Useful Notes / Planets
This Useful Notes
page deals with planets, both solar and extrasolar, the types of them, their properties and how to avoid obvious mistakes when making ones up.
(For specific planets orbiting our own star, see The Solar System
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.
It is entirely possible that planets are more common than previously thought. It was originally thought that most stars in the galaxy were solitary, with most stars having no planets orbiting them, or having only a single planet. As astronomic sciences become steadily more advanced, however, it's becoming increasingly common to hear that our Solar System may be on the small
side in terms of number of planets: some stars may have dozens
of planets, and there are thousands of billions
of stars in our galaxy. Therefore, it is theorized, all
planet types are more common than originally thought.
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
, Earth, and Mars
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
A lifeless Goldilocks planet is likely to have an atmosphere of nitrogen, carbon dioxide, and similar nonbreathable gases.
These planets are the most common ones described in fiction. Many sci-fi universes forget completely about other types and concentrate on the Goldilocks
. There's various reasons for this: humans can run around on them
, they can be easily mocked up in the backlot or 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 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, can provide cool and unique plot points and that sense of "Wow!" that's essential to good science fiction.
Non-Goldilocks terrestrial planets
Chthonian planets note
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.
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 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 terraforming
Marses (Desert worlds)
This subset of rockball is distinct enough for a closer look. They are intermediate in size between a true little barren rockball like Mercury or the Moon and a large terrestrial planet like Earth and Venus. That's why they start with a good budget of volatiles and have atmospheres and oceans early in their history, only to degenerate into almost-rockballs later, exterminating any life that may have been there already
. They have thin, worn, unbreathable atmospheres, sandy, desert-like landscapes and little to no water (it, if any is present, is likely to be buried in subsurface ice deposits).
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.
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 chthonians, with a monstrous atmosphere and chemistry absolutely unsuitable for life. In The Solar System
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 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
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
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 and possibly contain exotic marine alien life adapted to existence in a lightless but warm sea.
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 have been made that tholin could be a very useful resource to space explorers if we could bio-engineer micro-organisms (bacteria and algae) capable of processing this gunge into food and fertilizer.
One final point: Iceballs and icy rockballs ought to be distinguished from Goldilocks planets in a "Snowball" era; such planets are merely experiencing a severe Ice Age and remain in the Goldilocks zone, but are very cold, except for perhaps a band around the Equator (or equivalent region of highest insolation). Earth went through this
(although it may have been incomplete); multicellular life first appears in the fossil record right after the glaciation ended.
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 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 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.
Also note that ammonia worlds are only possible around cool, orange or red suns; hotter ones like our Sun or even bluer radiate too much ultraviolet, which tends to break ammonia down. The end result is just a big honking icy rockball with nitrogen/CO2
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.
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 and mine their motherlodes of valuable heavy and radioactive metals.
Larger and less radioactive super-volcanic worlds are likely to build up a dense sulfurous atmosphere of volcanic gases; but most often it gets simply blown away by radiation.
Similar to super-volcanics in conditions, but different in origin, primordial worlds are young planets and large planetesimals orbiting around freshly coalesced stars in discs of dust and debris. Certain types of stars (A and B type whites and blue-whites) can only hold primordial worlds since they are quite short-lived.
Every planetary system starts as a dust and debris disc, similar to a gas giant's ring system but much larger, around the star. Planetesimals, the seeds that grow into planets, are the largest fragments of debris that have gravity potent enough to attract dust and grow. In all respects and purposes, common planetesimals are rockballs and dirty iceballs, not much different from those already examined in this article. A very young system contains dozens of these in clumsy eccentric orbits; after half a billion years or so, most are ejected, ground into asteroids or lumped into planets.
A young planet, or a big planetesimal, in the inner system is what eventually becomes a Goldilock, a greenhouse or a desert world, depending on circumstances. But now they are all alike: large barren worlds lit by dim young suns, under black or dark indigo skies, covered by tenuous primordial atmospheres of nitrogen and carbon dioxide, constantly bombarded by smaller planetesimals and covered with erupting, furious volcanoes. That's what Hadean Earth was like, and Venus, and Mars too, probably. Once the "late heavy bombardment" phase is over, the crust stabilizes, the volcanism goes down and life starts to develop from primitive amino acids. Or doesn't.
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.
Okay, so what makes super-Earths different?
- Gravity, of course. 1.2 to 1.4 g is harsh for humans, but possible to adapt. Anything more requires genemodding Heavyworlder humans to survive and procreate on the surface of such a planet.
- Volatile retention. Heavier superearths have the ability to retain helium, which can compose a significant part of their atmospheres. A certain isotope of helium (Helium-3, to be precise) is a valuable fusion fuel, which can make superearths prime spots for helium extraction and thus colonization.
- Dense atmospheres. 5 to 10 bar seems to be the norm; if all of this is standard Earth air, it becomes unbreathable and toxic, since both oxygen and nitrogen have dangerous mind-altering properties (narcosis) in these pressures. Helium, on the other hand, can make even these pressures possible to adapt to, turning the air to something not unlike scuba diving mix.
- Strong volcanism. Not as strong as with super-volcanic worlds, but strong enough to pollute the atmosphere with toxic and sulfurous gases.
