History UsefulNotes / Planets

The end result is a cool, quite probably beautiful but hostile alien planet. Many scientists hypothesize that these 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.

Superearths are much like terrestrials, only heavier and thus have some specific features common terrestrials lack. In essence, a superearth is a planer that has a mass over three Earth masses; the upper limit is variable and depends on temperature, as colder worlds accumulate heavy atmospheres typical for gas giants much easier than hotter ones. Think the upper limit to be seven to ten or twelve Earth masses, more for hotter worlds, less for colder.

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. But keep in mind that colder superearths tend to gather hydrogen-helium atmospheres and grow seamlessly into the gas giant class...

Basically, planets come in three flavors: terrestrial planets, giant planets and dwarf planets. Most astronomers don't consider dwarf planets to be true planets, but they are similar enough to be described here, too.

These planets are the most common ones described in fiction. [[AllPlanetsAreEarthlike Many sci-fi universes forget completely about other types and concentrate on the Goldilocks]]. There's various reasons for this: [[MostWritersAreHuman humans can run around on them]], they can be easily mocked up in the backlot or [[BBCQuarry local quarry]], and they don't require a lot of expensive prop work. It could be imagined that in most universes, non-Earthlike planets are in fact quite common, but [[LawOfConservationOfDetail nobody cares about them and so we don't hear about it]]. It's a shame, since non-Goldilocks aren't boring at all, many of them have their own stern and inhospitable beauty and can provide cool and unique plot points.

These planets are the most common ones described in fiction. [[AllPlanetsAreEarthlike Many sci-fi universes forget completely about other types and concentrate on the Goldilocks]]. There's various reasons for this: [[MostWritersAreHuman humans can run around on them]], they can be easily mocked up in the backlot or [[BBCQuarry local quarry]], and they don't require a lot of expensive prop work. It could be imagined that in most universes, non-Earthlike planets are in fact quite common, but [[LawOfConservationOfDetail nobody cares about them and so we don't hear about it]].

The line between Goldilocks and desert worlds is thin and fuzzy. Young Marses can be welcoming and inviting small Goldilocks; very old Earth-like planets can degenerate into big lifeless deserts. On the other hand, these planets are one of the easiest targets of {{Terraforming}}; scientists speculate that we already can terraform Mars if only we manage to get over all this capitalist and militarist claptrap and unite our resources.

The most extreme super-volcanic worlds can possibly be found in the inner systems of neutron stars, which have very potent gravity, stimulating unimaginable levels of volcanism, and very potent radiation, showering the planets in X-rays (and the planets themselves are likely to be very rich in radioactives, since they formed from fresh supernova remnants). These planets should be fascinating if shown properly in a sci-fi story with characters crazy enough to visit them.

If the primary star emits enough radiation (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 are made that tholin can be a very useful resources to space explorers if we could bio-engineer micro-organisms (bacteria and algae) capable of processing this gunge into food and fertilizer.

An icy rockball is differentiated; it means that rock and metal, the usual planet stuff, is concentrated in its core and mantle, and the crust is made of ice. Full planet-sized bodies are very likely to be differentiated by volcanism; small gas giant moons are only differentiated if tidal fluctuations cook up their otherwise nonexistent volcanism, as in Europa's case. Iceballs aren't differentiated, they are made of mostly ice, with a possible small rocky core. Iceballs and small icy rockballs typically have little to no atmosphere, being the outer system equivalents to common rockballs, only having white ice and black starry skies of void - inhospitable indeed; larger ones can have dense atmospheres and oceans, making them ''alternate ocean worlds'' (see below).

An icy rockball is differentiated; it means that rock and metal, the usual planet stuff, is concentrated in its core and mantle, and the crust is made of ice. Full planet-sized bodies are very likely to be differentiated by volcanism; small gas giant moons are only differentiated if tidal fluctuations cook up their otherwise nonexistent volcanism, as in Europa's case. Iceballs aren't differentiated, they are made of mostly ice, with a possible small rocky core. Iceballs and small icy rockballs typically have little to no atmosphere, being the outer system equivalents to common rockballs; larger ones can have dense atmospheres and oceans, making them ''alternate ocean worlds'' (see below).

Iceballs are all very cold, at least twice as cold as the Antarctic can ever hope to be (140-150 Kelvin appears to be the typical snow line temperature, and further away from the star it can go all the way down to the ambient background radiation temp, which is close to absolute zero). But strong volcanism can partly melt the crust of icy rockballs, resulting in a great liquid ocean covered by massive pack ices (Europa is an example in our Solar system). Such oceans are considered good places for colonization.

There can be two types of greenhouse planets: wet and dry. Wet greenhouse planets still have lots of water vapor in their atmospheres, because they have magnetic fields that prevent atmosphere irradiation by solar wind and thus breakdown of water molecules. These are easy enough to terraform: chilling them up with shades causes water vapor to condense and turns down the heat. Dry greenhouses, like Venus, lack water altogether.

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/CO2 atmosphere.

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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 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 these 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).

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

''Alternate ocean worlds''

Small planets, too small to hold most gases except for the heaviest. They can have any temperature depending on where they are relatively to their star, but they have no water and no to almost no air. Mercury is a rockball, Earth's Moon is one, and UsefulNotes/{{Mars}}, though it used to be more similar to Earth, turned into a rockball-like desert planet by losing water and atmosphere. Though Mars is not the worst case of rockball, and can be recovered by terraforming.

An icy rockball is differentiated; it means that rock and metal, the usual planet stuff, is concentrated in its core and mantle, and the crust is made of ice. Full planet-sized bodies are very likely to be differentiated by volcanism; small gas giant moons are only differentiated if tidal fluctuations cook up their otherwise nonexistent volcanism, as in Europa's case. Iceballs aren't differentiated, they are made of mostly ice, with a possible small rocky core.

Iceballs are all very cold, at least twice as cold as the Antarctic can ever hope to be (140-150 Kelvin appears to be the typical snow line temperature). But strong volcanism can partly melt the crust of icy rockballs, resulting in a great liquid ocean covered by massive pack ices (Europa is an example in our Solar system). Such oceans are considered good places for colonization.

In the Solar system, all gas giants are cold. They all are behind the snow line, a radius beyond which ices can exist in solid form indefinitely. But in other star systems there were found gas giants really close to their stars. It's probably observer bias, as such planets are the easiest to detect from afar, but most known exoplanets are hot gas giants.

Small planets, too small to hold most gases except for the heaviest. They can have any temperature depending on where they are relatively to their star, but they have no water and no to almost no air. Mercury is a rockball, Earth's Moon is one, and Mars, though it used to be more similar to Earth, turned into a rockball-like desert planet by losing water and atmosphere. Though Mars is not the worst case of rockball, and can be recovered by terraforming.

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 CO2 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 the Solar system, Venus is an example.

These planets are the most common ones described in fiction. [[AllPlanetsAreEarthlike Many sci-fi universes forget completely about other types and concentrate on the Goldilocks]].

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 were 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, that goes for a hundred of AUs from the Sun at the farthest point of its orbit.

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

''Goldilocks planets''

''Non-Goldilocks terrestrial planets''

'''Chtonian planets'''

'''Rockball planets'''

'''Greenhouse planets'''

''Large and small gas giants''

''Cold and hot gas giants''

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.