--- title: Mixed world summary: A rocky planet with both continents and oceans, a temperate climate and liquid water at the surface. Earth is the only known example, and the model every search for habitable planets starts from. science_status: [observed, model, sim] categories: [Planets, Planet classes, Terrestrial planets, Habitability] aliases: [TMW, T2-T, Earth-like world, Earth-like planet, Earth analogue, Earth twin, Terrestrial mixed world, Temperate terrestrial world] infobox: type: planet_class name: Mixed world code: TMW legacy_code: T2-T level: type series: "Terrestrial with atmosphere (T)" science_status: [observed, model, sim] image: File:Mixed_world_sim.avif literature_equivalent: "Earth analogue; temperate rocky planet in the habitable zone with surface liquid water and exposed land" defining_criteria_sim: "Rocky planet with liquid surface water covering 18 to 90% of the surface, 0.3 to 3 bar of air and a mean surface temperature of roughly 250 to 310 K" mass_earth: {observed: "1 (Earth)", sim: "0.5 to 3.0"} radius_earth: {observed: "1 (Earth, 6,371 km)", sim: "R = M^0.27, 0.8 to 1.5"} density_g_cm3: {observed: "5.51 (Earth)"} surface_gravity_m_s2: {observed: "9.81 (Earth)"} escape_velocity_km_s: {observed: "11.2 (Earth)"} equilibrium_temperature_k: {observed: "255 (Earth, Bond albedo 0.3)"} surface_temperature_k: {observed: "288 (Earth, mean)", sim: "about 215 to 350 (5th to 95th percentile)"} bond_albedo: {observed: "0.30 (Earth)", sim: "0.10 to 0.55"} atmosphere_classes: [young_reducing, anoxic_biotic, transitional, mature_biotic, hyperoxic, high_co2_abiotic, abiotic_mature] surface_pressure_bar: {observed: "1.013 (Earth)", sim: "0.3 to 3"} dominant_gases: "Earth: N2 78%, O2 21%, Ar 0.9%, CO2 about 0.04%, variable H2O" magnetic_field: "Earth: dipole about 25 to 65 microtesla at the surface" interior: "Iron core (about a third of the mass), silicate mantle, thin crust; plate tectonics" ocean_cover: {observed: "71% (Earth)", sim: "18 to 90%"} typical_orbit: "Inside the habitable zone: roughly 0.95 to 1.7 AU for a Sun-like star" frequency_in_sim: "131 of 8,742 generated planets (1.5%), in the committed sheets as of 2026-09-27" real_examples: ["Earth", "Candidates of unknown nature: TRAPPIST-1 e, TOI-700 d, Kepler-186 f, Proxima Centauri b"] subtypes: ["TMW-A* land-dominated (ocean below 35%)", "TMW-M* mixed (35 to 65%)", "TMW-W* ocean-dominated (65% and above)", "TMW-BR brine seas", "TMW-TL tidally locked"] sim_source: "Temperate terrestrial physics engine, TMW subtype classifier, temperate atmosphere archetypes, life classifier; science set TMW_00 to TMW_19" last_verified: 2026-09-27 sim: entity: planet_class.TMW refs: - id: kasting1993 type: article-journal author: [{family: "Kasting", given: "James F."}, {family: "Whitmire", given: "Daniel P."}, {family: "Reynolds", given: "Ray T."}] title: "Habitable Zones around Main Sequence Stars" container-title: "Icarus" volume: "101" page: "108-128" issued: 1993 DOI: 10.1006/icar.1993.1010 - id: kopparapu2013 type: article-journal author: [{family: "Kopparapu", given: "Ravi Kumar"}, {family: "Ramirez", given: "Ramses"}, {family: "Kasting", given: "James F."}, {family: "Eymet", given: "Vincent"}, {family: "Robinson", given: "Tyler D."}, {family: "Mahadevan", given: "Suvrath"}, {family: "Terrien", given: "Ryan C."}, {family: "Domagal-Goldman", given: "Shawn"}, {literal: "et al."}] title: "Habitable zones around main-sequence stars: new estimates" container-title: "The Astrophysical Journal" volume: "765" page: "131" issued: 2013 DOI: 10.1088/0004-637x/765/2/131 - id: kopparapu2014 type: article-journal author: [{family: "Kopparapu", given: "Ravi Kumar"}, {family: "Ramirez", given: "Ramses M."}, {family: "SchottelKotte", given: "James"}, {family: "Kasting", given: "James F."