View source for Astronomy:Ocean planet

{{short description|Type of planet with a surface completely covered by an ocean of water}}
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An '''ocean planet''', '''ocean world''', '''water world''', '''aquaplanet''' or '''panthalassic planet''' is a type of [[Astronomy:Terrestrial planet|terrestrial planet]] that contains a substantial amount of [[Chemistry:Water|water]] either at its surface or subsurface.<ref>[http://www.omnilexica.com/?q=ocean+planet Definition of Ocean planet]. Retrieved 1 October 2017.</ref><ref name="Adams 2008">{{cite journal |title=Ocean Planet or Thick Atmosphere: On the Mass-Radius Relationship for Solid Exoplanets with Massive Atmospheres |journal=The Astrophysical Journal |date=1 February 2008 |last=Adams |first=E. R. |last2=Seager |first2=S. |last3=Elkins-Tanton |first3=L. |volume=673 |pages=1160–1164 |issue=2 |doi=10.1086/524925 |quote=A planet with a given mass and radius might have substantial water ice content (a so-called ocean planet), or alternatively a large rocky iron core and some H and/or He.|bibcode=2008ApJ...673.1160A |arxiv=0710.4941 }}</ref><ref name="Nimmo 2016">{{cite journal |title=Ocean worlds in the outer solar system |journal=Journal of Geophysical Research |volume=121 |issue=8 |pages=1378 |date=8 August 2016 |last=Nimmo |first=F. |last2=Pappalardo |first2=R. T. |doi=10.1002/2016JE005081 |url=https://websites.pmc.ucsc.edu/~fnimmo/website/ocean_worlds_new.pdf |accessdate=2017-10-01 |bibcode=2016JGRE..121.1378N }}</ref><ref name="Vance 2007">[http://online.liebertpub.com/doi/pdf/10.1089/ast.2007.0075 Hydrothermal Systems in Small Ocean Planets]. (PDF) Steve Vance, Jelte Harnmeijer, Jun Kimura, Hauke Hussmann, Brian deMartin, and J. Michael Brown. ''Astrobiology''. December 2007, 7(6): 987–1005. DOI: 10.1089/ast.2007.0075</ref> The term ''ocean world'' is also used sometimes for astronomical bodies with an ocean composed of a different fluid,<ref>[Ocean Worlds: The story of seas on Earth and other planets]. By Jan Zalasiewicz and Mark Williams. OUP Oxford, October 23, 2014. {{ISBN|019165356X}}, 9780191653568.</ref> such as [[Earth:Lava|lava]] (the case of [[Astronomy:Io (moon)|Io]]), [[Chemistry:Ammonia|ammonia]] (in a [[Physics:Eutectic|eutectic]] mixture with water, as is likely the case of [[Astronomy:Titan (moon)|Titan]]'s inner ocean) or  hydrocarbons like on [[Astronomy:Titan (moon)|Titan]]'s surface (which could be the most abundant kind of exosea).<ref>{{cite journal|author=F. J. Ballesteros|author2=A. Fernandez-Soto|author3=V. J. Martinez|title=Title: Diving into Exoplanets: Are Water Seas the Most Common?|date=2019|doi=10.1089/ast.2017.1720|journal=Astrobiology|pmid=30789285|volume=19|pages=642–654}}</ref>

[[Astronomy:Earth|Earth]] is the only astronomical object known to have bodies of ''liquid'' water on its surface, although several [[Astronomy:Exoplanet|exoplanet]]s have been found with the right conditions to support liquid water.<ref name="NOAA 2017">{{cite web |url=https://oceanservice.noaa.gov/facts/et-oceans.html |title=Are there oceans on other planets? |work=National Oceanic and Atmospheric Administration |date=6 July 2017 |accessdate=2017-10-03 }}</ref> For exoplanets, current technology cannot directly observe liquid surface water, so atmospheric water vapor
may be used as a proxy.<ref name="Seager 2013"/> The characteristics of ocean worlds—or ocean planets—provide clues to their history and the [[Astronomy:Formation and evolution of the Solar System|formation and evolution of the Solar System]] as a whole. Of additional interest is their potential to [[Biology:Abiogenesis|originate]] and [[Astronomy:Planetary habitability|host life]].

