Ocean world

{{Short description|Planet containing a significant amount of water or other liquid}} {{Hatnote group| {{Other uses|Ocean World (disambiguation)}} {{Distinguish|Ocean#World Ocean{{!}}World Ocean}} }}

[[File:Ocean world Earth.jpg|thumb|Earth's surface is dominated by the ocean, which forms 71% of Earth's surface. Thus, Earth can be considered a water world, although not a fully oceanic world.]]

An '''ocean world''', '''ocean planet''' or '''water world''' is a type of planet or natural satellite 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.<ref>{{cite web |url=http://www.omnilexica.com/?q=ocean+planet |title=Ocean planet definition/meaning |author=<!--Not stated--> |date=1 October 2017 |website=Omnilexica |access-date=1 October 2017 |archive-url=https://web.archive.org/web/20171002021758/http://www.omnilexica.com/?q=ocean+planet |archive-date=2 October 2017 |quote=An ocean planet is a hypothetical type of planet which has a substantial fraction of its mass made of water. The surface on such planets would be completely covered with an ocean of water hundreds of kilometers deep, much deeper than the oceans of Earth.}}</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 |last1=Adams |first1=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 |page=1378 |date=8 August 2016 |last1=Nimmo |first1=F. |last2=Pappalardo |first2=R. T. |doi=10.1002/2016JE005081 |bibcode=2016JGRE..121.1378N |doi-access=free }}</ref><ref name="Vance 2007">{{cite journal | doi = 10.1089/ast.2007.0075 | volume=7 | title=Hydrothermal Systems in Small Ocean Planets | year=2007 | journal=Astrobiology | pages=987–1005 | last1 = Vance | first1 = Steve | last2 = Harnmeijer | first2 = Jelte | last3 = Kimura | first3 = Jun | last4 = Hussmann | first4 = Hauke | last5 = Brown | first5 = J. Michael| issue=6 | pmid=18163874 | bibcode=2007AsBio...7..987V }}</ref> The term ''ocean world'' is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen,<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 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).<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|issue=5|pages=642–654|hdl=10261/213115 |hdl-access=free}}</ref> 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 subsurface oceans are suspected to exist on Jupiter's moons Europa and Ganymede and Saturn's moons Enceladus and Titan.<ref>{{cite web | url=https://science.nasa.gov/solar-system/ocean-worlds/ | title=Ocean Worlds: Water in the Solar System and Beyond - NASA Science | date=22 July 2023 }}</ref> Several exoplanets 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 |access-date=2017-10-03 }}</ref> There are also considerable amounts of subsurface water found on Earth, mostly in the form of aquifers.<ref>{{Cite web |title=Aquifers and Groundwater {{!}} U.S. Geological Survey |url=https://www.usgs.gov/special-topics/water-science-school/science/aquifers-and-groundwater |access-date=2023-05-02 |website=www.usgs.gov|date=9 October 2018 }}</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 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.<ref name="NASA-20200618" /><ref name="OceanPlanets">{{cite journal|last1=Quick|first1=Lynnae C.|author-link=Lynnae Quick|last2=Roberge|first2=Aki|last3=Barr Mlinar|first3=Amy|last4=Hedman|first4=Matthew M.|date=2020-06-18|title=Forecasting Rates of Volcanic Activity on Terrestrial Exoplanets and Implications for Cryovolcanic Activity on Extrasolar Ocean Worlds|journal=Publications of the Astronomical Society of the Pacific|volume=132|issue=1014|page=084402|doi=10.1088/1538-3873/ab9504|bibcode=2020PASP..132h4402Q |doi-access=free}}</ref>

==Overview== ===Definitions=== According to Lunine, "oceans" have been defined as "stable, globe-girdling bodies of liquid water."<ref name=Lunine>{{cite journal |last1=Lunine |first1=Jonathan I. |title=Ocean worlds exploration |journal=Acta Astronautica |date=February 2017 |volume=131 |pages=123–130 |doi=10.1016/j.actaastro.2016.11.017 |doi-access=free |bibcode=2017AcAau.131..123L }}</ref> In addition, "Ocean worlds is the label given to objects in the solar system that host stable, globe-girdling bodies of liquid water," in contrast to the terms "'ocean planet' and 'water world', both of which refer to exoplanets (planets orbiting other stars) with substantial mass fractions of water in their bulk compositions."<ref name=Lunine/>