}, {family: "Domagal-Goldman", given: "Shawn"}, {family: "Eymet", given: "Vincent"}] title: "Habitable Zones Around Main-sequence Stars: Dependence on Planetary Mass" container-title: "The Astrophysical Journal" volume: "787" page: "L29" issued: 2014 DOI: 10.1088/2041-8205/787/2/l29 - id: leconte2013 type: article-journal author: [{family: "Leconte", given: "Jérémy"}, {family: "Forget", given: "Francois"}, {family: "Charnay", given: "Benjamin"}, {family: "Wordsworth", given: "Robin"}, {family: "Pottier", given: "Alizée"}] title: "Increased insolation threshold for runaway greenhouse processes on Earth-like planets" container-title: "Nature" volume: "504" page: "268-271" issued: 2013 DOI: 10.1038/nature12827 - 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: colbourn2015 type: article-journal author: [{family: "Colbourn", given: "G."}, {family: "Ridgwell", given: "A."}, {family: "Lenton", given: "T. M."}] title: "The time scale of the silicate weathering negative feedback on atmospheric CO2" container-title: "Global Biogeochemical Cycles" volume: "29" page: "583-596" issued: 2015 DOI: 10.1002/2014gb005054 - id: morbidelli2000 type: article-journal author: [{family: "Morbidelli", given: "A."}, {family: "Chambers", given: "J."}, {family: "Lunine", given: "J. I."}, {family: "Petit", given: "J. M."}, {family: "Robert", given: "F."}, {family: "Valsecchi", given: "G. B."}, {family: "Cyr", given: "K. E."}] title: "Source regions and timescales for the delivery of water to the Earth" container-title: "Meteoritics & Planetary Science" volume: "35" page: "1309-1320" issued: 2000 DOI: 10.1111/j.1945-5100.2000.tb01518.x - 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: gumsley2017 type: article-journal author: [{family: "Gumsley", given: "Ashley P."}, {family: "Chamberlain", given: "Kevin R."}, {family: "Bleeker", given: "Wouter"}, {family: "Söderlund", given: "Ulf"}, {family: "de Kock", given: "Michiel O."}, {family: "Larsson", given: "Emilie R."}, {family: "Bekker", given: "Andrey"}] title: "Timing and tempo of the Great Oxidation Event" container-title: "Proceedings of the National Academy of Sciences" volume: "114" page: "1811-1816" issued: 2017 DOI: 10.1073/pnas.1608824114 - id: som2016 type: article-journal author: [{family: "Som", given: "Sanjoy M."}, {family: "Buick", given: "Roger"}, {family: "Hagadorn", given: "James W."}, {family: "Blake", given: "Tim S."}, {family: "Perreault", given: "John M."}, {family: "Harnmeijer", given: "Jelte P."}, {family: "Catling", given: "David C."}] title: "Earth's air pressure 2.7 billion years ago constrained to less than half of modern levels" container-title: "Nature Geoscience" volume: "9" page: "448-451" issued: 2016 DOI: 10.1038/ngeo2713 - id: heller2014 type: article-journal author: [{family: "Heller", given: "René"}, {family: "Armstrong", given: "John"}] title: "Superhabitable Worlds" container-title: "Astrobiology" volume: "14" page: "50-66" issued: 2014 DOI: 10.1089/ast.2013.1088 - id: schulzemakuch2020 type: article-journal author: [{family: "Schulze-Makuch", given: "Dirk"}, {family: "Heller", given: "René"}, {family: "Guinan", given: "Edward"}] title: "In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World" container-title: "Astrobiology" volume: "20" page: "1394-1404" issued: 2020 DOI: 10.1089/ast.2019.2161 - id: bryson2021 type: article-journal author: [{family: "Bryson", given: "Steve"}, {family: "Kunimoto", given: "Michelle"}, {family: "Kopparapu", given: "Ravi K."}, {family: "Coughlin", given: "Jeffrey L."}, {family: "Borucki", given: "William J."}, {family: "Koch", given: "David"}, {family: "Aguirre", given: "Victor Silva"}, {family: "Allen", given: "Christopher"}, {literal: "et al."}] title: "The Occurrence of Rocky Habitable-zone Planets around Solar-like Stars from Kepler Data" container-title: "The Astronomical Journal" volume: "161" page: "36" issued: 2021 DOI: 10.3847/1538-3881/abc418 - id: agol2021 type: article-journal author: [{family: "Agol", given: "Eric"}, {family: "Dorn", given: "Caroline"}, {family: "Grimm", given: "Simon L."}, {family: "Turbet", given: "Martin"}, {family: "Ducrot", given: "Elsa"}, {family: "Delrez", given: "Laetitia"}, {family: "Gillon", given: "Michaël"}, {family: "Demory", given: "Brice-Olivier"}, {literal: "et al."