Planets (useful notes)
This Useful Notes page deals with planets, both solar and extrasolar, the types of them, their properties and how to avoid Did Not Do the Research 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.
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. A lifeless Goldilocks planet is likely to have an atmosphere of nitrogen, carbon dioxide, methane 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.
Non-Goldilocks terrestrial planets
Chtonian planets
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.
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, but they have no water and no to almost no air. Mercury is a rockball, Earth's Moon is one, and Mars, though it used to be more similar to Earth, turned into a rockball-like desert planet by losing water and atmosphere. Though Mars is not the worst case of rockball, and can be recovered by terraforming.
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 CO2 in carbonate rock and condensing water into oceans). They become really hot, often hotter than the hottest rockballs and chtonians, with a monstruous atmosphere and chemistry absolutely unsuitable for life. In The Solar System, Venus is an example.
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.
Giant planets
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 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).
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 behind the snow line, a radius beyond which ices can exist in solid form indefinitely. But in other star systems there were found gas giants really close to their stars. It's probably observer bias, as such planets are the easiest to detect from afar, but most known exoplanets are hot gas giants.
A gas giant cannot form in the inner system, but it can migrate there. There are two types of inner-system gas giants: "eccentric Jupiters" and "epistellar Jupiters". The first type is in the process of migration, it occupies an eccentric, irregular orbit, coming closer to the sun at times, and further from it other times. They usually disrupt any formation of terrestrial planets by doing so. The second type is a planet firmly settled near the sun. They heat up, their atmospheres expand and, if they are heavy enough, they can become much larger than it's usually allowed for gas giants. These hot Jupiters are called "puffy planets" for their very low density. After that, their atmosphere is slowly grazed away by solar winds: many epistellar giants have enormous "tails" of gas being ejected from them stretching outwards. The end result is a chtonian planet.
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.