Circumbinary planets, as their name suggests, are planets that orbit two stars simultaneously. Because of the close proximity and orbit of some binary stars, the only way for planets to form is by forming outside the orbit of the two stars.
Gas giants are large planets that are not primarily composed of rock or other solid matter.
Pulsar planets are planets that are found orbiting pulsars, or rapidly rotating neutron stars.
A terrestrial planet is a planet that is composed primarily of silicate rocks or metals. Within a solar system, the terrestrial planets are among the inner planets closest to the Sun. Terrestrial planets will acquire water during their accretion, some of which will be buried in the magma ocean but most of it will go into a steam atmosphere, and when the atmosphere cools it will collapse onto the surface forming an ocean.
Coreless planet is a type of terrestrial planet that consists of silicate rock but has no metallic core. Coreless planets are believed to form farther from the star where volatile oxidizing material is more common.
Carbon planet (also called "diamond planet") is a class of planets, composed of a metal core surrounded by primarily carbon-based minerals. They may be considered a type of terrestrial planet if the metal content dominates.
A Chthonian planet is a class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a star. The remaining rocky or metallic core would resemble a terrestrial planet in many aspects.
Ice Planets are similar to carbon or terrestrial planets but more than 90% of their surface is covered by a thick layer ice and vast mountains.
Planetary objects that form in the outer solar system begin as a comet-like mixture of roughly half water and half rock by mass. Planets are likely to migrate inward or outward as they form, presenting the possibility that icy planets could move to orbits where their ice melts into liquid form, turning them into ocean planets. Such planets could therefore support life that would be aquatic.
The oceans on such planets would be hundreds of kilometers deep, much deeper than the ones on terrestrial planets. The immense pressures in the lower regions of these oceans could lead to the formation of a mantle of exotic forms of ice. This ice would not necessarily be as cold as conventional ice. If the planet is close enough to its sun that the water's temperature reaches the boiling point, the water will become supercritical and lack a well-defined surface.
Even on cooler water-dominated planets, the atmosphere can be quite thick, and composed largely of water vapor, producing a very strong greenhouse effect.
Smaller ocean planets have less dense atmospheres and lower gravity; thus, liquid could evaporate much more easily than on more massive ocean planets.
Theoretically, such planets could have higher waves than their more massive counterparts due to their lower gravity.
Extrasolar terrestrial planets that are extremely close to their parent star will be tidally locked and so one half of the planet will be a magma ocean. It is also possible that terrestrial planets had magma oceans at some point during their formation as a result of giant impacts.
Where there are suitable temperatures and pressure, volatile chemicals which might exist as liquids in abundant quantities on planets include ammonia, argon, carbon disulfide, ethane, hydrazine, hydrogen, hydrogen cyanide, hydrogen sulfide, methane, neon, nitrogen, nitric oxide, phosphine, silane, sulfuric acid, and water.
Hot Neptunes are planets orbiting close to their star (normally less than one astronomical unit away) and could lose their atmospheres via hydrodynamic escape, leaving behind their cores with various liquids on the surface.