===Solar System planetary bodies=== {{Further|Ocean Worlds Exploration Program}} [[File:PIA20013-Enceladus-SaturnMoon-ArtistConcept-20151026.jpg|thumb|upright=1.2|Diagram of the interior of Enceladus. {{legend|#bcc|Ice crust}} {{legend|#245|Global ocean}} {{legend|#555|Rocky core}} The southern polar region shows active jets.]] Ocean worlds are of interest to astrobiologists for their potential to develop life and sustain biological activity.<ref name="Vance 2007" /><ref name="Nimmo 2016" /> Major moons and dwarf planets in the Solar System thought to harbor subsurface oceans are of interest because they can be reached and studied by space probes, in contrast to exoplanets, which are light-years away, beyond the reach of current technology. The best-established water worlds in the Solar System, other than the Earth, are Callisto, Enceladus, Europa, Ganymede, and Titan.<ref name="Nimmo 2016" /><ref name="OW Roadmap 2019">{{cite journal | last1 = Hendrix | first1 = Amanda R. | last2 = Hurford | first2 = Terry A. | last3 = Barge | first3 = Laura M. | last4 = Bland | first4 = Michael T. | last5 = Bowman | first5 = Jeff S. | last6 = Brinckerhoff | first6 = William | last7 = Buratti | first7 = Bonnie J. | last8 = Cable | first8 = Morgan L. | last9 = Castillo-Rogez | first9 = Julie | last10 = Collins | first10 = Geoffrey C. | display-authors = etal | year = 2019| title = The NASA Roadmap to Ocean Worlds | journal = Astrobiology | volume = 19| issue = 1 | pages = 1–27| doi = 10.1089/ast.2018.1955 | pmid = 30346215 | pmc = 6338575 | bibcode = 2019AsBio..19....1H | doi-access = free }}</ref> Europa and Enceladus are considered compelling targets for exploration due to their thin outer crusts and cryovolcanic features.

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,<ref name='OW Roadmap 2019'/> Titania,<ref>{{Cite web|url=https://weather.com/en-IN/weather/today/l/39.0438,-77.4879|title=Weather forecast and conditions for Ashburn, VA, United States - The Weather Channel &#124; weather.com|website=The Weather Channel}}</ref><ref>{{cite web | url=https://www.nasa.gov/feature/jpl/new-study-of-uranus-large-moons-shows-4-may-hold-water | title=New Study of Uranus' Large Moons Shows 4 May Hold Water - NASA | date=4 May 2023 }}</ref> Umbriel,<ref name="space.com">{{cite web | url=https://www.space.com/uranus-four-biggest-moons-buried-oceans | title=Uranus' 4 biggest moons may have buried oceans of salty water | website=Space.com | date=5 May 2023 }}</ref> Ceres,<ref name="Nimmo 2016"/> Dione,<ref name="Marco2020"/> 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> Miranda,<ref name='OW Roadmap 2019'/> Oberon,<ref name="Vance 2007"/><ref name="Hussmann Sohl et al. 2006"/> Pluto,<ref name="Johnson2016"/> Triton,<ref name="schenk2021"/> Eris,<ref name="Vance 2007"/><ref name="glein2024"/> and Makemake.<ref name="glein2024"/>

===Exoplanets=== {{Further|List of extrasolar candidates for liquid water}} [[File:NASA-Exoplanet-WaterWorlds-20180817.jpg|thumb|center|upright=2|{{center|A set of exoplanets of varying size containing water, compared with the Earth (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 |access-date=17 August 2018 }}</ref>}}]] [[File:ExoplanetPopulations-20170616.png|thumb|upright=2|Exoplanet population with purely oceanic worlds as transition group with ice giants between gas giants and lava or rocky planets]] Outside the Solar System, exoplanets that have been described as candidate ocean worlds include GJ 1214 b,<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="planetmodels2">{{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 |doi = 10.1086/521346|bibcode = 2007ApJ...669.1279S|arxiv = 0707.2895 }}</ref> Kepler-22b, Kepler-62e, Kepler-62f,<ref>[https://web.archive.org/web/20100325211745/http://www.solstation.com/planets/water-worlds.htm Water Worlds and Ocean Planets]. 2012. Sol Company</ref><ref name="disco-charbonneaub">{{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 |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|access-date=26 November 2016}}</ref> and the planets of Kepler-11<ref name="dangelo_bodenheimer_2016"/> and 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 |access-date=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 |access-date=4 September 2017 }}</ref>

