north pole located? answer: Near the south pole, obviously. Remember, the north magnetic pole of a magnet is the one that points north, magnetic poles are attracted to their opposites, so the geographic north pole has a south magnetic pole. Confusion over.\\
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Many astronomical bodies are magnetic, planets included. In our own system, all planets have magnetic fields apart from Venus and Mars, and Jupiter's moon ganymede does as well. While some rocks, and iron, can be magnetized permanently, scientists think the large planetary magnetic fields are generated in a different way. The interiors of planets are hot, and too hot magnetic materials can't hold a field. Instead, planetary magnetic fields are generated through dynamos.\\
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These dynamos require two things: a rotating planet, and a conductive liquid inside the planet that is convecting. the magnetic field is generated by electric currents. Move an electrical conductor through a magnetic fieldin any direction not along the field lines, and an electric current is produced. This electric current generates its own magnetic field, this field generates currents in conductors that flow through it, etc.: organize everything correctly, and the result is a much larger magnetic field. Inside planets, conductive fluid supplies the conductive material, and convection keeps it moving (generating an electric current requires energy, which comes from the electrical conductor, which slows it down unless something forces it to keep moving.) Rotation organizes the flow: convecting material that isn't rotating would probably produce random fields that would cancel each other out instead of adding together.\\
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The result is a magnetic field that roughly aligns with a planet's rotation. However, convection is still somewhat chaotic, and this is where an odd property of conductive fluids makes its mark: Magnetic fields and the fluid they are in tend to move together. Either the field pushes the fluid to flow with it, the fluid pulls the field lines to move with it, or both. (Why this is takes some timeto describe, and this useful notes is long enough as is.) In fact, you could imagine the magnetic field in a conductive fluid as an object itself, with elastic field lines pulling, pushing, and being pulled and pushed, against the fluid it is in. Convection forces some fluid to move past field lines, but also drags the magnetic field with it, resulting in a field that doesn't quite match the simple dipole you think of from a bar magnet.\\
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Magnetic field as a result don't exactly match the axis of rotation: in our own system, this ranges from the relatively close field of Saturn to the around 50 degree offset fields of Neptune and Uranus (think a quarter to a third of the way from one pole to another). Monitoring this offset is important to navigators on Earth (or was when Compasses were common, at least): Compasses really don't ever point exactly North, and the exact direction shifts from year to year. Magnetic strength also varies from the smooth decrease and increase expected from a simple dipole; scientists use more complicated shapes (4 poles, 6 poles, etc.) added together to model the actual pattern. Occasionally, a reversed field may show up in some Earth areas; pieces of Earth's oceans have had such fields at some points. Earth's field is known to have reversed in the past: We don't know if other planet's fields have also reversed, but our sun does as well, so it is plausible.\\
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The presence or absence of a magnetic field tells something about a body's structure. Just about everything spins fast enough (even relatively slow Venus: currents in Earth's core move at about a few miles/kilometers a year, much slower than the thousands of kilometers a year rotation speed Venus would have), so the presence of a magnetic field shows convection and a conductive fluid, while the absence of a field points to the absence of a conductive fluid, or no convection within such a fluid. Venus and Mars lack fields while Earth does: It's thought that both have little activity in their mantles (Venus is too thick due to a lack of water/thick crust stopping heat loss, mars has cooled too much), meaning little flow, little heat lost from their conductive cores, and little convection compared to Earth. Jupiter's moon Ganymede has a field, while similar moons Io, Europa, Callisto, and titan do not. It's thought that Callisto and Titan haven't fully differentiated and no conductive iron layer exists: Io and Europa have such cores, but so much heat is generated tidally nearer the surface that the outer layers are hot and keep their cores from losing much heat.\\
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Once generated, a magnetic field extends far outside the planet, and organizes the space around a planet. This space is not completely empty (Really, no space is, matter just gets continually thinner and thinner, though its all a good vacuum by our standards, and matter gets ridiculously spread out at some locations), it has enough ionized material to conduct electricity. This ionized material forms a plasma, which conducts electricity, forming an electrically conductive fluid. The magnetic field and this plasma interact with each other, and like in cores, the magnetic field lines and the plasma tend to move together. Near a star, stellar winds are also a plasma, this wind drags a magnetic field out from the star, and blows the field of a planet into something like a comet tail shape, with a sperical section facing the star, and a long tail trailing away. Inside this area is the planet's magnetic field, outside is the field of the stelllar wind.\\
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It is said that a magnetic field protects from atmospheric loss by blocking a stellar wind, but this may not be true depending on the field. It blocks materials from directly flowing to a planet, but also captures energy and material from a much wider area and can channel this to a planet. Earth, Venus and mars are thought to lose atmosphere to solar wind at about the same rate, even though Earth has a field and the others do not.\\
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Magnetospheres do a lot of interesting things, but there are a couple that are more visible and important for anyone near a planet. Radiation belts are one of these: Magnetic field trap charged particles (Uniform fields cause changed particles to move in circles, planetary fields are not uniform but can still trap particles in some locations) Energetic particles are a type of radiation, and the high energy particles trapped can damage electronics, expose living things to radiation, effect material that travel through them, among other effects. the material in a magnetic field that forms this radiation has a number of sources: Stellar wind material can enter a magnetic field, a planet's atmosphere can shed a little bit of material, and moons can produce material. Anything that produces gas that can be ionized will work.\\
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The other effect is Auroras: glowing lines or curtains of light in the upper atmosphere. These are produced when energy added to a magnetic field produces electric currents. Currents moving across magnetic field lines are defected, but currents flowing along a field line can flow with no problem. Energy deposited at some point in the magnetic field as a result can flow along a field line, hit the upper atmosphere, and cause the gases there to glow. Since most field lines in a magnetosphere link to the polar regions of a planet, that's where auroras happen. Aurora color and appearance depends on the gas in the atmosphere, and on the amount of energy deposited.\\
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Sources of energy include stellar winds: as a stellar wind moves past a magnetic field, it can create turbulence as a non-plasma fluid does: this turbulence mixes plasma into the magnetic field and also deposits energy, which is stored in the field and released over time. The planet's rotation also means that the stellar wind's magnetic field and the planet's can sometimes interact, allowing material to flow into the planet's field. Some energy can come from a planet's rotation: a magnetic field rotates with a planet, and forces plasma to rotate with it, energizing the plasma. Most of the time, this rotation rate is faster than orbital speed, meaning plasma is pushed outward by centrifugal effects, where it cal be further energized. This energy is transferred by electric currents from the planet, and these currents generate auroras. Moons can help produce auroras as well: they can generate plasma, and/or disturb existing plasma, these disturbances can generate their own currents.\\

!![[{{Literature/Frankenstein}} Its Alive!]] Life

We're searching, but we haven't found it yet outside of Earth. Probably the biggest feature in media, with forest planets, grasslands, [[StarfishAliens weird aliens]], [[{{4X}} galactic empires fighting]], etc. all a form of life found elsewhere.\\
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So, what conditions lead to life? The answer is.....We don't actually know. (shrug shoulders). Obviously, Earthlike conditions work, with water, organic chemicals, a source of energy, but whether there are more conditions needed and/or other chemicals could do the job is unknown.\\
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Part of the issue is defining life, and figuring out what features are necessary to the stuff weird stuff walking and crawling and growing all around us (and [[TomatoInTheMirror Are Us!]]) Self organization, reproduction, the use of energy are important conditions, but how many possibilities exist within that is, not well known.\\
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In our own system, Earth has it, and Mars, Europa, and Enceladus are the most promising candidates. Mars has had surface water in the past and still might have some underground, while Europa and Enceladus have both subsurface oceans and tectonic activity for energy. The clouds of Venus are sometimes speculated as an environment, water exists there, the sun would supply energy, sulfuric acid could be adapted to, and Venus may have been more like Earth early in its history.\\
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Once life exists, what form does it take? Are there just bacteria like organisms? Insects? [[FungusHumongous Giant Mushrooms]] Once again, [[RunningGag We're not sure]]. Microbes are almost certainly much more common, they are obviously simpler, and Earth didn't have multicellular life for much of its history. Exact evoluton is trickier: It's likely multicellular life is similar in some level if it exists, since a lot of Earth life adaptations are functional in ways extraterrestrial life would have to be as well, and it responds to similar physical restraints. Which properties of Earth life are like this is speculated or guessed, but not known for sure. Life will probably compete for resources, consume those resources, move, support itself against gravity, among other activities.\\


[[/folder]]

[[folder:Planet tropes, Could they exist?]]

Are certain tropes possible? How would you generate certain types of planets? It turns out a lot of movie style planets are generally possible, though not nearly as common as in fiction. Read on to find out:

!!Ocean Planets
These likely exist. In fact, we live on one: Earth is mostly Ocean these days, and likely in its early history was covered in ocean apart from some small islands until continents began forming about 3 billion years ago. (Even earlier, Earth has a magma ocean while forming, but this is not the Ocean the trope is thinking of)\\
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A world covered in ocean could be anything from a rocky planet with lots of water to a planet where water makes up a big fraction of its mass. Also needed is the right combination of temperature and pressure: pressure comes from a thick enough atmosphere, the temperature can come from whatever combination of reflection, greenhouse warming, and energy and distance from a central star keeps water in the liquid range. With stretched definitions, an "oil planet" of organic chemical oceans could form as well, these could be colder or warmer than the standard ocean planet.\\
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Ocean planets are good candidates for life, since, well, Earth has oceans and lots of life. An interesting question is whether a planet has continents: Earth's continents are formed from different materials than rocks under the oceans, and formed through the unique process of plate tectonics. It is possible that such processes haven't occurred yet, or don't occur on some other planet, also possible is having so much water than no continents break the surface.\\

!![[LethalLavaLand Volcanic Planets]]

Volcanic planets also exist: See Jupiter's moon Io, with numerous volcanoes spitting plumes, lava, and other eruptions all over the moon. Or see a newly formed/young planet, either covered in magma, or still hot enough that the rocky outside layer is thin and lets through enormous amounts of eruption.\\
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Of course, whether this counts of not is up to you. An extremely volcanic world covered in magma flows, volcanoes everywhere, etc. probably only exists on newly formed planets, even Io is mostly covered in frost of sulfur dioxide, forming something like HailfirePeaks. Other planetary processes will also compete with volcanoes to shape the terrain.\\
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To form such a planet, pick a body that has a lot of internal heat (tidal heating really helps, but younger/larger also adds to its volcanism), have as little other stuff as possible, and you are good to go. for an interesting variation, repeat the above, but give the planet an ice layer on type, for a cryovolcanic world. Less heat is necessary for such a planet.\\

!!Desert Planet

In a sense, most solid bodies we know of are desert planets: no life, no liquids. However, a more movie style desert planet, sandy, thick atmosphere, hot and dry, is actually a [[https://en.wikipedia.org/wiki/Desert_planet plausible planet type]], possibly being habitable at a wider range of distances than Earth. Such a planet would be very dry, mostly rock and sand with occasional pools of water on the surface, and some sort of thick atmosphere.\\
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Even this type of plan kind of exists in our solar system, if you stretch things a bit. Saturn's moon Titan is effectively a very cold version of such a desert; it has dunes, seasonal rivers with shallow lakes, and small amounts of rain. Different chemicals substitute for Earth equivalents: hydrocarbons, mostly methane, act as the liquid, water ice is the bedrock, random organic chemicals for the "sand", the atmosphere is nitrogen with a small amount of hydrocarbons instead of nitrogen + oxygen + small fraction of water vapor. Only things missing are some methane moisture farmers, some raiders, a crime ridden spaceport.... Mars could also count, it has dust storms, water clouds, dusty soil, and possible occasional water flows, but the atmosphere is a lot thinner and long lasting water does not exist.\\

!![[SlippySlideyIceWorld Ice Planets]]

Like the other planets here, many world exist with icy surfaces: See most solid bodies in the outer solar system. A movie style icy planet, that is effectively a cold version of Earth, is also very, very possible: simply cooling an Earthlike planet until water freezes will do the job. It is thought that Earth went through a [[https://en.wikipedia.org/wiki/Snowball_Earth period like this]], on a smaller scale, northern parts of the Earth iced over during more recent ice ages.\\
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A planet covered with water ice is self stabilizing: ice and snow reflect large amounts of light, further cooling such a planet. Outer solar system icy bodies are some of the coldest places we know of, because of this reflection in addition to being far from the sun. If ice does melt or evaporate, however, the changes can be very fast, melting ice exposes darker terrain, which absorbs more light, which heats the planet more, melts more ice, etc.\\

!![[AllPlanetsAreEarthlike Earthlike planet]], Forest Planet, AgriWorld, [[JungleJapes Jungle Planet]], etc.

