History UsefulNotes / Planets

10th Jun '16 4:02:31 PM ScorpiusOB1
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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 hypothetical superearths that could have formed in the innermost Solar System would have [[ColonyDrop crashed among themselves]] or [[HurlIntoTheSun fallen into the Sun, along with the debris of those collisions]].

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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 that, among other things, hypothetical superearths that could have formed in the innermost Solar System would have [[ColonyDrop crashed among themselves]] or [[HurlIntoTheSun [[HurlItIntoTheSun fallen into the Sun, along with the debris of those collisions]].
10th Jun '16 3:59:46 PM ScorpiusOB1
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Added DiffLines:

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 hypothetical superearths that could have formed in the innermost Solar System would have [[ColonyDrop crashed among themselves]] or [[HurlIntoTheSun fallen into the Sun, along with the debris of those collisions]].
10th Feb '16 9:57:48 PM RainbowPhoenix
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* The location of the center of gravity, AKA the barycenter, is dependent on the relative size of the two bodies. If both planets are of roughly equal mass, the barycenter will be at approximately the halfway point between them. If there is a significant difference in mass, the barycenter will be closer to the larger planet while still being in the empty space above that planet's surface.
7th Feb '16 12:44:09 PM RainbowPhoenix
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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, 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. This configuration is possible and even experienced directly by our scientists: the dwarf planets Pluto and Charon are in this configuration.

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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, 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. This configuration is possible and even experienced directly by our scientists: the dwarf planets Pluto and Charon are in this configuration.




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* Any moons will orbit the same center of mass as the planets themselves. If the moons are small, irregular rocks, it won't be an issue, but a large moon will cause major tidal reactions in both planets.
2nd Feb '16 9:22:31 PM RainbowPhoenix
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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). Those smaller than Uranus and Neptune are sometimes called gas dwarfs.

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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.



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. This configuration is possible and even experienced directly by our scientists: the dwarf planets Pluto and Charon are in this configuration.

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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. mass which, unlike a typical planet-moon pairing such as Earth and its moon, 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. This configuration is possible and even experienced directly by our scientists: the dwarf planets Pluto and Charon are in this configuration.
configuration.
26th Dec '15 3:09:08 AM ScorpiusOB1
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Added DiffLines:

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.
25th Dec '15 2:33:09 AM ScorpiusOB1
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These are former gas giants that migrated too close to their stars and had all gas blown from them by streams of particles (solar wind). None exist in our solar system, but some of them were detected around other stars. These planets are like huge Mercuries: airless, rocky, with lead-melting heat on the day side and chilling cold on the night side.

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These are former gas giants that migrated too close to their stars and had all gas blown from them by streams of particles (solar wind). None exist in our solar system, but some of them were detected around other stars. These planets are like huge Mercuries: airless, rocky, with lead-melting -or higher- heat on the day side and chilling cold on the night side.



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. If the moon is tidally locked, it will hang in the same place, or oscillate around one spot, neither setting nor rising. Another are compact systems, where (large) planets are in astronomical terms very close to each other, such as the one of [[https://en.wikipedia.org/wiki/Gliese_876 Gliese 876]]. Seen from one of them the others would look instead as stars as full-fledged planets, even showing phases as the Moon from Earth.

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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. If the moon is tidally locked, it will hang in the same place, or oscillate around one spot, neither setting nor rising. Another are compact systems, where (large) planets are in astronomical terms very close to each other, such as the one of [[https://en.wikipedia.org/wiki/Gliese_876 Gliese 876]]. Seen from one of them the others would look instead as of stars as full-fledged planets, even showing phases as the Moon from Earth.
25th Dec '15 2:31:58 AM ScorpiusOB1
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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. If the moon is tidally locked, it will hang in the same place, or oscillate around one spot, neither setting nor rising.

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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. If the moon is tidally locked, it will hang in the same place, or oscillate around one spot, neither setting nor rising. Another are compact systems, where (large) planets are in astronomical terms very close to each other, such as the one of [[https://en.wikipedia.org/wiki/Gliese_876 Gliese 876]]. Seen from one of them the others would look instead as stars as full-fledged planets, even showing phases as the Moon from Earth.
5th Sep '15 1:22:33 AM HeraldAlberich
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!!Terrestrial planets

These are the ones with a solid surface, rocky crust and innards that can be either molten and hot or solid and cold. In our solar system, there are four: Mercury, Venus, Earth and Mars.

