--- title: Atmospheric escape summary: The loss of a planet's or moon's air to space, gas molecule by gas molecule or in a bulk wind driven by starlight. It decides which worlds keep an atmosphere, and it carves the radius valley and the hot Neptune desert. science_status: [observed, model, sim] categories: [Planets, Atmospheric science, Planet formation, Exoplanets] aliases: [Atmospheric loss, Atmospheric mass loss, Atmospheric stripping, Jeans escape, Jeans parameter, Hydrodynamic escape, Energy-limited escape, Photoevaporation, Core-powered mass loss, Boil-off, Exobase, Non-thermal escape] infobox: type: physics_concept name: Atmospheric escape image: File:Gliese_436b_hydrogen_tail_ESAHubble.jpg caption: "Artist's concept: the cloud of hydrogen escaping the warm Neptune GJ 436 b, measured by Hubble" definition: "The loss of gas from the top of an atmosphere to space, either thermally (Jeans escape, molecule by molecule; hydrodynamic escape, as a bulk wind) or by non-thermal processes driven by charged particles, photochemistry and impacts." formulae: - "Jeans parameter at the exobase: lambda = G M m / (k T r) = (v_esc / v_0)^2" - "Energy-limited escape: dM/dt = eta pi F_XUV R_p^3 / (G M_p K)" symbols: "M, M_p planet mass; m molecular mass; T exobase temperature; r exobase radius; v_0 = sqrt(2kT/m); F_XUV X-ray and extreme-ultraviolet flux; eta heating efficiency (of order 0.1); K tidal correction for the star's pull (1 or less)" validity: "Jeans escape applies when lambda is large and the gas leaks slowly; the energy-limited formula is an upper estimate for strongly irradiated hydrogen atmospheres, and becomes recombination-limited at the highest fluxes" worked_example: "Earth's exobase, about 500 km up at about 1,000 K: lambda is about 7 for hydrogen atoms and about 110 for oxygen atoms, so hydrogen leaks away slowly and oxygen stays." literature_schemes: [Jeans (thermal), hydrodynamic and energy-limited, recombination-limited, core-powered mass loss, boil-off, Roche-lobe overflow, non-thermal (sputtering, ion pickup, charge exchange, photochemical escape), impact erosion] key_quantities: "Escape velocity (Earth 11.2 km/s, Mars 5.0, Titan 2.6); exobase temperature; X-ray output, about a thousandth of a young Sun-like star's total light during its first 100 million years" observed_examples: [Mars (MAVEN), Venus (deuterium-to-hydrogen ratio), HD 209458 b, GJ 436 b, WASP-107 b (helium)] sim_use: "Every mini-Neptune the generator makes has its escape worked out: XUV flux from its star's luminosity, type and age, energy-limited and core-powered rates, a Roche-lobe check and an envelope lifetime. The fastest losers become evaporating mini-Neptunes (MNH-EV); stripped giant cores are chthonian planets." last_verified: 2026-10-07 refs: - id: catling2017 type: book author: [{family: "Catling", given: "David C."}, {family: "Kasting", given: "James F."}] title: "Atmospheric Evolution on Inhabited and Lifeless Worlds" publisher: Cambridge University Press issued: 2017 DOI: 10.1017/9781139020558 - id: catling2009 type: article-magazine author: [{family: "Catling", given: "David C."}, {family: "Zahnle", given: "Kevin J."}] title: "The Planetary Air Leak" container-title: Scientific American volume: "300" page: "36-43" issued: 2009 DOI: 10.1038/scientificamerican0509-36 - id: hunten1973 type: article-journal author: [{family: "Hunten", given: "Donald M."}] title: "The escape of light gases from planetary atmospheres" container-title: Journal of the Atmospheric Sciences volume: "30" page: "1481-1494" issued: 1973 DOI: 10.1175/1520-0469(1973)030<1481:TEOLGF>2.0.CO;2 - id: watson1981 type: article-journal author: [{family: "Watson", given: "Andrew J."}, {family: "Donahue", given: "Thomas M."}, {family: "Walker", given: "James C. G."}] title: "The dynamics of a rapidly escaping atmosphere: applications to the evolution of Earth and Venus" container-title: Icarus volume: "48" page: "150-166" issued: 1981 DOI: 10.1016/0019-1035(81)90101-9 - id: hunten1987 type: article-journal author: [{family: "Hunten", given: "Donald M."}, {family: "Pepin", given: "Robert O."}, {family: "Walker", given: "James C. G."