--- title: Ocean world summary: A planet whose surface is almost entirely water, from Earth-like worlds with a few scattered islands to true water worlds with oceans hundreds of kilometres deep. None is confirmed, but several planets are strong candidates. science_status: [model, sim] categories: [Planets, Planet classes, Terrestrial planets, Ocean worlds, Habitability] aliases: [TOW, T2-O, Water world, Waterworld, Ocean planet, Aquaplanet, Terrestrial ocean world, Oceanic world] infobox: type: planet_class name: Ocean world code: TOW legacy_code: T2-O level: type series: "Terrestrial with atmosphere (T)" science_status: [model, sim] image: File:Ocean_world_sim.avif literature_equivalent: "Ocean planet or water world (Léger et al. 2004); aquaplanet in climate modelling; 'hycean' world for the proposed hydrogen-atmosphere variant" defining_criteria_sim: "Rocky planet with 90% or more of its surface under liquid water" mass_earth: {sim: "0.5 to 2.5"} radius_earth: {sim: "R = 1.03 M^0.28, about 0.85 to 1.33"} water_mass_fraction: {observed: "0.02% (Earth)", model: "up to tens of per cent for planets formed beyond the snow line", sim: "0.1 to 5%"} equilibrium_temperature_k: {sim: "within the habitable zone"} surface_temperature_k: {sim: "about 250 to 355 (5th to 95th percentile)"} bond_albedo: {sim: "0.10 to 0.80 depending on atmosphere class"} atmosphere_classes: [archean, biotic, waterworld_co2, abiotic_mature, steam_rich] surface_pressure_bar: {sim: "0.3 to 50, by atmosphere class"} interior: "Rocky core and mantle; with more than about 0.5 to 1% water by mass on an Earth-sized planet, high-pressure ice can form at the bottom of the ocean" typical_orbit: "Habitable zone" frequency_in_sim: "173 of 8,742 generated planets (2.0%), in the committed sheets as of 2026-09-27" real_examples: ["None confirmed", "Candidates: LHS 1140 b, Kepler-138 d", "Disputed: K2-18 b (sub-Neptune)"] subtypes: ["TOW-AR Archean", "TOW-BI Biotic", "TOW-WC CO2 waterworld", "TOW-AM Abiotic mature", "TOW-SR Steam-rich"] sim_source: "Ocean world physics engine, ocean-world atmosphere and chemistry modules, TOW subtype classifier; science set TOW_00 to TOW_19" last_verified: 2026-09-27 sim: entity: planet_class.TOW refs: - id: leger2004 type: article-journal author: [{family: "Léger", given: "A."}, {family: "Selsis", given: "F."}, {family: "Sotin", given: "C."}, {family: "Guillot", given: "T."}, {family: "Despois", given: "D."}, {family: "Mawet", given: "D."}, {family: "Ollivier", given: "M."}, {family: "Labèque", given: "A."}, {literal: "et al."}] title: "A new family of planets? “Ocean-Planets”" container-title: "Icarus" volume: "169" page: "499-504" issued: 2004 DOI: 10.1016/j.icarus.2004.01.001 - id: raymond2004 type: article-journal author: [{family: "Raymond", given: "Sean N"}, {family: "Quinn", given: "Thomas"}, {family: "Lunine", given: "Jonathan I"}] title: "Making other earths: dynamical simulations of terrestrial planet formation and water delivery" container-title: "Icarus" volume: "168" page: "1-17" issued: 2004 DOI: 10.1016/j.icarus.2003.11.019 - id: luger2015 type: article-journal author: [{family: "Luger", given: "R."}, {family: "Barnes", given: "R."}] title: "Extreme Water Loss and Abiotic O2 Buildup on Planets Throughout the Habitable Zones of M Dwarfs" container-title: "Astrobiology" volume: "15" page: "119-143" issued: 2015 DOI: 10.1089/ast.2014.1231 - id: noack2016 type: article-journal author: [{family: "Noack", given: "L."}, {family: "Höning", given: "D."}, {family: "Rivoldini", given: "A."}, {family: "Heistracher", given: "C."}, {family: "Zimov", given: "N."}, {family: "Journaux", given: "B."}, {family: "Lammer", given: "H."}, {family: "Van Hoolst", given: "T."}, {literal: "et al."