}] title: "Refining the Transit-timing and Photometric Analysis of TRAPPIST-1: Masses, Radii, Densities, Dynamics, and Ephemerides" container-title: "The Planetary Science Journal" volume: "2" page: "1" issued: 2021 DOI: 10.3847/psj/abd022 - id: gilbert2020 type: article-journal author: [{family: "Gilbert", given: "Emily A."}, {family: "Barclay", given: "Thomas"}, {family: "Schlieder", given: "Joshua E."}, {family: "Quintana", given: "Elisa V."}, {family: "Hord", given: "Benjamin J."}, {family: "Kostov", given: "Veselin B."}, {family: "Lopez", given: "Eric D."}, {family: "Rowe", given: "Jason F."}, {literal: "et al."}] title: "The First Habitable-zone Earth-sized Planet from TESS. I. Validation of the TOI-700 System" container-title: "The Astronomical Journal" volume: "160" page: "116" issued: 2020 DOI: 10.3847/1538-3881/aba4b2 - id: quintana2014 type: article-journal author: [{family: "Quintana", given: "Elisa V."}, {family: "Barclay", given: "Thomas"}, {family: "Raymond", given: "Sean N."}, {family: "Rowe", given: "Jason F."}, {family: "Bolmont", given: "Emeline"}, {family: "Caldwell", given: "Douglas A."}, {family: "Howell", given: "Steve B."}, {family: "Kane", given: "Stephen R."}, {literal: "et al."}] title: "An Earth-Sized Planet in the Habitable Zone of a Cool Star" container-title: "Science" volume: "344" page: "277-280" issued: 2014 DOI: 10.1126/science.1249403 - id: faria2022 type: article-journal author: [{family: "Faria", given: "J. P."}, {family: "Suárez Mascareño", given: "A."}, {family: "Figueira", given: "P."}, {family: "Silva", given: "A. M."}, {family: "Damasso", given: "M."}, {family: "Demangeon", given: "O."}, {family: "Pepe", given: "F."}, {family: "Santos", given: "N. C."}, {literal: "et al."}] title: "A candidate short-period sub-Earth orbiting Proxima Centauri" container-title: "Astronomy & Astrophysics" volume: "658" page: "A115" issued: 2022 DOI: 10.1051/0004-6361/202142337 images_wanted: - file: File:Mixed_world_sim.avif subject: "A TMW planet from the sim: continents and oceans with cloud cover, ideally a biotic mixed world (TMW-MB or TMW-WB)" source: sim note: "Shot list: render a TMW plate on the lit sphere; label 'Sim render'. Confirm the plate generator's licence." - file: File:Earth_Apollo17_AS17-148-22727.jpg subject: "The Blue Marble: Earth from Apollo 17 on 7 December 1972, Africa and Antarctica in view" source: nasa page_url: https://images.nasa.gov/details/as17-148-22727 credit: "NASA" licence: "Public domain (NASA)" note: "AS17-148-22727. Label 'Observation'." - file: File:Carbonate_silicate_cycle.svg subject: "Diagram: the carbonate-silicate cycle, with volcanic outgassing, rain and weathering, carbonate burial and subduction, and the temperature feedback marked" source: other note: "To be drawn by us; label 'Diagram'." --- A **mixed world** is a rocky planet with both continents and oceans, a temperate climate and liquid water at the surface: a planet like Earth. It is the only kind of world known to host life, which makes it the reference point for the search for habitable planets, although Earth remains its one confirmed member. Planets of Earth's size have been found in the habitable zones of other stars, but none is yet known to have oceans, continents or an atmosphere like ours. Pax Abyssi generates the class throughout its galaxy as the Earth-like end of its habitable worlds. ::figure{src="File:Earth_Apollo17_AS17-148-22727.jpg" size=wide alt="The full Earth seen from space: Africa and Arabia under swirls of white cloud, blue ocean, and the Antarctic ice cap at the bottom." caption="Observation: Earth from Apollo 17 on 7 December 1972, on the way to the Moon. Oceans cover 71 per cent of the surface."