}] title: "Mass fractionation in hydrodynamic escape" container-title: Icarus volume: "69" page: "532-549" issued: 1987 DOI: 10.1016/0019-1035(87)90022-4 - id: erkaev2007 type: article-journal author: [{family: "Erkaev", given: "N. V."}, {family: "Kulikov", given: "Yu. N."}, {family: "Lammer", given: "H."}, {family: "Selsis", given: "F."}, {family: "Langmayr", given: "D."}, {family: "Jaritz", given: "G. F."}, {family: "Biernat", given: "H. K."}] title: "Roche lobe effects on the atmospheric loss from \"Hot Jupiters\"" container-title: Astronomy & Astrophysics volume: "472" page: "329-334" issued: 2007 DOI: 10.1051/0004-6361:20066929 - id: murrayclay2009 type: article-journal author: [{family: "Murray-Clay", given: "Ruth A."}, {family: "Chiang", given: "Eugene I."}, {family: "Murray", given: "Norman"}] title: "Atmospheric escape from hot Jupiters" container-title: The Astrophysical Journal volume: "693" page: "23-42" issued: 2009 DOI: 10.1088/0004-637X/693/1/23 - id: owen2016 type: article-journal author: [{family: "Owen", given: "James E."}, {family: "Alvarez", given: "Marcelo A."}] title: "UV driven evaporation of close-in planets: energy-limited, recombination-limited, and photon-limited flows" container-title: The Astrophysical Journal volume: "816" page: "34" issued: 2016 DOI: 10.3847/0004-637X/816/1/34 - id: wright2011 type: article-journal author: [{family: "Wright", given: "Nicholas J."}, {family: "Drake", given: "Jeremy J."}, {family: "Mamajek", given: "Eric E."}, {family: "Henry", given: "Gregory W."}] title: "The stellar-activity-rotation relationship and the evolution of stellar dynamos" container-title: The Astrophysical Journal volume: "743" page: "48" issued: 2011 DOI: 10.1088/0004-637X/743/1/48 - id: jackson2012 type: article-journal author: [{family: "Jackson", given: "A. P."}, {family: "Davis", given: "T. A."}, {family: "Wheatley", given: "P. J."}] title: "The coronal X-ray-age relation and its implications for the evaporation of exoplanets" container-title: Monthly Notices of the Royal Astronomical Society volume: "422" page: "2024-2043" issued: 2012 DOI: 10.1111/j.1365-2966.2012.20657.x - id: owen2013 type: article-journal author: [{family: "Owen", given: "James E."}, {family: "Wu", given: "Yanqin"}] title: "Kepler planets: a tale of evaporation" container-title: "The Astrophysical Journal" volume: "775" page: "105" issued: 2013 DOI: 10.1088/0004-637X/775/2/105 - id: lopez2013 type: article-journal author: [{family: "Lopez", given: "Eric D."}, {family: "Fortney", given: "Jonathan J."}] title: "The role of core mass in controlling evaporation: the Kepler radius distribution and the Kepler-36 density dichotomy" container-title: "The Astrophysical Journal" volume: "776" page: "2" issued: 2013 DOI: 10.1088/0004-637X/776/1/2 - id: owen2017 type: article-journal author: [{family: "Owen", given: "James E."}, {family: "Wu", given: "Yanqin"}] title: "The evaporation valley in the Kepler planets" container-title: "The Astrophysical Journal" volume: "847" page: "29" issued: 2017 DOI: 10.3847/1538-4357/aa890a - id: owenwu2016 type: article-journal author: [{family: "Owen", given: "James E."}, {family: "Wu", given: "Yanqin"}] title: "Atmospheres of low-mass planets: the \"boil-off\"" container-title: The Astrophysical Journal volume: "817" page: "107" issued: 2016 DOI: 10.3847/0004-637X/817/2/107 - id: ginzburg2018 type: article-journal author: [{family: "Ginzburg", given: "Sivan"}, {family: "Schlichting", given: "Hilke E."}, {family: "Sari", given: "Re'em"}] title: "Core-powered mass-loss and the radius distribution of small exoplanets" container-title: "Monthly Notices of the Royal Astronomical Society" volume: "476" page: "759-765" issued: 2018 DOI: 10.1093/mnras/sty290 - id: gupta2019 type: article-journal author: [{family: "Gupta", given: "Akash"}, {family: "Schlichting", given: "Hilke E."}] title: "Sculpting the valley in the radius distribution of small exoplanets as a by-product of planet formation: the core-powered mass-loss mechanism" container-title: "Monthly Notices of the Royal Astronomical Society" volume: "487" page: "24-33" issued: 2019 DOI: 10.1093/mnras/stz1230 - id: li2010 type: article-journal author: [{family: "Li", given: "Shu-Lin"}, {family: "Miller", given: "N."}, {family: "Lin", given: "Douglas N. C."