Anocean world,ocean planet orwater world is a type of planet ornatural satellite that contains a substantial amount ofwater in the form ofoceans, as part of itshydrosphere, either beneath thesurface, as subsurface oceans, or on the surface, potentially submerging alldry land.[1][2][3][4] The termocean world is also used sometimes forastronomical bodies with an ocean composed of a different fluid orthalassogen,[5] such aslava (the case ofIo),ammonia (in aeutectic mixture with water, as is likely the case ofTitan's inner ocean) or hydrocarbons (like on Titan's surface, which could be the most abundant kind of exosea).[6] The study of extraterrestrial oceans is referred to asplanetary 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 moonsEuropa andGanymede and Saturn's moonsEnceladus andTitan.[7] Severalexoplanets have been found with the right conditions to support liquid water.[8] There are also considerable amounts of subsurface water found on Earth, mostly in the form ofaquifers.[9] For exoplanets, current technology cannot directly observe liquid surface water, so atmospheric water vapor may be used as a proxy.[10] The characteristics of ocean worlds provide clues to their history and theformation and evolution of the Solar System as a whole. Of additional interest is their potential tooriginate andhost life.
In June 2020,NASA scientists reported that it is likely thatexoplanets with oceans are common in theMilky Way galaxy, based onmathematical modeling studies.[11][12]
According to Lunine, "oceans" have been defined as "stable, globe-girdling bodies of liquid water."[13] 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."[13]
Ocean worlds are of interest toastrobiologists for their potential todevelop life and sustain biological activity.[4][3]Major moons anddwarf planets in theSolar System thought to harborsubsurface oceans are of interest because they can be reached and studied byspace probes, in contrast toexoplanets, which are light-years away, beyond the reach of current technology. The best-established water worlds in the Solar System, other than theEarth, areCallisto,Enceladus,Europa,Ganymede, andTitan.[3][14] Europa and Enceladus are considered compelling targets for exploration due to their thin outer crusts andcryovolcanic 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, includingAriel,[14]Titania,[15][16]Umbriel,[17]Ceres,[3]Dione,[18]Mimas,[19][20]Miranda,[14]Oberon,[4][21]Pluto,[22]Triton,[23]Eris,[4][24] andMakemake.[24]
Outside the Solar System,exoplanets that have been described as candidate ocean worlds includeGJ 1214 b,[26][27]Kepler-22b,Kepler-62e,Kepler-62f,[28][29][30][31] and the planets ofKepler-11[32] andTRAPPIST-1.[33][34]
More recently, the exoplanetsTOI-1452 b,Kepler-138c, andKepler-138d have been found to have densities consistent with large fractions of their mass being composed of water.[35][36] Additionally, models of the massive rocky planetLHS 1140 b suggest its surface may be covered in a deep ocean.[37]
Although 70.8% of allEarth's surface is covered in water,[38] 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 asice V.[32] 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 becomesupercritical and lack a well-defined surface.[39] 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 stronggreenhouse effect. Such planets would have to be small enough not to be able to retain a thick envelope of hydrogen and helium,[40] or be close enough to their primary star to be stripped of these light elements.[32] Otherwise, they would form awarmer version of anice giant instead, likeUranus andNeptune.[citation needed]
Gravitational calculations suggested by the start of 20th century that Europa's composition was water rich, and Earth ground based observations byGerard Kuiper revealed 1957 the water ice composition.[41]
Important preliminary theoretical work was carried out prior to the planetary missions of the 1970s. In particular, Lewis showed in 1971 thatradioactive decay alone was likely sufficient to produce subsurface oceans in large moons, especially if ammonia (NH
3) were present. Peale and Cassen figured out in 1979 the important role oftidal heating (aka: tidal flexing) on satellite evolution and structure.[3] The first confirmed detection of an exoplanet was in 1992. Marc Kuchner in 2003 and Alain Légeret al figured in 2004 that a small number of icy planets that form in the region beyond thesnow line canmigrate inward to ~1AU, where the outer layers subsequently melt.[42][43]
The cumulative evidence collected by theHubble Space Telescope, as well asPioneer,Galileo,Voyager,Cassini–Huygens, andNew Horizons missions, strongly indicate that several outer Solar System bodies harbour internal liquid water oceans under an insulating ice shell.[3][44] Meanwhile, theKepler space observatory, launched on March 7, 2009, has discovered thousands of exoplanets, about 50 of them ofEarth-size in or nearhabitable zones.[45][46]
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 thecircumstellar disc from the planet's place of origin.[10] As of 30 October 2025, there are 6,128 confirmedexoplanets in 4,584planetary systems, with 1,017 systemshaving more than one planet.[47]
In June 2020,NASA scientists reported that it is likely thatexoplanets with oceans may be common in theMilky Way galaxy, based onmathematical modeling studies.[11]