}] title: "Water-rich planets: How habitable is a water layer deeper than on Earth?" container-title: "Icarus" volume: "277" page: "215-236" issued: 2016 DOI: 10.1016/j.icarus.2016.05.009 - id: journaux2020 type: article-journal author: [{family: "Journaux", given: "Baptiste"}, {family: "Kalousová", given: "Klára"}, {family: "Sotin", given: "Christophe"}, {family: "Tobie", given: "Gabriel"}, {family: "Vance", given: "Steve"}, {family: "Saur", given: "Joachim"}, {family: "Bollengier", given: "Olivier"}, {family: "Noack", given: "Lena"}, {literal: "et al."}] title: "Large Ocean Worlds with High-Pressure Ices" container-title: "Space Science Reviews" volume: "216" page: "7" issued: 2020 DOI: 10.1007/s11214-019-0633-7 - id: kite2018 type: article-journal author: [{family: "Kite", given: "Edwin S."}, {family: "Ford", given: "Eric B."}] title: "Habitability of Exoplanet Waterworlds" container-title: "The Astrophysical Journal" volume: "864" page: "75" issued: 2018 DOI: 10.3847/1538-4357/aad6e0 - id: hayworth2020 type: article-journal author: [{family: "Hayworth", given: "Benjamin P. C."}, {family: "Foley", given: "Bradford J."}] title: "Waterworlds May Have Better Climate Buffering Capacities than Their Continental Counterparts" container-title: "The Astrophysical Journal Letters" volume: "902" page: "L10" issued: 2020 DOI: 10.3847/2041-8213/abb882 - id: walker1981 type: article-journal author: [{family: "Walker", given: "James C. G."}, {family: "Hays", given: "P. B."}, {family: "Kasting", given: "J. F."}] title: "A negative feedback mechanism for the long-term stabilization of Earth's surface temperature" container-title: "Journal of Geophysical Research: Oceans" volume: "86" page: "9776-9782" issued: 1981 DOI: 10.1029/jc086ic10p09776 - id: zeng2019 type: article-journal author: [{family: "Zeng", given: "Li"}, {family: "Jacobsen", given: "Stein B."}, {family: "Sasselov", given: "Dimitar D."}, {family: "Petaev", given: "Michail I."}, {family: "Vanderburg", given: "Andrew"}, {family: "Lopez-Morales", given: "Mercedes"}, {family: "Perez-Mercader", given: "Juan"}, {family: "Mattsson", given: "Thomas R."}, {literal: "et al."}] title: "Growth model interpretation of planet size distribution" container-title: "Proceedings of the National Academy of Sciences" volume: "116" page: "9723-9728" issued: 2019 DOI: 10.1073/pnas.1812905116 - id: luque2022 type: article-journal author: [{family: "Luque", given: "Rafael"}, {family: "Pallé", given: "Enric"}] title: "Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars" container-title: "Science" volume: "377" page: "1211-1214" issued: 2022 DOI: 10.1126/science.abl7164 - id: rogers2023 type: article-journal author: [{family: "Rogers", given: "James G."}, {family: "Schlichting", given: "Hilke E."}, {family: "Owen", given: "James E."}] title: "Conclusive Evidence for a Population of Water Worlds around M Dwarfs Remains Elusive" container-title: "The Astrophysical Journal Letters" volume: "947" page: "L19" issued: 2023 DOI: 10.3847/2041-8213/acc86f - id: cadieux2024a type: article-journal author: [{family: "Cadieux", given: "Charles"}, {family: "Plotnykov", given: "Mykhaylo"}, {family: "Doyon", given: "René"}, {family: "Valencia", given: "Diana"}, {family: "Jahandar", given: "Farbod"}, {family: "Dang", given: "Lisa"}, {family: "Turbet", given: "Martin"}, {family: "Fauchez", given: "Thomas J."}, {literal: "et al."}] title: "New Mass and Radius Constraints on the LHS 1140 Planets: LHS 1140 b Is either a Temperate Mini-Neptune or a Water World" container-title: "The Astrophysical Journal Letters" volume: "960" page: "L3" issued: 2024 DOI: 10.3847/2041-8213/ad1691 - id: cadieux2024b type: article-journal author: [{family: "Cadieux", given: "Charles"}, {family: "Doyon", given: "René"}, {family: "MacDonald", given: "Ryan J."