}, {family: "Fortney", given: "Jonathan J."}] title: "WASP-12b as a prolate, inflated and disrupting planet from tidal dissipation" container-title: Nature volume: "463" page: "1054-1056" issued: 2010 DOI: 10.1038/nature08715 - id: vidalmadjar2003 type: article-journal author: [{family: Vidal-Madjar, given: A.}, {family: Lecavelier des Etangs, given: A.}, {family: Désert, given: J.-M.}, {literal: "et al."}] title: "An extended upper atmosphere around the extrasolar planet HD209458b" container-title: Nature volume: 422 page: 143-146 issued: 2003 DOI: 10.1038/nature01448 - id: ehrenreich2015 type: article-journal author: [{family: "Ehrenreich", given: "David"}, {family: "Bourrier", given: "Vincent"}, {family: "Wheatley", given: "Peter J."}, {family: "Lecavelier des Etangs", given: "Alain"}, {family: "Hébrard", given: "Guillaume"}, {family: "Udry", given: "Stéphane"}, {family: "Bonfils", given: "Xavier"}, {family: "Delfosse", given: "Xavier"}, {literal: "et al."}] title: "A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b" container-title: "Nature" volume: "522" page: "459-461" issued: 2015 DOI: 10.1038/nature14501 - id: oklopcic2018 type: article-journal author: [{family: Oklopčić, given: Antonija}, {family: Hirata, given: Christopher M.}] title: "A New Window into Escaping Exoplanet Atmospheres: 10830 Å Line of Helium" container-title: The Astrophysical Journal Letters volume: 855 page: L11 issued: 2018 DOI: 10.3847/2041-8213/aaada9 - id: spake2018 type: article-journal author: [{family: Spake, given: J. J.}, {family: Sing, given: D. K.}, {family: Evans, given: T. M.}, {literal: "et al."}] title: "Helium in the eroding atmosphere of an exoplanet" container-title: Nature volume: 557 page: 68-70 issued: 2018 DOI: 10.1038/s41586-018-0067-5 - id: hu2015 type: article-journal author: [{family: Hu, given: Renyu}, {family: Seager, given: Sara}, {family: Yung, given: Yuk L.}] title: "Helium Atmospheres on Warm Neptune- and Sub-Neptune-sized Exoplanets and Applications to GJ 436b" container-title: The Astrophysical Journal volume: 807 page: 8 issued: 2015 DOI: 10.1088/0004-637X/807/1/8 - id: jakosky2018 type: article-journal author: [{family: "Jakosky", given: "B.M."}, {family: "Brain", given: "D."}, {family: "Chaffin", given: "M."}, {family: "Curry", given: "S."}, {family: "Deighan", given: "J."}, {family: "Grebowsky", given: "J."}, {family: "Halekas", given: "J."}, {family: "Leblanc", given: "F."}, {literal: "et al."}] title: "Loss of the Martian atmosphere to space: Present-day loss rates determined from MAVEN observations and integrated loss through time" container-title: "Icarus" volume: "315" page: "146-157" issued: 2018 DOI: 10.1016/j.icarus.2018.05.030 - id: donahue1982 type: article-journal author: [{family: "Donahue", given: "T. M."}, {family: "Hoffman", given: "J. H."}, {family: "Hodges", given: "R. R."}, {family: "Watson", given: "A. J."}] title: "Venus Was Wet: A Measurement of the Ratio of Deuterium to Hydrogen" container-title: "Science" volume: "216" page: "630-633" issued: 1982 DOI: 10.1126/science.216.4546.630 - id: gunell2018 type: article-journal author: [{family: "Gunell", given: "H."}, {family: "Maggiolo", given: "R."}, {family: "Nilsson", given: "H."}, {family: "Stenberg Wieser", given: "G."}, {family: "Slapak", given: "R."}, {family: "Lindkvist", given: "J."}, {family: "Hamrin", given: "M."}, {family: "De Keyser", given: "J."}] title: "Why an intrinsic magnetic field does not protect a planet against atmospheric escape" container-title: Astronomy & Astrophysics volume: "614" page: "L3" issued: 2018 DOI: 10.1051/0004-6361/201832934 - id: zahnle2017 type: article-journal author: [{family: "Zahnle", given: "Kevin J."}, {family: "Catling", given: "David C."}] title: "The Cosmic Shoreline: The Evidence that Escape Determines which Planets Have Atmospheres, and what this May Mean for Proxima Centauri B" container-title: "The Astrophysical Journal" volume: "843" page: "122" issued: 2017 DOI: 10.3847/1538-4357/aa7846 - id: kreidberg2019 type: article-journal author: [{family: "Kreidberg", given: "Laura"}, {family: "Koll", given: "Daniel D. B."