All of these are variations on "earthlike planet" with life. Whether they exist, and what is needed for them to exist is...uncertain. (See the section on life in the above part of this useful notes) .\\
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What make a planet Earthlike is hard to say, but these are probably not common in RealLife. We're not sure just how many conditions are needed, and even Earth wasn't Earthlike through big chunks of its history (no continents for 1-2 billion years, no multicellular life until a few hundred million years ago out of several billion, no oxygen until about halfway through is history). That said, a world like Earth is a pretty good place for life: liquid water is a good solvent for organic chemicals, which are stable at appropriate temperatures, Earth's geologic activity recycles and controls materials useful for life, sunlight provides a lot of energy, among other conditions.\\

!!Colonizing

Can a planet be colonized? It depends on the technology available, obviously, but there are a few things to consider:

* Energy sources: What energy is available on a planet? If close to a star, solar power works, an active world can supply geothermal energy. If more exotic power sources exist, such as fission, fusion, black hole, magic super advanced quantum spinstring energy, etc., presumably this could be used. Atmospheres without oxygen cannot use most engines that we know of today: some could use an oxidizer carried as we would carry a fuel, some would need to carry fuel and oxidizer onboard if such an engine were wanted, other sources of power like electrified rails or batteries could be used. Ships would float more or less effectively on different types of liquid lakes or oceans. On the other hand, thinner atmospheres and/or gravity could allow surface vehicles to travel more easily, and thicker atmospheres and/or lower gravity would allow easier air transport and zeppelins/balloons with appropriate power.
* Transportation: If rockets are being used, transporting materials from other worlds is very, very expensive, and anything that can be made on world hugely saves the cost of building. If transportation is cheaper, more trade can occur between worlds.
* Environmental protection: Unless the planet is Earthlike (or really, even if it is), extreme conditions must be defended against. Obvious threats include radiation, cold, unbreathable or even corrosive atmospheres (or nonexistent atmospheres), and extreme weather, among others.
* Gravity: If gravity is too low or high, settlement will be difficult. High gravity slows movement down, and requires a more fit population, low gravity can cause health problems, bone decay in humans, and also requires extra health work to adapt too. Having ArtificialGravity is immensely helpful, possibly genetic engineering if it is advanced far enough.
* Available materials: For humans, water is extremely valuable: it can be split to supply oxygen, and the hydrogen can be used as fuel or in additional chemical reactions.Organic chemical would also be very useful for fuel, some nutrients, and specialty chemicals. Rocks could supply oxygen with a lot of energy, and also supply building materials, silicon, etc. Valuable metals on a body could be sold or used by a colony. solid material of any sort could be used for structures. The amount of processing needed to build a colony, and how that first binfrastructure is set up, are important considerations when building one.

More advanced than colonizing is terraforming: changing the surface conditions of a planet artificially. How much this can be done depends on available materials (Do the right chemicals for a good atmosphere exist? How to keep a planet at the right temperature? etc. Planetary science is a good guide to what changes need to be made, and what the results would be.

[[/folder]]

[[folder:Interesting Real Planets]]

Our own solar system has at least 8 full planets, at least 9 dwarf planets, dozens of moons large enough to be spherical, plus any undiscovered objects out there. Since the 1990's and 2000's, large numbers of extrasolar planets have been found as well. Here we write about the standouts among them, as well as some theoretical types of planets we may find in the future.\\

!![[{{UsefulNotes/Earth}} Earth]]

Yes, our own planet. Compared to the other planets we've found, Earth is really, really weird. most obviously, it is place known to have life, including the gelatinous thing fixed on a rocky support structure reading this right now. It is also the only one we know of with deep oceans and solid continents (some extrasolar planets may be water planets, but oceans they have would be too deep for land. titan has liquids, but in shallow lakes at most). it is the only place known to have plate tectonics: most worlds have so called stagnant lid convection, where the upper part of the planet stays in place, maybe getting pushed and pulled, but stays mostly separate from the creeping mantle below. In Earth's plate tectonics, the upper part takes part in convection: seafloor ridges draw material from several hundred miles down, while subducted material sinks all the way to Earths core. Earth's atmosphere is also unusual in having lots of oxygen, and the structure is slightly different: most atmospheres have a layer where temperature decreases with altitude, than a layer where it stays roughly constant, than increases. Earth has 4 layers, one where temperature decreases with altitude, than increases, than decreases, than increases. Earth's moon is also unusually large compared to its main planet. It has a magnetic field, like most planets but unlike its two most similar bodies Venus and Mars. It has all of the features listed in the second part of this article, apart from planetary rings.\\
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What makes Earth so unusual? Having the right conditions for liquid water is almost certainly a huge part. Water of course makes up oceans, and makes a good medium for life. It also softens rock, allowing Earth's plates and mantle to convect more easily, allowing plate tectonics. Faster mantle convection draws more heat from Earth's liquid iron outer core, creating a magnetic field. Water + sunlight is the most abundant source of hydrogen for life, and the leftover oxygen fills the atmosphere and allows even more types of life. Oxygen photochemically reacts to produce ozone, which absorbs radiation and further heats the middle atmosphere. What other features are necessary for an Earth, we do not know. Our moon may be important, it stabilizes Earth's rotation and prevents unstable seasons. The moon's formation may have also have changed the composition of Earth. Planetary migration in our solar system, and the presence of Jupiter, may have influenced Earth in other ways necessary for some of its features.\\
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[[CaptainObvious Because we live on it]], Earth is much easier to study than other planets, and the much greater knowledge we have about Earth is used to help understand similar worlds.\\

!![[UsefulNotes/TheSolarSystem Our Solar System]]

Is our solar system weird? It's hard to know for sure. Most systems we've found look very, very different. However, with the technology available, it is easier to detect certain types of planets than others, biasing for larger, closer in worlds to smaller stars. It's said that, if we tried to detect planets around the sun from nearby stars, we'd probably/possibly detect Jupiter, maybe Neptune as well, but other planets would be almost impossible to see, let alone the various moons, dwarf planets, etc.\\
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We don't know our fellow planets nearly as well as Earth, but know them a heck of a lot better than any extrasolar ones: They are far easier to see with telescopes, and we can send space probes.\\
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The standouts in particular areas are:

* Size: Jupiter is the big planet here. Matter in our system is concentrated towards heavier objects: Jupiter is heavier than everything (we know of) smaller than it put together, Saturn has more mass than everything smaller than it put together, Uranus and neptune are about the same size as each other, and bigger than everything smaller put together, the same applies to Earth and Venus. At Mars, mercury, and smaller this breaks down, but several moons are more massive than all smaller moons put goether, Ceres contains about a third of the asteroid belt, etc.
* Surface Gravity: Jupiter once again dominates here. The other outer planets, Earth, and Venus have similar surface gravity, Mars and Mrcury are about a third of that, and the strentch tails off as bodies get smaller.
* Moons: The outer planets all have a lot of moons, Jupiter or Saturn almost certainly have the most (Jupiter is a real showoff, isn't it?), neptune has the farthest orbiting moons (being so far from the sun, Objects can orbit Neptune further out without being pulled into orbit around the sun). Earth and Pluto have unusually large moons compared to their own size. Overall, there are 7 "large" moons, with a sizable gap towards the next 10 or so moons, plus lots of smaller ones not big enough to become spherical.
* Density: Most dense is Earth: it is made from metal and rock, already quite dense, and as the largest of the rocky bodies, it further compresses the stuff inside it. Mercury is very dense for its size, it has a much larger fraction of Iron than the other rocky planets. Saturn is the least dense: It is mostly hydrogen, the lightest stuff, and not being as big as Jupiter, does not compress it as much. It is less dense than water on Earth.
* Volcanoes and Tectonics: If you just want lots of activity, Jupiter's moon Io is the king here. lots and lots of tidal heating produces gigantic plumes, extremely hot, constant eruptions, plus a collection of mountains to go with the lava. If you want variety of features, Venus is our thing: most of the surface comes from volcanic eruptions several hundred million years ago, little erosion has left channels formed by lava, pancakes, mountains, and lots of weirdly shaped features seen nowhere else to fill the surface. For a weirder take, check out Europa, home of ice eruptions and tectonics.
* Weather: Most bodies with an atmosphere have some sort of interesting weather. For large storms, and fast winds, see the gas giants: Jupiter's great red spot is the longest lasting storm we know of, Saturn has a hexagon at the pole, Saturn and Neptune have ridiculously fast winds, all have various vortexes and turbulence.
* Extreme Atmospheres: Earth has this wildly energetic, corrosive gas making up about a fifth of its atmosphere (Better not go there.) Venus is the hottest, thanks to an extremely strong greenhouse effect. The coldest are probably Pluto, Triton, or more likely some further out world: reflective ice and no sunlight keep the atmosphere cold enough to freeze out at times.
* Liquids: Earth and titan are the places to go. Earth gets more rain and has oceans, but otherwise these are very similar, with rivers, lakes, channels, clouds with rain, and such.
* Glaciers: Pluto is the place to go, with a great variety of ice types (nitrogen, carbon monoxide, methane, among others) flowing all over the body.
* Rings: all gas giants have them, but Saturn has the brightest, most massive once, as you can see in most pictures of the planet.
* Magnetic field: Jupiter is the king here, a metallic hydrogen layer taking up most of a gigantic planet with plenty of heat for convection produces an enormously powerful field, with powerful auroras, radio emissions, and radiation belts to match. the field's strength, plus plasma from eruptions on Io and energized by Jupiter's rotation, inflates a huge magnetosphere that is far larger than the sun, with a tail that stretches to Saturn's orbit, and would be the biggest object in our sky if we could see it.
* Craters: Many bodies in our system are crater covered worlds without much else going on. For large craters, the standout is probably Caloris basin on Mercury, taking up a sizable fraction of the surface, and so powerful that the opposite side of Mercury has shocked terrain from shockwaves converging. [[RunningGag Jupiter]] has had the most powerful impacts seen today, the [[https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9 Shoemaker Levy impact]] was predicted and watched worlwide when it hit Jupiter in several gigantic explosions.

!!Extrasolar Planets: How to Find Them

Telescopes can barely make out features of other stars, how on Earth (or space, or your body of choice) do you spot them? there are a couple of important ways: transits, and the effect on their star's motion. When a planet crosses a star, the star's light dims very slightly, sensitive enough instruments can detect this dimming. Depending on what type of light is dimmed and the amount dimmed, scientists can estimate the volume of a planet, and get some idea of what art of it is made of. As a planet orbits a star, its varying gravity causes the star to wobble back and forth, this change in velocity, and in theory position, can be measured by sensitive enough instruments, and used to determine a planet's mass and orbit. in theory, planetary radiation could be detected, and other detection methods have been proposed, but discoveries have not yet been made this way.\\
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From size, mass, orbit, and spectrum data, scientist can take a guess at what planets might be like. the information is very limited, and a lot must be guessed from knowledge of how weather, physics, etc. work.\\
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Rogue planets cannot be detected in these ways, but being away from stars, could in theory be detected directly by light they emit. several proposed rogue planets have been seen, but not confirmed yet.\\

!!51 Pegasi b and other Hot Jupiters

51 Pegasi B was the first exoplanet around another main sequence star. It used the so called Radial velocity method: measuring using red shift a star's movement back and forth as it is pulled by the planet orbiting it. Further observations of the planet have nailed down its details: It's a bit less than half the mass of Jupiter, orbits in 4 days around its star, and water has been detected spectroscopically in the atmosphere.\\
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Since its discovery, more Hot Jupiters have been found: planets roughly the mass of Jupiter, almost certainly gas giants, orbiting extremely close to their stars, closer than Mercury. These planets are easier to detect than other types: being large, close, and fast moving, they cause their stars to wobble more, and cover more of their star during transits. Hot Jupiters didn't match planetary formation models for our own system: Plants too close in were expected to not grow big enough to pull in hydrogen and helium and form such a giant. Planetary scientists have recently come up with models that can explain this, with gas giants either migrating closer to the their stars thanks to gravitational interaction with gas around the star, and/or with small bodies they encounter, or forming from Super Earths that we've found around other stars. How common these are is hard to say: they could be the most common type of gas giant, or could be not that common but simply much easier to detect.\\
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Hot Jupiters are obviously hot, thanks to being so clase to a are. They have ther extreme properties as well: most will be tidally locked that close, resulting in a much hotter atmosphere facing the star and a much colder one facing away. This temperature difference will create extremely fast winds, and the heat will puff up the planet's atmosphere: most hot Jupiters probably have a much larger radius than Jupiter despite not having similar masses. Rocks can form clouds and an equivalent of snow or rain in these high temperatures. The heat may strip hot Jupiter atmospheres, though the high mass of a planet, and high gravity, means this probably happens slowly.\\

