History UsefulNotes / Planets

This is the name for planets that are most similar to Earth: not too hot and not too cold, and thus suitable for life as we know it. These planets have to be located in a biozone (the area around a star that has comfortable insulation level). They are likely to develop their own life; note that they can only have oxygen in the air if they have life, because oxygen is a very active gas that quickly reacts with something and gets depleted if there aren't any organisms that produce it[[note]]The other telltale sign of life on a planet is methane[[/note]]. A lifeless Goldilocks planet is likely to have an atmosphere of nitrogen, carbon dioxide, and similar nonbreathable gases.

''Alternate ocean worlds ("Alien Goldilocks", and we [[StarfishAliens really do mean alien]])''

This is the name for planets that are most similar to Earth: not too hot and not too cold, and thus suitable for life as we know it. These planets have to be located in a biozone (the area around a star that has comfortable insulation level). They are likely to develop their own life; note that they can only have oxygen in the air if they have life, because oxygen is a very active gas that quickly reacts with something and gets depleted if there aren't any organisms that produce it[[note]]The other telltale sign of life on a planet is methane[[/note]]. A lifeless Goldilocks planet is likely to have an atmosphere of nitrogen, carbon dioxide, methane and similar nonbreathable gases.

This is the name for planets that are most similar to Earth: not too hot and not too cold, and thus suitable for life as we know it. These planets have to be located in a biozone (the area around a star that has comfortable insolation level). They are likely to develop their own life; note that they can only have oxygen in the air if they have life, because oxygen is a very active gas that quickly reacts with something and gets depleted if there aren't any organisms that produce it. A lifeless Goldilocks planet is likely to have an atmosphere of nitrogen, carbon dioxide, methane and similar nonbreathable gases.

The climate will vary between the hemispheres, and the difference will be more drastic if the atmosphere is less dense. If the atmosphere is Earth-like or thicker, it's enough to level most of the difference, producing raging winds by the way; however, with little or no atmosphere the night side can freeze all the way to the ambient background temp, which is close to the absolute zero, and the day side will be scalding hot. On the other side, tidal lock may actually be beneficial for habitability, if it's a Mars-like cold world around a dim star. The day hemisphere, which otherwise would be very cold, will be heated to a comfortable temperature, and the night side will serve as a motherlode of easily accessible frozen volatiles. Some calculations suggest that this "half-Mars" could be a very common planet type around red dwarf stars.

!!MegaEarths

A Megaearth as [[http://en.wikipedia.org/wiki/Kepler-10c Kepler-10c]], that is around seventeen times more massive than the Earth and twice as large, is basically a superearth UpToEleven, with heavier mass -thus with more gravity- and larguer. Going even further we've [[http://en.wikipedia.org/wiki/HD_149026_b]], that has a mass similar to that of Saturn but its 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 elemets leaving just the lightest ones.

These planets have atmospheres and oceans, like Earth, but the catch is that the oceans are not made of water. Two commonly hypothesized types of alternate oceans are ammonium hydroxide (ammonia-water solution/compound) and hydrocarbons (the latter type exists in our system on Titan, the moon of Saturn). The atmosphere, too, is alien, suffocating and, quite possibly, toxic and corrosive. These worlds are colder than Earth (ammonia worlds are not much colder, Antarctic or Mars level, and hydrocarbon worlds are full-blown icy rockballs beyond the snow line).

The end result is a cool, quite probably beautiful but hostile alien planet. Many scientists hypothesize that alien life with unusual biochemistry can arise on these planets. Those aliens would need spacesuits to survive in the scalding-hot room temperature and toxic, corrosive oxygen atmosphere of Earth; in other words, our world would be as exotic, hostile and dangerous to them as Titan is to us. If no aliens are present, a hydrocarbon world is an inviting place to colonize because, despite its severe nature, its seas are made of fucking GASOLINE! (liquefied natural gas, to be precise). This is one of the most attractive selling points of space colonization, since unsealing the hydrocarbon storehouses of Titan can solve all peak oil problems. However, such hydrocarbon worlds would of course lack any oxygen in their atmospheres to actually ''burn'' the hydrocarbons with. (If oxygen ''had'' been present, the hydrocarbons would have combusted with it long ago.) But anyways, when we reach Titan, no one will be burning hydrocarbons that can be used for organic chemistry, and the fact that somebody used to ''burn'' valuable chemical resources will be taught to children in history classes.

* Mighty oceans. Many superearths are theoretized to contain much more water than regular Goldilocks, making them waterworlds or, for hotter ones, something intermediate between waterworlds and greenhouses (imagine vast seas that are kept from boiling by great air pressure, like planet-sized pressure cookers).

Gas giants vary in size. The smallest ones, like Uranus and 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).