}, {family: "Turbet", given: "Martin"}, {family: "Artigau", given: "Étienne"}, {family: "Lim", given: "Olivia"}, {family: "Radica", given: "Michael"}, {family: "Fauchez", given: "Thomas J."}, {literal: "et al."}] title: "Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS" container-title: "The Astrophysical Journal Letters" volume: "970" page: "L2" issued: 2024 DOI: 10.3847/2041-8213/ad5afa - id: piaulet2023 type: article-journal author: [{family: "Piaulet", given: "Caroline"}, {family: "Benneke", given: "Björn"}, {family: "Almenara", given: "Jose M."}, {family: "Dragomir", given: "Diana"}, {family: "Knutson", given: "Heather A."}, {family: "Thorngren", given: "Daniel"}, {family: "Peterson", given: "Merrin S."}, {family: "Crossfield", given: "Ian J. M."}, {literal: "et al."}] title: "Evidence for the volatile-rich composition of a 1.5-Earth-radius planet" container-title: "Nature Astronomy" issued: 2022 DOI: 10.1038/s41550-022-01835-4 - id: piaulet2024 type: article-journal author: [{family: "Piaulet-Ghorayeb", given: "Caroline"}, {family: "Benneke", given: "Björn"}, {family: "Radica", given: "Michael"}, {family: "Raul", given: "Eshan"}, {family: "Coulombe", given: "Louis-Philippe"}, {family: "Ahrer", given: "Eva-Maria"}, {family: "Kubyshkina", given: "Daria"}, {family: "Howard", given: "Ward S."}, {literal: "et al."}] title: "JWST/NIRISS Reveals the Water-rich “Steam World” Atmosphere of GJ 9827 d" container-title: "The Astrophysical Journal Letters" volume: "974" page: "L10" issued: 2024 DOI: 10.3847/2041-8213/ad6f00 - id: madhu2021 type: article-journal author: [{family: "Madhusudhan", given: "Nikku"}, {family: "Piette", given: "Anjali A. A."}, {family: "Constantinou", given: "Savvas"}] title: "Habitability and Biosignatures of Hycean Worlds" container-title: "The Astrophysical Journal" volume: "918" page: "1" issued: 2021 DOI: 10.3847/1538-4357/abfd9c - id: madhu2023 type: article-journal author: [{family: "Madhusudhan", given: "Nikku"}, {family: "Sarkar", given: "Subhajit"}, {family: "Constantinou", given: "Savvas"}, {family: "Holmberg", given: "Måns"}, {family: "Piette", given: "Anjali A. A."}, {family: "Moses", given: "Julianne I."}] title: "Carbon-bearing Molecules in a Possible Hycean Atmosphere" container-title: "The Astrophysical Journal Letters" volume: "956" page: "L13" issued: 2023 DOI: 10.3847/2041-8213/acf577 - id: wogan2024 type: article-journal author: [{family: "Wogan", given: "Nicholas F."}, {family: "Batalha", given: "Natasha E."}, {family: "Zahnle", given: "Kevin J."}, {family: "Krissansen-Totton", given: "Joshua"}, {family: "Tsai", given: "Shang-Min"}, {family: "Hu", given: "Renyu"}] title: "JWST Observations of K2-18b Can Be Explained by a Gas-rich Mini-Neptune with No Habitable Surface" container-title: "The Astrophysical Journal Letters" volume: "963" page: "L7" issued: 2024 DOI: 10.3847/2041-8213/ad2616 - id: shorttle2024 type: article-journal author: [{family: "Shorttle", given: "Oliver"}, {family: "Jordan", given: "Sean"}, {family: "Nicholls", given: "Harrison"}, {family: "Lichtenberg", given: "Tim"}, {family: "Bower", given: "Dan J."}] title: "Distinguishing Oceans of Water from Magma on Mini-Neptune K2-18b" container-title: "The Astrophysical Journal Letters" volume: "962" page: "L8" issued: 2024 DOI: 10.3847/2041-8213/ad206e - id: luque2025 type: article-journal author: [{family: "Luque", given: "R."}, {family: "Piaulet-Ghorayeb", given: "C."}, {family: "Radica", given: "M."}, {family: "Xue", given: "Q."}, {family: "Zhang", given: "M."}, {family: "Bean", given: "J. L."}, {family: "Samra", given: "D."}, {family: "Steinrueck", given: "M. E."