==Overview==
[[File:NASA-Exoplanet-WaterWorlds-20180817.jpg|thumb|center|600px|<center>Exoplanets containing water (artist concept; 17 August 2018)<ref name="PHYS-20180817">{{cite web |title=Water-worlds are common: Exoplanets may contain vast amounts of water |url=https://phys.org/news/2018-08-water-worlds-common-exoplanets-vast-amounts.html |date=17 August 2018 |work=Phys.org |accessdate=17 August 2018 }}</ref></center>]]
Water worlds are of extreme interest to [[Astronomy:Astrobiology|astrobiologists]] for their potential to [[Biology:Abiogenesis|develop life]] and sustain biological activity over geological timescales.<ref name="Vance 2007"/><ref name="Nimmo 2016"/>  The four best established water worlds in the Solar System include [[Astronomy:Europa (moon)|Europa]], Enceladus, [[Astronomy:Ganymede (moon)|Ganymede]], and [[Astronomy:Callisto (moon)|Callisto]].<ref name="Nimmo 2016"/> A host of other bodies in the outer Solar System are inferred by a single type of observation or by theoretical modeling to have subsurface oceans, and these include: Dione, [[Astronomy:Pluto|Pluto]], [[Astronomy:Triton (moon)|Triton]], and [[Astronomy:Ceres (dwarf planet)|Ceres]],<ref name="Nimmo 2016"/><ref name="McEwen presentation 2016">{{cite web |url=http://www.lpi.usra.edu/opag/meetings/feb2016/presentations/day-1/08-Roadmap-Ocean-Worlds-McEwen.pdf |title=Roadmaps to Ocean Worlds (ROW) |last=McEwen |first=Alfred |work=Lunar and Planetary Institute |date=1 February 2016 |accessdate=2017-09-30 }}</ref><ref name="Creech 2016">{{cite web |url=https://ntrs.nasa.gov/search.jsp?R=20160011102 |title=Ocean World Exploration and SLS: Enabling the Search for Life |last=Creech |first=Stephen D |last2=Vane |first2=Greg |work=Nasa Technical Reports Server |publisher=NASA |archivedate= 30 July 2016 |accessdate=2017-09-30 }}</ref><ref name="Scott 2015">{{cite news |last=Anderson |first=Paul Scott |url=http://www.americaspace.com/2015/05/20/ocean-worlds-exploration-program-new-budget-proposal-calls-for-missions-to-europa-enceladus-and-titan/ |title='Ocean Worlds Exploration Program': New Budget Proposal Calls for Missions to Europa, Enceladus, and Titan |work=AmericaSpace |date=15 May 2015 |accessdate=2017-09-30 }}</ref><ref name="Wenz 2015">{{cite news |last=Wenz |first=John |url=http://www.popularmechanics.com/space/a15632/nasa-ocean-exploration/ |title=NASA Wants to go Underwater Exploring on Ocean Moons |work=Popular Mechanics |date=19 May 2015 |accessdate=2017-09-30 }}</ref><ref name="Berger 2015">{{cite news |last=Berger |first=Eric |url=http://blog.chron.com/sciguy/2015/05/the-house-budget-for-nasa-plants-the-seeds-of-a-program-to-finally-find-life-in-the-outer-solar-system/ |title=The House budget for NASA plants the seeds of a program to finally find life in the outer solar system |work=Chron |date=19 May 2015 |accessdate=2017-09-30 }}</ref> as well as Mimas,<ref>[https://www.jpl.nasa.gov/infographics/infographic.view.php?id=11262 Ocean Worlds]. JPL, NASA.</ref><ref>[https://www.nasa.gov/specials/ocean-worlds/ Ocean Worlds Exploration Program]. NASA</ref> [[Astronomy:Eris (dwarf planet)|Eris]],<ref name="Vance 2007"/><ref name="Hussmann Sohl et al. 2006">{{cite journal| doi = 10.1016/j.icarus.2006.06.005| last1 = Hussmann| first1 = Hauke| last2 = Sohl| first2 = Frank| last3 = Spohn| first3 = Tilman| date = November 2006| title = Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects| journal = Icarus| volume = 185| issue = 1| pages = 258–273| url = https://www.researchgate.net/publication/225019299| bibcode = 2006Icar..185..258H| ref = {{sfnRef|Hussmann Sohl et al.|2006}}}}</ref> and [[Astronomy:Oberon (moon)|Oberon]].<ref name="Vance 2007"/><ref name="Hussmann Sohl et al. 2006"/>

==History==

Important preliminary theoretical work was carried prior to the planetary missions launched starting in the 1970s. In particular, Lewis showed in 1971 that [[Physics:Radioactive decay|radioactive decay]] alone was likely sufficient to produce subsurface oceans in large moons, especially if ammonia ([[Chemistry:Ammonia|{{chem|NH|3}}]]) was present. Peale and Cassen figured out in 1979 the important role of [[Astronomy:Tidal heating|tidal heating]] (aka: tidal flexing) on satellite evolution and structure.<ref name="Nimmo 2016"/> The first confirmed detection of an exoplanet was in 1992. Alain Léger ''et al'' figured in 2004 that a small number of icy planets that form in the region beyond the [[Physics:Frost line (astrophysics)|snow line]] can [[Astronomy:Planetary migration|migrate]] inward to ∼1 [[Astronomy:Astronomical unit|AU]], where the outer layers subsequently melt.<ref name="Kennedy 2008"/><ref name=Léger/>

The cumulative evidence collected by the [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]], as well as [[Astronomy:Pioneer program|''Pioneer'']], ''[[Engineering:Galileo (spacecraft)|Galileo]]'', ''[[Astronomy:Voyager program|Voyager]]'', ''[[Astronomy:Cassini–Huygens|Cassini–Huygens]]'', and ''[[Astronomy:New Horizons|New Horizons]]'' missions, strongly indicate that several outer Solar System bodies harbour internal liquid water oceans under an insulating ice shell.<ref name="Nimmo 2016"/><ref name="greenberg">Greenberg, Richard (2005)  ''Europa: The Ocean Moon: Search for an Alien Biosphere'', Springer + Praxis Books, {{ISBN|978-3-540-27053-9}}.</ref> Meanwhile, the [[Engineering:Kepler (spacecraft)|''Kepler'' space observatory]], launched in March 7, 2009, has discovered thousands of exoplanets, about 50 of them of [[Astronomy:Terrestrial planet|Earth-size]] in or near [[Astronomy:Habitable zone|habitable zone]]s.<ref name="NYT-20140512">{{cite news |last=Overbye |first=Dennis |title=Finder of New Worlds |url=https://www.nytimes.com/2014/05/13/science/finder-of-new-worlds.html |date=May 12, 2013 |work={{wipe|The New York Times}} |accessdate=May 13, 2014}}</ref><ref name="NYT-20150106-DB">{{cite news |last=Overbye |first=Dennis |authorlink=Dennis Overbye |title=As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next |url=https://www.nytimes.com/2015/01/07/science/space/as-ranks-of-goldilocks-planets-grow-astronomers-consider-whats-next.html |date=January 6, 2015 |work={{wipe|The New York Times}} |accessdate=January 6, 2015}}</ref>

