# Habitable zone <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *The habitable zone and the frost line (Solar System explorer)* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?view=zones&embed=1" data-title="The habitable zone and the frost line (Solar System explorer)"></div> *The Solar System explorer locked on this article's state (`?view=zones`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: set the speed to 1 year/s and watch Earth run around the inner part of the habitable-zone band while Venus circles outside it, too close to the Sun; follow Mars through a few of its years and see its oval orbit carry it from the middle of the band at perihelion to the band's outer edge near 1.67 AU at aphelion; then drag outward to the thin frost-line ring near 2.7 AU, beyond the band, where water stayed frozen when the planets formed.* The **habitable zone** (HZ), also called the **circumstellar habitable zone** or informally the **Goldilocks zone**, is the range of distances from a star at which a planet with a suitable atmosphere could keep liquid [[Water|water]] on its surface.[^cruz2013][^brownlee2004] Its inner edge is set by the point at which starlight, amplified by greenhouse gases, would drive a planet's water into the atmosphere and eventually into space; its outer edge by the point at which even a thick carbon dioxide atmosphere could no longer stop the water from freezing.[^kasting1993] Because an exoplanet's orbital distance is usually among the first things measured, the zone is the standard first screen in the search for habitable worlds, although lying in it guarantees neither water nor life.[^tasker2017] Estimates for the [[Sun]] depend strongly on the climate model, from a narrow band hugging [[Earth]]'s orbit to one reaching past [[Jupiter]].[^hart1979][^pierrehumbert2011] The zone moves outward as a star brightens, is much closer to cool red dwarfs than to the Sun, and has been extended to moons, to subsurface oceans outside it and to other solvents than water. Surveys suggest that Earth-sized planets in the habitable zones of Sun-like stars are common, with about one Sun-like star in five hosting one.[^petigura2013] The explorer at the top of this page draws the Sun's habitable zone as a band from 0.99 to 1.67 [[Astronomical_unit|AU]], with Earth and [[Mars]] inside it and [[Venus]] closer to the Sun than its inner edge, together with the [[Frost_line_(astrophysics)|frost line]] near 2.7 AU.[^kopparapu2013][^martin2012] ## History The idea is older than the name. Alexander Winchell discussed planetary habitability in 1883 in terms close to a habitable zone for complex life, and Edward Maunder's 1913 book *Are the Planets Inhabited?* contains what may be the earliest use of the phrase "habitable zone".[^lingam2021][^lorenz2020] In 1953 Hubertus Strughold, writing on the prospects for life on [[Mars]], described an "ecosphere" around the Sun, and Harlow Shapley in the same year wrote of a "liquid water belt"; both put liquid water at the centre.[^huggett1995][^kasting2010] Su-Shu Huang developed the modern concept of circumstellar habitable zones in papers of the late 1950s and 1960s.[^huang1959][^lingam2021] Stephen Dole's 1964 RAND study *Habitable Planets for Man* worked through the conditions humans would need and estimated some 600 million such planets in the [[Milky_Way|Milky Way]].[^dole1964] The nickname "Goldilocks zone", for a region where temperatures are "just right" for liquid water, came into use in the 1970s.[^nasa-goldilocks] The working definition used today comes from James Kasting and colleagues, who in 1993 computed the zone's edges for stars of different luminosity with a one-dimensional climate model in which water vapour and carbon dioxide are the greenhouse gases.[^kasting1993] Ravi Kopparapu and colleagues revised that model in 2013 with new absorption data and introduced the now-standard distinction between conservative and optimistic limits.[^kopparapu2013] The concept became practical once telescopes could find small planets: NASA's Kepler mission, launched in 2009, was designed to measure how often Earth-sized planets orbit in habitable zones, and TESS continued the search from 2018.[^borucki2013][^tess] ### Extensions of the concept In 2001 Guillermo Gonzalez, Donald Brownlee and Peter Ward proposed a galactic habitable zone: a ring in a galaxy where stars are rich enough in heavy elements to build rocky planets but far enough from the crowded centre to escape frequent sterilising events.[^gonzalez2001] Others proposed zones for hypothetical life using solvents other than water, and in 2013 a circumplanetary "habitable edge" was defined for moons, inside which tidal heating would boil away a moon's water.