} ## Characteristics ### Temperature and the habitable zone Earth receives 1,361 W/m² of sunlight and reflects about 30 per cent of it. On those numbers alone it would settle at an equilibrium temperature of 255 K, well below freezing (see [[Planet classification]]). Its mean surface temperature is about 288 K because water vapour, carbon dioxide and other gases absorb and re-emit infrared heat before it escapes: the greenhouse effect. The range of orbits in which a planet like Earth could keep liquid water is the [[Habitable zone]]. Its inner edge is set by the **runaway greenhouse**: once the surface is warm enough, evaporating oceans add so much water vapour that the planet cannot shed the extra heat, and the oceans boil away. For a planet of Earth's mass around the Sun, one-dimensional climate models put that limit at about 0.95 AU :cite[kopparapu2014]; three-dimensional models, in which dry subtropical air lets more heat escape, move it somewhat closer to the star :cite[leconte2013]. The outer edge is the **maximum greenhouse** limit near 1.7 AU, beyond which adding more carbon dioxide cools a planet, because the gas starts to scatter sunlight and condense into clouds :cite[kasting1993] :cite[kopparapu2013]. ### The thermostat A planet at the right distance still needs something to hold its climate steady over billions of years, while its star brightens by tens of per cent. On Earth that job is done by the **carbonate-silicate cycle**. Volcanoes release carbon dioxide. Rain dissolves it into a weak acid that weathers silicate rocks on land, and the dissolved products wash into the sea, where they are locked into carbonate minerals and buried on the seafloor. In simplified form, $$ \mathrm{CaSiO_3} + \mathrm{CO_2} \rightarrow \mathrm{CaCO_3} + \mathrm{SiO_2}. $$ Weathering runs faster when the climate is warm and wet, so a warming planet draws down more carbon dioxide and cools, and a cooling one lets volcanic carbon dioxide build up and warms :cite[walker1981]. The feedback is slow, responding over hundreds of thousands of years :cite[colbourn2015], but over geological time it keeps the surface in the liquid-water range. **Plate tectonics** closes the loop by carrying carbonate-rich seafloor down into the mantle, where it is heated and the carbon returns through volcanoes. This is where a mixture of land and sea matters. Exposed continents give rain rock to weather, and oceans give the products somewhere to be deposited. A planet with almost no land, or almost no water, must regulate its climate differently or not at all (see [[Ocean world]] and [[Dry habitable world]]). ### Atmosphere and life Earth's present air, 78 per cent nitrogen and 21 per cent oxygen, is not the atmosphere it formed with. The early atmosphere had almost no free oxygen, and the air pressure 2.7 billion years ago was less than half of today's :cite[som2016]. Oxygen made by photosynthesising microbes began to accumulate in the Great Oxidation Event about 2.43 to 2.46 billion years ago :cite[gumsley2017]. On a living mixed world, the atmosphere is therefore partly a product of biology, and a spectrum showing oxygen alongside methane would be among the strongest signs of life a telescope could detect. ### Size Not every rocky planet in the habitable zone is equally promising. A small planet like Mars cools quickly, loses its magnetic field and much of its air. Heller and Armstrong (2014) argued that a planet somewhat larger and older than Earth, orbiting a K-type star that lives longer and is steadier than the Sun, might be even more hospitable; they called such planets **superhabitable** :cite[heller2014]. A later search of known planets and candidates found two dozen that fit some of those criteria, none of them confirmed as habitable :cite[schulzemakuch2020]. ## Formation Earth-like planets form in the inner part of their star's disc, where it is too warm for water ice to condense, so they start out dry. Their water has to be delivered. Earth's oceans weigh about $1.4 \times 10^{21}$ kg, only about 0.02 per cent of the planet's mass, and the ratio of deuterium to hydrogen in seawater matches carbonaceous asteroids from the outer asteroid belt rather than most comets. Dynamical models show how the growing giant planets