Planets of almost all masses, sizes, and orbits have been detected, illustrating not only the variable nature of planet formation but also a subsequent migration through the [[Astronomy:Circumstellar disc|circumstellar disc]] from the planet's place of origin.<ref name="Seager 2013"/> {{Extrasolar planet counts|full}}

==Formation==
[[File:HL Tau protoplanetary disk.jpg|thumb|[[Astronomy:Atacama Large Millimeter Array|Atacama Large Millimeter Array]] image of HL Tauri, a protoplanetary disk ]]

Planetary objects that form in the outer [[Astronomy:Solar System|Solar System]] begin as a [[Astronomy:Comet|comet]]-like mixture of roughly half water and half rock by mass, displaying a density lower than that of rocky planets.<ref name=Léger>{{cite journal|doi=10.1016/j.icarus.2004.01.001|title=A new family of planets? "Ocean-Planets"|journal=Icarus|volume=169|issue=2|pages=499–504|year=2004|last1=Léger|first1=A.|last2=Selsis|first2=F.|last3=Sotin|first3=C.|last4=Guillot|first4=T.|last5=Despois|first5=D.|last6=Mawet|first6=D.|last7=Ollivier|first7=M.|last8=Labèque|first8=A.|last9=Valette|first9=C.|last10=Brachet|first10=F.|last11=Chazelas|first11=B.|last12=Lammer|first12=H.|arxiv=astro-ph/0308324|bibcode=2004Icar..169..499L}}</ref> Icy planets and moons that form near the [[Physics:Frost line (astrophysics)|frost line]] should contain mostly {{chem|H|2|O}} and [[Chemistry:Silicate|silicate]]s. Those that form farther out can acquire ammonia ({{chem|NH|3}}) and methane ({{chem|CH|4}}) as hydrates, together with [[Chemistry:Carbon monoxide|CO]], [[Chemistry:Nitrogen|{{chem|N|2}}]], and [[Chemistry:Carbon dioxide|{{chem|CO|2}}]].<ref name="apj"/>

Planets that form prior to the dissipation of the gaseous [[Astronomy:Accretion disk|circumstellar disk]] experience strong torques that can induce rapid inward migration into the habitable zone, especially for planets in the terrestrial mass range.<ref name=Luger/><ref name="apj"/> Since water is highly soluble in [[Biology:Magma|magma]], a large fraction of the planet's water content will initially be trapped in the [[Earth:Mantle (geology)|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 [[Physics:Solid solution#Exsolution|exsolved]] to form a steam atmosphere, which may eventually condense to form an ocean.<ref name=Luger/>  Ocean formation requires [[Astronomy:Planetary differentiation|differentiation]], and a heat source, either [[Physics:Radioactive decay|radioactive decay]], [[Astronomy:Tidal heating|tidal heating]], or the early luminosity of the parent body.<ref name="Nimmo 2016"/> Unfortunately, the initial conditions following [[Physics:Accretion (astrophysics)|accretion]] are theoretically incomplete.

Planets that formed in the outer, water-rich regions of a [[Astronomy:Accretion disk|disk]] and migrated inward are more likely to have abundant water.<ref>[https://arxiv.org/ftp/arxiv/papers/0710/0710.2366.pdf New Worlds on the Horizon: Earth-Sized Planets Close to Other Stars]. Eric Gaidos1, Nader Haghighipour, Eric Agol, David Latham, Sean Raymond, and John Rayner. Science  12 Oct 2007: Vol. 318, Issue 5848, pp. 210–213. DOI: 10.1126/science.1144358</ref> Conversely, planets that formed close to their host stars are less likely to have water because the primordial disks of gas and dust are thought to have hot and dry inner regions. So if a water world is found close to a [[Astronomy:Star|star]], it would be strong evidence for [[Astronomy:Planetary migration|migration]] and ''ex situ'' formation,<ref name="dangelo_bodenheimer_2016" /> because insufficient volatiles exist near the star for ''in situ'' formation.<ref name="Adams 2008"/> Simulations of [[Astronomy:Formation and evolution of the Solar System|Solar System formation]] and of [[Astronomy:Exoplanet|extra-solar system formation]] have shown that planets are likely to [[Astronomy:Planetary migration|migrate]] inward (i.e., toward the star) as they form.<ref name=tanaka_etal_2002>{{cite journal|last=Tanaka|first=H.|author2=Takeuchi, T.| author3=Ward, W. R.|title=Three-Dimensional Interaction between a Planet and an Isothermal Gaseous Disk. I. Corotation and Lindblad Torques and Planet Migration |journal=The Astrophysical Journal|date=2002|volume=565|issue=2|pages=1257–1274|doi= 10.1086/324713|bibcode = 2002ApJ...565.1257T }}</ref><ref name=dangelo_lubow_2010>{{cite journal|last=D'Angelo|first=G.|author2=Lubow, S. H. |title=Three-dimensional Disk-Planet Torques in a Locally Isothermal Disk|journal=The Astrophysical Journal|date=2010|volume=724|issue=1|pages=730–747|doi=10.1088/0004-637X/724/1/730|arxiv = 1009.4148 |bibcode = 2010ApJ...724..730D }}</ref><ref name=li2011>{{cite book|last=Lubow|first=S. H.|author2=Ida, S. |chapter=Planet Migration |bibcode=2011exop.book..347L| title=Exoplanets |publisher=University of Arizona Press, Tucson, AZ| editor=S. Seager. |pages=347–371|date=2011|chapter-url=http://www.uapress.arizona.edu/Books/bid2263.htm| arxiv=1004.4137 }}</ref>  Outward migration may also occur under particular conditions.<ref name="li2011" />  Inward migration presents 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 astronomical literature by Marc Kuchner<ref name="apj">{{Cite journal|arxiv=astro-ph/0303186|title=Volatile-rich Earth-Mass Planets in the Habitable Zonene |first=Marc |last=Kuchner| journal=Astrophysical Journal| date=2003|volume=596|issue=1 | pages=L105–L108|doi=10.1086/378397| bibcode=2003ApJ...596L.105K}}</ref> and Alain Léger in 2004.<ref name="physorg">{{Cite journal| arxiv=astro-ph/0308324|title=A New Family of Planets ? "Ocean Planets"| first=Alain| last=Léger| date=2004| doi=10.1016/j.icarus.2004.01.001| journal=Icarus|volume=169| issue=2| pages=499–504| bibcode=2004Icar..169..499L}}</ref>