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.<ref name="Cadieux2022"/><ref name="Piaulet2022"/> Additionally, models of the massive rocky planet LHS 1140 b suggest its surface may be covered in a deep ocean.<ref name="Lillo-Box2020"/>

Although 70.8% of all 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 could be so deep and dense that even at high temperatures the pressure would turn the water into ice. The immense pressures of many thousands of bar in the lower regions of such oceans, could lead to the formation of a mantle of exotic forms of ice such as ice V.<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 |doi-access=free }}</ref> 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.<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> 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,<ref name=dangelo_bodenheimer_2013>{{cite journal|last=D'Angelo|first=G.|author2= Bodenheimer, P. |title=Three-Dimensional Radiation-Hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks|journal=The Astrophysical Journal|year=2013|volume=778|issue=1|pages=77 (29 pp.)|doi=10.1088/0004-637X/778/1/77|arxiv = 1310.2211 |bibcode = 2013ApJ...778...77D }}</ref> 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 warmer version of an ice giant instead, like Uranus and Neptune.{{Citation needed|date=February 2022}}

==History== Gravitational calculations suggested by the start of 20th century that Europa's composition was water rich, and Earth ground based observations by Gerard Kuiper revealed 1957 the water ice composition.<ref name="q533">{{cite web | last=Yazgin | first=Evrim | title=Astronomers believe that Jupiter's moon Callisto is a water world | website=Cosmos | date=February 22, 2025 | url=https://cosmosmagazine.com/space/astronomy/callisto-jupiter-moon-water/ | access-date=May 7, 2025}}</ref>

Important preliminary theoretical work was carried out prior to the planetary missions of the 1970s. In particular, Lewis showed in 1971 that radioactive decay alone was likely sufficient to produce subsurface oceans in large moons, especially if ammonia ({{chem|NH|3}}) were present. Peale and Cassen figured out in 1979 the important role of tidal heating (aka: tidal flexing) on satellite evolution and structure.<ref name="Nimmo 2016"/> The first confirmed detection of an exoplanet was in 1992. Marc Kuchner in 2003 and Alain Léger ''et al'' figured in 2004 that a small number of icy planets that form in the region beyond the snow line can migrate inward to ~1 AU, where the outer layers subsequently melt.<ref name="Kennedy 2008"/><ref name="Léger"/>

The cumulative evidence collected by the Hubble Space Telescope, as well as ''Pioneer'', ''Galileo'', ''Voyager'', ''Cassini–Huygens'', and ''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 ''Kepler'' space observatory, launched on March 7, 2009, has discovered thousands of exoplanets, about 50 of them of Earth-size in or near habitable zones.<ref name="NYT-20140512">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Finder of New Worlds |url=https://www.nytimes.com/2014/05/13/science/finder-of-new-worlds.html |date=May 12, 2013 |work=The New York Times |access-date=May 13, 2014}}</ref><ref name="NYT-20150106-DB">{{cite news |last=Overbye |first=Dennis |author-link=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=The New York Times |access-date=January 6, 2015}}</ref>

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

In June 2020, NASA scientists reported that it is likely that exoplanets with oceans may be common in the Milky Way galaxy, based on mathematical modeling studies.<ref name="NASA-20200618">{{cite news |author=Shekhtman, Lonnie |display-authors=et al. |title=Are Planets with Oceans Common in the Galaxy? It's Likely, NASA Scientists Find |url=https://www.nasa.gov/feature/goddard/2020/are-planets-with-oceans-common-in-the-galaxy-it-s-likely-nasa-scientists-find |date=18 June 2020 |work=NASA |access-date=20 June 2020 }}</ref>