!!The First Extrasolar Planet: PSR B1257+12 A, B, and C and other pulsar planets

Pulsar planets are strange beasts: Scientist expect that supernovas would destroy any planets nearby. Some of these planets formed from leftover supernova material that stayed near the star after the explosion, or possibly from large planets that somehow weren't completely destroyed. It is also possible that planets are captured from other stars. Collisions of smaller bodies to form a neutron star may also produce planets by throwing material off, that accretes into a new body.\\
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It is perhaps not surprising that a pulsar planet was the first discovered. Pulsars rotate with a very, very exact timing, and small changes in timing due to a planet's orbit effecting the pulsar allow them to be detected.\\
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Pulsar planets will have unusual environments, to say the least. The intense radiation from the pulsar almost certainly sterilizes the planet (unless it has a huge amount of atmosphere to block it), and the intensely focused radiation beams likely creates lines in the planet.\\

!!Super Earths

Planets made of similar material as Earth, but larger. Seems simple enough, and they probably have the same range of properties any other rocky world has, with more gravity, volcanic activity than similar small worlds, a denser possible atmosphere due to greater gravity, etc. For more unusual properties, super Earths, when heavy enough, may retain helium atmospheres. This may make them harder to detect, such a helium atmosphere by some predictions may extend far out from the planet, making it look like a less dense planet (Uranus/Neptune type icy planet) rather than a concentrated rocky planet with a thin but detectable atmosphere. Super Earths may also, depending on the model make plate tectonics either easier or more difficult to form (Science is fun, isn't it?) If Easier, Earth may be joined by other planets in having this unusual feature.\\
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Lots of these types of planets have been found, by one of or a combination of being easier to detect and being common in general. It's suggested that our own solar system may have formed larger rocky planets, but Jupiter's migration early on scattered and absorbed material from the inner solar system, leaving the existing sized planets instead. (In this model, Mar's small size is also due to this migration: without Jupiter moving close to its orbit, Mars would have had enough material to be about Earth/Venus sized.)\\

With enough exoplanet discoveries, scientists are finding a good number of roughly Earth sized ones within their star's habitable zones. Unfortunately, without measurements of their atmospheres, temperatures, and such, actually seeing if these planets have water, let alone life, is difficult. (Detecting water by itself is not enough, water vapor can exist in an atmosphere without liquid on the surface.)\\
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The biggest wrinkle for many habitable zone exoplanets: Around smaller stars, the theoretical habitable zone is close enough to tidally lock a planet. This most likely created a possibly habitable thin zone between the light and dark hemisphres, rather than a fully habitable planet.\\

!!Alpha Centauri System

[[UsefulNotes/LocalStars The Closest star system to our sun]]... has a planet! At least the red dwarf proxima Centauri does. We thought we'd found on in 2012 around, orbiting close to Alpha Centauri B, but this was found spurious some time later. Other planets have been theorized, but none detected around the central stars. Proxima Centauri, the red dwarf, does have a confirmed planet around it. the planet is around Earth sized, and is in what would be the habitable zone...but Proxima Centauri has such strong solar winds that any atmosphere is likely stripped.\\
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It may be that Alpha Centauri A and B cannot have planets, some simulations suggest that the two stars would disrupt any planets formed, or have disrupted any accretion dick which may have formed. Of course, this may not be the case, and as the closest star system, planetary searches continue there. Whether we can [[VideoGame/Civilization send a ship there]] and [[VideoGame/SidMeiersAlphaCentauri explore the native life]] is still up for grabs.\\

!!Carbon Planets

A hypothetical planet, mentioned way at the beginning of this useful notes and than forgotten. These in theory form in systems with a high carbon to oxygen ratio: Our own system has a higher oxygen to carbon ratio, so none of these have formed. Our technology is still not good enough to detect these in other systems: we'd need good spectroscopy to detect the expected chemicals, carbon planets by mass/size just look like icy or rocky or mixed planets.\\
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Such a planet likely has various carbide compounds in its interior, possibly large amounts of diamonds as well. On any surface, hydrocarbons, tars, waxes, and such would dominate. Water would be nonexistent, but organic chemical oceans are plausible with the right conditions.\\
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Gas giants and icy planets in a carbon rich system wouldn't be too different from our own, apart from less water, more methane and hydrocarbons.\\

[[/folder]]

Planets, moons, and similar bodies revolve around a star or other larger body, and [[EverythingsBetterWithSpinning rotate]] as well. They don't necessarily orbit where they form, a big discovery in the past few decades is that planetary orbits can move quite a bit over time. Planet can fling smaller bodies in or out, or can interact with gas in their accretion disks, or each other, to change orbits and move closer or farther from a star. In our own solar system, it's thought that Neptune, Saturn, and Uranus have moved outwards from where they were formed. Small bodies thrown outwards make up the kuiper belt and scattered dick, Probably Oort cloud also. In other systems, gas giants are thought to move in, forming hot Jupiters. It is possible for some planets to get thrown out of a system, becoming rogue planets that float through interstellar space near no star.\\

Why these materials? Why not, say, a planet made of solid gold, or a uranium core with an aluminum shell? Or a purely dry ice planet? The reasons are a combination of elemental abundances, chemistry, and phase separation.


In the universe, the vast majority of all elements are hydrogen and helium: remaining lighter elements are formed by fusion in usually older stars. As described in UsefulNotes/Stars, stars through most of their lives fuse hydrogen into helium and stop there, only when the star runs out of hydrogen in the center do they start to create heavier elements. Going down the periodic table, Lithium Beryllium, and Boron are often consumed in nuclear reactions, so as stars start to fuse helium at the end of their lives, carbon, oxygen, and some nitrogen become the next elements created, Many stars stop there, only heavier stars continue to fuse further, creating sodium, magnesium, aluminum, silicon, etc. As the elements get heavier, they get less common, and generally even numbered ones are more common. At Iron, something unusual happens: Iron is the most stable element, with the lowest energy per nucleon. Fusing iron absorbs energy, as does breaking it down. As a result, Iron is the stopping point for stars, and it is much more common than its atomic number would suggest.

Heavier elements are created in supernovas, or in neutron star collisions, which are quite rare.//

The result of all this together: Hydrogen and helium are the most common, to a large excess, next more common are Oxygen, Carbon, Nitrogen, with the next row of Sodium, Magnesium, etc. following, and Iron also somewhat common, with other elements being rare. Now, chemistry takes over.

Hydrogen is available in a large excess, as is helium, and helium doesn't react with anything anyway. Oxygen, Nitrogen, and Carbon can react with hydrogen and somewhat each other: the resulting chemicals freeze at roughly similar temperatures (compared to anything else common) and often dissolve in each other, creating the volatiles/ices. Joining them are some compounds of sulfur, phosphorus, and a few others. Silicon and many lighter metals are very energetic when binding to oxygen, forming rocks. Iron usually travels with rocks in space, but when combined into a planet, excess Iron can separate out, forming its own material. If Carbon is more common than Oxygen, the excess carbon can form its own wide array of compounds, as you can see in crude oil, tar, or overcooked food char and grunge when cooking: this is what may make carbon planets.

Finally, phase separation takes place. Each component condenses/freezes at different temparatures, and has a different density. Gases are light and condense at extremely low temperatures. Volatiles are denser and condense at higher temperatures. Rocks are denser and condense hotter still, and Iron is the most dense, though it melts at lower temperatures than rocks. Like also mixes/dissolves with like: Anything in gas form can mix, Volatiles can bind together, rock chemistry is somewhat flexible. Minor elements tend to bind to whatever major component they mix with best: Neon, for example, is low boiling and unreactive like hydrogen and helium, Uranium is very reactive in a similar way to light metals and mostly ends up in Rock, metal sulfides and some metals mix well with Iron. On Earth and likely other rocky planets, the [[https://en.wikipedia.org/wiki/Goldschmidt_classification goldschmidt]] classification gives a rough guide of which elements tend to go where. As a result, planet forming materials separate into the 4-5 types of materials described above. Within a planet, these materials are often separated in different layers, lighter towards the surface, heavier inside.

To the best of our knowledge, planets form along with stars. Their formation begins with gas and dust clouds in space. If these clouds get dense enough, or something disturbs them (supernova shocks are common guesses), their gravity is strong enough to pull them inward. As it collapses, the cloud does two things: heat up, and spins. The heat comes from gravity: As the cloud is pulled together, the gravitational energy of material falling in must go somewhere: friction and compression mean a lot of it ends up as heat. The spin could come from outside gravitational or friction forces, or from random motion: if its motion adds up to any kind of rotation at all, it will cause the cloud to spin up as it is pulled in (see the common analogy of an ice skater/dancer drawing arms in).

Most material in such a collapsing cloud will go to the central star, or a couple of central stars, but the fact that the cloud is spinning means some will orbit instead. This orbiting material forms what is called an accretion disk: centrifugal effects mean that the gas and dust is pulled outward at the equator of spinning, and flattened. The center is hotter: infalling material has fallen farther, releasing more gravitational energy, and is nearer the hot still-forming star in the middle. Gas fills the space in the disk, but solid material forms dust particles, and these particles are what will form comets, asteroids, planets, and any other bodies.

Smaller dust grains clump together, mostly from friction or static electricity, forming somewhat larger, boulder sized particles. Scientists are still not sure how these larger particles stick together, collisions would be expected to destroy them faster than they form, but obviously something must stick together to form planets. When the material gets large enough, gravity gets strong enough to hold the bodies together. Some of these bodies will stop growing and form comets, asteroids, or other small bodies. Others keep growing, becoming so called planetesimals. As bodies grow larger, their gravity effects the orbits of anything further and further out, these destablizations lead to further collisions, throwing some material and bodies into the central star, other material and bodies into distant orbits, or ejections from the system. The end result of this process: large bodies that dominate their orbital regions, or smaller bodies in belts or clusters. The larger bodies are planets, the smaller ones dwarf planets, or asteroids, comets, etc.

The material a planet is made of depends on where it is formed. Nearer the star, only silicates can solidify, and more rocky/metallic planets form. Further out, volatiles can more easily freeze, and become a larger mix within a planet. If enough material accumulates, a planet can pull in hydrogen and helium. Once formed, collisions and impacts can further change composition: volatiles may be brought in from further out regions, impacts can remove outer layers, and other changes occur.

Moons can form in a number of ways. Some may be captured: impacts, or gas around a forming planet slow them down enough to be caught in orbit. Tidal interactions in a double pair of objects may split the pair, also capturing a moon. Large gas giants form their own accretion disks when forming, and moons can form in a similar way as planets.

Planets, moons, and similar bodies revolve around a star or other larger body, and [[EverythingsBetterWithSpinning rotate]] as well. They don't necessarily orbit where they form, a big discovery in the past few decades is that planetary orbits can move quite a bit over time. Planet can fling smaller bodies in or out, or can interact with gas in their accretion disks, or each other, to change orbits and move closer or farther from a star. In our own solar system, it's thought that Neptune, Saturn, and Uranus have moved outwards from where they were formed. Small bodies thrown outwards make up the kuiper belt and scattered dick, Probably Oort cloud also. In other systems, gas giants are thought to move in, forming hot Jupiters. It is possible for some planets to get thrown out of a system, becoming rogue planets that float through interstellar space near no star.

More orbits of anything are not exactly circular, elliptical is more common. Hugely elliptical orbits are found with some small bodies such as comets in our system, their outer points are thousands, millions, or even more time further out than their inner points. Most larger bodies, including planets, are more circular: a more elliptical orbit gives a great chance to collide with something, and big planets will attract each other to make such a collision more likely.