[^villard2011][^hadhazy2013] ## Definition A planet's position relative to the zone depends on two numbers: the stellar flux it receives, set by the star's luminosity and the planet's orbital radius, and the climate response of its atmosphere to that flux. Climate models with the absorption properties of water vapour and carbon dioxide turn these into edges.[^kopparapu2013] The conservative zone runs from the "moist greenhouse" or water-loss limit, where the stratosphere becomes wet and hydrogen escapes, to the "maximum greenhouse" limit, beyond which adding carbon dioxide cools the planet instead of warming it. The optimistic zone is wider, bounded by empirical limits: a "recent Venus" edge, the flux [[Venus]] received about a billion years ago, since when it appears to have had no surface water, and an "early Mars" edge, the flux Mars received about 3.8 billion years ago, when it apparently did have liquid water.[^kopparapu2013] ### Habitable planets A planet in the zone may still be uninhabitable. Whether it holds water depends on its mass, composition, atmosphere, history and much else that the definition leaves out, and some astronomers have argued that "habitable" should not be used for such planets at all.[^tasker2017][^patel2019] Only the remote detection of signs of life could show a planet to be inhabited.[^patel2019] Dole listed much stricter conditions for human habitability, including surface gravity below about 1.5 g and a breathable oxygen atmosphere.[^dole1964] ### Solar System estimates Published limits for the Sun span more than an order of magnitude: | Inner edge (AU) | Outer edge (AU) | Study | Basis | |---|---|---|---| | 0.958 | 1.004 | Hart 1979[^hart1979] | Evolution of Earth's atmosphere; very narrow | | 0.95 | 1.37–1.67 | Kasting et al. 1993[^kasting1993] | 1-D climate model with H₂O and CO₂ | | 0.75 | — | Abe et al. 2011[^abe2011] | Dry "desert" planets | | — | 10 | Pierrehumbert and Gaidos 2011[^pierrehumbert2011] | Primordial hydrogen greenhouse | | 0.38 | — | Zsom et al. 2013[^zsom2013] | Most favourable humidity and reflectivity | | 0.95 | — | Leconte et al. 2013[^leconte2013] | 3-D climate model | | 0.99 | 1.70 | Kopparapu et al. 2013[^kopparapu2013] | Updated water-loss and maximum-greenhouse limits | | 0.95 | 2.4 | Ramirez and Kaltenegger 2017[^ramirez2017] | Volcanic hydrogen added | On the Kopparapu limits, Earth at 1.00 AU lies just inside the zone, close to its inner edge, and Mars, at a mean 1.52 AU, well inside.[^kopparapu2013][^nasa-mars] The explorer draws the zone to 1.67 AU, the maximum-greenhouse edge of the Kasting model, slightly inside Kopparapu's 1.70 AU.[^kopparapu2013] Mars's eccentric orbit, from 1.38 AU at perihelion to 1.67 AU at aphelion, then just touches the outer edge (derived).[^nasa-mars] ## Effect of climate model A planet needs a source of water before any climate can keep it liquid. The origin of Earth's water is still debated: impacts of icy bodies, outgassing from the interior and water bound in the minerals that built the planet are all candidates.[^drake2005] Planets formed beyond the frost line and later moved inward could be "ocean planets" with water hundreds of kilometres deep.[^kuchner2003] The atmosphere matters as much as the distance. Giovanni Vladilo and colleagues found in 2013 that the zone widens as surface pressure rises and that below about 15 millibars no liquid water is stable, because small changes in pressure or temperature push water out of the liquid range of its [[Phase_diagram|phase diagram]].[^vladilo2013] The greenhouse gases also change the result. When Ramses Ramirez and Lisa Kaltenegger added volcanically supplied [[Hydrogen|hydrogen]] to carbon dioxide and water vapour, the Sun's outer edge moved out to 2.4 AU.[^ramirez2017] Raymond Pierrehumbert and Eric Gaidos went further, dropping the carbon dioxide–water model entirely: a young planet holding tens to hundreds of bars of primordial hydrogen from the [[Protoplanetary_disk|protoplanetary disk]] could stay warm as far out as 10 AU, though it would lose that hydrogen within some millions to tens of millions of years unless something replaced it.[^pierrehumbert2011] ### Desert planets A dry "desert" planet has little water vapour and so a weaker greenhouse effect; it can keep liquid water in cool polar oases closer to its star than an ocean planet could. It also has less bright ice and snow to reflect sunlight, so its outer edge lies farther out. Yutaka Abe and colleagues placed the inner edge of such planets at about 0.75 AU for the Sun.[^abe2011] ## Effect of stellar luminosity The Solar System results are carried to other stars through the inverse-square law: a planet receives the same flux as Earth at a distance d = √(L/S_eff) AU, where L is the star's luminosity in solar units and S_eff is the flux, relative to Earth's, that produces a given edge.