In August 2022, TOI-1452 b, a super-Earth exoplanet with potential deep oceans that is 99 light-years from Earth, was discovered by the Transiting Exoplanet Survey Satellite.<ref name="Cadieux2022"/>

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

Planetary objects that form in the outer Solar System begin as a 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 frost line should contain mostly {{chem|H|2|O}} and silicates. Those that form farther out can acquire ammonia ({{chem|NH|3}}) and methane ({{chem|CH|4}}) as hydrates, together with CO, {{chem|N|2}}, and {{chem|CO|2}}.<ref name="apj"/>

Planets that form prior to the dissipation of the gaseous 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 magma, a large fraction of the planet's water content will initially be trapped in the 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 exsolved to form a steam atmosphere, which may eventually condense to form an ocean.<ref name=Luger/> Ocean formation requires differentiation, and a heat source, either radioactive decay, tidal heating, or the early luminosity of the parent body.<ref name="Nimmo 2016"/> Unfortunately, the initial conditions following accretion are theoretically incomplete.

Planets that formed in the outer, water-rich regions of a disk and migrated inward are more likely to have abundant water.<ref>{{cite journal | arxiv= 0710.2366 | doi = 10.1126/science.1144358 | volume=318 | title=New Worlds on the Horizon: Earth-Sized Planets Close to Other Stars | year=2007 | journal=Science | pages=210–213 | last1 = Gaidos | first1 = E. | last2 = Haghighipour | first2 = N. | last3 = Agol | first3 = E. | last4 = Latham | first4 = D. | last5 = Raymond | first5 = S. | last6 = Rayner | first6 = J.| issue = 5848 | pmid = 17932279 | bibcode = 2007Sci...318..210G }}</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 star, it would be strong evidence for 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 Solar System formation and of extra-solar system formation have shown that planets are likely to 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 |doi-access=free}}</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=2010exop.book..347L| title=WATER-PLANETS IN THE HABITABLE ZONE: ATMOSPHERIC CHEMISTRY, OBSERVABLE FEATURES, AND THE CASE OF KEPLER-62 ''e'' AND -62 ''f'' |journal=The Astrophysical Journal |publisher=University of Arizona Press, Tucson, AZ| editor=S. Seager. |pages=347–371|date=2011|volume=775 |issue=2 |doi=10.1088/2041-8205/775/2/L47 |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"/> in 2003.

==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 differentiation (separation into rock-ice layers) or not. Shape or gravity measurements can in some cases be used to infer the moment of inertia – if the body is in hydrostatic equilibrium (i.e. behaving like a fluid on long timescales). 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, librations, axial tilt, tidal response, radar sounding, compositional evidence, and surface features.<ref name="Nimmo 2016"/>

[[File:Ganymede diagram.svg|thumb|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 icy moon will consist of a water layer sitting atop a 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 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 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 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 |publisher=Harvard-Smithsonian Center for Astrophysics |access-date=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 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, radiogenic heating, or tidal flexing.<ref name="Vance 2007"/> Where fluid-rock interactions propagate slowly into a deep brittle layer, thermal energy from 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"/> Nevertheless, imaging data from the Voyager 2, Cassini-Huygens, ''Galileo'' and New Horizons spacecraft revealed cryovolcanic surface features on several of the icy bodies in our own solar system. Recent studies suggest that cryovolcanism may occur on ocean planets that harbor internal oceans beneath layers of surface ice as it does on the icy moons Enceladus and Europa in our own solar system.<ref name="NASA-20200618"/><ref name="OceanPlanets"/>

Liquid water oceans on extrasolar planets could be significantly deeper than the Earth's ocean, which has an average depth of 3.7&nbsp;km.<ref name="Charette2010"/> Depending on the planet's gravity and surface conditions, exoplanet oceans could be up to hundreds of times deeper. For example, a planet with a 300 K surface can possess liquid water oceans with depths from 30 to 500&nbsp;km, depending on its mass and composition.<ref name="Nixon2021"/> {{Further|Super-dense water}}

