An oceania[1], aquaplanet or water world is a type of exoplanet wherein the surface is completely covered by water, and a substantial portion of its mass is water. During planet formation it occurs that icy planets move inward (i.e., toward the star) as they form. Inward migration presents the possibility that they move to orbits where their ice melts into liquid form, turning them into ocean planets. In some oceaniae more than 10% of the total planetary mass is water. These planets have oceans hundreds of kilometres deep. Their abyssal depths are so deep and dense that even at high temperatures the pressure turns the water into ice. The immense pressures in the lower regions of these oceans lead to the formation of a mantle of exotic forms of ice. This ice is not necessarily as cold as conventional ice. If the planet is close enough to its star that the water reaches its boiling point, the water becomes supercritical and lacks 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 have to be small enough not to be able to retain a thick envelope of hydrogen and helium, otherwise they would form a warmer version of an ice giant instead, like Uranus and Neptune.

Smaller ocean planets have less dense atmospheres and lower gravity; thus, liquid can evaporate much more easily than on more massive ocean planets. Such planets have higher waves than their more massive counterparts due to their lower gravity.

View of Ordanne, a hot oceania teeming with marine organisms.

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