There's nothing impossible in a planet having several moons. Mars, for example, has two, and Pluto has five: one huge, Charon, and four smaller ones. However, you should remember that more than one ''major'' moon is rare, and three big, round ones around a terrestrial planet is blatantly space-operatic. Multiple moon configurations can realistically contain several small moonlets, like Deimos and Phobos.

It's pretty much obvious that you can see an alien sun from a planet of an alien sun. For main sequence stars, the following rule applies: the dimmer the star, the larger it appears in the sky of a habitable planet, and vice versa. A red dwarf will look like a huge, reddish-orange circle, gently warming but not hurting your eyes, completely safe to stare directly into (unless it's currently flaring). A bluish-white A star will look like tiny, almost point-like, but piercingly, painfully bright sun, setting your eyeballs on fire even on a cursory glance. The reason for that is simple: distance of the habitable zone. A habitable planet around a red dwarf is very close to the star, close enough that it appears huge; a habitable planet of a bright star is far away.

Finally, you can also have skies with celestial bodies unlike any ones found on Earth, or even in the Solar System. The most obvious example is a gas giant in the skies of its habitable moon. It will appear like a huge stormy, stripey circle hanging in the sky in the same place, or oscillating around one spot, neither setting nor rising.

Also note that ammonia worlds are only possible around cool, orange or red suns; hotter ones like our Sun or even bluer radiate too many ultraviolet, which tends to break ammonia down. The end result is just a big honking icy rockball with nitrogen/CO[[subscript:2]] atmosphere.

There's nothing impossible in a planet having several moons. Mars, for example, has two, and Pluto has four: one huge, Charon, and three smaller ones. However, you should remember that more than one ''major'' moon is rare, and three big, round ones around a terrestrial planet is blatantly space-operatic. Multiple moon configurations can realistically contain several small moonlets, like Deimos and Phobos.

''Chthonian planets''[[hottip:*:Pronounced like "though" with a K at the front, or "li'''ke so'''" with a lisp. Don't blame us, it's Greek.]]

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

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

Yes, there are {{Cloudcuckoolander}}s among planets. A planet is eccentric if its orbit is eccentric, that is, elliptic, with the primary in one of the ellipse's foci. Such a planet will experience the ''other'' kind of seasons, not found on Earth, seasons affecting the entire planet. If seasons of eccentricity are combined with seasons of axial tilt, it may result in a weird interplay of climates, with one hemisphere where the seasons of two types cancelling each other, resulting in a mild climate, and one hemisphere where the seasons of two types reinforce each other, resulting in very harsh annual changes in weather.

Yes, there are {{Cloudcuckoolander}}s among planets. A planet is eccentric if its orbit is eccentric, that is, elliptic, with the primary in one of the ellipse's foci. Such a planet will experience the ''other'' kind of seasons, not found on Earth, seasons affecting the entire planets. If seasons of eccentricity are combined with seasons of axial tilt, it may result in a weird interplay of climates, with one hemisphere where the seasons of two types cancelling each other, resulting in a mild climate, and one hemisphere where the seasons of two types reinforce each other, resulting in very harsh annual changes in weather.

== Tidally locked and resonant planets ==

== High tilt planets ==

== Double (binary) planets ==

== Trojan planets ==

The end result is a cool, quite probably beautiful but hostile alien planet. Many scientists hypothesize that alien life with unusual biochemistry can arise on these planets. Those aliens would need spacesuits to survive in the scalding-hot room temperature and toxic, corrosive oxygen atmosphere of Earth; in other words, our world would be as exotic, hostile and dangerous to them as Titan is to us. If no aliens are present, a hydrocarbon world is an inviting place to colonize because, despite its severe nature, its seas are made of fucking GASOLINE! (liquefied natural gas, to be precise). This is one of the most attractive selling points of space colonization, since unsealing the hydrocarbon storehouses of Titan can solve all peak oil problems. However, such hydrocarbon worlds would of course lack any oxygen in their atmospheres to actually ''burn'' the hydrocarbons with. (If oxygen ''had'' been present, the hydrocarbons would have combusted with it long ago.)

The end result is a cool, quite probably beautiful but hostile alien planet. Many scientists hypothesize that alien life with unusual biochemistry can arise on these planets. Those aliens would need spacesuits to survive in the scalding-hot room temperature and toxic, corrosive oxygen atmosphere of Earth; in other words, our world would be as exotic, hostile and dangerous to them as Titan is to us. If no aliens are present, a hydrocarbon world is an inviting place to colonize because, despite its severe nature, its seas are made of fucking GASOLINE! (liquefied natural gas, to be precise). This is one of the most attractive selling points of space colonization, since unsealing the hydrocarbon storehouses of Titan can solve all peak oil problems.