'''Goldilocks 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 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[[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.

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!!Terrestrial ----
!Terrestrial
planets

These are the ones with a solid surface, rocky crust and innards that can be either molten and hot or solid and cold. In our solar system, there are four: Mercury, Venus, Earth UsefulNotes/{{Mercury}}, UsefulNotes/{{Venus}}, Earth, and Mars.

'''Goldilocks planets'''

UsefulNotes/{{Mars}}.

!!Goldilocks 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 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[[note]]The it. [[note]]The other telltale sign of life on a planet is methane[[/note]]. methane.[[/note]] A lifeless Goldilocks planet is likely to have an atmosphere of nitrogen, carbon dioxide, and similar nonbreathable gases.



'''Non-Goldilocks terrestrial planets'''

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

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'''Non-Goldilocks !!Non-Goldilocks terrestrial planets'''

''Chthonian planets''[[note]]Pronounced
planets

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



''Rockball planets''

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 (the upper limit is the melting point of rock, 1000 to 1500 K, the lower limit is the snow line - see below), 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.

''Marses (Desert worlds)''

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''Rockball planets''

!!!Rockball planets

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 (the upper limit is the melting point of rock, 1000 to 1500 K, the lower limit is the snow line - see line--see below), but they have no water and no to almost no air. Mercury UsefulNotes/{{Mercury}} is a rockball, Earth's Moon [[UsefulNotes/TheMoon 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 could be recovered by terraforming.

''Marses
{{terraform}}ing.

!!!Marses
(Desert worlds)''
worlds)



The line between Goldilocks and desert worlds is thin and fuzzy. Young Marses can be welcoming and inviting small Goldilocks with air, water and all the kit and kaboodle; 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.

''Greenhouse 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 chthonians, with a monstruous atmosphere and chemistry absolutely unsuitable for life. In UsefulNotes/TheSolarSystem, UsefulNotes/{{Venus}} is an example.

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 and are really tough nuts to terraform; on the other hand, they are good places for a [[TheEmpireStrikesBack cloud city]].

''Iceballs and Icy Rockballs''

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; see below) 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.

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 or a mixture of ice, sand and dust, with a possible small, fuzzy 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).

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The line between Goldilocks and desert worlds is thin and fuzzy. Young Marses can be welcoming and inviting small Goldilocks with air, water and all the kit and kaboodle; 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}}; {{terraform}}ing; scientists speculate that we already can terraform Mars UsefulNotes/{{Mars}} if only we manage to get over all this capitalist and militarist claptrap and unite our resources.

''Greenhouse planets''

!!!Greenhouse 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 chthonians, with a monstruous monstrous atmosphere and chemistry absolutely unsuitable for life. In UsefulNotes/TheSolarSystem, UsefulNotes/{{Venus}} is an example.

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 and are really tough nuts to terraform; on the other hand, they are good places for a [[TheEmpireStrikesBack [[Film/TheEmpireStrikesBack cloud city]].

''Iceballs !!!Iceballs and Icy Rockballs''

Rockballs

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; see below) 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 [[UsefulNotes/TheMoonsOfJupiter Europa]] and Titan [[UsefulNotes/TheMoonsOfSaturn Titan]] are icy rockballs, Callisto and all [[UsefulNotes/TheMoonsOfUranus moons of Uranus Uranus]] are dirty iceballs. In other star systems, particularly those without gas giants, bodies of this sort can be true planets.

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 or a mixture of ice, sand and dust, with a possible small, fuzzy 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 void--inhospitable indeed. Larger ones can have dense atmospheres and oceans, making them ''alternate ocean worlds'' (see below).



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 resource to space explorers if we could bio-engineer micro-organisms (bacteria and algae) capable of processing this gunge into food and fertilizer.

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If the primary star emits enough radiation [[UsefulNotes/{{stars}} (it's a G or bluer, or a severely flaring M), 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 have been made that tholin can 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.



''[[AlienSea Alternate ocean worlds]] ("Alien Goldilocks")''

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). Hot liquid sulfur, extremely cold liquid nitrogen and even colder liquid hydrogen have been also considered. 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.) And maybe, 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.