==Atmospheric models== {{Further|Extraterrestrial atmosphere}} [[File:Hyceanplanet.jpg|thumb|Artist depiction of a hycean planet, a large ocean world with a hydrogen atmosphere]]

To allow surface water to be liquid for long periods of time, a planet—or moon—must orbit within the habitable zone (HZ), possess a protective 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 name="PierrehumbertGaidos2011">{{cite journal|last1=Pierrehumbert|first1=Raymond|last2=Gaidos|first2=Eric|title=Hydrogen Greenhouse Planets Beyond the Habitable Zone|journal=The Astrophysical Journal|volume=734|issue=1|year=2011|pages=L13 |doi=10.1088/2041-8205/734/1/L13|arxiv=1105.0021|bibcode=2011ApJ...734L..13P|doi-access=free}}</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 atmospheric pressure.<ref name="NOAA 2017"/> If the planet's gravity cannot sustain that, then all the water will eventually evaporate into outer space. A strong planetary magnetosphere, maintained by internal dynamo action in an electrically conducting fluid layer, is helpful for shielding the upper atmosphere from 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 protoplanetary nebula. The surface temperature on an exoplanet is governed by the atmosphere's greenhouse gases (or lack thereof), so an atmosphere can be detectable in the form of upwelling 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 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 |last1=Kennedy |first1=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="apj"/> 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>{{cite journal | doi = 10.1089/ast.2014.1231 | volume=15 | title=Extreme Water Loss and Abiotic O 2 Buildup on Planets Throughout the Habitable Zones of M Dwarfs | year=2015 | journal=Astrobiology | pages=119–143 | last1 = Luger | first1 = R.| issue=2 | pmc=4323125 | bibcode=2015AsBio..15..119L | pmid=25629240 | arxiv=1411.7412 }}</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 runaway greenhouse effect, water vapor reaches the stratosphere, where it is easily broken down (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 M-type stars.<ref name="apj"/>

Scientists have proposed Hycean planets, ocean planets with a thick atmosphere made mainly of hydrogen. Those planets would have a wide range area around their star where they could orbit and have liquid water. However, those models worked on rather simplistic approaches to the planetary atmosphere. More complex studies showed that hydrogen reacts differently to starlight's wavelengths than heavier elements like nitrogen and oxygen. If such a planet, with an atmospheric pressure 10 to 20 heavier than Earth's, was located at 1 astronomical unit (AU) from their star their water bodies would boil. Those studies now place the habitable zone of such worlds at 3.85 AU, and 1.6 AU if it had a similar atmospheric pressure to Earth.<ref>{{cite web |url= https://www.space.com/hycean-exoplanets-may-not-support-life|title= Hycean exoplanets may not be able to support life after all|author= Paul Sutter|date= May 2, 2023|publisher= Space.com |access-date=May 5, 2023}}</ref>

===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 spectral features.<ref name="Atmos chemistry">{{cite journal |last1=Kaltenegger |first1=L. |last2=Sasselov |first2=D. |last3=Rugheimer |first3=S. |title=WATER-PLANETS IN THE HABITABLE ZONE: ATMOSPHERIC CHEMISTRY, OBSERVABLE FEATURES, AND THE CASE OF KEPLER-62 e AND -62 f |journal=The Astrophysical Journal |date=18 September 2013 |volume=775 |issue=2 |pages=L47 |doi=10.1088/2041-8205/775/2/L47 |arxiv=1304.5058 |bibcode=2013ApJ...775L..47K }}</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 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, redox state of the planetary surface and interior, acidity levels of the oceans, planetary albedo, and surface gravity.<ref name="Seager 2013"/><ref>{{cite journal | last1 = Kasting | first1 = F. | last2 = Catling | first2 = D. | year = 2003 | title = Evolution of a Habitable Planet| journal = Annu. Rev. Astron. Astrophys. | volume = 41 | page = 429 | doi = 10.1146/annurev.astro.41.071601.170049 | bibcode = 2003ARA&A..41..429K }}</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 planetesimals that assemble into water planets is similar to that of comets: mostly water ({{chem|H|2|O}}), and some ammonia ({{chem|NH|3}}), and 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.| issue=4 | bibcode=2005M&PS...40..519D | doi-access=free }}</ref>