In August 2022,TOI-1452 b, a super-Earth exoplanet with potential deep oceans that is 99 light-years from Earth, was discovered by theTransiting Exoplanet Survey Satellite.[35]
Planetary objects that form in the outerSolar System begin as acomet-like mixture of roughly half water and half rock by mass, displaying a density lower than that of rocky planets.[43] Icy planets and moons that form near thefrost line should contain mostlyH
2O andsilicates. Those that form farther out can acquire ammonia (NH
3) and methane (CH
4) as hydrates, together withCO,N
2, andCO
2.[48]
Planets that form prior to the dissipation of the gaseouscircumstellar disk experience strong torques that can induce rapid inward migration into the habitable zone, especially for planets in the terrestrial mass range.[49][48] Since water is highly soluble inmagma, a large fraction of the planet's water content will initially be trapped in themantle. 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) areexsolved to form a steam atmosphere, which may eventually condense to form an ocean.[49] Ocean formation requiresdifferentiation, and a heat source, eitherradioactive decay,tidal heating, or the early luminosity of the parent body.[3] Unfortunately, the initial conditions followingaccretion are theoretically incomplete.
Planets that formed in the outer, water-rich regions of adisk and migrated inward are more likely to have abundant water.[50] 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 astar, it would be strong evidence formigration andex situ formation,[32] because insufficient volatiles exist near the star forin situ formation.[2] Simulations ofSolar System formation and ofextra-solar system formation have shown that planets are likely tomigrate inward (i.e., toward the star) as they form.[51][52][53] Outward migration may also occur under particular conditions.[53] Inward migration presents the possibility thaticy 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 byMarc Kuchner[48] in 2003.
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 undergonedifferentiation (separation into rock-ice layers) or not. Shape orgravity measurements can in some cases be used to infer the moment of inertia – if the body is inhydrostatic 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.[3] Specific techniques to detect inner oceans includemagnetic induction,geodesy,librations,axial tilt,tidal response,radar sounding, compositional evidence, and surface features.[3]
A genericicy moon will consist of a water layer sitting atop asilicate core. For a small satellite likeEnceladus, an ocean will sit directly above the silicates and below a solid icy shell, but for a larger ice-rich body likeGanymede, 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.[3] An important difference between these two cases is that for the small satellite the ocean is in direct contact with the silicates, which may providehydrothermal and chemical energy and nutrients to simple life forms.[3] Because of the varyingpressure at depth, models of a water world may include "steam, liquid, superfluid, high-pressure ices, and plasma phases" of water.[54] Some of the solid-phase water could be in the form ofice VII.[55]
Maintaining a subsurface ocean depends on the rate of internal heating compared with the rate at which heat is removed, and thefreezing point of the liquid.[3] 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 byhydrothermal activity,radiogenic heating, ortidal flexing.[4] Where fluid-rock interactions propagate slowly into a deep brittle layer, thermal energy fromserpentinization may be the primary cause of hydrothermal activity in small ocean planets.[4] 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 whichcryovolcanism occurs is a subject of some debate, as water, being denser than ice by about 8%, has difficulty erupting under normal circumstances.[3] Nevertheless, imaging data from theVoyager 2,Cassini-Huygens,Galileo andNew Horizons spacecraft revealedcryovolcanic surface features on several of the icy bodies in our own solar system. Recent studies suggest thatcryovolcanism may occur on ocean planets that harbor internal oceans beneath layers of surface ice as it does on the icy moonsEnceladus andEuropa in our own solar system.[11][12]
Liquid water oceans on extrasolar planets could be significantly deeper than the Earth's ocean, which has an average depth of 3.7 km.[56] 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 30–500 km, depending on its mass and composition.[57]
To allow surface water to be liquid for long periods of time, a planet—or moon—must orbit within thehabitable zone (HZ), possess a protectivemagnetic field,[58][59][10] and have the gravitational pull needed to retain an ample amount ofatmospheric pressure.[8] If the planet's gravity cannot sustain that, then all the water will eventually evaporate into outer space. A strong planetarymagnetosphere, maintained by internaldynamo action in an electrically conducting fluid layer, is helpful for shielding the upper atmosphere fromstellar wind mass loss and retaining water over long geological time scales.[58]
