An ocean planet, or waterworld, is a type of planet whose surface is completely covered with an ocean of water or other liquid.
Planets that form in the outer fringes of a star system begin as a comet-like mixture of rock and ice. Some icy planets migrate inward to orbits where their ice melts, turning them into ocean planets. Such planets can support aquatic life.
Rock with ocean
These planets consist mostly of rock covered with an ocean, and that ocean (or oceans) must cover at least 90% of the surface to classify it as an ocean planet. Such worlds often have a few islands or even a small continent, but the oceans themselves don't typically exceed twenty miles in depth. These planets are commonly called ocean worlds but are not true ocean worlds in the technical sense.
Ocean with rock
The oceans on true waterworlds are often fifty to a hundred miles deep (sometimes hundreds), much deeper than the typical oceans of water-bearing planets such as Earth. The immense pressures in the abyssal depths of these oceans often form a mantle of exotic forms of ice. This ice is not synonymous with conventional water-ice and is closer to being a hot, near-metallic solid. 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 (in other words, the term "surface of the ocean" would not apply). Even on cooler water-dominated planets, the atmosphere can be much thicker than that of worlds of analogous size and are usually composed largely of water vapor, lending to a very strong greenhouse effect.
Smaller ocean planets generally have lighter atmospheres and lower gravity; thus, liquid evaporates more easily than on larger ocean planets. Smaller ocean planets also have higher waves than their more massive counterparts due to their lower gravity.
Other types of ocean
Oceans and other bodies of liquid can consist of liquids other than water, such as hydrocarbon lakes on planets rich in fuel or seas of nitrogen on cryogenic planets. Underneath the thick atmospheres of gas giants exist oceans of hot, high-density fluid mixtures of water, ammonia and other volatiles. Their gaseous outer layers transition smoothly into oceans, usually of liquid hydrogen. Other worlds possess icy shells floating on oceans of very dense liquid water or water-ammonia. Some rocky planets have such high-pressure atmospheres that gases such as carbon dioxide become a supercritical fluid at the surface. Planets that are extremely close to their parent star are usually tidally-locked and possess a lava ocean on the sunny side. Other terrestrial planets develop lava oceans a result of massive impacts or surface-rending warfare.
Where there are suitable temperatures and pressures, volatile chemicals which might exist as liquids in abundant quantities on planets may include: ammonia, argon, carbon disulfide, ethane, hydrazine, hydrogen, hydrogen cyanide, hydrogen sulfide, methane, neon, nitrogen, nitric oxide, phosphine, silane, and sulfuric acid. Close-orbiting gas giants generally lose their atmospheres via hydrodynamic escape, leaving behind their cores, sometimes with various liquids on the surface (see chthonian planet).
Terrestrial planets acquire water during their formation; some is buried in the mantle but most of it goes into a steam atmosphere and falls to the surface as it cools, forming an ocean; outgassing of water from the mantle as the magma solidifies also occurs. Even planets with a low percentage of water can produce water oceans from outgassing.