Atmospheric models for Kepler-62f show that an atmospheric pressure of between 1.6 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"/>

== Oceanography == It is suggested that strong ocean currents exist in Enceladus, Titan, Ganymede, and Europa.<ref name=":0">{{Cite journal |last=Soderlund |first=Krista M. |date=2019-08-16 |title=Ocean Dynamics of Outer Solar System Satellites |journal=Geophysical Research Letters |volume=46 |issue=15 |pages=8700–8710 |doi=10.1029/2018GL081880 |arxiv=1901.04093 |bibcode=2019GeoRL..46.8700S }}</ref><ref>{{Cite journal |last1=Soderlund |first1=Krista M. |last2=Rovira-Navarro |first2=Marc |last3=Le Bars |first3=Michael |last4=Schmidt |first4=Britney E. |last5=Gerkema |first5=Theo |date=2024-01-17 |title=The Physical Oceanography of Ice-Covered Moons |journal=Annual Review of Marine Science |language=en |volume=16 |issue=1 |pages=25–53 |doi=10.1146/annurev-marine-040323-101355 |pmid=37669566 |doi-access=free }}</ref> In Enceladus, oceanic heat flux inferred from ice shell thickness suggests the upwelling of warm water at the poles and downwelling of colder water at low latitudes.<ref>{{Cite journal |last1=Čadek |first1=Ondřej |last2=Souček |first2=Ondřej |last3=Běhounková |first3=Marie |last4=Choblet |first4=Gaël |last5=Tobie |first5=Gabriel |last6=Hron |first6=Jaroslav |date=2019-02-01 |title=Long-term stability of Enceladus' uneven ice shell |journal=Icarus |volume=319 |pages=476–484 |doi=10.1016/j.icarus.2018.10.003 |bibcode=2019Icar..319..476C }}</ref><ref>{{Cite journal |last1=Choblet |first1=Gaël |last2=Tobie |first2=Gabriel |last3=Sotin |first3=Christophe |last4=Běhounková |first4=Marie |last5=Čadek |first5=Ondřej |last6=Postberg |first6=Frank |last7=Souček |first7=Ondřej |date=December 2017 |title=Powering prolonged hydrothermal activity inside Enceladus |journal=Nature Astronomy |volume=1 |issue=12 |pages=841–847 |doi=10.1038/s41550-017-0289-8 |bibcode=2017NatAs...1..841C }}</ref> Europa is predicted to have an equatorial upwelling of warm water with greater heat transfer at low latitudes.<ref name=":0" /> Global scale currents are organized into three zonal and two equatorial circulation cells, convecting internal heat toward the surface, especially in equatorial regions.<ref>{{Cite journal |last1=Heimpel |first1=Moritz |last2=Aurnou |first2=Jonathan |date=2007-04-01 |title=Turbulent convection in rapidly rotating spherical shells: A model for equatorial and high latitude jets on Jupiter and Saturn |journal=Icarus |volume=187 |issue=2 |pages=540–557 |doi=10.1016/j.icarus.2006.10.023 |bibcode=2007Icar..187..540H |url=https://escholarship.org/uc/item/4dw436f1 |url-access=subscription }}</ref><ref>{{Cite journal |last1=Aurnou |first1=Jonathan |last2=Heimpel |first2=Moritz |last3=Allen |first3=Lorraine |last4=King |first4=Eric |last5=Wicht |first5=Johannes |date=June 2008 |title=Convective heat transfer and the pattern of thermal emission on the gas giants |journal=Geophysical Journal International |volume=173 |issue=3 |pages=793–801 |doi=10.1111/j.1365-246x.2008.03764.x |doi-access=free |bibcode=2008GeoJI.173..793A }}</ref><ref>{{Cite journal |last1=Soderlund |first1=K. M. |last2=Schmidt |first2=B. E. |last3=Wicht |first3=J. |last4=Blankenship |first4=D. D. |date=January 2014 |title=Ocean-driven heating of Europa's icy shell at low latitudes |journal=Nature Geoscience |language=en |volume=7 |issue=1 |pages=16–19 |doi=10.1038/ngeo2021 |bibcode=2014NatGe...7...16S }}</ref> Titan and Ganymede are hypothesized to behave as a non-rotating system and have no coherent heat transfer patterns.<ref name=":0" />