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''[[AlienSea !!![[AlienSea Alternate ocean worlds]] ("Alien Goldilocks")''

Goldilocks")

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 [[UsefulNotes/TheMoonsOfSaturn Titan, the moon of Saturn).Saturn]]). Hot liquid sulfur, extremely cold liquid nitrogen and even colder liquid hydrogen have been also considered. 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 could 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 could 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.) And maybe, 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.



''Super-volcanic worlds''

These planets and moons have very powerful volcanism which is their defining feature. They are constantly wracked in fiery eruptions and quakes, and no surface feature on them is permanent. Such high levels of volcanism are caused by tidal interactions with their primaries, which can be stars or gas giants. Super-volcanic planets are likely to orbit close to compact, dense stars like red or brown dwarfs, surrounded by more distant planets in resonating orbits; super-volcanic moons are found in similar positions around the heaviest of gas giants, like Jupiter's moon Io. A super-volcanic world is a dangerous place to visit, not only because of its volcanism, but also because of its position: it orbits close to a potent radiation source (dwarf star) or in strong radiation belts of a heavy gas giant, making it very severely irradiated.

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''Super-volcanic worlds''

!!!Super-volcanic worlds

These planets and moons have very powerful volcanism volcanism, which is their defining feature. They are constantly wracked in fiery eruptions and quakes, and no surface feature on them is permanent. Such high levels of volcanism are caused by tidal interactions with their primaries, which can be stars or gas giants. Super-volcanic planets are likely to orbit close to compact, dense stars like red or brown dwarfs, surrounded by more distant planets in resonating orbits; super-volcanic moons are found in similar positions around the heaviest of gas giants, like [[UsefulNotes/TheMoonsOfJupiter Jupiter's moon Io.Io]]. A super-volcanic world is a dangerous place to visit, not only because of its volcanism, but also because of its position: it orbits close to a potent radiation source (dwarf star) or in strong radiation belts of a heavy gas giant, making it very severely irradiated.



''Primordial worlds''

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''Primordial worlds''
!!!Primordial worlds



!!Superearths

Superearths are much like terrestrials, only heavier and thus have some specific features common terrestrials lack. In essence, a superearth is a planet 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.

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!!Superearths

!Superearths

Superearths are much like terrestrials, only heavier heavier, and thus have some specific features common terrestrials lack. In essence, a superearth is a planet 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.colder.



!!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 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.

!!Giant planets

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!!Megaearths

!Megaearths

A Megaearth such 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 mass--thus with more gravity- and larguer.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 Saturn, but its 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 elemets elements, leaving just the lightest heaviest ones.

!!Giant !Giant planets



'''Large and small gas giants'''

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). These smaller than Uranus and Neptune are sometimes called gas dwarfs.

Larger gas giants, like Saturn, have much greater volumes of gas, and their cores become less significant. These medium-size giants tend to be light for their size; Saturn, for example, has less average density than water.

Once gas giants reach a maximum to their size (that is about the size of Jupiter), making them more massive increases their mass but not their size (but see Puffy Planets below). Large gas giants are all of the same size, but it is their mass that matters. They become more dense, accumulate thicker mantles of liquid metallic hydrogen and develop more powerful magnetic fields that bend solar winds into deadly radiation belts. Close orbits around large, massive gas giants are very radiation-hostile places.

And once a gas giant reaches an even larger size (13 Jupiter masses) it ceases to be a planet. It starts its own fusion reaction (usually deuterium-deuterium) and becomes a star - a brown dwarf. But brown dwarfs still exhibit some properties of planets, so they are sometimes classified as planets too, especially if they orbit other stars like gas giants do. They kinda sit on the fence.

'''Cold and hot gas giants'''

In the Solar system, all gas giants are cold. They all are beyond the snow line. 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.

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'''Large !!Large and small gas giants'''

giants

Gas giants vary in size. The smallest ones, like Uranus UsefulNotes/{{Uranus}} and Neptune, 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). These Those smaller than Uranus and Neptune are sometimes called gas dwarfs.

Larger gas giants, like Saturn, UsefulNotes/{{Saturn}}, have much greater volumes of gas, and their cores become less significant. These medium-size giants tend to be light for their size; Saturn, for example, has less average density than water.