}] title: "Insufficient evidence for DMS and DMDS in the atmosphere of K2-18 b" container-title: "Astronomy & Astrophysics" volume: "700" page: "A284" issued: 2025 DOI: 10.1051/0004-6361/202555580 - id: nasa_k218b type: webpage author: [{literal: "NASA Science"}] title: "Webb discovers methane, carbon dioxide in atmosphere of K2-18 b" container-title: "NASA Science: Webb" issued: 2023-09-11 accessed: 2026-09-27 URL: https://science.nasa.gov/missions/webb/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18-b/ images_wanted: - file: File:Ocean_world_sim.avif subject: "A TOW planet from the sim: a deep blue globe with a scattering of island arcs and bands of cloud, ideally TOW-BI or TOW-AM" source: sim note: "Shot list: render a TOW plate on the lit sphere; label 'Sim render'. The in-game ocean surface (JONSWAP spectrum) could supply a second, close-up shot. Confirm the plate generator's licence." - file: File:K2-18b_Webb_illustration.jpg subject: "Artist's concept of K2-18 b based on JWST data (September 2023)" source: nasa page_url: https://science.nasa.gov/missions/webb/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18-b/ credit: "Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI); Science: Nikku Madhusudhan (IoA)" licence: "Public domain (NASA/STScI release; follow the caption)" note: "Label 'Artist's concept'. The caption must say K2-18 b is a sub-Neptune and that an ocean is one disputed interpretation." - file: File:Ocean_world_layers.svg subject: "Diagram: interiors of Earth, a deep-ocean world with a high-pressure ice floor, and a hycean world with a hydrogen envelope, to scale" source: other note: "To be drawn by us; label 'Diagram'." --- An **ocean world** is a planet whose surface is almost entirely water. The term covers a range: at one end, Earth-sized planets like our own but with more water, where a few islands break an otherwise global sea; at the other, true **water worlds** whose oceans are hundreds of kilometres deep and make up a large fraction of the planet's mass. No ocean world is confirmed outside the Solar System, but formation models predict many, several known planets have densities that leave room for them, and they are central to the question of how common habitable conditions are. ## Characteristics ### How much water Earth is a dry planet by mass. Its oceans weigh about $1.4 \times 10^{21}$ kg, only about 0.02 per cent of the planet. Spread evenly over a smooth sphere, the depth of a water layer of mass fraction $f$ on a planet of mass $M$ and radius $R$ would be about $$ d \approx \frac{f\,M}{4\pi R^2 \rho_w}, $$ where $\rho_w \approx 1{,}000$ kg/m³ is the density of water. For Earth that gives 2.7 km. On an Earth-sized planet with 1 per cent water, it gives more than 100 km, and the pressure at the bottom would be around a gigapascal, ten thousand times atmospheric pressure. That changes what the ocean floor is made of. ### Ice at the bottom of the sea Water under enough pressure freezes even when it is warm, into dense forms of ice (ice VI, ice VII) that sink rather than float. On an Earth-sized planet with an ocean more than about 50 to 100 km deep, roughly half a per cent to one per cent of the planet's mass in water, a layer of high-pressure ice can form between the ocean and the rock beneath it, with the exact depth depending on the ocean's temperature :cite[noack2016]. That matters for life and climate, because the seafloor is where water meets rock: where minerals dissolve, where hydrothermal vents supply chemical energy, and where carbon is locked away. Whether an ice layer fully seals the ocean off is still debated; heat from below can melt it, and salts may pass through it :cite[journaux2020]. ### Climate without continents On Earth, the weathering of rock