==Astrobiology== {{Further|Astrobiology|Planetary habitability|Circumstellar habitable zone}} The characteristics of ocean worlds or ocean planets provide clues to their history, and the formation and evolution of the Solar System as a whole. Of additional interest is their potential to form and 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 |access-date=1 September 2018 |archive-date=1 September 2018 |archive-url=https://web.archive.org/web/20180901114708/https://www.eurekalert.org/pub_releases/2018-08/uoc-wwc083018.php }}</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=The Astrophysical Journal |volume=864 |issue=1 |page=75 |doi=10.3847/1538-4357/aad6e0 |arxiv=1801.00748 |bibcode=2018ApJ...864...75K |doi-access=free }}</ref>

An ocean world's 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 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 |last1=Franck |first1=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 |bibcode=2003IJAsB...2...35F }}</ref><ref>[https://web.archive.org/web/20100325211745/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 five Earth oceans' worth of water indicate the water would not contain enough phosphorus and other nutrients for Earth-like oxygen-producing ocean organisms such as 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 |doi-access=free }}</ref>

On the other hand, small bodies such as Europa and Enceladus are regarded as particularly habitable environments because the theorized locations of their oceans would almost certainly leave them in direct contact with the underlying silicate 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 tholins, formed by solar ultraviolet irradiation of simple organic compounds such as methane or 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 photosynthesis by life forms, so although encouraging, {{chem|O|2}} is not a reliable biosignature.<ref name="physorg"/><ref name=Luger/><ref>{{cite journal | doi = 10.1038/srep13977 | volume=5 | title=Titania may produce abiotic oxygen atmospheres on habitable exoplanets | year=2015 | journal=Scientific Reports | article-number=13977 | last1 = Narita | first1 = Norio| doi-access=free | bibcode=2015NatSR...513977N | pmid=26354078 | pmc=4564821 | arxiv=1509.03123 }}</ref><ref name="Seager 2013"/> In fact, planets with high concentration of {{chem|O|2}} in their atmosphere may be uninhabitable.<ref name=Luger/> Abiogenesis in the presence of massive amounts of atmospheric oxygen could be difficult because early organisms relied on the free energy available in 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}} * List of ocean worlds * {{annotated link|Circumstellar habitable zone}} * {{annotated link|Desert planet}} * {{annotated link|Earth analog}} * {{annotated link|Extraterrestrial liquid water}} * {{annotated link|Ice planet}} * {{annotated link|List of extrasolar candidates for liquid water}} * {{annotated link|Ocean#Extraterrestrial oceans|Ocean §&thinsp;Extraterrestrial oceans}} * {{annotated link|Panthalassa}} * {{Annotated link|Subnautica|''Subnautica''}} * {{annotated link|TOI-1452 b}}

'''Astrobiology mission concepts to water worlds in the outer Solar System:''' * {{annotated link|Enceladus Explorer|abbreviatio=EnEx}} * {{annotated link|Enceladus Life Finder|abbreviation=ELF}} * {{annotated link|Europa Lander}} * {{annotated link|Explorer of Enceladus and Titan|abbreviation=E²T}} * {{annotated link|Journey to Enceladus and Titan|abbreviation=JET}} * {{annotated link|Jupiter Icy Moons Explorer|abbreviation=JUICE}} * {{annotated link|Laplace-P}} * {{annotated link|Life Investigation For Enceladus|abbreviation=LIFE}} * {{annotated link|Oceanus (Titan orbiter)|''Oceanus''}} * {{annotated link|THEO|Testing the Habitability of Enceladus's Ocean|abbreviation=THEO}} * {{annotated link|Titan Lake In-situ Sampling Propelled Explorer|abbreviation=TALISE}} * {{annotated link|Titan Mare Explorer|abbreviation=TiME}} * {{annotated link|Triton Hopper|''Triton Hopper''}} {{div col end}}

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== 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 }}

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