}, {family: "Morley", given: "Caroline"}, {family: "Hu", given: "Renyu"}, {family: "Schaefer", given: "Laura"}, {family: "Deming", given: "Drake"}, {family: "Stevenson", given: "Kevin B."}, {family: "Dittmann", given: "Jason"}, {literal: "et al."}] title: "Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b" container-title: "Nature" volume: "573" page: "87-90" issued: 2019 DOI: 10.1038/s41586-019-1497-4 - id: greene2023 type: article-journal author: [{family: "Greene", given: "Thomas P."}, {family: "Bell", given: "Taylor J."}, {family: "Ducrot", given: "Elsa"}, {family: "Dyrek", given: "Achrène"}, {family: "Lagage", given: "Pierre-Olivier"}, {family: "Fortney", given: "Jonathan J."}] title: "Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST" container-title: "Nature" volume: "618" page: "39-42" issued: 2023 DOI: 10.1038/s41586-023-05951-7 --- **Atmospheric escape** is the loss of a planet's or moon's air to space. Every atmosphere leaks from the top, where the gas is so thin that a fast molecule heading upward can leave without hitting anything on the way. For Earth the leak is slow and mostly hydrogen. For a planet close to a young, active star it can be a wind that carries away a whole envelope of gas in a few hundred million years. Escape explains why Mars has thin air and Venus has almost no water, why some rocky exoplanets are bare, and why two of the clearest patterns in the planet census exist: the [[Radius valley|radius valley]] among small planets and the [[Hot Neptune desert|hot Neptune desert]] close to their stars. In Pax Abyssi the generator works out escape for every mini-Neptune it makes, and the planets losing their air fastest form a type of their own. ::figure{src="File:Gliese_436b_hydrogen_tail_ESAHubble.jpg" size=wide alt="An illustration of a planet in front of a red star, trailing a vast pale cloud of escaping hydrogen." caption="Artist's concept: the cloud of hydrogen escaping the warm Neptune GJ 436 b, measured by Hubble. The cloud is far larger than the planet and trails it like a comet's tail."} ## Where gas leaves Low in an atmosphere, molecules collide constantly and none gets far. Higher up the air thins until, at a level called the **exobase**, a molecule is more likely to fly off on a ballistic path than to hit another one. Earth's exobase is about 500 km up, in the thermosphere, where sunlight in the extreme ultraviolet heats the thin gas to around 1,000 K, more in years of high solar activity :cite[catling2017]. Whether a molecule at the exobase escapes depends on how its speed compares with the planet's escape velocity there. ## Thermal escape ### Jeans escape At any temperature the molecules of a gas have a spread of speeds, and a few in the fast tail of the distribution always exceed escape velocity. Their steady trickle away is **Jeans escape**, after James Jeans, who worked it out early in the twentieth century. Its strength is set by one number, the **Jeans parameter**, the ratio of a molecule's gravitational binding energy at the exobase to its thermal energy: $$ \lambda = \frac{G M m}{k T r} = \left(\frac{v_\mathrm{esc}}{v_0}\right)^2 , $$ where $M$ is the planet's mass, $m$ the mass of the molecule, $T$ the exobase temperature, $r$ the exobase radius and $v_0 = \sqrt{2kT/m}$ the most probable thermal speed. The escaping flux falls roughly as $(1+\lambda)\,e^{-\lambda}$, so a modest change in $\lambda$ makes an enormous difference :cite[catling2017]. At Earth's exobase, $\lambda$ is about 7 for hydrogen atoms and about 110 for oxygen atoms. Hydrogen therefore leaks away, while oxygen, sixteen times heavier, is held for the life of the planet. Earth loses about 3 kg of hydrogen and some 50 g of helium every second, through Jeans escape and the non-thermal routes described below :cite[catling2009]. The hydrogen comes mostly from water vapour and methane broken apart high in the atmosphere, and its loss is limited less by the exobase than by how fast hydrogen-bearing molecules can diffuse up to it, the **diffusion limit** :cite[hunten1973]. ### Hydrodynamic escape If the upper atmosphere is heated hard enough, $\lambda$ falls toward a few and escape changes character. The gas no longer leaks molecule by molecule but expands outward as a bulk flow, accelerating through the