on land draws carbon dioxide out of the air faster when the climate is warm, a thermostat that has kept the surface temperate for billions of years :cite[walker1981] (see [[Mixed world]]). An ocean world has little or no land to weather, and whether it can regulate its climate at all has been argued both ways. For water worlds with ten to a thousand times Earth's water, Kite and Ford (2018) found that the pressure at the seafloor shuts down the exchange of carbon between ocean and mantle, so the climate is set by how much carbon the planet started with; many such worlds still stay temperate for more than a billion years :cite[kite2018]. For planets with shallower oceans, weathering of the seafloor itself can take over, and one study found it may even stabilise climate better than continental weathering does :cite[hayworth2020]. ### What they would look like An ocean reflects little sunlight, so a cloud-free water world would be dark, with a Bond albedo of about 0.06 for the open sea. Clouds and sea ice change that completely. A warm ocean world would carry more water vapour in its air, a stronger greenhouse effect and more cloud; a cold one could freeze over and become a [[Subsurface ocean world]]. ## Formation Rocky planets that form inside their star's snow line start dry and gain water later. Planets that form beyond it, or whose building blocks do, can incorporate ice by the tonne: tens of per cent of their mass. Léger and colleagues named these hypothetical "ocean planets" in 2004, picturing a planet of a few Earth masses, half water, with a global ocean about 100 km deep :cite[leger2004]. Simulations of planet formation show that water delivery can vary by orders of magnitude between systems, depending on where the giant planets are and how they move, so ocean-covered planets could be common outcomes rather than oddities :cite[raymond2004]. Around red dwarfs the story can run the other way. These stars are very bright in their youth, and a planet now in the habitable zone may have spent its first hundreds of millions of years in a runaway greenhouse, losing oceans' worth of water to space :cite[luger2015]. Whether the typical rocky planet around a red dwarf is a water world or a desert is one of the open questions in the field. ## How we know No ocean has been seen on an exoplanet. The evidence so far comes from density, which says what a planet could be made of but rarely settles it. A planet with a lot of water is larger than a rocky planet of the same mass, because water is less dense than rock. Mass-radius models show where the water-rich compositions lie :cite[zeng2019]. The difficulty is **degeneracy**: a planet that is too large to be pure rock can be explained by water, by a thin envelope of hydrogen, or by both. Luque and Pallé (2022) argued that small planets around red dwarfs fall into three groups by density, one of them consistent with half rock and half water :cite[luque2022]; others have shown that the same planets can be explained by rock with a little hydrogen, so the water-world population is not yet established :cite[rogers2023]. See [[Radius valley]]. Spectra are starting to break the tie. JWST has found an atmosphere dominated by water vapour on GJ 9827 d, a planet about twice Earth's size, too hot for a liquid ocean: a "steam world" :cite[piaulet2024]. Kepler-138 d, about 1.5 times Earth's radius, has too low a density for rock and is best explained by a large fraction of volatiles, plausibly water :cite[piaulet2023]. ### Hycean worlds and K2-18 b Madhusudhan and colleagues proposed a further kind of ocean world: a **hycean** planet, with a liquid water ocean beneath a thick hydrogen atmosphere, which could stay temperate over a wider range of