Planetary objects that form in the outer Solar System begin as a comet-like mixture of roughly half water and half rock by mass. 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 could move to orbits where their ice melts into liquid form, turning them into ocean planets. This possibility was first discussed in the professional astronomical literature by Marc Kuchner and Alain Léger in 2003. Such planets could therefore theoretically support life that would be aquatic.
The oceans on such planets would be hundreds of kilometers deep, much deeper than the oceans of Earth. The immense pressures in the lower regions of these oceans could lead to the formation of a mantle of exotic forms of ice. This ice would not necessarily be as cold as conventional 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. 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.
The extrasolar planet GJ 1214 b is the most likely known candidate for an ocean planet. Many more such objects are expected to be discovered by the ongoing Kepler spacecraft mission, such as the recently discovered ocean planet candidate Kepler-22b.
Smaller ocean planets would have less dense atmospheres and lower gravity; thus, liquid could evaporate much more easily than on more massive ocean planets. Theoretically, such planets could have higher waves than their more massive counterparts due to their lower gravity.
Other types of ocean
Oceans, seas, lakes, etc., can be composed of liquids other than water: e.g. the hydrocarbon lakes on Titan. The possibility of seas of nitrogen on Triton was also considered but ruled out. Underneath the thick atmospheres of Uranus and Neptune it is expected that these planets are composed of oceans of hot high-density fluid mixtures of water, ammonia and other volatiles. The gaseous outer layers of Jupiter and Saturn transition smoothly into oceans of liquid hydrogen. There is evidence that the icy surfaces of the moons Ganymede, Callisto, Europa, Titan and Enceladus are shells floating on oceans of very dense liquid water or water-ammonia. Our own planet Earth is often called the ocean planet since it is 70% covered in water. The atmosphere of Venus is 96.5% carbon dioxide and at the surface the pressure makes the CO2 a supercritical fluid. Extrasolar terrestrial planets that are extremely close to their parent star will be tidally locked and so one half of the planet will be a magma ocean. It is also possible that terrestrial planets had magma oceans at some point during their formation as a result of giant impacts. Where there are suitable temperatures and pressures, volatile chemicals which might exist as liquids in abundant quantities on planets include ammonia, argon, carbon disulfide, ethane, hydrazine, hydrogen, hydrogen cyanide, hydrogen sulfide, methane, neon, nitrogen, nitric oxide, phosphine, silane, sulfuric acid, and water. Hot Neptunes close to their star could lose their atmospheres via hydrodynamic escape, leaving behind their cores with various liquids on the surface.
Terrestrial planets will acquire water during their accretion, some of which will be buried in the magma ocean but most of it will go into a steam atmosphere, and when the atmosphere cools it will collapse on to the surface forming an ocean. There will also be outgassing of water from the mantle as the magma solidifies - this will happen even for planets with a low percentage of their mass composed of water, so "super-Earth exoplanets may be expected to commonly produce water oceans within tens to hundreds of millions of years of their last major accretionary impact."
- In Olaf Stapledon's Last and First Men, Venus is presented as an ocean planet lacking atmospheric oxygen. It is home to a semi-intelligent form of marine life deriving energy from nuclear power rather than metabolism. These beings are exterminated and human beings (genetically engineered into a flying species) settle the planet.
- Stanisław Lem's Solaris is a planet entirely covered by a sentient ocean. The protagonists in vain try to communicate with the ocean.
- Kamino in Star Wars Episode II: Attack of the Clones, an ocean planet inhabited by an indigenous sentient species.
- Thalassa in The Songs of Distant Earth, a short story by Arthur C. Clarke. The planet was settled by colonists from the Earth in the 28th century, who created a utopian culture.
- Hydros in The Face of the Waters by Robert Silverberg is an ocean planet populated by people who live on artificial floating islands.
- Earth after melting the polar ice caps in Waterworld by Kevin Costner (impossible because the caps do not contain enough water).
- Alpha, a planet orbiting Alpha Centauri in Foundation and Earth by Isaac Asimov, settled by remnants of the population of the dying Earth. Its only dry land is an island 250 kilometers long and 65 kilometers wide.
- Jonathan L. Howard's Russalka Chronicles novels are set on the ocean planet Russalka, named after the mythical mermaid by its Russian colonists.
- Kuchner, Marc (2003). "Volatile-rich Earth-Mass Planets in the Habitable Zone". Astrophysical Journal 596: L105–L108. arXiv:astro-ph/0303186. Bibcode:2003ApJ...596L.105K. doi:10.1086/378397.
- Léger, Alain (2004). "A New Family of Planets ? "Ocean Planets"". Icarus 169 (2): 499–504. arXiv:astro-ph/0308324. Bibcode:2004Icar..169..499L. doi:10.1016/j.icarus.2004.01.001.
- "Stars and Habitable Planets - Notable Extra-Solar Planets" (HTML) (in English). http://www.solstation.com/: SolStation.com. Archived from the original on 2011. Retrieved 2012-02-26.