speed of sound like the solar wind. This is **hydrodynamic escape** :cite[watson1981]. A strong enough outflow of hydrogen drags heavier atoms with it, and because lighter species are carried more easily, it leaves the remaining gas enriched in heavy isotopes, a fingerprint that survives billions of years :cite[hunten1987]. The energy for the flow comes from the star's X-ray and extreme-ultraviolet light (together, XUV), absorbed high in the atmosphere. If a fixed fraction of that energy goes into lifting gas out of the planet's gravity well, the mass-loss rate is $$ \dot{M} \approx \frac{\eta\,\pi F_\mathrm{XUV} R_p^3}{G M_p K}, $$ the **energy-limited** rate, where $F_\mathrm{XUV}$ is the XUV flux at the planet, $R_p$ and $M_p$ its radius and mass, $\eta$ a heating efficiency of order 0.1 and $K$, at most 1, a correction for the star's tidal pull, which helps gas over the top :cite[watson1981] :cite[erkaev2007]. At the highest fluxes the formula overestimates: much of the absorbed energy is radiated away by hydrogen recombining with electrons, and the rate grows only as the square root of the flux, the **recombination-limited** regime :cite[murrayclay2009] :cite[owen2016]. ### Starlight that fades The XUV output of a star changes with age. A young Sun-like star spins fast, and its magnetic activity holds its X-ray output near a ceiling of about a thousandth of its total light :cite[wright2011]. It stays there for about its first 100 million years, then fades as the star's spin slows :cite[jackson2012]. Low-mass red dwarfs spin down more slowly and stay at that ceiling for longer, which makes their planets the hardest places to keep an atmosphere :cite[wright2011]. Most escape driven by starlight therefore happens early, and a planet's atmosphere today is the record of what survived its star's youth. ## Stripping whole envelopes Close-in planets with hydrogen-helium envelopes are the most vulnerable, because hydrogen is light and a puffy planet presents a large target with weak gravity at its top. Several processes act on them, at different times. - **Boil-off.** A planet that forms inside its star's gas disc is hot and swollen. When the disc disperses, the pressure holding its outer envelope in place disappears, and a young planet can lose a large share of its envelope in a rapid episode as it contracts :cite[owenwu2016]. - **Photoevaporation.** Over the next few hundred million years, XUV heating drives an energy-limited wind. Owen and Wu (2013) and Lopez and Fortney (2013) showed that it should strip the envelopes from small cores on close orbits and spare heavier cores or more distant planets, predicting a gap in planet sizes before it was found :cite[owen2013] :cite[lopez2013] :cite[owen2017]. - **Core-powered mass loss.** The heat left from a planet's formation also escapes through its envelope, and the star's ordinary light keeps the outer layers warm. Together they can drive the envelope away over about a billion years with no help from XUV at all :cite[ginzburg2018] :cite[gupta2019]. - **Roche-lobe overflow.** A planet so close to its star that its atmosphere reaches the point where the star's gravity takes over loses gas directly across that boundary. The hot Jupiter WASP-12 b is distorted by tides and probably overflowing in this way :cite[li2010]. The first two predict almost the same split of small planets into stripped rocky cores and survivors with envelopes, the [[Radius valley|radius valley]]. For larger planets, the same erosion helps empty the [[Hot Neptune desert|hot Neptune desert]]. ## Seeing it happen Escaping gas is too thin to image, but it can be seen in silhouette. During a transit, a planet with an extended cloud of escaping hydrogen absorbs the star's ultraviolet light in the Lyman-alpha line over an area far larger than the planet itself. The first detection, around the hot Jupiter HD 209458 b in 2003, showed hydrogen extending beyond the planet's Roche lobe :cite[vidalmadjar2003]. The warm Neptune GJ 436 b trails a hydrogen cloud large enough to block about half the star's Lyman-alpha light, in a tail that follows the planet like a comet's :cite[ehrenreich2015]. Lyman-alpha light is largely absorbed by interstellar