Large orbiting bodies strongly effect smaller bodies nearby: part of the definition of a planet is something that clears (dominates would be a better term) its orbital neighborhood. Most obviously, larger bodies can have moons. The volume things can orbit in is called a hill sphere, a region where the smaller body's gravity dominates the effects of the larger body it is circling around. This sphere gets larger when the planet/orbiting body gets more massive, the star/central body less massive, and the further away from the star/central body. So, in our own system, big gas giants further away from the sun can have more moons than smaller planets further in.

Big bodies also disturb smaller bodies further away. At so called lagrange points (stable points for a small body near two larger ones.), most planets in our system have so called "trojan asteroids". Jupiter shapes the asteroid belt, keeping it from clumping and keeping it in a band, Neptune plays a similar role with the kuiper belt, and possibly further out. Some moons do the same with planetary rings. However, two large bodies in the same orbit is a no-no, such systems are unstable, and most likely lead to a collision of some sort, possibly a change in orbits or ejection for one of the bodies relatively quickly.

An important feature of orbits are so called resonances: where one body and anonther body complet whole numbers of orbits in the same amount of time. (for example, a 2:1 resonance means one body orbits once for each 2 orbits of the other). Resonances can do interesting things: transferring energy between bodies, making orbits more elliptical, avoiding collisions, are among the examples.

Planetary rotation is not well understood. All (or 99.999...% of) bodies rotate, but the rotation speeds of many are not understood, apart from tidal locking (see next paragraph and the one after.). Most likely, bodies rotate in the same general direction as the whole system rotates when they formed, and being big, planets and moons don't change direction easily. However, gravity from other planets, or impacts, or changing mass distribution within a planet can change their rotation speed and axis.

If two bodies are close enough, tides become an important effect. The cause: the slightly different gravy between near, far sides, and the section around the middle of the body, cause it to distort and become egg shaped with two bulges on the near and far sides, and the circle in the middle squeezed in. Earth tides are caused this way by the noon: the bulges are high tides, the squeezed circle creates low tides.

Tides can cause tidal locking of moons, and planets too close to a star: as the moon rotates, a tidal bulge on it is moved away from the central body. the central body pulls back in the tidal bulge: the effect is to slow or speed the rotation of the smaller body until the tidal bulge is always facing the central body, which happens when the planet/moon rotates as fast as it orbits. they can also change the orbits of bodies in a similar way: if a body orbits faster than the tidal bulge in whatever it is orbiting, that tidal bulge pulls back, lowering the orbit and causing the central body to rotate more in the direction of that orbit. The opposite happens if a body orbits more slowly than a tidal bulge. Bodies in low orbits as a result will crash into whatever they are orbiting as a result, bodies in higher orbits may spiral away.

Tides do a lot more interesting things, but that's found in the next big section.

Obviously, the more material a planet has, the more massive and, usually, larger it is. The exact size and mass depend on the type of material, the same mass of more dense material obviously forms a smaller planet (by volume) than one made of less dense material.

Planetary density also depends on the size of the planet: larger planets compress their insides more, making them dense. This gets extreme at about Jupiter size: compression works so well that planets at about this volume stay at about a similar size as more material gets added. (They can get larger if heated, however, the heat expands their atmospheres.) Material inside can be squeezed and just get smaller, or can even chemically change under high pressure.

A body's surface gravity is proportional to its mass, and inversely proportional to the radius squared. As a result, gravity increases with mass, usually, but not as quickly as the actual mass does. Planets made of less dense materials can have weaker gravity than more dense but smaller planets: examples in our own system include Uranus and Earth (Uranus is slightly smaller), and Mercury and Mars (Almost the same gravity despite mercury being about 1/3 as massive).

Defining a body's radius, size, or zero altitude isn't as obvious as you might think. Rotating planets bulge at their equators, cause by centrifugal effects (The equator rotates further out than the poles, so centrifugal effects pull it further out), planetary radius's will be a bit higher near equators as a result. A solid body has mountains, valleys, highlands, and other higher and lower terrain, so some arbitrary altitude must be picked as the radius, based in some criteria (Average radius if a body is well mapped maybe the go to option). A Gas Giant may be either no surface, or a surface so deep that you'd lose a huge fraction of the planet by defining it there. For our own solar system, the radius where atmospheric pressure equals earth pressure is used. Atmospheres don't have an upper edge, they instead smoothly get thinner and change into whatever is in surrounding space, so are not any help. Planets with large oceans have the easiest measurement, simply measure an average sea level. (This is how Earth altitudes are done.)

To have geological activity, a planetary body must have a couple of things: an internal heat source, and to be made of the right materials. Obviously, a planet must have a solid surface to have geology: internal heat escaping in a liquid or gaseous planet creates ocean currents or weather instead. The material is important in another way: material that softens and melts at lower temperatures means less heat is needed to create geology.

Where does internal heat come from? Scientists know of three major sources today. A heat source every body has is heat from formation. The heat released from a planet sized mass of material pulling itself together is enormous: to get an idea, think of a reentering spacecraft or asteroid impacts, the heat released when going from space to settling on a planet could vaporize or destroy a spacecraft if not properly protected. Larger impacts create craters, and even melt material. When a planet forms, a good amount of this type of heat gets contained within.

Closely related is heat from differentiation: if a planet is warm enough, heavier material can sink through lighter material, releasing gravitational energy that further heats the body. Related chemical and physical changes can occur through the lifetime of a planet, material can freeze or melt, dissolve or come out of solution, and the resulting material can sink or rise, often resulting in further energy release. The freezing of Earth's core, for example, releases some energy.

Radioactivity is the second well known source of heat. Radioactive elements tend to concentrate in rocks, so within a system, the more rocks a body has, the more radioactive heat it generates. The amount of radioactive material likely changes from system to system, depending on the history of where it formed, and how many heavy elements were around when it formed.

Most complex is tidal heating, requiring three things: a body with large tides, an elliptical orbit, and material that can deform but resists deformation (hot rock, not to cold ice, and most liquids all work). If a body orbits elliptically, the size of the tide will change as it obits, and the tidal bulge will move back and forth a bit as well (the body rotates at a constant speed, but orbits at a slightly changing speed, pulling the tidal bulge a bit ahead of or behind the central body as it moves). If made of the right material ,with some plastic or fluid properties, the resistance to this motion creates heat. To last enough time, some outside force must keep the orbit elliptical, otherwise this resistance would make the orbit circular.

All else equal, larger and younger bodies should be more active. Larger bodies have something like the SquareCubeLaw occurring: Radioactive heating is proportional to mass (if made of the same substances), accretionary heating is proportional to the square of mass (more stuff pulled into the planet * more gravity pulling each piece in), tidal heating increases with radius to the fifth power (a few effects that add together). The amount of surface area to escape from, however, increases as radius squared, or a bit less than mass to the 2/3 power. Younger bodies haven't cooled as much from accretion, and their radioactive elements haven't decayed as much, generating more radiation.

If this heat can melt material close to the surface, volcanoes form. If the material has lots of gas, or is under great pressure in another way, explosive eruptions or plumes can occur, throwing material great distances. with less gas,liquid can flow out more smoothly, creating lava flows, channels, tunnels, or other such formations. Material building up over time can build up mountains. Some eruptions might come at spreading areas or cracks, creating fissures or ridges.

Internal heat also can create tectonic activity: if material inside convects, the movement distorts the surface. Rising plumes can create highlands, downward movement can create depressions, sideways movement can compress differnt regions to form mountains, or extend regions to form valleys or plains. This type of movement is a major source of quakes, as surface material is pushed and pulled, it can build up stress: the release of stress with sudden movement is a quake.

Before we begin: the Earth's mantle (and that of most rocky planets, most likely) is solid, not magma. Pop culture often confuses this, for understandable reasons: it is very hot down there, magma does come from there, and lots of descriptions of flow and convection sure sounds liquid like. What is actually down there is a solid, but plastic material. Like a lot of materials, rock melts at higher temperatures under higher pressures; mantle rock would melt if brought to the surface, but is under enough pressure to stay solid. the rock is plastic, technically: it would feel solid to us if we could feel it, but over time and with enough force can creep (this is the actual materials science term), allowing convection. A similar material is tar, the [[https://en.wikipedia.org/wiki/Pitch_drop_experiment Pitch Drop experiment]] shows how a seemingly solid material can look liquid like over long enough periods. A good analogy is fudge: when cold, it feels mostly solid, but put something heavy enough and let it sit for long enough, and it will deform. [[TheCore The Cor]]...uh, [[FanonDiscontinuity a movie that was not made in 2003 and does not exist]] had this as one of its many, many, many scientific issues, the liquid mantle also made some other phenomena even less realistic (not that they were that realistic anyway.)

While rocky planets may have magma layers, it's likely most are mostly made of solid rock. The reason is that liquids flow easily, and transport heat very, very quickly: unless a huge amount of heat is produced, or the planet is recently formed, or close enough to a star to melt on the surface, a molten interior will very quickly lose that heat until much of it freezes and the resulting solid rock insulates the interior.

How does anyone know what the interiors of planets are like? We can't drill directly and see: Even on Earth we've only gotten a few miles underground at most, let alone other bodies. Scientists have a few ways. Just knowing the size and mass of a planet tells you roughly what materials it is made of. Further details are provided by the shape of a body and the details of its rotation. Spinning bodies are slightly larger at the equator than at the poles due to centrifugal effects, the size of this bulge, and the change in gravity it causes, tell how material is distributed in a body. Tidal bulges can provide similar information, as can changes in rotation rate as a body interacts with other bodies. Finer scale gravity measurements can also give details of convection or temperature distribution, smaller than expected gravity over and area shows lighter, probably hotter, material is underneath it, the opposite for higher than expected gravity. A magnetic field shows that something electrically conductive is inside the body, tectonics shows heat exists, among other features.

Sound waves or seismic waves can travel through a body and provide information. On Earth, seismic waves from earthquakes have been the main way to see what the interior is like: Different types of waves speed up or slow down as they pass through different material, or even stopped. Seeing how long waves from an earthquake (or [[BreadEggsMilkSquick nuclear test]]) take to reach different regions tells something about the material in between, with more powerful computers and enough earthquakes and seismometers, [[https://en.wikipedia.org/wiki/Seismic_tomography relatively detailed maps]] can be created. Sound waves can be used for gaseous bodies, the sun's interior was figured out in this way.

If you can access the surface of a body, material from the inside is sometimes available. On Earth, mantle material has been brought to the surface in a few places through volcanoes or tectonics. Foreign material trapped in diamonds, which are formed in Earth's mantle, has provided some details of history and chemistry. On gaseous or liquid bodies, currents may bring deeper material to the surface.

Finally, scientists may use physics/chemistry knowledge to find out what materials do inside planets. they may extrapolate from known material properties, or use quantum mechanics/atomic/molecular level calculations to see what materials do at the temperatures and pressures inside planets. Using special equipment, they can study materials directly: a common method these days is the diamond anvil cell, which uses diamonds to concentrate force to create extremely high pressures, than use laser heating if needed to heat the material. Diamonds are clear, allowing examination of the results. New types of minerals were found this way, with the right properties to explain seismic studies. Scientists can also copy from planet to planet: planets with similar compositions and sizes probably have similar interiors.

Still, scientists might know what an interior is mostly like, but many details will still be unknown. Even on well studied Earth, scientists have only recently discovered evidence of high pressure ice in parts of the mantle. The existence of diamonds and jade (formed in the mantle) would be hard to spot from space. Scientists guessed that mantle convection existed, but the discovery of [[https://en.wikipedia.org/wiki/Large_low-shear-velocity_provinces two giant piles of....something]] was a surprise. If you are writing a story, the insides of planets gives much room for speculation. In Jupiter, the core was recently (from when this page was written) found to be mixed with metallic hydrogen above it, not a completely separate layer or nonexistent as had been predicted.

So, what are planetary interiors like? Whatever a body is made of, the weight of mile/tens/hundreds of miles of stuff creates enormous pressures, often changing material properties. many bodies are also hot inside. Almost always the further in you go, the hotter it gets. (for a simple reason, things cool from the outside.) Only planets still forming might break this, since the inner material accreted as a smaller body with less gravity, it may be cooler than shallower material. Big enough bodies will have different layers of material. Some layers form as the same material changes under different pressures and temperatures. Others form from different chemicals: if a bodies forms from different material at different times, or gets warm and soft enough for heavier material to sink, and the body is not mixed afterward, that body will have layers of heavier material towards the center and lighter material towards the surface.