[^kasting1993] Because cooler stars emit more of their light in the infrared, which water and carbon dioxide absorb more strongly and which is scattered less by the atmosphere, S_eff depends on the star's effective temperature; Kopparapu and colleagues give it as a fourth-order polynomial in temperature for each edge, valid from 2,600 to 7,200 K.[^kopparapu2013] For a red dwarf with 1% of the Sun's luminosity, the distance scale shrinks by a factor of ten, putting the zone at roughly 0.1–0.17 AU before the temperature correction, well inside Mercury's orbit (derived). ### Spectral types and star-system characteristics Stellar type affects more than the zone's position. In binary stars the zone has a different shape and orbital stability must also be checked.[^cuntz2014] Andrea Buccino and colleagues found that only about 40% of the stars they studied, including the Sun, have zones in which liquid water and tolerable ultraviolet flux overlap.[^buccino2006] Planets in the zones of red dwarfs orbit so close that they are likely tidally locked, and strong tidal heating might turn some into scorched "tidal Venuses".[^barnes2013] Climate models from 2013, however, show that clouds on the star-facing side of a locked planet can raise its reflectivity and keep its climate moderate, widening the zone.[^yang2013] Eric Agol has argued that white dwarfs could host short-lived zones, and red dwarfs still contracting towards the main sequence can have zones lasting up to billions of years.[^agol2011][^ramirez2014] ## Evolution A star's zone moves as the star ages. Massive O-type stars leave the main sequence in less than 10 million years, too quickly for their zones to host long-lived biospheres, while red dwarfs burn for hundreds of billions of years.[^carroll2007][^richmond2004] Main-sequence stars also brighten slowly, pushing their zones outward: the young Sun gave only about 75% of its present light in the Archean eon, and continued brightening will take Earth out of the zone before the Sun becomes a red giant.[^kasting1986][^franck2002] For this reason a "continuously habitable zone" is defined for the region that stays habitable over a chosen span of time.[^franck2002] Young red dwarfs are violent: flares can double a star's brightness within minutes, and starspots can cover a fifth of its surface, enough to strip a planet's atmosphere.[^croswell2001][^alekseev2002] Their activity declines with age, and by about 1.2 billion years many are calm enough for life to develop.[^alpert2005] After the main sequence the zone moves far out. Once a Sun-like star settles into helium burning on the horizontal branch, its zone could lie between about 7 and 22 AU, placing [[Titan_(moon)|Titan]] near its centre, for roughly a billion years.[^lopez2005][^lorenz1997] Ramirez and Kaltenegger found, however, that the strong winds of such stars would strip the atmospheres of bodies as small as Titan; life predating that phase might still be detected as the ice of such worlds melts.[^ramirez2016] ## Exoplanets in habitable zones Estimates of how many planets lie in habitable zones follow the data available. Kepler results led Eric Petigura and colleagues to conclude that about 22% of Sun-like stars have an Earth-sized planet in the zone, which implies tens of billions of such planets in the Milky Way.[^petigura2013][^overbye2013] For the red dwarfs observed by Kepler, Kopparapu put the fraction with an Earth-sized planet in the zone at about 0.48.[^kopparapu2013l] Earlier estimates were lower: a 2011 analysis of early Kepler data found that between 1.4% and 2.7% of Sun-like stars host an Earth analogue.[^catanzarite2011] A 2023 catalogue of all planets found in habitable zones shows how far most of them are from Earth analogues: it includes giant planets, strongly eccentric orbits and active or evolved host stars.[^hill2023] ## Habitable planets A 2015 review ranked Kepler-62f, Kepler-186f and Kepler-442b among the best candidates for habitability.[^gilster2015] The nearest planet in a habitable zone is Proxima Centauri b, about 4.2 light-years away in the [[Alpha_Centauri|Alpha Centauri]] system, which has at least 1.3 Earth masses and orbits a red dwarf in 11.2 days.[^anglada2016] ### Early findings The first planets found in habitable zones, in the late 1990s, were gas giants, several on eccentric orbits; large moons of such planets might still hold liquid water.[^jones2006] 70 Virginis b was first nicknamed "Goldilocks", but later work showed it too hot.[^extrasolar70vir] HD 28185 b, announced in 2001, stays entirely inside its star's zone on a nearly circular orbit.