hydrogen, so only nearby stars can be studied this way. In 2018 Oklopčić and Hirata proposed a line of helium at 1,083 nanometres, in the near-infrared, which passes through interstellar space and can be observed from the ground :cite[oklopcic2018]. Helium escaping from the super-Neptune WASP-107 b was detected with it the same year :cite[spake2018]. Because hydrogen escapes more readily than helium, slow escape could leave some warm Neptunes with atmospheres rich in helium :cite[hu2015]; see [[Exotic worlds]]. ## Non-thermal escape Atoms and molecules can also be given escape speed without being heated as a gas. Sunlight and particles from the solar wind ionise the upper atmosphere, and ions can be picked up by the passing wind's magnetic field and swept away (**ion pickup**), or hit neutral atoms hard enough to knock them out (**sputtering**). A fast ion can steal an electron from a slow atom and leave as a fast neutral (**charge exchange**). Molecules broken apart by sunlight can leave fragments with enough energy to escape (**photochemical escape**) :cite[catling2017]. These processes dominate on small, cool worlds. NASA's MAVEN orbiter has measured these processes at Mars, which loses about 2 to 3 kg of gas a second today; extrapolated over the Sun's history, escape removed at least 0.8 bar of carbon dioxide and enough water to cover the planet 23 metres deep :cite[jakosky2018]. See [[Arid world]]. Whether a global magnetic field protects an atmosphere is less clear than often assumed. A field deflects the solar wind, but it also channels energy into the polar regions and opens other escape routes, and Earth, Mars and Venus lose ions at comparable rates today :cite[gunell2018]. ## Who keeps an atmosphere Across the Solar System, the bodies with substantial atmospheres are those whose escape velocity is high for the light they receive. Zahnle and Catling traced a **cosmic shoreline** separating them from airless bodies, in which the insolation needed to strip a body rises roughly as the fourth power of its escape velocity :cite[zahnle2017]. Mercury and the Moon fall on the airless side; [[Titan]], with a lower escape velocity than Mercury's but far from the Sun, keeps a thick one. Venus kept its carbon dioxide but lost its water: the ratio of deuterium to ordinary hydrogen in its atmosphere is about 100 times Earth's, the heavy isotope left behind as hydrogen escaped :cite[donahue1982]. See [[Greenhouse world]]. The shoreline is now being tested on rocky exoplanets, most of them around red dwarfs. The hot rocky planet LHS 3844 b has no thick atmosphere :cite[kreidberg2019], and the James Webb Space Telescope found that TRAPPIST-1 b's dayside is as hot as a bare rock's :cite[greene2023]. See [[Barren rock world]]. :::callout{type=sim title="In Pax Abyssi"} **IN THE GAME.** Every mini-Neptune in the game's generated star systems was made with its escape worked out. The generator estimates the XUV flux at the planet from its star's luminosity, type and age; computes the energy-limited photoevaporation rate, with the tidal correction for the star's pull, and the core-powered rate from the planet's own cooling; keeps the stronger of the two; and checks whether the planet overfills its Roche lobe. From the rate it derives how long the envelope will last and how the loss would look from afar: how far the hydrogen cloud reaches, how long a helium tail trails behind, and whether a comet-like tail and a bow shock form. A hot mini-Neptune that is losing more than a tenth of its mass every billion years, or overflowing its Roche lobe, or hotter than 1,200 K, becomes an **evaporating mini-Neptune** (MNH-EV). A census of the game's generated systems on 7 October 2026 counts 87 of them. Giants stripped down to their cores are **chthonian planets**, and each carries the process that stripped it, chosen from photoevaporation, core-powered mass loss, Roche-lobe overflow and a giant impact; the same census counts 272. See [[Mini-Neptune]] and [[Exotic worlds]]. ::: ## See also - [[Radius valley]] - [[Hot Neptune desert]] - [[Mini-Neptune]] - [[Hot Jupiter]] - [[Exotic worlds]] - [[Barren rock world]] - [[Arid world]] - [[Greenhouse world]] - [[Titan]] - [[Red dwarf]]