Iron layers may be liquid or solid depending on the temperature and pressure it is at. Rocky layers may be liquid or solid as well. The minerals also may change deeper into a planet, the same mix of elements can form denser minerals as pressure increases, and change properties with temperature changes. Water can form liquid water or several types of ice. Higher pressure ices can stay solid at boiling temperatures on earth. Put enough pressure on water, and it breaks down, separating the hydrogen and oxygen and forming ionic or superionic water. Hydrogen when compressed becomes liquid, than metallic. Carbon under higher pressures forms diamonds, carbon reacting with metals or rocky elements forms carbides, either liquid or solid.

Smaller components also do unusual things. At some pressures and temperatures, hydrogen and helium are thought to separate, the helium forms a separate layer. Volatile chemicals react in unusual ways, hydrocarbons may separate and form carbon, creating diamonds. Rocks may be able to hold water or other volatiles, creating resevoirs of such materials. Gaseous or water rich planets may have blocks of solid materials. Minor components are where the speculation can run wild; scientists can make some predictions, but data from gravity or surface features reveals little.

If the interior of a planet is hot enough (how hot depends on the materials), it will convect. Convection can mix the interior of a planet, bringing material to the surface, and/or bringing surface material deeper where the hotter/high pressure conditions cause it to react in unexpected ways. On solid bodies, it creates tectonics, and brings heat to the surface to create volcanoes. In liquids and gases, it creates weather, and (probably) ocean currents.

Can we breathe on a random planet? [[AllPlanetsAreEarthlike Are all planets Earthlike]]? The answer is almost certainly no, for a smple reason: Oxygen is an extremely reactive and energetic gas. Good for powering an energetic warm blooded body, bad for lasting a long time in an atmosphere. Carbon, hydrogen, [[BreadEggsBreadedEggs hydrocarbons]], most other organic chemicals, ammonia, carbon monoxide, sulfur, sulfides, silicon, free metals including iron, and many more chemicals will react within extremely short timeframes with oxygen if available. Even rock can react: some rocks are not as oxidized as they could be, and with lots of rock making up a planet, there is plenty to react with wisps of oxygen in the air. The only way to maintain an oxygen atmosphere is to continuously replenish it, and we know of only three cases: Earth, where oxygen is produced by photosynthesis in life, and Europa and Ganymede, with atmospheres in name only (detectable by instruments, still pretty much a vacuum.) where wisps of oxygen are produced by ultraviolet rays and radiation from the occasional water molecule sputtered off the surface by more radiation.

Even with oxygen, the air may not be breathable. the air may be too thick or thin (too much oxygen is poisonous over time), or have other toxic gases.

So what is in an atmosphere, and where can I get one? Many bodies are surrounded by gas, but to hold a decent atmosphere for a good length of time, a body must be large enough and cold enough. Gases are held in place by gravity, but can escape if molecules are given enough energy to reach escape velocity, or are blown off the planet in some other way. Larger planets have higher escape velocities (even if surface gravity isn't as high, the total energy needed to leave a large body's influence is higher than for a small body), meaning gas can be more easily trapped. Hotter atmospheres mean individual molecules have more energy, and are more likely to reach escape velocity. Atmospheres can also be exposed to other energy sources, such as radiation or impacts, that under the right conditions can cause atmospheric escape. In a sense, no atmosphere is really permanently bound: a few molecules here and there can always escape by random chance, but there is still a vast difference between a long lasting, thick, tightly bound collection of gas and a barely there, almost a vacuum, escapes as fast as it is produced gas envelope that some bodies have.

The ease of escape also effects what kind of gases could make up an atmosphere. Colder and/or larger planets can hold hydrogen and helium. Warmer/smaller than this and these gases escape, but volatiles and gases made from them (nitrogen, methane, water, carbon dioxide, etc.) can be held. Vaporized rock and iron could conceivably exist if the body gets hot enough. This brings us to the other end of the scale: if too cold, an atmosphere will freeze out, although wisps of gas will always exist in equilibrium with the solid surface, and this gas will be bound to the body. Some outer solar system bodies have atmospheres like this, where nitrogen, methane, and/or other components freeze into the surface when farther away from the sun, and evaporate more when closer.

Atmospheres can be formed in a few ways. Some, called primary atmospheres,form along with the planet. Gas giants in our solar system are like this. Earth, Venus, and mars have atmospheres formed mostly from outgassing: chemicals in the interior are released as gases and create one. Impacts can also create atmospheres, either bringing material themselves, or causing material in the body to be released.

What gases are expected? If the body can hold helium and/or hydrogen, these gases will probably make up most of its atmosphere, being so common. Among the volatiles, nitrogen, carbon dioxide, methane, possibly water if hot enough are quite common, other gases are often found in trace amounts. The exact mix of gases will change from body to body, but chemical reactions will likely create these gases. Nitrogen mostly comes from ammonia, stellar radiation tends to break ammonia down into nitrogen, and nitrogen is itself quite stable, doesn't react easily, and doesn't react energetically with much. Carbon dioxide is similar, it is a relatively stable molecule. Methane and water can be broken down similar to ammonia, but are also quite common and often replenished.Other gases are usually more reactive, and therefore found in trace quantities, as they react very quickly after being formed. Iron or silicate vapors are not found within our system, nothing is anywhere close to hot enough except perhaps deep inside gas giants where we can't see anyway, so their properties are speculative if they exist.

If some gases can condense, clouds can form from small particles of liquid or solid. All solid and liquid substances have a so called "vapor pressure": let a system go to equilibrium, and there will be some pressure of evaporated material in equlibrium with the solid or liquid stuff, freezing or condensing as fast as evaporation happens. If the concentration of a gas is higher than the vapor pressure, and has any kind of small particle or disturbance to condense on to, it will form a cloud. Since vapor pressure almost always decreases at lower temperatures, this most often happens when air cools. Clouds of water, ammonia, and methane have been seen directly, and ammonium sulfide, iron, and rocks are likely on some bodies.

Closely related are organic hazes: These are created when organic chemicals react in an atmosphere, usually driven by photochemistry from starlight or ultraviolet rays, forming chemicals that are condensed at the temperatures they form at, creating a fog. On Earth, this process makes smog. Less damaging to respiratory systems, outer planets and some moons also seem to have hazes. Similar photochemical reactions likely help create Vanus's sulfuric acid clouds.

And finally, we get to temperature. If you ever see someone arguing that global warming is exaggerated or doesn't exist, give them a good slap. The basic physics works not just for Earth, but also for any planetary body. The main challenge to models is working out the effects, Earth is a complicated planet with lots of systems effected (ice, life, etc.), but there is no question that some warming will occur, and the general predictions of most models are happening as predicted.

The temperature of a planet roughly depends on the amount of heat coming in, and the amount going out. Planetary bodies gain heat from their stars light, and some from internal heating: Starlight is by far the most important for planets near a star, internal heating gets more important as you move further away. The amount of light coming in depends on how close a planet is to a star, how bright the star is, and how reflective the body is: bodies close to luminous stars with non-reflecting materials will absorb lots of light, bodies far from dim stars made of reflective materials will be very cold. Reflection can come from clouds and hazes, or from surface features if a planet has them. Particularly reflective are Venus's sulfuric acid clouds, and snow and clean ice (this is one of several reasons melting ice caps are an issue, losing them means more light gets absorbed).

If a planet's temperature of stable, it is losing heat as quickly as it gains it. Space being the vacuum it is, heat loss happens by thermal radiation: the body gives off energy in electromagnetic radiation, the exact mix depends on temperature. Thermal radiation generates the light from stars (see [[UsefulNotes/Stars]]), at most planetary temperatures, infrared emission is the main form emitted. Greenhouse gases work their magic here: They absorb some radiation, than emit the energy in all directions, sending some back down to the planet. Being high in the atmosphere, they are also often colder, and emit less radiation overall. As a result, the planet emits less radiation overall, and heats up until a balance is restored. Due to the workings of molecules, most gases made of more than one element absorb infrared radiation but do nothing to visible light, meaning they retain heat on a planet and don't reflect anything, heating a planet without cooling it.

Different gases absorb different types of infrared, and a single gas has diminishing returns in how much infrared it can absorb. Absorption is also effected by interactions with other gases nearby. the result is that a mix of gases absorbs more than the same amount of a single gas, and traces in an atmosphere can have a seemingly disproportionate effect on temperature. Without any greenhouse effect at all, Earth would be freezing, but slightly too much of one, and lots of issues happen.

If you've ever been outside (not a sure thing for a TV Troper, obviously) or heard a weather report, you'll know that atmospheres do things. Wind blows, rain falls, lighting strikes, clouds form and disappear. Look at other worlds and you'll see even more exotic weather: storms lasting decades or centuries, bands of wind, turbulence.

Ultimately, weather is driven by heat. Temperature differences of various sorts: lower atmosphere to upper, equator to poles, night side to day side are the main possibilities, are what ultimately drive weather. Our good friend thermal convection shows up here, if you read this entire useful notes all the way through, you'll notice convection is [[{{Understatement}} somewhat important]] in planetary science.

Also important is rotation: this creates centrifugal effects and something called the coriolis effect. Centrifugal effects are the familiar "spinning objects tend to pull outwards". The coriolis effect occurs as air moves between different latitudes. While all of a planetary body rotates at the same rate of spinning, the actual speed the planet moves depends on the latitude: near an equator, the surface moves the fastest, it has farther to go in the same time period. he poles are the slowest, staying in the same location as the planet spins. Air moving towards the equator keeps the same velocity, but appears to by moving slower compared to something rotating with the planet. The opposite happens as air moves poleward. Combine centrifugal and coriolis effects, and moving winds tend to rotate, atmospheric scientists model this by pretending an extra force exists, defecting air as it moves.

Temperature differences between lower and higher in the atmosphere cause the familiar bouyant convection you know about: hot air is less dense than colder air, so tends to float upwards. This rising air draws in surrounding air, which creates wind at whatever level it is rising from. Horizontal temperature differences also create winds: because air is less dense, less hotter air is above a particular point than equivalent colder air. The change in air pressure between different heights is equal to the weight of air between the two heights. A column of hotter air, as a result, has slower pressure changes than a column of cooler air, this means that at some point in the atmosphere, a pressure difference will exist between air of different temperatures. This pressure difference will drive winds. Actual examples include day to night winds, and winds between the usually hotter equator and the colder poles.

Once winds are created, the Coriolis and centrifugal effects cause them to deflect. Air drawn in to the convective system described above will rotate as it is drawn in, and leaving will also rotate. Winds moving towards the poles will deflect towards the equator, until the pressure difference caused by different temperatures balances the coriolis effect. Winds can also shear against each other, creating turbulence and more rotation. These, and other coriolis effects, cause lots of rotating vortexes in an atmosphere: if allowed to spin ,these vortexes can transfer energy into each other, sometimes merging and becoming larger, sometimes driving further winds.

Heat sources can include both internal heating and heating from a central star. The strength of the winds does '''not''' simply depend on the amount of energy available: in our own system, Earth probably gets the most energy per atmosphere, but has relatively weak winds. Having a solid surface obviously slows winds through friction, gas giants have longer lasting and stronger winds than solid bodies in our system. It also may be that more heat creates turbulence that disrupts winds and slows them down. Thicker atmospheres also experience more friction, and are harder to get up to speed as well.

Rising and falling air doesn't stay at the same temperature. Air pressure in an area is equal to the weight of air above it: as height increases, less air is above that height, and the air pressure is lower. Rising air as a result is moving from higher to lower pressure: the pressure decreases, the pocket of air pushes the surrounding air outwards, doing work. The energy to do this comes from their air's heat, as a result the temperature of the rising air drops. The opposite happens with falling air, which heats up. The expected temperature change with height is called a lapse rate: in an active atmosphere, with lots of rising and sinking air, the temperature at different heights will follow the lapse rate, cooler up high, warmer down below, the change occurring roughly at a constant rate with height.