- Mighty oceans. Many superearths are theoretized to contain much more water than regular Goldilocks, making them waterworlds or, for hotter ones, something intermediate between waterworlds and greenhouses (imagine vast seas that are kept from boiling by great air pressure, like planet-sized pressure cookersnote ).
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 Chthonian superearth, an exaggerated super-Venus with a pressure of 500 bar, an alien ocean superearth with seas of ammonia, a titanic icy rockball, a giant raging ball of volcanic fire. But keep in mind that colder superearths tend to gather superdense hydrogen-helium atmospheres and grow seamlessly into the gas giant class...
Even if they seem to be abundantnote
, no superearths have been found in our Solar System. it has recently been proposed
the early Solar System could
have had some of them, but Jupiter's interactions and movement within the protoplanetary disk caused such a huge mess that, among other things, hypothetical superearths that could have formed in the innermost Solar System would have crashed among themselves
or fallen into the Sun, along with the debris of those collisions
A Megaearth such as Kepler-10c
, that is around seventeen times more massive than the Earth and twice as large, is basically a superearth Up to Eleven
, with heavier mass—thus with more gravity—and larger. Going even further we've HD 149026 b
, that has a mass similar to that of Saturn, but it's two thirds as large, suggesting a rocky-icy core of 60 Earth mass or even more below a (still) massive hydrogen atmosphere. If you don't have enough with this, some theoretical research suggests really
massive solid planets, up to several thousand times the mass of the Earth, could form in the protoplanetary disks of metal-rich massive stars whose strong UV evaporation and stellar winds would strip the lightest elements, leaving just the heaviest ones.
These planets are so huge that they are dominated by powerful hydrogen atmospheres they are capable of holding. Under high pressure, hydrogen gradually liquefies and then solidifies, forming the mantle of such a planet. It lacks a clear line between atmosphere, ocean and mantle because all three are hydrogen with some other gases in the mix. That's why such planets are also known as gas giants.
Large and small gas giants
Gas giants vary in size. The smallest ones, like Uranus
, 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.
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 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.
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 chthonian planet.
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 giantnote
. 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.
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 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.
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 have been confirmed around other stars. Kuiper belt dwarf planets are very cold and covered by frozen nitrogen and methane that could be their atmospheres if they were a little warmer. In our system, Pluto is a typical example, along with Eris, Haumea, Makemake and the most extreme of them, Sedna, which ranges out to a hundred AUs from the Sun at the farthest point of its orbit.
Tidally locked and resonant planets
Tidal lock is a very common phenomenon; more common with smaller 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.
A tidally locked planet always faces its primary with the same side, leaving the other side in perpetual darkness. Depending on the axial tilt, there may be a sizable zone of normal day cycle between the everlight and everdark hemispheres, or a thin line called a terminator
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. The possibility also exists of the terminator being a comfortable zone between a raging hot dayside and a frozen nightside.
Another possibility are "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 from space they 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 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. This is 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 tilt planets
Seasonal cycles as we know them are produced by axial tilt: the axis of planetary rotation remains the same regardless of where the planet is in its orbit, and 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 result:
- Seasons are very strong.
- The entire planet experiences the "polar" day and night cycle, namely a very long day in summer and a very long night in winter.
Yes, there are Cloudcuckoolanders
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.
Double (binary) planets
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 moonnote
, 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.
However, there are several wrinkles with double planets:
- Their primary has to be a heavy, bright star, if you want a double Goldilock. Smaller, dimmer stars require putting the planets too close to the star, where tidal effects will disrupt the balance.
- They can't be too close to each other. Putting them too close will result in their gravities affecting each other with strong tides that will heat them from inside and turn them into volcanic hellholes.
- 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.
- 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.
There's also more than one way to put two planets into one orbit. Those two planets could be in the Lagrange points of a very heavy body, such as a brown dwarf (which is either a very dim stillborn star or a very heavy, hot gas giant planet, depending on who you ask). You can replace the brown dwarf with a usual gas giant, but in this case you can put only small, Moon-sized bodies in its Lagrange points.
Alien Sky, and how to make it realistic
is perhaps the most important visual cue of another planet. However, it's also a common way of making an astronomy goof
. This section contains tips on what cool alien skies can other planets have.
There's nothing impossible about 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 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.
See also UsefulNotes.Stars
It's pretty much obvious that you can see an alien sun from a planet of an alien sun. For main sequence stars, the following rule applies: the dimmer the star, the larger it appears in the sky of a habitable planet, and vice versa. A red dwarf will look like a huge, reddish-orange circle, gently warming but not hurting your eyes, completely safe to stare directly into (unless it's currently flaring). A bluish-white A star will look like tiny, almost point-like, but piercingly, painfully bright sun, setting your eyeballs on fire even on a cursory glance. The reason for that is simple: distance of the habitable zone. A habitable planet around a red dwarf is very close to the star, close enough that it appears huge; a habitable planet of a bright star is far away. It can also be explained in other way: brightness of a source of light per unit of apparent area is constant as we move from it (until this source becomes a point), so brighter (hotter and in reality bigger) stars must be visually smaller for the total amount of light given be suitable.
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 far away not to cause too 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 alien celestial bodies
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 Gliese 876
. Seen from one of them the others would look instead of stars as full-fledged planets, even showing phases as the Moon from Earth.