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Planet classes

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planet classObservedModel
Arid world
A rocky planet with a thin, cold atmosphere and no stable liquid water at the surface, only ice and the traces of ancient rivers. Mars is the Solar System's example.
planet classObservedModel
Barren rock world
A rocky planet with no real atmosphere, whose surface lies bare to starlight, cosmic rays and meteorites. Mercury is the Solar System's example, and JWST has found several around other stars.
planet classObservedModel
Cold gas giant
A gas giant far enough from its star that ammonia freezes into clouds high in its atmosphere, like Jupiter and Saturn. Its own internal heat rivals the sunlight it absorbs, and it can keep large families of moons and rings.
planet classModelSim
Dry habitable world
A rocky planet with liquid water on its surface but only a little of it, in lakes, brines and polar seas rather than oceans. Climate models suggest such land planets can stay habitable over a wider range of distances than an Earth-like world.
planet classObservedSpeculative
Exotic worlds
Four kinds of planet that physics allows but the Solar System lacks. Iron-rich super-Mercuries are observed; carbon planets, helium-atmosphere planets and chthonian planets (the stripped cores of giants) remain hypotheses with candidates but no confirmed member.
planet classObservedModel
Gas giant
A planet made mostly of hydrogen and helium, with no solid surface, from about a tenth of Jupiter's mass up to the brown dwarf boundary near 13 Jupiter masses. Jupiter and Saturn are the Solar System's two; thousands more are known around other stars.
planet classObservedModel
Greenhouse world
A rocky planet smothered by a thick carbon dioxide atmosphere whose greenhouse effect bakes the surface far above the boiling point of water. Venus is the Solar System's example, a near-twin of Earth in size.
planet classObservedSim
Hot Jupiter
A gas giant orbiting so close to its star that its year lasts days and its dayside is hotter than lava. Hot Jupiters were the first planets found around Sun-like stars and remain the best-studied exoplanet atmospheres, though fewer than one Sun-like star in a hundred has one.
planet classObservedModel
Ice giant
A giant planet made mostly of water, methane and ammonia rather than hydrogen and helium, like Uranus and Neptune. Their interiors are hot, dense fluids, their magnetic fields are lopsided, and planets of about their mass may be among the commonest in the Galaxy.
planet classObservedModel
Ice world
A cold world whose surface is made of frozen water, nitrogen, methane and carbon monoxide, like Pluto and Triton, where ices behave like rock and even the atmosphere can freeze out.
planet classObservedModel
Lava world
A rocky planet so close to its star that its dayside rock is molten, often a magma ocean under a thin atmosphere of vaporised rock. Dozens are known, most on orbits shorter than a day.
planet classObservedModel
Mini-Neptune
A planet between about 1.7 and 4 times Earth's radius with a rocky or icy core wrapped in a thin envelope of hydrogen and helium. The most common kind of planet Kepler found, and one the Solar System lacks.
planet classObservedModel
Mixed world
A rocky planet with both continents and oceans, a temperate climate and liquid water at the surface. Earth is the only known example, and the model every search for habitable planets starts from.
planet classModelSim
Ocean world
A planet whose surface is almost entirely water, from Earth-like worlds with a few scattered islands to true water worlds with oceans hundreds of kilometres deep. None is confirmed, but several planets are strong candidates.
physics conceptObservedModel
Planet classification
How astronomers sort planets by size, mass, temperature and composition, and how Pax Abyssi files every world it generates under one of 37 coded types.
planet classObservedModel
Subsurface ocean world
A world with a global ocean of liquid water sealed beneath a shell of ice, kept from freezing by tidal flexing and radioactive heat. Europa and Enceladus are the best-studied examples.
planet classModelSim
Sudarsky classification
A theoretical scheme from 2000 that sorts giant planets into five classes by the temperature of their upper atmospheres, which decides what clouds can form and so how much light, and what colour, the planet reflects.
planet classObservedModel
Super-Earth
A planet more massive than Earth but lighter than Neptune, usually rocky, with no counterpart in the Solar System. Super-Earths are among the most common planets found around other stars.
planet classObservedModel
Temperate gas giant
A gas giant warmer than Jupiter but cooler than a hot Jupiter, roughly 150 to 800 K, where ammonia clouds give way to water clouds and then to clear air. It spans the widest range of looks of any giant, from brilliant white to dark blue.
planet classObservedModel
Volcanic world
A rocky world whose surface is continually remade by eruptions, powered by internal heat far above Earth's. Jupiter's moon Io, heated by tides, is the most volcanic body known.

Pages in this category

Arid world

Barren rock world

Cold gas giant

Dry habitable world

Exotic worlds

Gas giant

Greenhouse world

Hot Jupiter

Ice giant

Ice world

Lava world

Mini-Neptune

Mixed world

Ocean world

Planet classification

Subsurface ocean world

Sudarsky classification

Super-Earth

Temperate gas giant

Volcanic world