Ocean world
An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of
Earth is the only astronomical object known to presently have bodies of liquid water on its surface, although several exoplanets have been found with the right conditions to support liquid water.[7] There are also considerable amounts of subsurface water found on Earth, mostly in the form of aquifers.[8] For exoplanets, current technology cannot directly observe liquid surface water, so atmospheric water vapor may be used as a proxy.[9] 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.[10][11]
Overview
Solar System planetary bodies
Ocean worlds are of extreme interest to
A host of 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
Exoplanets
Outside the Solar System, exoplanets that have been described as candidate ocean worlds include GJ 1214 b,[26][27] Kepler-22b, Kepler-62e, Kepler-62f,[28][29][30][31] and the planets of Kepler-11[32] and TRAPPIST-1.[33][34]
More recently, the exoplanets
Although 70.8% of all Earth'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 1,000 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.[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 become supercritical 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 strong greenhouse 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 a warmer version of an ice giant instead, like Uranus and Neptune.[citation needed]
History
Important preliminary theoretical work was carried prior to the planetary missions launched starting in 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 (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.[3] 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 snow line can migrate inward to ~1 AU, where the outer layers subsequently melt.[41][42]
The cumulative evidence collected by the
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 circumstellar disc from the planet's place of origin.[9] As of 1 March 2024, there are 5,640 confirmed exoplanets in 4,155 planetary systems, with 895 systems having more than one planet.[46]
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.[10]
In August 2022, TOI-1452 b, a nearby super-Earth exoplanet with potential deep oceans, was discovered by the Transiting Exoplanet Survey Satellite.[35]
Formation
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.[42] Icy planets and moons that form near the frost line should contain mostly H
2O and silicates. Those that form farther out can acquire ammonia (NH
3) and methane (CH
4) as hydrates, together with CO, N
2, and CO
2.[47]
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.[48][47] 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.[48] Ocean formation requires differentiation, and a heat source, either radioactive decay, tidal heating, or the early luminosity of the parent body.[3] Unfortunately, the initial conditions following accretion are theoretically incomplete.
Planets that formed in the outer, water-rich regions of a
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.[3] Specific techniques to detect inner oceans include magnetic induction, geodesy, librations, axial tilt, tidal response, radar sounding, compositional evidence, and surface features.[3]
A generic
Maintaining a subsurface ocean depends on the rate of internal heating compared with the rate at which heat is removed, and the
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
Liquid water oceans on extrasolar planets could be significantly deeper than the Earth’s ocean, which has an average depth of 3.7 km.[55] 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.[56]
Atmospheric models
To allow surface water to be liquid for long periods of time, a planet—or moon—must orbit within the
A planet's atmosphere forms from outgassing during planet formation or is gravitationally captured from the surrounding
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.[48] 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.[48] Volatile-rich planets should be more common in the habitable zones of young stars and M-type stars.[47]
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.[59]
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
2 and for atmospheric relative humidity, redox state of the planetary surface and interior, acidity levels of the oceans, planetary albedo, and surface gravity.[9][61]
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.[60] 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.[60] 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 (H
2O), and some ammonia (NH
3), and carbon dioxide (CO
2).[60] 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.[60][62]
Atmospheric models for Kepler-62f show that an atmospheric pressure of between 1.6 bar and 5 bar of 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.[60]
Astrobiology
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,[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.[63][64]
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.[65][66] 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.[67]
On the other hand, small bodies such as
Oxygen
Molecular oxygen (O
2) can be produced by geophysical processes, as well as a byproduct of photosynthesis by life forms, so although encouraging, O
2 is not a reliable biosignature.[39][48][69][9] In fact, planets with high concentration of O
2 in their atmosphere may be uninhabitable.[48] 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 O
2-rich planet, organisms would have to compete with the oxygen for this free energy.[48]
See also
- Circumstellar habitable zone– Orbits where planets may have liquid surface water
- Desert planet – Rocky planet with very little water
- Earth analog – Planet with environment similar to Earth's
- Extraterrestrial liquid water – Liquid water naturally occurring outside Earth
- Ice planet – Planet with an icy surface
- List of extrasolar candidates for liquid water – Possible existence of liquid water beyond Earth
- Ocean § Extraterrestrial oceans – Body of salt water covering the majority of Earth
- Panthalassa – Prehistoric superocean that surrounded Pangaea
Astrobiology mission concepts to water worlds in the outer Solar System:
- Enceladus Explorer
- Enceladus Life Finder (ELF) – Proposed NASA mission to a moon of Saturn
- Europa Lander – Proposed NASA lander for Europa
- Explorer of Enceladus and Titan (E2T) – NASA/ESA Saturnian moon probe concept
- Journey to Enceladus and Titan (JET) – Proposed space mission
- Jupiter Icy Moons Explorer (JUICE) – European Space Agency spacecraft
- Laplace-P – Proposed Russian spacecraft to study the Jovian moon system and land on Ganymede
- Life Investigation For Enceladus (LIFE)
- Oceanus
- Testing the Habitability of Enceladus's Ocean (THEO) – Orbiter mission to Enceladus
- Titan Lake In-situ Sampling Propelled Explorer (TALISE) – Proposed space mission
- Titan Mare Explorer (TiME) – Proposed spacecraft lander design
- TOI-1452 b – Super-Earth orbiting TOI-1452
- Triton Hopper – Proposed NASA Triton lander space probe
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