distances from its star :cite[madhu2021]. Their leading example is **K2-18 b**, a planet of 8.6 Earth masses and 2.6 Earth radii in the habitable zone of a red dwarf about 120 light years away, which by size and mass is a sub-Neptune. In 2023, JWST found methane and carbon dioxide in its atmosphere and no ammonia, which the team argued fits a hycean world; they also reported a possible, weaker signal of dimethyl sulfide, a gas that on Earth is made by life :cite[madhu2023] :cite[nasa_k218b]. Other groups showed that the same spectrum can be explained by a gas-rich mini-Neptune with no habitable surface :cite[wogan2024] or by a magma ocean under hydrogen :cite[shorttle2024], and a joint reanalysis found insufficient evidence for dimethyl sulfide :cite[luque2025]. K2-18 b is a sub-Neptune whose nature is unresolved; it is not a confirmed ocean world. See [[Mini-Neptune]]. ::figure{src="File:K2-18b_Webb_illustration.jpg" size=wide alt="An illustration of a blue planet with a hazy atmosphere next to a small red star." caption="Artist's concept: K2-18 b, a sub-Neptune eight times Earth's mass. A deep ocean under a hydrogen atmosphere is one proposed interpretation of JWST's data; a gas-rich mini-Neptune with no surface is another."} ## Notable candidates | Planet | Radius | Mass | Notes | |---|---|---|---| | LHS 1140 b | 1.73 R⊕ | 5.6 M⊕ | too light for pure rock; either a water world or a mini-Neptune :cite[cadieux2024a]. JWST ruled out a hydrogen-rich atmosphere, leaving a water world with a thin atmosphere as a viable option :cite[cadieux2024b] | | Kepler-138 d | about 1.5 R⊕ | about 2 M⊕ | low density; volatile-rich, plausibly water :cite[piaulet2023] | | GJ 9827 d | about 2 R⊕ | about 3 M⊕ | steam atmosphere; too hot for a liquid ocean :cite[piaulet2024] | | K2-18 b | 2.6 R⊕ | 8.6 M⊕ | sub-Neptune; hycean interpretation disputed :cite[madhu2023] :cite[wogan2024] | LHS 1140 b is currently the best candidate for a temperate world with liquid water. It orbits a quiet red dwarf 49 light years away, receives about half the sunlight Earth does, and if it is a water world, water makes up 9 to 19 per cent of its mass :cite[cadieux2024a]. With one hemisphere facing its star, it could be frozen over except for an ocean beneath the point where its star stands overhead :cite[cadieux2024b]. :::callout{type=sim title="In Pax Abyssi"} The sim's ocean worlds are rocky planets of 0.5 to 2.5 Earth masses with at least 90 per cent of the surface under water: Earth-sized worlds with far more ocean than Earth, rather than the half-water planets of the formation models. Water makes up 0.1 to 5 per cent of their mass, so many have oceans tens to hundreds of kilometres deep, and the sim flags a high-pressure ice floor where the pressure at the bottom is high enough. Five subtypes follow the atmosphere, which the sim builds from the planet's age, temperature, ocean cover and whether life arose: **steam-rich** worlds (TOW-SR) above about 340 K, with air full of water vapour; **carbon-dioxide waterworlds** (TOW-WC) whose oceans cover more than 95 per cent of the surface and fail to draw carbon dioxide down; **Archean** worlds (TOW-AR), young and oxygen-free like the early Earth; **biotic** worlds (TOW-BI) with oxygen from photosynthesis; and the default, **abiotic mature** worlds (TOW-AM). In the committed system sheets, 173 of 8,742 generated planets are ocean worlds, and 19 of them carry life. In the game each is drawn as a prebaked texture plate chosen by its subtype code. ::: ## See also - [[Mixed world]] - [[Subsurface ocean world]] - [[Mini-Neptune]] - [[Super-Earth]] - [[Radius valley]] - [[Habitable zone]] - [[Hycean worlds and K2-18 b]] - [[Ocean waves on other worlds]] - [[Planet classification]]