If their air has something that can condense, it forms clouds when cool: rising air cooling down is a common location for clouds to form. Condensation or freezing has another effect: it releases heat. As a result, rising air with condensation doesn't cool as quickly as dry rising air. one resulting effect: if the cloud remains behind and the air packet is forced back down in some way, it gets compressed and heated to a higher temperature than a dry air packet following the same path. This has been seen on Earth and jupiter, it creates so called chinook winds on Earth. If the air is allowed to rise, and has enough condensable material, it's upward speed will increase, generating an extremely tall cloud with strong winds. In other words, a storm.

Which brings us to lighting. Lightning is effectively very powerful static electricity, and is probably generated the same way, by two types of particles rubbing each other in an atmosphere, becoming charged, and being separated. the different charges build up an electric field, is something lets electricity flow, it discharges to create the lightning. On Earth, water ice and liquid water in thunderstorms cause this. On other planets, we're not sure of the source, though it has been detected. It's probably safe to say that a planet with lots of clouds and active weather has some sort of lightning.

Most weather happens in the lower atmospheres of planets. As height increases, the air thins, and the air gets hit with radiation and incoming sources of energy. At some point, this energy is enough to heat the air up significantly, and temperature decreases less, or stops decreasing with height. Hotter air above colder air does not allow convection: rising air becomes heavier than surrounding air, and sinks, higher air that sinks becomes much warmer and lighter than surrounding air, and is forced up again. At around this height in the atmosphere, most weather stops, winds still exist but activity is otherwise slower and calmer. The structure of the atmosphere at this point depends on the amount of radiation absorbed and the resulting temperatures.

Atmospheres don't really have a ceiling, they instead smoothly get thinner, and change to resemble whatever is in surrounding space. At some point, an atmosphere gets thin enough that winds cannot mix it, an the chemistry begins to change significantly with height. As the air gets thinner and more radiation comes up, most atmospheres start to heat with altitude above a certain point. Radiation also ionizes some gas, and their air is thin enough that recombination doesn't occur quickly. This air can conduct electricity, and is called an ionosphere, and a number of interesting things happen here.

As the trope describes, and alien sky is a great way to show you aren't on Earth. What would the sky from other bodies actually look like?

If a planet is covered in clouds or haze, probably pretty boring. You'd just see the haze, of whatever color. Possibly it would be lit if enough starlight got through, or dark is facing away, light may also be diffused throughout the clouds, reducing the day/night contrast. Rain of some sort may fall, or lightning if a planet had some.

Clearer skies make life more interesting. Clouds on other planets, if they exist, are probably variable like they are on earth, with lumpy, flats, filaments, and other familiar shapes. Planets with multiple types of clouds might have different colors in sky sky, see Jupiter's colored bands and turbulence and imagine from lower down. Faster winds if they exist may visibly distort or move clouds, and turbulence would create interesting sights also.

The day or night sky in a clear atmosphere, or a nonexistent one, would allow views of astronomical objects. Different types of central stars would appear to be different colors, see [[UsefulNotes/Stars]] for more. Obviously, stars would appear larger and brighter the close a body was. Binary stars, depending on the size of their orbits, would appear to be separate bodies, more or less close to each other. A rogue planet would obviously have no nearby stars, the sky would always appear like a night sky.

Other nearby bodies would appear in the sky much like Earth's moon does. A gas giant would appear large in its moon's sky, though depending on orbital distance might not seem that large. Rings, unless looking from the equator, would fill a large chunk of their sky if visible, or would block out stars if not.

The stars seen from a sky would change from system to system. Planets in the space between galaxies would have little visible, just faraway galaxies. Planets in a galaxy would see it in the sky much like the milky way is visible from Earth (in rural areas without too much light.) Planets in star clusters would have a very bright night sky, possibly with gas visible.

When you think of Rings, you probably think of Saturn's big bright disk. However, all gas giants in our system have rings, Jupiter, Neptune, and /Uranus's are smaller and made of darker/dustier materials. All rings are made from dispersed small bits of material: dust, ice particles, carbonaceous material. The material can come from a number of sources: a destroyed body, impacts, material already floating from outside the system, eruptions, anything that produces dusty material. The particles as they orbit clump together and get knocked apart by impacts.

Rings exist because of tidal forces. Normally, a ring of material around a body would come together into a body, this is how planets and moons originally formed. However, within a certain distance of a body, tidal forces are stronger than the gravitational force holding such a body together. These tidal forces could destroy a body that gets too close (It's thought that Saturn's rings may have formed this way), and also prevent a body from accumulating around a planet. This distance is larger the higher gravity from the central body, and also changes with material (A denser material resists tidal forces more, it is smaller for the same mass, and this means lower tidal forces and higher surface gravity to keep the orbiting body together.)

Once formed, rings can be organized and energized by resonances with other orbiting bodies. Moon can act something similar to Jupiter with our system's asteroid belt, interacting with the dust particles, keeping them together, and adding energy to the system. Planetary rings will orbit near their equator, or near the plane of whatever system they are in. A planet's equatorial bulge pull ring material towards it, material also tends to align with a system's plain through gravity.

Ultimately, rings are expected to decay over time. some material may evaporate, or get changed through radiation. Rings also create drag on each other: ring material orbiting slightly further in moves faster, further out moves slower, and these materials drag on each other, slowing the inner material and causing its orbit to drop, while further out material is pulled faster and its orbit moves further out. These effects cause rings to spread out, eventually, material will collide with something, merge with a central planet, and overall become too faint or dispersed to be a structure anymore. This process will happen faster in heavier ring systems, lighter in fainter/dispersed ones.

Of the common phases of matter, liquids exist at the narrowest range of conditions. Below a certain pressure or temperature, only solids and gases can exist, lower pressure/higher temperature leads to gases, higher pressure/lower temperature to solids. Above this temperature and pressure, liquids can exist in a range of temperatures between the boiling and freezing points, both of which change as pressure changes. However, above a certain temperature and pressure, liquids and gases merge into something called a supercritical fluid, which does not act completely like a liquid.

The smaller window of conditions means surface liquid isn't likely as common as atmospheres or fully solid surfaces. To have such a liquid, a body must have an atmosphere with a high enough pressure, have a temperature not too high or low, and have enough material that can be liquid at the body's conditions. How much material exists determines what liquid features are found. Small amounts give river, creeks, and such. Larger amounts give deeper and deeper oceans. If the liquid makes up a large fraction of a planet's mass, the ocean is itself a major layer of the body as a whole.

Most likely, this liquid will be a volatile of some kind: their liquid ranges are the most likely to exist on the surfaces of planets. Our own solar system has water oceans and rivers (on Earth), and hydrocarbon rivers and lakes (on titan), these are likely common materials elsewhere: water is very common in the universe, and carbon compounds are also common, with a variety that can liquify at very different conditions. More exotic materials like ammonia can also be liquid at the right conditions, but is easily destroyed by photochemical reactions, so would need some protection or replenishment to last long. Magma oceans are a lot less common, but certainly exist somewhere: forming rocky planets will have a magma ocean if large enough, thanks to their heats of formation/impacts raining down. How long this ocean lasts is still not known for sure, ranges of hundreds or thousands to millions of years are estimated by scientists. Planets very close to hot stars would have magma oceans also, but amount of orbits where this an occur is very small. Iron and hydrogen/helium oceans are far less likely: iron can melt, but is so heavy it will sink to the middle of a planet unless it is all metal. Hydrogen/helium boil at extremely low temperatures, and if they condense out, no other materials exist to maintain an atmosphere to create needed pressures for liquids.//

What do surface liquids do? If rivers exist on a solid surface, they erode the surrounding terrain, creating channels and valleys. On earth, rivers tend to start as creeks and small flows, merging into larger structures as they flow to low lying terrain, and this shape likely occurs on other planets as well. Liquids can also chemically interacts with surface solids: preferentially dissolving some materials, possibly chemically reacting with others, and creating new types of material as a result.

As bodies of liquid get larger and deeper, they are able to carry large amounts of heat. Liquids will not flow as fast as gases, but being much denser they can carry a large amount of heat in a smaller space. On Earth, this creates weird weather changes (El Nino is the most famous), and oceans help transfer heat from the equator to the poles. Our good friend convection will create currents in an ocean, driven by temperature differences on the surface or by internal heat, other currents may be created by wind or tides. Large bodies of water also chemically influence and are influenced by their atmospheres and deeper processes. Absorption and release of gases buffers an atmosphere, removing gases that dissolve easily, and dissolving deeper materials or volcanic gases effect the oceans and change materials further down.

Planets made of solid material are not necessarily completely solid on their surfaces: instead, powders/dust are common. Some solid material is due to weathering, the material being broken up in various ways. Impacts from space break down material on any planet. Wind and surface liquid help as well on planets with these hings. Temperature changes may also break things down through thermal expansion and contraction.

An unusual form of erosion is found on some icy moons: darker areas absorb light and heat up, while lighter areas reflect light and stay cool. ice evaporates from the darker areas and deposits in the lighter areas, leaving other (often darker) chemicals behind, weakening the structure and further darkening it to continue he process.

(In technical terms, weathering is the breaking down of material, while erosion is the carrying away of material. Just in case you are talking to the scientists in question.)

Surface dust or soil may not be the same material as the bulk of the planet. Chemical reactions may produce new materials that coat the surface, atmospheric aerosols are a common way to do this. Material can freeze, producing equivalents of frost or snow if conditions are right. Life on Earth helps create soil, and may do so on other worlds with life as well.

Dust on the surface may simply build up, creating a layer on top of other material. If an atmosphere exists, wind will transport the dust: This can create dune or other structures if wind is constant enough, sometimes dust storms form with strong enough intermittent winds and light enough dust. Dust can participate in weathering: wind, glaciers, liquids, or anything else carrying it causes the dust to hit material on the surface, sometimes breaking it apart, or to scrape against nearby material. Given long enough, built up dust can form new rocks: pressure and/or heat may bind the particles together.

A more dramatic form of movement of called mass wasting: in common terms, and avalanche or landslide. This can happen any time solid material is undermined or builds up in too big a pile: if the structure is weakened in some way, material will slide down to some sort of lower point. Mass wasting, given enough time, can erode large structures, flattening the terrain out. The evaporation of ice on icy moons has eroded craters in this way, for example, as the leftover debris cannot support itself as well.

And finally, there are glaciers and ice caps. On Earth, glaciers are water ice on a rocky planet, but other worlds can have their own version of a lower melting/evaporating material forming large structures. Local ice can build up in colder areas: Poles, shadowed craters, mountains are common sites in our system. If the ice is on a higher place, it can creep (This is the actual term from materials science, the material deforms in the direction of force. This is the same process causing convection in solid planetary interiors), flowing over time. Glaciers can make *fantastic* eroders and weatherers: being solid, they can hold dust and larger blocks of material much better than liquids or atmospheres, and scrape away almost anything underneath them. Ices, if more or less reflective than other surface material, can influence weather, forming cold or warm spots which can create winds, even cooling or warming an entire planet.

Sometimes, a planet encounters something else in space. Sometimes, these two things get quite close together. Sometimes, thy get close enough to meet, and given speeds in space and the force of gravity, these meetings are very, very rough. How rough depends on the speed of impact, the size of the objects, and any gravity between them that pulls them together faster.

Impacts from space are extremely energetic: to get an idea, think of a reentering spacecraft, most must be shielded to prevent them burning up. The energy comes from a combination of Earth's gravity and the speed of the orbit (A spacecraft coming back from another world would have even more energy). Gravity may be smaller or larger on a planet (but still pretty strong), while speeds in space are almost certainly greater (remember, most spacecraft are orbiting Earth and moving with it as a result, a body moving in a star system is most likely not moving with a planet, and that difference in direction and/or speed creates a larger speed difference, adding energy to the collision) If you are reading through this entire useful notes, the energy of impact is how accretion energy/heat of formation is added to a planet.

Impacts can range from little dust grains though planet sized bodies. Small enough objects will burn up in a planet's atmosphere if it has one, or if moving slowly enough, may more gently become some of the dust that probably already exists. Smaller impactors that reach the surface contribute to weathering, hitting the ground and helping to break it up.