One important feature of gas giants is that they are large enough to have their own mini-solar systems of large moons that can rival true planets in size. In the Solar system all gas giants are cold and boast subsystems of iceballs, icy rockballs, one rocky super-volcanic world (Io) and one cold alternate ocean world (Titan); warm and hot gas giants around other stars can have moons of hotter types, like rockballs, Marses, greenhouses or even Goldilocks like ''{{Avatar}}'''s Pandora. The catch is that a gas giant that's firmly settled in a circular Goldilock orbit is rare indeed, and eccentric giants tend to have moons with hope-crushingly severe annual heat changes. Imagine living on a planet where the winter is colder than Antarctic and the summer melts tin and lead. Also, the closer a gas giant is to the star, then less likely it has moons because of tidal interactions between the giant and its primary. Epistellar giants never have any moons for this reason.

Other than that, superearths can take most of the forms regular terrestrials can, barring, of course, the size-related types like rockball or Mars. You can have a searing-hot Chtonian superearth, an exaggerated super-Venus with a pressure of 500 bar, an alien ocean superearth with seas of ammonia, a titanic icy rockball, a giant raging ball of volcanic fire. But keep in mind that colder superearths tend to gather superdense hydrogen-helium atmospheres and grow seamlessly into the gas giant class...

A gas giant cannot form in the inner system, but it can migrate there. There are two types of inner-system gas giants: "eccentric Jupiters" and "epistellar Jupiters". The first type is in the process of migration, it occupies an eccentric, irregular orbit, coming closer to the sun at times, and further from it other times. They usually disrupt any formation of terrestrial planets by doing so. The second type is a planet firmly settled near the sun. They heat up, their atmospheres expand and, if they are heavy enough, they can become much larger than it's usually allowed for gas giants. These hot Jupiters are called "puffy planets" for their very low density. After that, their atmosphere is slowly grazed away by solar winds: many epistellar giants have enormous "tails" of gas being ejected from them stretching outwards. The end result is a chtonian planet.

''Chtonian planets''

They start like Goldilocks, but they are soon dominated by a runaway greenhouse effect and fail to overcome it by the way Earth did in its early history (trapping CO[[subscript:2]] in carbonate rock and condensing water into oceans). They become really hot, often hotter than the hottest rockballs and chtonians, with a monstruous atmosphere and chemistry absolutely unsuitable for life. In UsefulNotes/TheSolarSystem, UsefulNotes/{{Venus}} is an example.

A "snow line" or "ice line" is an imaginary line (actually, a sphere) around a star, beyond which solid ice can exist indefinitely without evaporating. A planet orbiting beyond the snow line can never have liquid water or water vapor; only ice can exist, which is treated like a rock rather than a volatile. A terrestrial planet formed in such circumstances is an icy rockball (if differentiated) or a dirty iceball (if not). Pure iceballs, containing little to no silicates, are possible, too. In our solar system, the region beyond the snow line is dominated by giant planets, but iceballs and icy rockballs exist as their moons and far-fringe dwarf planets; Europa and Titan are icy rockballs, Callisto and all moons of Uranus are dirty iceballs. In other star systems, particularly those without gas giants, bodies of this sort can be true planets.

The end result is a cool, quite probably beautiful but hostile alien planet. Many scientists hypothesize that alien life with unusual biochemistry can arise on these planets. Those aliens will need spacesuits to survive in scalding-hot room temperature and toxic, corrosive oxygen atmosphere; in other words, Earth will be as exotic, hostile and dangerous to them as Titan is to us. If no aliens are present, a hydrocarbon world is an inviting place to colonize because, despite its severe nature, its seas are made of fucking GASOLINE! (liquefied natural gas, to be precise). This is one of the most attractive selling points of space colonization, since unsealing the hydrocarbon storehouses of Titan can solve all peak oil problems.

One important feature of gas giants is that they are large enough to have their own mini-solar systems of large moons that can rival true planets in size. In the Solar system all gas giants are cold and boast subsystems of iceballs, icy rockballs and one cold alternate ocean world (Titan); warm and hot gas giants around other stars can have moons of hotter types, like rockballs, Marses, greenhouses or even Goldilocks like ''{{Avatar}}'''s Pandora. The catch is that a gas giant that's firmly settled in a circular Goldilock orbit is rare indeed, and eccentric giants tend to have moons with hope-crushingly severe annual heat changes. Imagine living on a planet where the winter is colder than Antarctic and the summer melts tin and lead. Also, the closer a gas giant is to the star, then less likely it has moons because of tidal interactions between the giant and its primary. Epistellar giants never have any moons for this reason.

This UsefulNotes page deals with planets, both solar and extrasolar, the types of them, their properties and how to avoid DidNotDoTheResearch when making ones up.