Once gas giants reach a maximum to their size (that is about the size of Jupiter), UsefulNotes/{{Jupiter}}), making them more massive increases their mass but not their size (but see Puffy Planets below). Large gas giants are all of the same size, but it is their mass that matters. They become more dense, accumulate thicker mantles of liquid metallic hydrogen and develop more powerful magnetic fields that bend solar winds into deadly radiation belts. Close orbits around large, massive gas giants are very radiation-hostile places.

And once a gas giant reaches an even larger size (13 Jupiter masses) it ceases to be a planet. It starts its own fusion reaction (usually deuterium-deuterium) and becomes a star - a UsefulNotes/{{star|s}}--a brown dwarf. But brown dwarfs still exhibit some properties of planets, so they are sometimes classified as planets too, especially if they orbit other stars like gas giants do. They kinda sit on the fence.

'''Cold !!Cold and hot gas giants'''

giants

In the Solar system, all gas giants are cold. They all are beyond the snow line. But in other star systems there were we have found gas giants really very 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.



'''Gas giant moons'''

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 ''Film/{{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.


!!Dwarf planets

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.
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.

!!Special Cases

'''Tidally locked and resonant planets'''

Tidal lock is a very common phenomenon, the more common the smaller is the primary. Habitable planets are likely to be in tidal lock if their primary is a K; if it's an M or dimmer, they will always be tidally locked. All moons (with very few exceptions) are tidally locked.

to:

'''Gas !!Gas giant moons'''

moons

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) ([[UsefulNotes/TheMoonsOfJupiter Io]]) and one cold alternate ocean world (Titan); ([[UsefulNotes/TheMoonsOfSaturn Titan]]); warm and hot gas giants around other stars can have moons of hotter types, like rockballs, Marses, greenhouses greenhouses, or even Goldilocks like ''Film/{{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.


!!Dwarf !Dwarf planets

What's different about dwarf planets is that they aren't massive enough to dominate their orbits. So orbits, so they are found in belts of various space debris: asteroid belts, Kuiper belts and scattered discs.
discs.

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 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, that goes for which ranges out to a hundred of [=AUs=] from the Sun at the farthest point of its orbit.

!!Special
orbit.


!Special
Cases

'''Tidally !!Tidally locked and resonant planets'''

planets

Tidal lock is a very common phenomenon, the phenomenon; more common the with smaller is the primary.primary stars. Habitable planets are likely to be in tidal lock if their primary is a K; if it's an M or dimmer, they will always be tidally locked. All moons (with very few exceptions) are tidally locked.



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, existing too the possibility of the terminator being a comfortable zone between a raging hot dayside and a frozen nightside.
Another possibility are [[http://storiesbywilliams.com/2014/01/13/news-from-space-full-model-of-exoplanet-created/ "eyeball planets"]], planets covered by ice except an ocean located in the planet's subsolar point (the point of the planet where the star is directly overhead all the time) and named so because [[CaptainObvious from space would resemble a big eye]]

Tidal lock also may be beneficial for habitability if the planet is hopelessly hot. For example, Alpha Centauri Bb, the first discovered planet in the Alpha Centauri system, is very hot, being so close to its primary; however, it's also very likely to be tidally locked, which limits the raging sea of lava to only one hemisphere and allowing us to safely land on the other, ice-bound one, and mine valuable ores.

The alternative to tidal lock is the 3:2 spin-orbital resonance. It is the situation in which Mercury is: the planet has discernible day and night, but they are twice as long as the Mercurian year. Planets with eccentric enough orbits may end up in resonance if they otherwise would be tidally locked.

'''High tilt planets'''

Seasonal cycles as we know them are produced by axial tilt: the axis of planetary rotation remains the same regardless of where is the planet on its orbit, and it results in the star heating one hemisphere better than the other one, then vice versa. The higher the tilt, the more pronounced are the seasonal effects. If the planet has a very high tilt, the following effects affect it:

to:

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 difference (producing raging winds by the way; 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, existing too the stars. The possibility also exists of the terminator being a comfortable zone between a raging hot dayside and a frozen nightside.
Another possibility are [[http://storiesbywilliams.com/2014/01/13/news-from-space-full-model-of-exoplanet-created/ "eyeball planets"]], planets covered by ice except for an ocean located in the planet's subsolar point (the point of the planet where the star is directly overhead all the time) and named so because [[CaptainObvious from space they would resemble a big eye]]

eye]].