===Structure===

The internal structure of an icy astronomical body is generally deduced from measurements of its bulk density, gravity moments,
and shape. Determining the moment of inertia of a body can help assess whether it has undergone [[Astronomy:Planetary differentiation|differentiation]] (separation into rock-ice layers) or not. Shape or [[Physics:Gravimetry|gravity measurements]] can in some cases be used to infer the moment of inertia – if the body is in [[Astronomy:Hydrostatic equilibrium#Planetary geology|hydrostatic equilibrium]] (i.e. behaving like a fluid on long timescales). However, proving that a body is in hydrostatic equilibrium is extremely difficult, but by using a combination of shape and gravity data, the hydrostatic contributions can be deduced.<ref name="Nimmo 2016"/> Specific techniques to detect inner oceans include magnetic induction, [[Geodesy|geodesy]], [[Astronomy:Libration|libration]]s, [[Astronomy:Axial tilt|axial tilt]], [[Astronomy:Tidal acceleration|tidal response]], [[Earth:Radioglaciology|radar sounding]], compositional evidence, and surface features.<ref name="Nimmo 2016"/>

[[File:Ganymede diagram.svg|thumb|400px|right|Artist's cut-away representation of the internal structure of Ganymede, with a liquid water ocean "sandwiched" between two ice layers. Layers drawn to scale.]]
A generic [[Astronomy:Icy moon|icy moon]] will consist of a water layer sitting atop a [[Astronomy:Planetary core|silicate core]]. For a small satellite like Enceladus, an ocean will sit directly above the silicates and below a solid icy shell, but for a larger ice-rich body like [[Astronomy:Ganymede (moon)|Ganymede]], pressures are sufficiently high that the ice at depth will transform to higher pressure phases, effectively forming a "water sandwich" with an ocean located between ice shells.<ref name="Nimmo 2016"/> An important difference between these two cases is that for the small satellite the ocean is in direct contact with the silicates, which may provide [[Earth:Hydrothermal circulation|hydrothermal]] and chemical energy and nutrients to simple life forms.<ref name="Nimmo 2016"/> Because of the varying pressure at depth, models of a water world may include "steam, liquid, superfluid, high-pressure ices, and plasma phases" of water.<ref name="arxiv0912">
{{cite journal |author1=Rogers, L.A. |author2=Seager, S. | title=Three Possible Origins for the Gas Layer on GJ 1214b | journal=The Astrophysical Journal | type=abstract | year=2010 | volume= 716| issue= 2| pages= 1208–1216| arxiv=0912.3243|bibcode = 2010ApJ...716.1208R |doi = 10.1088/0004-637X/716/2/1208 }}</ref> Some of the solid-phase water could be in the form of [[Earth:Ice VII|ice VII]].<ref name="harvardpress">{{cite web |url=http://www.cfa.harvard.edu/news/2009/pr200924.html |title=Astronomers Find Super-Earth Using Amateur, Off-the-Shelf Technology |author=David A. Aguilar |date=2009-12-16 |website= |publisher=Harvard-Smithsonian Center for Astrophysics |accessdate=December 16, 2009}}</ref>

Maintaining a subsurface ocean depends on the rate of internal heating compared with the rate at which heat is removed, and the [[Physics:Freezing point|freezing point]] of the liquid.<ref name="Nimmo 2016"/> Ocean survival and tidal heating are thus intimately linked.

Smaller ocean planets would have less dense atmospheres and lower gravity; thus, liquid could evaporate much more easily than on more massive ocean planets. Simulations suggest that planets and satellites of less than one Earth mass could have liquid oceans driven by hydrothermal activity, [[Physics:Radiogenic nuclide#Radiogenic heating|radiogenic heating]], or [[Astronomy:Tidal heating|tidal flexing]].<ref name="Vance 2007"/> Where fluid-rock interactions propagate slowly into a deep brittle layer, thermal energy from [[Earth:Serpentinization|serpentinization]] may be the primary cause of hydrothermal activity in small ocean planets.<ref name="Vance 2007"/> The dynamics of global oceans beneath tidally flexing ice shells represents a significant set of challenges which have barely begun to be explored. The extent to which cryovolcanism occurs is a subject of some debate, as water, being denser than ice by about 8%, has difficulty erupting under normal circumstances.<ref name="Nimmo 2016"/>