Make an object large and energetic enough, and it can form an impact crater. Craters are not simply formed by the impactor punching downward, it is instead an explosion. the fast moving impactor quickly compresses anything in its way, creating a shockwave. The compressed material than explodes outward, throwing stuff out of the area surrounding the impact. Hit with enough energy, and some material vaporizes or melts. If thrown hard enough, material may leave the planet: as likely, it falls back to the surface, possibly creating rays leading away from a crater, possibly coating the surface far away with ejected material, effecting the entire planet in a large enough impact. Shock waves from the impact will travel through the planet, possibly creating quakes as they pass, and possibly disrupting the opposite side of the crater as the shocks converge.

Hitting liquid or atmosphere obviously doesn't leave a crater, but is energetic all the same. Breaking up in an atmosphere is effectively like an air exploded bomb: impacts of Jupiter create visible fireballs and smoke clouds afterwards, and Earth has had its own explosions in the [[https://en.wikipedia.org/wiki/Tunguska_event Tunguska Event]] and [[https://en.wikipedia.org/wiki/Chelyabinsk_meteor Chelyabinsk meteor]] (Meteors love Russia I guess. A large land area does mean it is more likely to get hit) Hitting a liquid will vaporize and throw liquid over the surface, and create enormous waves from the impact site.

A general rule for craters: the more craters a planet has within a system, the less other stuff (volcanoes, weather, glaciers, etc.) it has. Craters form at a rate that does not depend on anything else happening on a planet (apart from, say, atmospheres burning up smaller impactors), and build up relatively slowly, giving time for other processes to wear them down/fill them in/smooth them out. A dead, airless body shouldby a ball of craters, a highly active planet should have almost none.

Two planet sized bodies hitting each other is a [[ApocalypseHow Planetary level Apocalypse event]]. Such srtong shockwaves and so much heat are released thatanything on a surface or atmosphere (If the planets have one) are destroyed, material will melt and/or vaporize all over. Large amounts of material will be ejected, but often not as much as you'd think: gravity is strong enough that such collisions likely lead to mergers rather than destruction. It's thought that collisions like this helped form our moon, broke up Jupiter's core, and changed the rotation rate/angle of Venus and Uranus. Merging bodies obvious can have different compositions than the original bodies did, both by mixing and by

Back to a smaller scale, craters can have different forms depending on how strong the impact was, how strong the material it is made from is, and other material properties. Lower energy impacts form simple bowl craters, more energetic impacts can form peaks in the center and/or rings within the bowl, called complex craters. the process is similar to a water drop falling in water and producing ripples and a peak, which got frozen in place, though the exact mechanism is a bit different. Craters in stronger material will of course last longer, craters in weaker material will tend to slump over time, leaving flat circular features if they slump far enough. Even if eroded or slumped, traces of the impact still exist underground, shocked minerals and other circular patterns will show that an impact happened.

Wnat to trip some people up? Question: Where is Earth's magnetic north pole located? answer: Near the south pole, obviously. Remember, the north magnetic pole of a magnet is the one that points north, magnetic poles are attracted to their opposites, so the geographic north pole has a south magnetic pole. Confusion over.

Many astronomical bodies are magnetic, planets included. In our own system, all planets have magnetic fields apart from Venus and Mars, and Jupiter's moon ganymede does as well. While some rocks, and iron, can be magnetized permanently, scientists think the large planetary magnetic fields are generated in a different way. The interiors of planets are hot, and too hot magnetic materials can't hold a field. Instead, planetary magnetic fields are generated through dynamos.

These dynamos require two things: a rotating planet, and a conductive liquid inside the planet that is convecting. the magnetic field is generated by electric currents. Move an electrical conductor through a magnetic fieldin any direction not along the field lines, and an electric current is produced. This electric current generates its own magnetic field, this field generates currents in conductors that flow through it, etc.: organize everything correctly, and the result is a much larger magnetic field. Inside planets, conductive fluid supplies the conductive material, and convection keeps it moving (generating an electric current requires energy, which comes from the electrical conductor, which slows it down unless something forces it to keep moving.) Rotation organizes the flow: convecting material that isn't rotating would probably produce random fields that would cancel each other out instead of adding together.

The result is a magnetic field that roughly aligns with a planet's rotation. However, convection is still somewhat chaotic, and this is where an odd property of conductive fluids makes its mark: Magnetic fields and the fluid they are in tend to move together. Either the field pushes the fluid to flow with it, the fluid pulls the field lines to move with it, or both. (Why this is takes some timeto describe, and this useful notes is long enough as is.) In fact, you could imagine the magnetic field in a conductive fluid as an object itself, with elastic field lines pulling, pushing, and being pulled and pushed, against the fluid it is in. Convection forces some fluid to move past field lines, but also drags the magnetic field with it, resulting in a field that doesn't quite match the simple dipole you think of from a bar magnet.

Magnetic field as a result don't exactly match the axis of rotation: in our own system, this ranges from the relatively close field of Saturn to the around 50 degree offset fields of Neptune and Uranus (think a quarter to a third of the way from one pole to another). Monitoring this offset is important to navigators on Earth (or was when Compasses were common, at least): Compasses really don't ever point exactly North, and the exact direction shifts from year to year. Magnetic strength also varies from the smooth decrease and increase expected from a simple dipole; scientists use more complicated shapes (4 poles, 6 poles, etc.) added together to model the actual pattern. Occasionally, a reversed field may show up in some Earth areas; pieces of Earth's oceans have had such fields at some points. Earth's field is known to have reversed in the past: We don't know if other planet's fields have also reversed, but our sun does as well, so it is plausible.

The presence or absence of a magnetic field tells something about a body's structure. Just about everything spins fast enough (even relatively slow Venus: currents in Earth's core move at about a few miles/kilometers a year, much slower than the thousands of kilometers a year rotation speed Venus would have), so the presence of a magnetic field shows convection and a conductive fluid, while the absence of a field points to the absence of a conductive fluid, or no convection within such a fluid. Venus and Mars lack fields while Earth does: It's thought that both have little activity in their mantles (Venus is too thick due to a lack of water/thick crust stopping heat loss, mars has cooled too much), meaning little flow, little heat lost from their conductive cores, and little convection compared to Earth. Jupiter's moon Ganymede has a field, while similar moons Io, Europa, Callisto, and titan do not. It's thought that Callisto and Titan haven't fully differentiated and no conductive iron layer exists: Io and Europa have such cores, but so much heat is generated tidally nearer the surface that the outer layers are hot and keep their cores from losing much heat.

Once generated, a magnetic field extends far outside the planet, and organizes the space around a planet. This space is not completely empty (Really, no space is, matter just gets continually thinner and thinner, though its all a good vacuum by our standards, and matter gets ridiculously spread out at some locations), it has enough ionized material to conduct electricity. This ionized material forms a plasma, which conducts electricity, forming an electrically conductive fluid. The magnetic field and this plasma interact with each other, and like in cores, the magnetic field lines and the plasma tend to move together. Near a star, stellar winds are also a plasma, this wind drags a magnetic field out from the star, and blows the field of a planet into something like a comet tail shape, with a sperical section facing the star, and a long tail trailing away. Inside this area is the planet's magnetic field, outside is the field of the stelllar wind.

It is said that a magnetic field protects from atmospheric loss by blocking a stellar wind, but this may not be true depending on the field. It blocks materials from directly flowing to a planet, but also captures energy and material from a much wider area and can channel this to a planet. Earth, Venus and mars are thought to lose atmosphere to solar wind at about the same rate, even though Earth has a field and the others do not.

Magnetospheres do a lot of interesting things, but there are a couple that are more visible and important for anyone near a planet. Radiation belts are one of these: Magnetic field trap charged particles (Uniform fields cause changed particles to move in circles, planetary fields are not uniform but can still trap particles in some locations) Energetic particles are a type of radiation, and the high energy particles trapped can damage electronics, expose living things to radiation, effect material that travel through them, among other effects. the material in a magnetic field that forms this radiation has a number of sources: Stellar wind material can enter a magnetic field, a planet's atmosphere can shed a little bit of material, and moons can produce material. Anything that produces gas that can be ionized will work.

The other effect is Auroras: glowing lines or curtains of light in the upper atmosphere. These are produced when energy added to a magnetic field produces electric currents. Currents moving across magnetic field lines are defected, but currents flowing along a field line can flow with no problem. Energy deposited at some point in the magnetic field as a result can flow along a field line, hit the upper atmosphere, and cause the gases there to glow. Since most field lines in a magnetosphere link to the polar regions of a planet, that's where auroras happen. Aurora color and appearance depends on the gas in the atmosphere, and on the amount of energy deposited.

Sources of energy include stellar winds: as a stellar wind moves past a magnetic field, it can create turbulence as a non-plasma fluid does: this turbulence mixes plasma into the magnetic field and also deposits energy, which is stored in the field and released over time. The planet's rotation also means that the stellar wind's magnetic field and the planet's can sometimes interact, allowing material to flow into the planet's field. Some energy can come from a planet's rotation: a magnetic field rotates with a planet, and forces plasma to rotate with it, energizing the plasma. Most of the time, this rotation rate is faster than orbital speed, meaning plasma is pushed outward by centrifugal effects, where it cal be further energized. This energy is transferred by electric currents from the planet, and these currents generate auroras. Moons can help produce auroras as well: they can generate plasma, and/or disturb existing plasma, these disturbances can generate their own currents.

We're searching, but we haven't found it yet outside of Earth. Probably the biggest feature in media, with forest planets, grasslands, [[StarfishAliens weird aliens]], [[{{4X}} galactic empires fighting]], etc. all a form of life found elsewhere.

So, what conditions lead to life? The answer is.....We don't actually know. (shrug shoulders). Obviously, Earthlike conditions work, with water, organic chemicals, a source of energy, but whether there are more conditions needed and/or other chemicals could do the job is unknown.

Part of the issue is defining life, and figuring out what features are necessary to the stuff weird stuff walking and crawling and growing all around us (and [[TomatoInTheMirror Are Us!]]) Self organization, reproduction, the use of energy are important conditions, but how many possibilities exist within that is, not well known.

In our own system, Earth has it, and Mars, Europa, and Enceladus are the most promising candidates. Mars has had surface water in the past and still might have some underground, while Europa and Enceladus have both subsurface oceans and tectonic activity for energy. The clouds of Venus are sometimes speculated as an environment, water exists there, the sun would supply energy, sulfuric acid could be adapted to, and Venus may have been more like Earth early in its history.

Once life exists, what form does it take? Are there just bacteria like organisms? Insects? [[FungusHumongous Giant Mushrooms]] Once again, [[RunningGag We're not sure]]. Microbes are almost certainly much more common, they are obviously simpler, and Earth didn't have multicellular life for much of its history. Exact evoluton is trickier: It's likely multicellular life is similar in some level if it exists, since a lot of Earth life adaptations are functional in ways extraterrestrial life would have to be as well, and it responds to similar physical restraints. Which properties of Earth life are like this is speculated or guessed, but not known for sure. Life will probably compete for resources, consume those resources, move, support itself against gravity, among other activities.

These likely exist. In fact, we live on one: Earth is mostly Ocean these days, and likely in its early history was covered in ocean apart from some small islands until continents began forming about 3 billion years ago. (Even earlier, Earth has a magma ocean while forming, but this is not the Ocean the trope is thinking of)


A world covered in ocean could be anything from a rocky planet with lots of water to a planet where water makes up a big fraction of its mass. Also needed is the right combination of temperature and pressure: pressure comes from a thick enough atmosphere, the temperature can come from whatever combination of reflection, greenhouse warming, and energy and distance from a central star keeps water in the liquid range. With stretched definitions, an "oil planet" of organic chemical oceans could form as well, these could be colder or warmer than the standard ocean planet


Ocean planets are good candidates for life, since, well, Earth has oceans and lots of life. An interesting question is whether a planet has continents: Earth's continents are formed from different materials than rocks under the oceans, and formed through the unique process of plate tectonics. It is possible that such processes haven't occurred yet, or don't occur on some other planet, also possible is having so much water than no continents break the surface.

Volcanic planets also exist: See Jupiter's moon Io, with numerous volcanoes spitting plumes, lava, and other eruptions all over the moon. Or see a newly formed/young planet, either covered in magma, or still hot enough that the rocky outside layer is thin and lets through enormous amounts of eruptions.


Of course, whether this counts of not is up to you. An extremely volcanic world covered in magma flows, volcanoes everywhere, etc. probably only exists on newly formed planets, even Io is mostly covered in frost of sulfur dioxide, forming something like HailfirePeaks. Other planetary processes will also compete with volcanoes to shape the terrain.