[^jones2006] ### Super-Earths in a habitable zone Kepler showed that most Sun-like stars have close-in planets between Earth and [[Neptune]] in size, often called super-Earths, a label that has been criticised as misleading.[^moore2017] Many of the larger ones keep hydrogen envelopes while smaller ones are stripped cores, and those in or near habitable zones tend to resemble small Neptunes more than large Earths.[^bean2021] Kepler-452b, announced in 2015, is about 1.6 Earth radii and orbits a Sun-like G2 star in about 385 days.[^jenkins2015] ### Near Earth-sized planets and Solar analogs Kepler-62e and Kepler-62f, announced in 2013, have radii of 1.6 and 1.4 times Earth's.[^borucki2013] Kepler-186f, about 1.1 Earth radii, was the first Earth-sized planet confirmed in a habitable zone, but its star is a red dwarf, not a solar analogue.[^morelle2014] LHS 1140 b, a dense super-Earth of about 6.6 Earth masses 39 light-years away, transits a quiet red dwarf, which allows its atmosphere to be studied.[^dittmann2017] In 2016 three planets were reported around the ultracool dwarf TRAPPIST-1, raising the prospect that small, cool, nearby stars host many temperate planets.[^eso1615] ### Moons Moons of giant planets could also be habitable. A moon must orbit far enough from its planet to escape runaway tidal heating like that of [[Io_(moon)|Io]], yet stay inside the planet's [[Hill_sphere|Hill sphere]]; around red dwarfs of less than about a fifth of a solar mass the two conditions cannot both be met.[^hadhazy2013][^hamilton1992] ## Alternative habitable zones Liquid water exists well outside the Sun's habitable zone. [[Europa_(moon)|Europa]], [[Ganymede_(moon)|Ganymede]], [[Titan_(moon)|Titan]] and [[Enceladus]] are thought to hold oceans beneath their ice, heated by tides and radioactive decay.[^torres2012][^reynolds1987] Giant planets can thus have "tidally heated habitable zones" of their own, and Europa is the leading example.[^reynolds1987] Dorian Abbot and Eric Switzer suggested that [[Radioactive_decay|radiogenic heat]] under a thick ice shell could keep water liquid even on planets drifting between the stars.[^techreview2011] Christopher McKay proposed that liquid methane might host a "cryolife", with the Sun's methane zone centred about 1.6 billion km, roughly 11 AU, from the Sun (derived), close to Titan's orbit at 9.5 AU.[^villard2011][^nasa-saturn] ## Significance for complex and intelligent life The classical zone asks only for liquid water, enough for microbes. Complex animal life needs more, including limits on atmospheric carbon dioxide and carbon monoxide, which define a narrower "habitable zone for complex life".[^schwieterman2019] The Rare Earth hypothesis argues that complex life is rare and that the zone is only one of many requirements, including plate tectonics and [[Photosynthesis|photosynthesis]] to supply [[Oxygen|oxygen]].[^brownlee2004][^decker2011] Searches for extraterrestrial intelligence start with stars that have habitable zones: a catalogue of nearby habitable stellar systems was compiled for SETI target selection in 2003, and radio surveys have since targeted Kepler planets in habitable zones.[^turnbull2003][^siemion2013] In the Drake equation, the number of habitable-zone planets per star is a key term.[^brownlee2004] ## See also - [[Frost_line_(astrophysics)]] - [[Planetary_system]] - [[Earth's_energy_budget]] - [[Titan_(moon)]] · [[Europa_(moon)]] · [[Enceladus]] - Habitability of red dwarf systems · Habitable zone for complex life ## References [^cruz2013]: Cruz, M.; Coontz, R. 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A search for narrow-band emission from select targets". *The Astrophysical Journal* 767: 94. https://doi.org/10.1088/0004-637X/767/1/94 ## Further reading - Kasting, J. (2010). *How to Find a Habitable Planet*. Princeton University Press. ISBN 978-0-691-13805-3. - Kopparapu, R. K.; Ramirez, R. M.; SchottelKotte, J.; et al. (2014). "Habitable zones around main-sequence stars: dependence on planetary mass". *The Astrophysical Journal Letters* 787: L29. https://doi.org/10.1088/2041-8205/787/2/L29 ## External links - NASA Exoplanet Archive — https://exoplanetarchive.ipac.caltech.edu/ - Virtual Planetary Laboratory habitable-zone calculator (Kopparapu et al.) — https://depts.washington.edu/naivpl/content/hz-calculator - NASA Science: "The habitable zone" — https://science.nasa.gov/exoplanets/habitable-zone/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Habitable_zone) : [Wikitube](https://en.wikitube.io/wiki/Habitable_zone) · pinned revision [1374517544](https://en.wikipedia.org/w/index.php?oldid=1374517544) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-006 · explorer state `?view=zones`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->