==Atmospheric models==
{{further|Astronomy:Extraterrestrial atmosphere}}

To allow water to be liquid for long periods of time, a planet —or moon— must orbit within the [[Astronomy:Circumstellar habitable zone|habitable zone]] (HZ), possess a protective [[Astronomy:Magnetosphere|magnetic field]],<ref name="Driscoll 2011">{{cite journal |title=Optimal dynamos in the cores of terrestrial exoplanets: Magnetic field generation and detectability |journal=Icarus |date=May 2011 |last=Driscoll |first=Peter |volume=213 |issue=1 |pages=12–23 |doi=10.1016/j.icarus.2011.02.010 |bibcode=2011Icar..213...12D }}</ref><ref>[http://iopscience.iop.org/article/10.1088/2041-8205/734/1/L13/meta Hydrogen green house planets beyond the Habitable Zone]. Raymond Pierrehumbert, and Eric Gaidos. ''The Astrophysical Journal Letters''. 19 May 2011.</ref><ref name="Seager 2013">{{cite journal |title=Exoplanet Habitability |journal=Science |year=2013 |last=Seager |first=Sara |volume=340 |issue=577 |pages=577–581 |doi=10.1126/science.1232226 |pmid=23641111 |bibcode=2013Sci...340..577S }}</ref> and have the gravitational pull needed to retain an ample amount of [[Physics:Atmospheric pressure|atmospheric pressure]].<ref name="NOAA 2017"/> If the planet gravity cannot sustain that, then all the water will eventually evaporate into outer space.  A strong planetary [[Astronomy:Magnetosphere|magnetosphere]], maintained by internal [[Physics:Dynamo theory|dynamo action]] in an electrically conducting fluid layer, is helpful for shielding the upper atmosphere from [[Astronomy:Stellar wind|stellar wind]] mass loss and retaining water over long geological time scales.<ref name="Driscoll 2011"/>

A planet's atmosphere forms from outgassing during planet formation or is gravitationally captured from the surrounding [[Astronomy:Protoplanetary nebula|protoplanetary nebula]]. The surface temperature on an exoplanet is governed by the atmosphere's [[Chemistry:Greenhouse gas|greenhouse gas]]es (or lack thereof), so an atmosphere can be detectable in the form of upwelling [[Astronomy:Infrared astronomy|infrared radiation]] because the greenhouse gases absorb and re-radiate energy from the host star.<ref name="Seager 2013"/> Ice-rich planets that have migrated inward into orbit too close to their host stars may develop thick steamy atmospheres but still retain their volatiles for billions of years, even if their atmospheres undergo slow [[Earth:Atmospheric escape|hydrodynamic escape]].<ref name="Kennedy 2008">{{cite journal |title=Planet Formation around Stars of Various Masses: The Snow Line and the Frequency of Giant Planets |journal=The Astrophysical Journal |date=20 January 2008 |last=Kennedy |first=Grant M. |last2=Kenyon |first2=Scott J |volume=673 |issue=1 |pages=502–512 |doi=10.1086/524130 |bibcode=2008ApJ...673..502K |arxiv=0710.1065 }}</ref><ref name="Kuchner 2010">{{cite journal |title=Volatile-rich Earth-Mass Planets in the Habitable Zone |journal=The Astrophysical Journal Letters |date=10 October 2003 |last=Kuchner |first=Marc J. |volume=506 |arxiv=astro-ph/0303186 |doi=10.1086/378397 |bibcode=2003ApJ...596L.105K |pages=L105–L108 }}</ref><ref name="apj"/>  [[Physics:Ultraviolet|Ultraviolet]] photons are not only biologically harmful but can drive fast atmospheric escape that leads to the erosion of planetary atmospheres;<ref name=Luger/><ref name="apj"/> photolysis of water vapor, and hydrogen/oxygen escape to space can lead to the loss of several Earth oceans of water from planets throughout the habitable zone, regardless of whether the escape is energy-limited or diffusion-limited.<ref name=Luger>[http://online.liebertpub.com/doi/full/10.1089/ast.2014.1231 Extreme Water Loss and Abiotic O2 Buildup on Planets Throughout the Habitable Zones of M Dwarfs]. Luger R. and Barnes R. ''Astrobiology''. 14 February 2015, Vol 15, Issue 2; pages 119–143. DOI: 10.1089/ast.2014.1231</ref> The amount of water lost seems proportional with the planet mass, since the diffusion-limited hydrogen escape flux is proportional to the planet surface gravity.

During a [[Earth:Runaway greenhouse effect|runaway greenhouse effect]], water vapor reaches the stratosphere, where it is easily broken down ([[Astronomy:Photodissociation|photolyzed]]) by ultraviolet radiation (UV). Heating of the upper atmosphere by UV radiation can then drive a hydrodynamic wind that carries the hydrogen (and potentially some of the oxygen) to space, leading to the irreversible loss of a planet's surface water, oxidation of the surface, and possible accumulation of oxygen in the atmosphere.<ref name=Luger/> The fate of a given planet's atmosphere strongly depends on the extreme ultraviolet flux, the duration of the runaway regime, the initial water content, and the rate at which oxygen is absorbed by the surface.<ref name=Luger/>  Volatile-rich planets should be more common in the habitable zones of young stars and [[Astronomy:Stellar classification#Class M|M-type stars]].<ref name="apj"/>