To form such a planet, pick a body that has a lot of internal heat (tidal heating really helps, but younger/larger also adds to its volcanism), have as little other stuff as possible, and you are good to go. for an interesting variation, repeat the above, but give the planet an ice layer on type, for a cryovolcanic world. Less heat is necessary for such a planet.

In a sense, most solid bodies we know of are desert planets: no life, no liquids. However, a more movie style desert planet, sandy, thick atmosphere, hot and dry, is actually a [[https://en.wikipedia.org/wiki/Desert_planet plausible planet type]], possibly being habitable at a wider range of distances than Earth. Such a planet would be very dry, mostly rock and sand with occasional pools of water on the surface, and some sort of thick atmosphere.

Even this type of plan kind of exists in our solar system, if you stretch things a bit. Saturn's moon Titan is effectively a very cold version of such a desert; it has dunes, seasonal rivers with shallow lakes, and small amounts of rain. Different chemicals substitute for Earth equivalents: hydrocarbons, mostly methane, act as the liquid, water ice is the bedrock, random organic chemicals for the "sand", the atmosphere is nitrogen with a small amount of hydrocarbons instead of nitrogen + oxygen + small fraction of water vapor. Only things missing are some methane moisture farmers, some raiders, a crime ridden spaceport.... Mars could also count, it has dust storms, water clouds, dusty soil, and possible occasional water flows, but the atmosphere is a lot thinner and long lasting water does not exist.

Like the other planets here, many world exist with icy surfaces: See most solid bodies in the outer solar system. A movie style icy planet, that is effectively a cold version of Earth, is also very, very possible: simply cooling an Earthlike planet until water freezes will do the job. It is thought that Earth went through a [[https://en.wikipedia.org/wiki/Snowball_Earth period like this]], on a smaller scale, northern parts of the Earth iced over during more recent ice ages.


A planet covered with water ice is self stabilizing: ice and snow reflect large amounts of light, further cooling such a planet. Outer solar system icy bodies are some of the coldest places we know of, because of this reflection in addition to being far from the sun. If ice does melt or evaporate, however, the changes can be very fast, melting ice exposes darker terrain, which absorbs more light, which heats the planet more, melts more ice, etc.

All of these are variations on "earthlike planet" with life. Whether they exist, and what is needed for them to exist is...uncertain. (See the section on life in the above part of this useful notes)


What make a planet Earthlike is hard to say, but these are probably not common in RealLife. We're not sure just how many conditions are needed, and even Earth wasn't Earthlike through big chunks of its history (no continents for 1-2 billion years, no multicellular life until a few hundred million years ago out of several billion, no oxygen until about halfway through is history). That said, a world like Earth is a pretty good place for life: liquid water is a good solvent for organic chemicals, which are stable at appropriate temperatures, Earth's geologic activity recycles and controls materials useful for life, sunlight provides a lot of energy, among other conditions.

Our own solar system has at least 8 full planets, at least 9 dwarf planets, dozens of moons large enough to be spherical, plus any undiscovered objects out there. Since the 1990's and 2000's, large numbers of extrasolar planets have been found as well. Here we write about the standouts among them, as well as some theoretical types of planets we may find in the future.

Yes, our own planet. Compared to the other planets we've found, Earth is really, really weird. most obviously, it is place known to have life, including the gelatinous thing fixed on a rocky support structure reading this right now. It is also the only one we know of with deep oceans and solid continents (some extrasolar planets may be water planets, but oceans they have would be too deep for land. titan has liquids, but in shallow lakes at most). it is the only place known to have plate tectonics: most worlds have so called stagnant lid convection, where the upper part of the planet stays in place, maybe getting pushed and pulled, but stays mostly separate from the creeping mantle below. In Earth's plate tectonics, the upper part takes part in convection: seafloor ridges draw material from several hundred miles down, while subducted material sinks all the way to Earths core. Earth's atmosphere is also unusual in having lots of oxygen, and the structure is slightly different: most atmospheres have a layer where temperature decreases with altitude, than a layer where it stays roughly constant, than increases. Earth has 4 layers, one where temperature decreases with altitude, than increases, than decreases, than increases. Earth's moon is also unusually large compared to its main planet. It has a magnetic field, like most planets but unlike its two most similar bodies Venus and Mars. It has all of the features listed in the second part of this article, apart from planetary rings.


What makes Earth so unusual? Having the right conditions for liquid water is almost certainly a huge part. Water of course makes up oceans, and makes a good medium for life. It also softens rock, allowing Earth's plates and mantle to convect more easily, allowing plate tectonics. Faster mantle convection draws more heat from Earth's liquid iron outer core, creating a magnetic field. Water + sunlight is the most abundant source of hydrogen for life, and the leftover oxygen fills the atmosphere and allows even more types of life. Oxygen photochemically reacts to produce ozone, which absorbs radiation and further heats the middle atmosphere. What other features are necessary for an Earth, we do not know. Our moon may be important, it stabilizes Earth's rotation and prevents unstable seasons. The moon's formation may have also have changed the composition of Earth. Planetary migration in our solar system, and the presence of Jupiter, may have influenced Earth in other ways necessary for some of its features.

[[CaptainObvious Because we live on it]], Earth is much easier to study than other planets, and the much greater knowledge we have about Earth is used to help understand similar worlds.

Is our solar system weird? It's hard to know for sure. Most systems we've found look very, very different. However, with the technology available, it is easier to detect certain types of planets than others, biasing for larger, closer in worlds to smaller stars. It's said that, if we tried to detect planets around the sun from nearby stars, we'd probably/possibly detect Jupiter, maybe Neptune as well, but other planets would be almost impossible to see, let alone the various moons, dwarf planets, etc.


We don't know our fellow planets nearly as well as Earth, but know them a heck of a lot better than any extrasolar ones: They are far easier to see with telescopes, and we can send space probes.

Telescopes can barely make out features of other stars, how on Earth (or space, or your body of choice) do you spot them? there are a couple of important ways: transits, and the effect on their star's motion. When a planet crosses a star, the star's light dims very slightly, sensitive enough instruments can detect this dimming. Depending on what type of light is dimmed and the amount dimmed, scientists can estimate the volume of a planet, and get some idea of what art of it is made of. As a planet orbits a star, its varying gravity causes the star to wobble back and forth, this change in velocity, and in theory position, can be measured by sensitive enough instruments, and used to determine a planet's mass and orbit. in theory, planetary radiation could be detected, and other detection methods have been proposed, but discoveries have not yet been made this way.


From size, mass, orbit, and spectrum data, scientist can take a guess at what planets might be like. the information is very limited, and a lot must be guessed from knowledge of how weather, physics, etc. work.


Rogue planets cannot be detected in these ways, but being away from stars, could in theory be detected directly by light they emit. several proposed rogue planets have been seen, but not confirmed yet.

51 Pegasi B was the first exoplanet around another main sequence star. It used the so called Radial velocity method: measuring using red shift a star's movement back and forth as it is pulled by the planet orbiting it. Further observations of the planet have nailed down its details: It's a bit less than half the mass of Jupiter, orbits in 4 days around its star, and water has been detected spectroscopically in the atmosphere.


Since its discovery, more Hot Jupiters have been found: planets roughly the mass of Jupiter, almost certainly gas giants, orbiting extremely close to their stars, closer than Mercury. These planets are easier to detect than other types: being large, close, and fast moving, they cause their stars to wobble more, and cover more of their star during transits. Hot Jupiters didn't match planetary formation models for our own system: Plants too close in were expected to not grow big enough to pull in hydrogen and helium and form such a giant. Planetary scientists have recently come up with models that can explain this, with gas giants either migrating closer to the their stars thanks to gravitational interaction with gas around the star, and/or with small bodies they encounter, or forming from Super Earths that we've found around other stars. How common these are is hard to say: they could be the most common type of gas giant, or could be not that common but simply much easier to detect.


Hot Jupiters are obviously hot, thanks to being so clase to a are. They have ther extreme properties as well: most will be tidally locked that close, resulting in a much hotter atmosphere facing the star and a much colder one facing away. This temperature difference will create extremely fast winds, and the heat will puff up the planet's atmosphere: most hot Jupiters probably have a much larger radius than Jupiter despite not having similar masses. Rocks can form clouds and an equivalent of snow or rain in these high temperatures. The heat may strip hot Jupiter atmospheres, though the high mass of a planet, and high gravity, means this probably happens slowly.

Pulsar planets are strange beasts: Scientist expect that supernovas would destroy any planets nearby. Some of these planets formed from leftover supernova material that stayed near the star after the explosion, or possibly from large planets that somehow weren't completely destroyed. It is also possible that planets are captured from other stars. Collisions of smaller bodies to form a neutron star may also produce planets by throwing material off, that accretes into a new body.


It is perhaps not surprising that a pulsar planet was the first discovered. Pulsars rotate with a very, very exact timing, and small changes in timing due to a planet's orbit effecting the pulsar allow them to be detected.


Pulsar planets will have unusual environments, to say the least. The intense radiation from the pulsar almost certainly sterilizes the planet (unless it has a huge amount of atmosphere to block it), and the intensely focused radiation beams likely creates lines in the planet.

Planets made of similar material as Earth, but larger. Seems simple enough, and they probably have the same range of properties any other rocky world has, with more gravity, volcanic activity than similar small worlds, a denser possible atmosphere due to greater gravity, etc. For more unusual properties, super Earths, when heavy enough, may retain helium atmospheres. This may make them harder to detect, such a helium atmosphere by some predictions may extend far out from the planet, making it look like a less dense planet (Uranus/Neptune type icy planet) rather than a concentrated rocky planet with a thin but detectable atmosphere. Super Earths may also, depending on the model make plate tectonics either easier or more difficult to form (Science is fun, isn't it?) If Easier, Earth may be joined by other planets in having this unusual feature.


Lots of these types of planets have been found, by one of or a combination of being easier to detect and being common in general. It's suggested that our own solar system may have formed larger rocky planets, but Jupiter's migration early on scattered and absorbed material from the inner solar system, leaving the existing sized planets instead. (In this model, Mar's small size is also due to this migration: without Jupiter moving close to its orbit, Mars would have had enough material to be about Earth/Venus sized.)

With enough exoplanet discoveries, scientists are finding a good number of roughly Earth sized ones within their star's habitable zones. Unfortunately, without measurements of their atmospheres, temperatures, and such, actually seeing if these planets have water, let alone life, is difficult. (Detecting water by itself is not enough, water vapor can exist in an atmosphere without liquid on the surface.)


The biggest wrinkle for many habitable zone exoplanets: Around smaller stars, the theoretical habitable zone is close enough to tidally lock a planet. This most likely created a possibly habitable thin zone between the light and dark hemisphres, rather than a fully habitable planet.

[[UsefulNotes/LocalStars The Closest star system to our sun]]... has a planet! At least the red dwarf proxima Centauri does. We thought we'd found on in 2012 around, orbiting close to Alpha Centauri B, but this was found spurious some time later. Other planets have been theorized, but none detected around the central stars. Proxima Centauri, the red dwarf, does have a confirmed planet around it. the planet is around Earth sized, and is in what would be the habitable zone...but Proxima Centauri has such strong solar winds that any atmosphere is likely stripped.


It may be that Alpha Centauri A and B cannot have planets, some simulations suggest that the two stars would disrupt any planets formed, or have disrupted any accretion dick which may have formed. Of course, this may not be the case, and as the closest star system, planetary searches continue there. Whether we can [[VideoGame/Civilization send a ship there]] and [[VideoGame/SidMeiersAlphaCentauri explore the native life]] is still up for grabs.

A hypothetical planet, mentioned way at the beginning of this useful notes and than forgotten. These in theory form in systems with a high carbon to oxygen ratio: Our own system has a higher oxygen to carbon ratio, so none of these have formed. Our technology is still not good enough to detect these in other systems: we'd need good spectroscopy to detect the expected chemicals, carbon planets by mass/size just look like icy or rocky or mixed planets.


Such a planet likely has various carbide compounds in its interior, possibly large amounts of diamonds as well. On any surface, hydrocarbons, tars, waxes, and such would dominate. Water would be nonexistent, but organic chemical oceans are plausible with the right conditions.


Gas giants and icy planets in a carbon rich system wouldn't be too different from our own, apart from less water, more methane and hydrocarbons.