could have scattered water-rich bodies of that kind inward to the young Earth :cite[morbidelli2000]. Simulations of planet formation find that the amount of water a planet in the habitable zone ends up with can vary by orders of magnitude, depending on where the giant planets are and how they move :cite[raymond2004]. A planet with Earth's orbit could as easily be a desert or a water world. Earth's balance of land and sea may be a matter of chance. ## How we know Only Earth has been studied as a mixed world, but the question of how many others exist is now statistical. Combining Kepler's detections with Gaia-refined stellar properties, Bryson and colleagues estimated that between about a third and two-thirds of Sun-like stars host a rocky planet in the conservative habitable zone, with large uncertainties because Kepler found few such planets directly :cite[bryson2021]. A handful of known planets have roughly Earth's size or mass and receive roughly Earth's sunlight. None has a measured atmosphere, so none can yet be called Earth-like: | Planet | Host star | Size or mass | Sunlight received | Notes | |---|---|---|---|---| | TRAPPIST-1 e | ultracool red dwarf | 0.92 R⊕, 0.69 M⊕ | about 0.65 × Earth's | density slightly below Earth's :cite[agol2021] | | TOI-700 d | red dwarf | about 1.2 R⊕ | 0.86 × Earth's | first Earth-sized habitable-zone planet from TESS :cite[gilbert2020] | | Kepler-186 f | red dwarf | about 1.1 R⊕ | about a third of Earth's | first Earth-sized planet found in a habitable zone :cite[quintana2014] | | Proxima Centauri b | nearest star, a red dwarf | at least 1.07 M⊕ | about 0.65 × Earth's | minimum mass from radial velocity; no transit :cite[faria2022] | All four orbit red dwarfs, whose planets are easiest to find. Such planets are probably tidally locked, and their stars flare and may have stripped their early atmospheres, so whether any of them keeps oceans and air is an open question (see [[Red dwarf habitability]]). ## Notable examples **Earth** is the only confirmed mixed world: 1 Earth mass, 5.51 g/cm³, 71 per cent ocean, 1 bar of nitrogen and oxygen, a mean surface temperature of 288 K, active plate tectonics and a global magnetic field. The first life appeared within a billion years or so of its formation; oxygen-producing life took far longer to change the air. **Early Mars** may once have come close. Valley networks and lake beds more than 3.5 billion years old show that liquid water flowed on its surface, before the planet lost most of its atmosphere (see [[Arid world]]). :::callout{type=sim title="In Pax Abyssi"} Mixed worlds are the sim's Earth-like class: rocky planets of 0.5 to 3 Earth masses in or near the habitable zone, with oceans covering 18 to 90 per cent of the surface, 0.3 to 3 bar of air and mean surface temperatures of roughly 250 to 310 K. The subtype code records two things. The first letter is the share of ocean: **A** below 35 per cent, **M** from 35 to 65 per cent and **W** above that, so Earth itself, at 71 per cent, would be a W world; brine-sea worlds are **BR** and tidally locked worlds **TL**. The second letter follows the atmosphere, which in the sim is shaped by life: **A** abiotic, **Y** young and oxygen-free, **B** biotic, and **P** once photosynthesis is under way. Mixed worlds are wired to the sim's life system, which moves a planet through eight stages from lifeless to complex life on land, each with a minimum age taken from Earth's history. In the committed system sheets, 131 of 8,742 generated planets are mixed worlds; 46 of them carry microbial or complex life, 15 of it complex. The sim scores such planets as potentially habitable; it does not declare any of them habitable. In the game each is drawn as a prebaked texture plate chosen by its subtype code. ::: ## See also - [[Habitable zone]] - [[Ocean world]] - [[Dry habitable world]] - [[Super-Earth]] - [[Superhabitable worlds]] - [[Life in Pax Abyssi]] - [[Colony and Habitat Viability Indices]] - [[Sol]] - [[Planet classification]]