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Ocean planets are an as-yet hypothetical type of planet whose surface is completely covered with an ocean of water.

The mechanism by which an ocean planet could form was first proposed in 2003 by Alain Léger of the University of Paris-Sud.^[1] Planetary objects that form in the outer solar system begin as a comet-like mixture of roughly 50% water and 50% rock by weight, and if they grow large enough (about ten Earth masses) begin capturing hydrogen and helium to become a gas giant. Recent simulations of solar system formation have shown that planets are likely to migrate inward or outward as they form, presenting the possibility that icy planets which fell short of developing into full gas giants could wind up in an orbit where its water becomes liquid.

Unlike the oceans of Earth, the oceans on such planets would be hundreds of kilometers deep. The immense pressures in the lower regions of these oceans could lead to the formation of a mantle of exotic forms of ice. 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.^[1]

1. ^ ^{a b} "Ocean Planets on the Brink of Detection", Physorg.com, 2007-02-02. Retrieved on 2007-02-02.

• Léger, Alain; F.Selsis, C.Sotin, T.Guillot, D.Despois, H.Lammer, M.Ollivier, F.Brachet, A.Labeque, C.Valette (2004). "A new family of planets? 'Ocean Planets'". Icarus 169: 499L. arXiv:astro-ph/0308324. 
• Selsis, F.; B. Chazelas, P. Borde, M. Ollivier, F. Brachet, M. Decaudin, F. Bouchy, D. Ehrenreich, J.-M. Griessmeier, H. Lammer, C. Sotin, O. Grasset, C. Moutou, P. Barge, M. Deleuil, D. Mawet, D. Despois, J. F. Kasting, A. Leger (2007). "Could we identify hot Ocean-Planets with CoRoT, Kepler and Doppler velocimetry?". Icarus. arXiv:astro-ph/0701608.