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English: An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. 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 study of extraterrestrial oceans is referred to as planetary oceanography.

Earth is the only astronomical object known to presently have bodies of liquid water on its surface, although several exoplanets have been found with the right conditions to support liquid water. There are also considerable amounts of subsurface water found on Earth, mostly in the form of aquifers. For exoplanets, current technology cannot directly observe liquid surface water, so atmospheric water vapor may be used as a proxy. The characteristics of ocean worlds 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.

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.

Ocean worlds are of extreme interest to astrobiologists for their potential to develop life and sustain biological activity over geological timescales. Major moons and dwarf planets in the Solar System thought to harbor subsurface oceans are of substantial interest because they can realistically be reached and studied by space probes, in contrast to exoplanets, which are tens if not hundreds or thousands of light-years away, far beyond the reach of current human technology. The best-established water worlds in the Solar System, other than the Earth, 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, Titania, Umbriel, Ceres, Dione, Eris, Mimas, Miranda, Oberon, Pluto, and Triton.

Outside the Solar System, exoplanets that have been described as candidate ocean worlds include GJ 1214 b, Kepler-22b, Kepler-62e, Kepler-62f, and the planets of Kepler-11 and TRAPPIST-1.

More recently, the exoplanets TOI-1452 b, Kepler-138c, and Kepler-138d have been found to have densities consistent with large fractions of their mass being composed of water. Additionally, models of the massive rocky planet LHS 1140 b suggest its surface may be covered in a deep ocean.

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. If the planet is close enough to its star that the water reaches its boiling point, the water will become supercritical and lack a well-defined surface. Even on cooler water-dominated planets, the atmosphere can be much thicker than that of Earth, and composed largely of water vapor, producing a very strong greenhouse effect. Such planets would have to be small enough not to be able to retain a thick envelope of hydrogen and helium, or be close enough to their primary star to be stripped of these light elements. Otherwise, they would form a warmer version of an ice giant instead, like Uranus and Neptune.

Image created by Pablo Carlos Budassi in 2023 (pablocarlosbudassi.com)