===Composition models===

There are challenges in examining an exoplanetary surface and its atmosphere, as cloud coverage influences the atmospheric temperature, structure as well as the observability of [[Physics:Astronomical spectroscopy|spectral features]].<ref name="Atmos chemistry">[http://iopscience.iop.org/article/10.1088/2041-8205/775/2/L47/meta Water planets in the habitable zone: Atmospheric chemistry observable features, and the case of Kepler-62e and -62f]</ref> However, planets composed of large quantities of water that reside in the habitable zone (HZ) are expected to have distinct geophysics and geochemistry of their surface and atmosphere.<ref name="Atmos chemistry"/> For example, in the case of exoplanets Kepler-62e and -62f, they could possess a liquid ocean outer surface, a steam atmosphere, or a full cover of surface Ice I, depending on their orbit within the HZ and the magnitude of their [[Earth:Greenhouse effect|greenhouse effect]]. Several other surface and interior processes affect the atmospheric composition, including but not limited to the ocean fraction for dissolution of {{chem|CO|2}} and for atmospheric relative humidity, [[Chemistry:Redox|redox]] state of the planetary surface and interior, acidity levels of the oceans, planetary [[Earth:Albedo|albedo]], and surface gravity.<ref name="Seager 2013"/><ref>F. Kasting, D. Catling, ''Annu. Rev. Astron. Astrophys.'' 41, 429 (2003).</ref>

The atmospheric structure, as well as the resulting HZ limits, depend on the density of a planet's atmosphere, shifting the HZ outward for lower mass and inward for higher mass planets.<ref name="Atmos chemistry"/><!--(see also: Kopparapu et al. 2013)--> Theory, as well as computer models suggest that atmospheric composition for water planets in the habitable zone (HZ) should not differ substantially from those of land-ocean planets.<ref name="Atmos chemistry"/> For modeling purposes, it is assumed that the initial composition of icy [[Astronomy:Planetesimal|planetesimal]]s that assemble into water planets is similar to that of comets: mostly water ({{chem|H|2|O}}), and some ammonia ([[Chemistry:Ammonia|{{chem|NH|3}}]]), and carbon dioxide ([[Chemistry:Carbon dioxide|{{chem|CO|2}}]]).<ref name="Atmos chemistry"/> An initial composition of ice similar to that of comets leads to an atmospheric model composition of 90% {{chem|H|2|O}}, 5% {{chem|NH|3}}, and 5% {{chem|CO|2}}.<ref name="Atmos chemistry"/><ref>{{cite journal|doi=10.1111/j.1945-5100.2005.tb00960.x | volume=40 | title=Origin of water in the terrestrial planets | year=2005 | journal=Meteoritics & Planetary Science | pages=519–527 | last1 = Drake | first1 = Michael J.}}</ref>

Atmospheric models for Kepler-62f show that an atmospheric pressure of between 1.6 [[Bar (unit)|bar]] and 5 bar of {{chem|CO|2}} are needed to warm the surface temperature above freezing, leading to a scaled surface pressure of 0.56–1.32 times Earth's.<ref name="Atmos chemistry"/>

==Exoplanets==
{{See|List of extrasolar candidates for liquid water}}

[[File:Ocean world.png|thumb|Artist's illustration of a hypothetical ocean planet with two natural satellites]]
Outside the Solar System, [[Astronomy:Kepler-11|Kepler-11]],<ref name=dangelo_bodenheimer_2016>{{cite journal|last=D'Angelo|first=G.|author2= Bodenheimer, P. |title=In Situ and Ex Situ Formation Models of Kepler 11 Planets|journal=The Astrophysical Journal|year=2016|volume=828|issue=1|pages=in press|doi=10.3847/0004-637X/828/1/33|arxiv = 1606.08088 |bibcode = 2016ApJ...828...33D }}</ref> [[Astronomy:GJ 1214 b|GJ 1214 b]], [[Astronomy:Kepler-22b|Kepler-22b]], [[Astronomy:Kepler-62f|Kepler-62f]], [[Astronomy:Kepler-62e|Kepler-62e]]<ref>[http://www.solstation.com/planets/water-worlds.htm Water Worlds and Ocean Planets]. 2012. Sol Company</ref><ref name="disco-charbonneau">{{cite journal |author=David Charbonneau |display-authors=4 |author2=Zachory K. Berta |author3=Jonathan Irwin|author4=Christopher J. Burke |author5=Philip Nutzman|author6=Lars A. Buchhave |author7=Christophe Lovis|author8=Xavier Bonfils |author9=David W. Latham|author10=Stéphane Udry |author11=Ruth A. Murray-Clay|author12=Matthew J. Holman |author13=Emilio E. Falco|author14=Joshua N. Winn |author15=Didier Queloz|author16=Francesco Pepe |author17=Michel Mayor|author18=Xavier Delfosse |author19=Thierry Forveille |date=2009 |title=A super-Earth transiting a nearby low-mass star |journal=Nature |volume=462 |issue=17 December 2009 |pages=891–894 |doi=10.1038/nature08679 |pmid=20016595 |bibcode=2009Natur.462..891C|arxiv = 0912.3229 }}</ref><ref name="planetmodels">{{cite journal |last1= Kuchner|first1= Seager |first2=M.|last2=Hier-Majumder |first3=C. A.|last3=Militzer |date=2007 |title=Mass–radius relationships for solid exoplanets |journal=The Astrophysical Journal |volume=669 |issue=2 |pages=1279–1297 |url=http://www.iop.org/EJ/abstract/0004-637X/669/2/1279/ |doi=10.1086/521346 |bibcode=2007ApJ...669.1279S|arxiv = 0707.2895 }}</ref><ref>{{cite news|url=https://www.bbc.co.uk/news/science-environment-19008908|title=A home from home: Five planets that could host life|last=Rincon|first=Paul|date=5 December 2011|work=BBC News|accessdate=26 November 2016}}</ref> and the planets of [[Astronomy:TRAPPIST-1|TRAPPIST-1]]<ref name="HST-20170831">{{cite news |last1=Bourrier |first1=Vincent |last2=de Wit |first2=Julien |last3=Jäger |first3=Mathias |title=Hubble delivers first hints of possible water content of TRAPPIST-1 planets |url=http://www.spacetelescope.org/news/heic1713/ |date=31 August 2017 |work=www.SpaceTelescope.org |accessdate=4 September 2017 }}</ref><ref name="TIE-20170904">{{cite news |author=PTI |title=First evidence of water found on TRAPPIST-1 planets – The results suggest that the outer planets of the system might still harbour substantial amounts of water. This includes the three planets within the habitable zone of the star, lending further weight to the possibility that they may indeed be habitable. |url=http://indianexpress.com/article/technology/science/first-evidence-of-water-found-on-trappist-1-planets-4827977/ |date=4 September 2017 |work=The Indian Express |accessdate=4 September 2017 }}</ref> are some of most likely known candidates for an extrasolar ocean planet. Many more such objects are expected to be discovered by [[Engineering:Kepler (spacecraft)|''Kepler'']].

