An ocean planet (also called a waterworld) is a type of planet whose surface is completely covered with an ocean of water.

Planetary objects which form in the outer Solar System begin as a comet-like mixture of roughly half water and half rock by mass. Simulations of Solar System formation have shown that planets are likely to migrate inward and outward as they form, presenting the possibility that icy planets could move to orbits where their ice melts into liquid form, turning these planets into ocean planets. 

The oceans on these planets would be hundreds of kilometers deep, much deeper than the oceans on Earth. 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 so close to its host star that the waters temperature reaches the boiling point, the water will become supercritical and lack a well-defined surface. Even on a cooler water dominated planet, the atmosphere can be much thicker than that of Earth, and composed largely of water vapor, producing an extremely strong greenhouse effect.

Ocean planets are known to exist. The extrasolar planet GJ 1214 b is the most likely known candidate for an ocean planet. Kepler-22b is also speculated to be an ocean planet.

Small ocean planets would have less dense atmospheres and lower gravity; thus, liquid could evaporate much more easily than on a more massive ocean planet. Theoretically, these planets would or could have much higher waves than their more massive counterparts due to the lower gravity.

Other types of oceansEdit

Oceans, seas, lakes, etc. can be composed of liquids other than water: e.g. the hydrocarbon lakes on Titan. The possibility of seas of nitrogen on Triton was also considered but was ruled out. Underneath the thick atmospheres of Uranus and Neptune, it is expected that these planets are composed of oceans of hot high-density fluid mixtures or water, ammonia, and other volatiles. here is evidence that the icy surfaces of the moons Ganymede, Callisto, Europa, Titan and Enceladus are shells floating on oceans of very dense liquid water or water-ammonia. The atmosphere of Venus is 96.5% carbon dioxide and at the surface the pressure makes the CO2 a supercritical fluid. 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 pressures, 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 close to their host star could lose their atmospheres via hydrodynamic escape, leaving behind their cores with various liquids on the surface.

See alsoEdit