A planet's atmosphere forms from outgassing during planet formation or is gravitationally captured from the surroundingprotoplanetary nebula. The surface temperature on an exoplanet is governed by the atmosphere'sgreenhouse gases (or lack thereof), so an atmosphere can be detectable in the form of upwellinginfrared radiation because the greenhouse gases absorb and re-radiate energy from the host star.[10] 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 slowhydrodynamic escape.[42][48]Ultraviolet photons are not only biologically harmful but can drive fast atmospheric escape that leads to the erosion of planetary atmospheres;[49][48]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.[49] 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 arunaway 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.[49] 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.[49] Volatile-rich planets should be more common in the habitable zones of young stars andM-type stars.[48]
Scientists have proposedHycean 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 1astronomical 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.[60]
There are challenges in examining an exoplanetary surface and its atmosphere, as cloud coverage influences the atmospheric temperature, structure as well as the observability ofspectral features.[61] 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.[61] 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 surfaceIce I, depending on their orbit within the HZ and the magnitude of theirgreenhouse effect. Several other surface and interior processes affect the atmospheric composition, including but not limited to the ocean fraction for dissolution ofCO
2 and for atmospheric relative humidity,redox state of the planetary surface and interior, acidity levels of the oceans, planetaryalbedo, and surface gravity.[10][62]
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.[61] 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.[61] For modeling purposes, it is assumed that the initial composition of icyplanetesimals that assemble into water planets is similar to that of comets: mostly water (H
2O), and some ammonia (NH
3), and carbon dioxide (CO
2).[61] An initial composition of ice similar to that of comets leads to an atmospheric model composition of 90%H
2O, 5%NH
3, and 5%CO
2.[61][63]
Atmospheric models for Kepler-62f show that an atmospheric pressure of between 1.6bar and 5 bar ofCO
2 are needed to warm the surface temperature above freezing, leading to a scaled surface pressure of 0.56–1.32 times Earth's.[61]
It is suggested that strongocean currents exist inEnceladus,Titan,Ganymede, andEuropa.[64][65] InEnceladus, oceanic heat flux inferred from ice shell thickness suggests theupwelling of warm water at the poles anddownwelling of colder water at low latitudes.[66][67]Europa is predicted to have an equatorialupwelling of warm water with greater heat transfer at low latitudes.[64] Global scale currents are organized into three zonal and two equatorial circulation cells, convecting internal heat toward the surface, especially in equatorial regions.[68][69][70]Titan andGanymede are hypothesized to behave as a non-rotating system and have no coherentheat transfer patterns.[64]
The characteristics of ocean worlds or ocean planets provide clues to their history, and theformation and evolution of the Solar System as a whole. Of additional interest is their potential toform andhost 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,[3] that may offer the possibility for sustaining simple biological activity over geological timescales.[3][4] In August 2018, researchers reported that water worlds could support life.[71][72]
An ocean world'shabitation 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 rockymantle.[73][74] Simulations of a hypothetical ocean world covered by five Earth oceans' worth of water indicate the water would not contain enoughphosphorus and other nutrients for Earth-like oxygen-producing ocean organisms such asplankton 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.[75]
On the other hand, small bodies such asEuropa andEnceladus are regarded as particularly habitable environments because the theorized locations of their oceans would almost certainly leave them in direct contact with the underlying silicatecore, a potential source of both heat and biologically important chemical elements.[3] 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 asorganic molecules from comets ortholins, formed by solarultraviolet irradiation of simpleorganic compounds such asmethane orethane, often in combination with nitrogen.[76]
Molecular oxygen (O
2) can be produced by geophysical processes, as well as a byproduct ofphotosynthesis by life forms, so although encouraging,O
2 is not a reliablebiosignature.[39][49][77][10] In fact, planets with high concentration ofO
2 in their atmosphere may be uninhabitable.[49]Abiogenesis in the presence of massive amounts of atmospheric oxygen could be difficult because early organisms relied on the free energy available inredox reactions involving a variety of hydrogen compounds; on anO
2-rich planet, organisms would have to compete with the oxygen for this free energy.[49]
Astrobiology mission concepts to water worlds in the outer Solar System:
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.
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.
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