Tidal lock also may be beneficial for habitability if the planet is hopelessly hot. For example, Alpha Centauri Bb, the first discovered planet in the Alpha Centauri system, is very hot, being so close to its primary; however, it's also very likely to be tidally locked, which limits the raging sea of lava to only one hemisphere and allowing allows us to safely land on the other, ice-bound one, and mine valuable ores.

The alternative to tidal lock is the 3:2 spin-orbital resonance. It This is the situation in which Mercury is: UsefulNotes/{{Mercury}}'s situation: the planet has discernible day and night, but they are twice as long as the Mercurian year. Planets with eccentric enough orbits may end up in resonance if they otherwise would be tidally locked.

'''High !!High tilt planets'''

planets

Seasonal cycles as we know them are produced by axial tilt: the axis of planetary rotation remains the same regardless of where is the planet on is in its orbit, and it this results in the star heating one hemisphere better than the other one, then vice versa. The higher the tilt, the more pronounced are the seasonal effects. If the planet has a very high tilt, the following effects affect it:result:



'''Eccentric planets'''

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 cancel 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.

'''Double (binary) planets'''

A long-standing staple in space opera, a double planet is a system of two planets acting like each other's moons. They will orbit a common centre of mass. This configuration is possible and even experienced directly by our scientists: the dwarf planets Pluto and Charon are in this configuration.

to:

'''Eccentric planets'''

!!Eccentric planets

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 each type cancel each other, resulting in a mild climate, and one hemisphere where the seasons of two types each type reinforce each other, resulting in very harsh annual changes in weather.

'''Double !!Double (binary) planets'''

planets

A long-standing staple in space opera, a double planet is a system of two planets acting like as each other's moons. They will orbit a common centre of mass. This configuration is possible and even experienced directly by our scientists: the dwarf planets Pluto and Charon are in this configuration.



'''Trojan planets'''

to:

'''Trojan planets'''
!!Trojan planets



!! AlienSky, and how to make it realistic

to:

!! AlienSky,
!AlienSky,
and how to make it realistic



'''Alien moons'''

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. Besides on a habitable moon of a gas giant, other moons would look like multiple big moons on the sky.

Also note that the potential number of moons around the planet is greater if the planet is further away from its sun. Yes, it's tidal interactions again. Tidal interactions between star, planet and moon tend to disrupt the system if the star is too close. If the planet ends up tidally locked to the star, orbits of any moons will decay and they will eventually fall or be destroyed and turned into debris rings. However, if the planet is far enough from its primary, it will be able to hold several moons. That means, habitable planets with many moons can only be found around bright stars; dim stars' habitability zones are a little too close for lunar comfort.

''' Alien suns '''

to:

'''Alien moons'''

!!Alien moons

There's nothing impossible in about a planet having several moons. Mars, UsefulNotes/{{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. Besides Besides, on a habitable moon of a gas giant, other moons would look like multiple big moons on the sky.

Also note that the potential number of moons around the planet is greater if the planet is further away from its sun. Yes, it's tidal interactions again. Tidal interactions between star, planet and moon tend to disrupt the system if the star is too close. If the planet ends up tidally locked to the star, orbits of any moons will decay and they will eventually fall or be destroyed and turned into debris rings. However, if the planet is far enough from its primary, it will be able to hold several moons. That means, means that habitable planets with many moons can only be found around bright stars; dim stars' habitability zones are a little too close for lunar comfort.

''' Alien suns '''
!!Alien suns

See also UsefulNotes.{{Stars}}



Double and multiple suns are also possible, but in practice only two variants are plausible: we can have twin suns staying near each other on the sky (if our planet orbits both) or (if our planet orbits one the second must be fur away not to cause too big perturbations) a bright point star, in practice having properties of moon (small but probably considerable amount of light) and Jupiter (slowly wandering between fixed stars on the sky).

'''Completely alien celestial bodies'''

to:

[[BinarySuns Double and multiple suns suns]] are also possible, but in practice only two variants are plausible: we can have twin suns staying near each other on the sky (if if our planet orbits both) or (if both; or, if our planet orbits one one, the second must be fur far away not to cause too big perturbations) a extreme perturbations--a bright point star, in practice having properties of the moon (small but probably considerable amount of light) and Jupiter (slowly wandering between fixed stars on the sky).

'''Completely !!Completely alien celestial bodies'''
bodies



23rd Mar '15 7:09:24 PM HalcyonDayz
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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.

to:

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 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[[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.
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