Ocean planet is a type of terrestrial planet that contains a substantial amount of water as hydrosphere at its surface or within a subsurface ocean. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava (the case of Io), ammonia (in a eutectic mixture with water, as is likely the case of Titan’s inner ocean) or hydrocarbons like on Titan’s surface (which could be the most abundant kind of exosea).
The characteristics of ocean worlds—or ocean planets—provide clues to their history and the formation and evolution of the Solar System as a whole. Of additional interest is their potential to originate and host life.
Planets that form prior to the dissipation of the gaseous circumstellar disk experience strong torques that can induce rapid inward migration into the habitable zone, especially for planets in the terrestrial mass range.
Since water is highly soluble in magma, a large fraction of the planet’s water content will initially be trapped in the mantle. As the planet cools and the mantle begins to solidify from the bottom up, large amounts of water (between 60% and 99% of the total amount in the mantle) are exsolved to form a steam atmosphere, which may eventually condense to form an ocean.
Ocean formation requires differentiation, and a heat source, either radioactive decay, tidal heating, or the early luminosity of the parent body. Unfortunately, the initial conditions following accretion are theoretically incomplete.
Major moons and dwarf planets in the Solar System thought to harbor subsurface oceans are of substantial interest because they can be reached and studied by space probes, in contrast to exoplanets. The best established water worlds in the Solar System are Callisto, Enceladus, Europa, Ganymede, and Titan. Europa and Enceladus are considered among the most compelling targets for exploration due to their comparatively thin outer crusts and observations of cryovolcanism.
A host of other bodies in the Solar System are considered candidates to host subsurface oceans based upon a single type of observation or by theoretical modeling, including Ariel, Ceres, Dione, Eris, Mimas, Miranda, Oberon, Pluto and Triton.
Several exoplanets have been found with the right conditions to support liquid water. For exoplanets, current technology cannot directly observe liquid surface water, so atmospheric water vapor may be used as a proxy. In June 2020, NASA scientists reported that it is likely that exoplanets with oceans are common in the Milky Way galaxy, based on mathematical modeling studies. Some of most likely known candidates for an extrasolar ocean planet Kepler-11, GJ 1214 b, Kepler-22b, Kepler-62f, Kepler-62e and the planets of TRAPPIST-1 are.
Although 70.8% of all Earth’s surface is covered in water, water accounts for only 0.05% of Earth’s mass. An extraterrestrial ocean could be so deep and dense that even at high temperatures the pressure would turn the water into ice. The immense pressures in the lower regions of such oceans could lead to the formation of a mantle of exotic forms of ice such as ice V. This ice would not necessarily be as cold as conventional ice.