Although 70.8% of all [[Astronomy:Earth|Earth]]'s surface is covered in water,<ref>Pidwirny, M. [http://www.physicalgeography.net/fundamentals/8o.html "Surface area of our planet covered by oceans and continents. (Table 8o-1)"]. University of British Columbia, Okanagan. 2006. Retrieved May 13, 2016.</ref> water accounts for only 0.05% of Earth's mass. An extraterrestrial ocean's depth would 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 these oceans could lead to the formation of a mantle of exotic forms of ice such as [[Earth:Ice V|ice V]].<ref name=dangelo_bodenheimer_2016 /> 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 [[Physics:Supercritical fluid|supercritical]] and lack a well-defined surface.<ref name="physorg"/> 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 [[Earth:Greenhouse effect|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.<ref name=dangelo_bodenheimer_2016 /> Otherwise, they would form a [[Astronomy:Hot Neptune|warmer version]] of an [[Astronomy:Ice giant|ice giant]] instead, like [[Astronomy:Uranus|Uranus]] and [[Astronomy:Neptune|Neptune]].

==Astrobiology==

{{Further|Astronomy:Astrobiology|Astronomy:Planetary habitability|Astronomy:Habitable zone}}
The characteristics of ocean worlds or ocean planets provide clues to their history, and the [[Astronomy:Formation and evolution of the Solar System|formation and evolution of the Solar System]] as a whole. Of additional interest is their potential to [[Biology:Abiogenesis|form]] and [[Astronomy:Planetary habitability|host life]]. Life as we know it requires liquid water, a source of energy, and nutrients, and all three key requirements can potentially be satisfied within some of these bodies,<ref name="Nimmo 2016"/> that may offer the possibility for sustaining simple biological activity over geological timescales.<ref name="Nimmo 2016"/><ref name="Vance 2007"/> In August 2018, researchers reported that water worlds could support life.<ref name="EA-2018901">{{cite web |author=Staff |title=Water worlds could support life, study says - Analysis by UChicago, Penn State scientists challenges idea that life requires 'Earth clone' |url=https://www.eurekalert.org/pub_releases/2018-08/uoc-wwc083018.php |date=1 September 2018 |work=EurekAlert |accessdate=1 September 2018 }}</ref><ref name="APJ-20180831">{{cite journal |last1=Kite |first1=Edwin S. |last2=Ford |first2=Eric B. |title=Habitability of Exoplanet Waterworlds |date=31 August 2018 |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=864 |pages=75 |doi=10.3847/1538-4357/aad6e0 |arxiv=1801.00748 }}</ref>

An ocean world's [[Astronomy:Planetary habitability|habitation]] by Earth-like life is limited if the planet is completely covered by liquid water at the surface, even more restricted if a pressurized, solid ice layer is located between the global ocean and the lower rocky [[Earth:Mantle (geology)|mantle]].<ref name="Franck 2003">{{cite journal |title=The habitable zone of Earth-mass planets around 47 UMa: results for land and water worlds |journal=International Journal of Astrobiology |date=January 2003 |last=Franck |first=S. |last2=Cuntz |first2=M. |last3=von Bloh |first3=W. |last4=Bounama |first4= C. |volume=2 |issue=1 |pages=35–39 |doi=10.1017/S1473550403001368 |url=https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/the-habitable-zone-of-earth-mass-planets-around-47-uma-results-for-land-and-water-worlds/618868C81B8CE5D1EB9CEFF30926D6A4 |accessdate=2017-10-01 |bibcode=2003IJAsB...2...35F }}</ref><ref>[http://www.solstation.com/planets/water-worlds.htm "Water Worlds and Ocean Planets"]. ''Solsation.com''. 2013. Retrieved January 7, 2016.</ref> Simulations of a hypothetical ocean world covered by 5 Earth oceans' worth of water indicate the water would not contain enough [[Chemistry:Phosphorus|phosphorus]] and other nutrients for Earth like oxygen-producing ocean organisms such as [[Earth:Plankton|plankton]] to evolve. On Earth, phosphorus is washed into the oceans by rainwater hitting rocks on exposed land so the mechanism would not work on an ocean world. Simulations of ocean planets with 50 Earth oceans' worth of water indicate the pressure on the sea floor would be so immense that the planet's interior would not sustain plate tectonics to cause volcanism to provide the right chemical environment for terrestrial life.<ref name="Witze 2017">{{cite journal |title=Exoplanet hunters rethink search for alien life |journal=Nature |date=23 November 2017 |last=Witze |first=Alexandra |volume=551 |issue=23 November 2017 |pages=421–422 |doi=10.1038/nature.2017.23023 |pmid=29168837 |bibcode=2017Natur.551..421W }}</ref>

