History UsefulNotes / Planets

6th Aug '17 5:05:53 PM RainbowPhoenix
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In ice giants, the innermost layer of water tends to be highly comprised, and is mixed with gasses such as ammonia that condenses in the upper atmosphere and rains down into the interior. This highly compressed water-ammonia mixture produces magnetic fields significantly stronger than those seen in terrestrial planets. Both of the ice giants in our solar system have rather lopsided magnetic fields compared to their axial tilt and mass distributions. There is currently no way to tell if this is typical of ice giants, or if something strange happened to ours in their early histories.

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In ice giants, the innermost layer of water tends to be highly comprised, compressed, and is mixed with gasses such as ammonia that condenses in the upper atmosphere and rains down into the interior. This highly compressed water-ammonia mixture produces magnetic fields significantly stronger than those seen in terrestrial planets. Both of the ice giants in our solar system have rather lopsided magnetic fields compared to their axial tilt and mass distributions. There is currently no way to tell if this is typical of ice giants, or if something strange happened to ours in their early histories.
31st Jul '17 2:10:59 PM RainbowPhoenix
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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. UsefulNotes/{{Mercury}} is a rockball, Earth's [[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 could be recovered by {{terraform}}ing.

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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. UsefulNotes/{{Mercury}} is a rockball, Earth's [[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.atmosphere due to the solidification of its outer core. Though Mars is not the worst case of rockball, and could be recovered by {{terraform}}ing.



The state of these planets is due to the fact that the smaller an object is, the faster it cools down. As a result, the interiors of these desert worlds solidify much earlier than those of larger terrestrial planets, robbing them of both the liquid mantle needed to replenish their atmospheres via volcanic activity and the magnetic field that protects their surfaces from solar radiation.



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 [[Film/TheEmpireStrikesBack cloud city]].

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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. They also have the potential to evolve into a different type of terrestrial planet on their own depending on their distance from the parent star and the presence of a magnetic field. Earth and Venus both are hypothesized to have been wet greenhouses early in their histories. Dry greenhouses, like Venus, lack water altogether and are really tough nuts to terraform; on the other hand, they are good places for a [[Film/TheEmpireStrikesBack cloud city]].



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

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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. Stars actually have multiple snow lines, one for each type of ice. 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; [[UsefulNotes/TheMoonsOfJupiter Europa]] and [[UsefulNotes/TheMoonsOfSaturn Titan]] are icy rockballs, Callisto and all [[UsefulNotes/TheMoonsOfUranus moons of Uranus]] are dirty iceballs. In other star systems, particularly those without gas giants, bodies of this sort can be true planets.



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 [[UsefulNotes/TheMoonsOfJupiter 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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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. In some cases, the volcanism is so extreme that they don't even have a true crust. 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]]. 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.



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.

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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.
dwarfs. Regardless of which type a gaseous planet falls into, they often have intense storms that are visible from space.

In ice giants, the innermost layer of water tends to be highly comprised, and is mixed with gasses such as ammonia that condenses in the upper atmosphere and rains down into the interior. This highly compressed water-ammonia mixture produces magnetic fields significantly stronger than those seen in terrestrial planets. Both of the ice giants in our solar system have rather lopsided magnetic fields compared to their axial tilt and mass distributions. There is currently no way to tell if this is typical of ice giants, or if something strange happened to ours in their early histories.



Once gas giants reach a maximum to their size (that is about the size of 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.

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Once gas giants reach a maximum to their size (that is about the size of 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.
places. Metallic hydrogen produces the strongest magnetic fields of any planet.



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

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In the Solar system, all gas giants are cold. They all are beyond the snow line. But in other star systems we have found gas giants 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.
giants. These planets are variously called "Hot Jupiters", "Hot Saturns", or "Hot Neptunes" depending on their mass.




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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.

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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. \n 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.




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In our solar system, this is Uranus's claim to fame.



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

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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 each type cancel each other, resulting in a mild climate, and one hemisphere where the seasons of each type reinforce each other, resulting in very harsh annual changes in weather.
weather. This phenomenon is seen on Mars, which has relatively mild seasons on its northern hemisphere, but extremely harsh seasons on its southern.



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. 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[[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.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.



* 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.

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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.
surface. Once the barycenter slips below the surface of the more massive partner, it is no longer considered a double-planet, but a planet-moon pair.



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

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There's nothing impossible about a planet having several moons. UsefulNotes/{{Mars}}, for example, has two, and Pluto has five: one huge, Charon, which many argue should be considered a dwarf planet in its own right, 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.
22nd Oct '16 1:35:51 PM TheWhistleTropes
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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. Sometimes superearths (intermediate planets between terrestrials and gas giants, big terrestrials heavier than Earth) are viewed as a distinct subset of terrestrials.

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Basically, planets come in three flavors: terrestrial planets, giant planets 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. Sometimes superearths (intermediate planets between terrestrials and gas giants, big terrestrials heavier than Earth) are viewed as a distinct subset of terrestrials.
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.

to:

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.

to:

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.

to:

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.
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