- Charbonneau, David; Zachory K. Berta, Jonathan Irwin, Christopher J. Burke, Philip Nutzman, Lars A. Buchhave, Christophe Lovis, Xavier Bonfils, David W. Latham, Stéphane Udry, Ruth A. Murray-Clay, Matthew J. Holman, Emilio E. Falco, Joshua N. Winn, Didier Queloz, Francesco Pepe, Michel Mayor, Xavier Delfosse, Thierry Forveille (2009). "A super-Earth transiting a nearby low-mass star". Nature 462 (17 December 2009): 891–894. arXiv:0912.3229. Bibcode:2009Natur.462..891C. doi:10.1038/nature08679. PMID 20016595. Retrieved 2009-12-15.
- Kuchner, Seager; M., Hier-Majumder, C. A., Militzer (2007). "Mass–radius relationships for solid exoplanets". The Astrophysical Journal 669 (2): 1279–1297. arXiv:0707.2895. Bibcode:2007ApJ...669.1279S. doi:10.1086/521346.
- Page 485, Encyclopedia of the solar system By Lucy-Ann Adams McFadden, Paul Robert Weissman, Torrence V. Johnson
- Atreya, S.; Egeler, P.; Baines, K. (2006). "Water-ammonia ionic ocean on Uranus and Neptune?". Geophysical Research Abstracts 8: 05179. Bibcode:2005AGUFM.P11A0088A.
- Guillot, T. (1999). "A comparison of the interiors of Jupiter and Saturn". Planetary and Space Science 47 (10–11): 1183–200. arXiv:astro-ph/9907402. Bibcode:1999P&SS...47.1183G. doi:10.1016/S0032-0633(99)00043-4.
- Lang, Kenneth R. (2003). "Jupiter: a giant primitive planet". NASA. Retrieved 2007-01-10.
- Coustenis, A.; Lunine, J.; Lebreton, J.; Matson, D.; Erd, C.; Reh, K.; Beauchamp, P.; Lorenz, R. et al. (2008). The Titan Saturn System Mission. "American Geophysical Union, Fall Meeting 2008, abstract #P21A-1346". American Geophysical Union 21: 1346. Bibcode:2008AGUFM.P21A1346C. "the Titan system, rich in organics, containing a vast subsurface ocean of liquid water"
- Nimmo, F; Bills, B. G. (2010). "Shell thickness variations and the long-wavelength topography of Titan". Icarus 208 (2): 896–904. Bibcode:2010Icar..208..896N. doi:10.1016/j.icarus.2010.02.020. "observations can be explained if Titan has a floating, isostatically-compensated ice shell"
- Goldreich, Peter M.; Mitchell, Jonathan L. (2010). "Elastic ice shells of synchronous moons: Implications for cracks on Europa and non-synchronous rotation of Titan". Icarus 209 (2): 631–638. arXiv:0910.0032. Bibcode:2010Icar..209..631G. doi:10.1016/j.icarus.2010.04.013. "A number of synchronous moons are thought to harbor water oceans beneath their outer ice shells. A subsurface ocean frictionally decouples the shell from the interior"
- "Study of the ice shells and possible subsurface oceans of the Galilean satellites using laser altimeters on board the Europa and Ganymede orbiters JEO and JGO" (PDF). Retrieved 2011-10-14.
- "Tidal heating and the long-term stability of a subsurface ocean on Enceladus" (PDF). Retrieved 2011-10-14.
- USA (2011-10-03). "The ocean planet". Ncbi.nlm.nih.gov. Retrieved 2011-10-14.
- Irrigating Crops with Seawater; August 1998; Scientific American
- Schaefer, Laura; Fegley, Bruce, Jr. (2009). "Chemistry of Silicate Atmospheres of Evaporating Super-Earths". The Astrophysical Journal Letters 703 (2): L113–L117. arXiv:0906.1204. Bibcode:2009ApJ...703L.113S. doi:10.1088/0004-637X/703/2/L113.
- Fluid Dynamics of a Terrestrial Magma Ocean, V. S. Solomatov, 2000
- Tables 3 and 4 in Many Chemistries Could Be Used to Build Living Systems, WILLIAM BAINS, ASTROBIOLOGY, Volume 4, Number 2, 2004
- Atmospheric Loss of Sub-Neptune’s and Implications for Liquid Phases of Different Solvents on Their Surfaces, J.J. Leitner (1), H. Lammer (2), P. Odert (3), M. Leitzinger (3), M.G. Firneis (1) and A. Hanslmeier (3), EPSC Abstracts, Vol. 4, EPSC2009-542, 2009, European Planetary Science Congress
- Elkins-Tanton (2010). "Formation of Early Water Oceans on Rocky Planets". Astrophysics and Space Science 332 (2): 359–364. arXiv:1011.2710. Bibcode:2011Ap&SS.332..359E. doi:10.1007/s10509-010-0535-3.
- "What If All the Ice Melts?" Myths and Realities
- 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 191 (2): 453. arXiv:astro-ph/0701608. Bibcode:2007Icar..191..453S. doi:10.1016/j.icarus.2007.04.010.