On the other hand, small bodies such as [[Astronomy:Europa (moon)|Europa]] and [[Astronomy:Enceladus|Enceladus]] are regarded as particularly habitable environments because their oceans are in direct contact with the underlying silicate [[Astronomy:Planetary core|core]], a potential source of both heat, and biologically important chemical elements.<ref name="Nimmo 2016"/> The surface geological activity of these bodies may also lead to the transport to the oceans of biologically-important building blocks implanted at the surface, such as organic molecules from comets or [[Astronomy:Tholin|tholin]]s —formed by solar [[Physics:Ultraviolet|ultraviolet]] irradiation of simple [[Chemistry:Organic compound|organic compound]]s such as [[Chemistry:Methane|methane]] or [[Chemistry:Ethane|ethane]], often in combination with nitrogen.<ref name="Hörst">Sarah Hörst, [http://www.planetary.org/blogs/guest-blogs/2015/0722-what-in-the-worlds-are-tholins.html "What in the world(s) are tholins?"], Planetary Society, July 23, 2015. Retrieved 30 Nov 2016.</ref>

===Oxygen===

Molecular oxygen ({{chem|O|2}}) can be produced by geophysical processes, as well as a byproduct of [[Earth:Photosynthesis|photosynthesis]] by life forms, so although encouraging, {{chem|O|2}} is not a reliable [[Astronomy:Biosignature|biosignature]].<ref name="physorg"/><ref name=Luger/><ref>[https://www.nature.com/articles/srep13977 Titania may produce abiotic oxygen atmospheres on habitable exoplanets]. Norio Narita, Takafumi Enomoto, Shigeyuki Masaoka, and Nobuhiko Kusakabe. ''Scientific Reports'' 5, Article number: 13977 (2015); doi:10.1038/srep13977</ref><ref name="Seager 2013"/>  In fact, planets with high concentration of {{chem|O|2}} in their atmosphere may be uninhabitable.<ref name=Luger/> [[Biology:Abiogenesis|Abiogenesis]] in the presence of massive amounts of atmospheric oxygen could be difficult because early organisms relied on the free energy available in [[Chemistry:Redox|redox reactions]] involving a variety of hydrogen compounds; on an {{chem|O|2}}-rich planet, organisms would have to compete with the oxygen for this free energy.<ref name=Luger/>

== See also ==
{{Div col|colwidth=26em}}

* {{annotated link|Astronomy:Circumstellar habitable zone}}
* {{annotated link|Astronomy:Desert planet}}
* {{annotated link|Astronomy:Earth analog}}
* {{annotated link|Astronomy:Extraterrestrial liquid water}}
* {{annotated link|Astronomy:Ice planet}}
* {{annotated link|Astronomy:List of extrasolar candidates for liquid water}}
'''[[Astronomy:Astrobiology|Astrobiology]] mission concepts to water worlds in the outer Solar System:'''
* {{annotated link|Astronomy:Enceladus Explorer|abbreviatio=EnEx}}
* {{annotated link|Astronomy:Enceladus Life Finder|abbreviation=ELF}}
* {{annotated link|Astronomy:Europa Lander (NASA)|Europa Lander}}
* {{annotated link|Astronomy:Explorer of Enceladus and Titan|abbreviation=E<sup>2</sup>T}}
* {{annotated link|Astronomy:Journey to Enceladus and Titan|abbreviation=JET}}
* {{annotated link|Astronomy:Jupiter Icy Moons Explorer|abbreviation=JUICE}}
* {{annotated link|Astronomy:Laplace-P}} 
* {{annotated link|Astronomy:Life Investigation For Enceladus|abbreviation=LIFE}}
* {{annotated link|Astronomy:Oceanus (Titan orbiter)|Oceanus}}
* {{annotated link|Astronomy:THEO|Testing the Habitability of Enceladus's Ocean|abbreviation=THEO}}
* {{annotated link|Astronomy:Titan Lake In-situ Sampling Propelled Explorer|abbreviation=TALISE}}
* {{annotated link|Astronomy:Triton Hopper}}
{{div col end}}

== References ==
{{Reflist|30em}}

== External links ==
* {{cite journal| title=Could we identify hot Ocean-Planets with CoRoT, Kepler and Doppler velocimetry?| author=F. Selsis| display-authors=4| author2=B. Chazelas| author3=P. Borde| author4=M. Ollivier| author5=F. Brachet| author6=M. Decaudin| author7=F. Bouchy| author8=D. Ehrenreich| author9=J.-M. Griessmeier| author10=H. Lammer| author11=C. Sotin| author12=O. Grasset| author13=C. Moutou| author14= P. Barge| author15=M. Deleuil| author16= D. Mawet| author17=D. Despois| author18=J. F. Kasting| author19=A. Leger| date=2007| journal=Icarus| arxiv=astro-ph/0701608 | doi=10.1016/j.icarus.2007.04.010 | volume=191| issue=2 | pages=453–468 | bibcode=2007Icar..191..453S}}

{{exoplanet}}
{{astrobiology}}

[[Category:Oceans]]
[[Category:Terrestrial planets|*]]
[[Category:Types of planet]]

{{Sourceattribution|Ocean planet|1}}