--- title: Hot Jupiter summary: A gas giant orbiting so close to its star that its year lasts days and its dayside is hotter than lava. Hot Jupiters were the first planets found around Sun-like stars and remain the best-studied exoplanet atmospheres, though fewer than one Sun-like star in a hundred has one. science_status: [observed, sim] categories: [Gas giants, Planet classes, Giant planets] aliases: [Hot Jupiters, GGH, G2-H, Hot gas giant, Hot gas giants, Ultra-hot Jupiter, Ultra-hot Jupiters, Roaster, Roasters, Pegasean planet, Hot Saturn] infobox: type: planet_class name: Hot Jupiter image: File:Hot_Jupiter_temperature_sequence_sim.png caption: "Sim renders: five hot giants from about 700 K to above 1,900 K, each shown near full phase and as a crescent (the generator's temperature-class looks)" code: GGH legacy_code: G2-H level: Type series: G (gas giant) science_status: "observed; sim (subtypes)" literature_equivalent: "Hot Jupiter (orbital period under about 10 days); ultra-hot Jupiter (dayside above about 2,000 K); Sudarsky classes IV and V" defining_criteria: "Sim: equilibrium temperature above 800 K. Subtypes by dayside temperature: MJ below 950 K, SS 950 to 1,200, FC 1,200 to 1,600, OP 1,600 to 1,700, AS 1,700 to 2,200, UD above 2,200 K (sim names)" mass: {value: "about 0.3 to 13", unit: M_Jup, source: observed, note: "Sim: log-normal about 1.3 M_Jup, clamped 0.31 to 7.9"} radius: {value: "about 0.8 to 2; most above about 1,000 K are inflated", unit: R_Jup, source: observed} bulk_density: {value: "about 0.1 to 2 (inflated planets can be less dense than Saturn)", unit: g/cm³, source: observed} equilibrium_temperature: {value: "about 800 to 4,000; KELT-9 b's dayside about 4,600", unit: K, source: observed} bond_albedo: {value: "mostly below about 0.1; geometric albedos mostly below 0.25 in the Kepler band", unit: dimensionless, source: observed} dominant_gases: "H2, He; H2O, CO, CO2; Na and K atoms; in ultra-hot planets atomic H, H- ions, Fe, Ti and other metals" cloud_and_haze_species: "Sulfides and salts (cooler), silicates and iron (hotter), mostly on the nightside and morning limb; none on ultra-hot daysides" wind_speeds: {value: "superrotating equatorial jets of kilometres per second (models); hot spots shifted east", unit: km/s, source: model} interior_structure: "As for other giants, with extra heat deposited in the interior that inflates the radius" tidal_state: "Tidally locked: one hemisphere always faces the star" typical_orbit: "About 0.015 to 0.1 AU; periods of about 1 to 10 days" rings_moons: "None expected: the star's tides and the small Hill sphere forbid them" frequency_in_sim: "53 of the 8,742 planets in the 5,159 generated systems" real_examples: "51 Pegasi b, HD 209458 b, HD 189733 b, WASP-12 b, WASP-39 b, WASP-76 b, WASP-121 b, KELT-9 b, TrES-2 b, Kepler-7 b" subtypes: "GGH-MJ, GGH-SS, GGH-FC, GGH-OP, GGH-AS, GGH-UD (sim names, by dayside temperature)" sim_source: "Hot gas giant physics engine and properties module; the hot gas giant science set (20 topic files)" rendered_example: "Temperature-class looks HF, HA, HS, HO, HU (sim renders, procedural gas giant generator)" last_verified: "2026-09-27, writer B" sim: entity: planet_type.GGH refs: - id: mayor1995 type: article-journal author: [{family: Mayor, given: Michel}, {family: Queloz, given: Didier}] title: "A Jupiter-mass companion to a solar-type star" container-title: Nature volume: 378 page: 355-359 issued: 1995 DOI: 10.1038/378355a0 - id: dawson2018 type: article-journal author: [{family: Dawson, given: Rebekah I.}, {family: Johnson, given: John Asher}] title: "Origins of Hot Jupiters" container-title: Annual Review of Astronomy and Astrophysics volume: 56 page: 175-221 issued: 2018 DOI: 10.1146/annurev-astro-081817-051853 - id: fressin2013 type: article-journal author: [{family: Fressin, given: François}, {family: Torres, given: Guillermo}, {family: Charbonneau, given: David}, {literal: "et al."}] title: "The False Positive Rate of Kepler and the Occurrence of Planets" container-title: The Astrophysical Journal volume: 766 page: 81 issued: 2013 DOI: 10.1088/0004-637X/766/2/81 - id: wright2012 type: article-journal author: [{family: Wright, given: J. T.}, {family: Marcy, given: G. W.}, {family: Howard, given: A. W.}, {literal: "et al."}] title: "The Frequency of Hot Jupiters Orbiting Nearby Solar-type Stars" container-title: The Astrophysical Journal volume: 753 page: 160 issued: 2012 DOI: 10.1088/0004-637X/753/2/160 - id: fischer2005 type: article-journal author: [{family: Fischer, given: Debra A.}, {family: Valenti, given: Jeff}] title: "The Planet-Metallicity Correlation" container-title: The Astrophysical Journal volume: 622 page: 1102-1117 issued: 2005 DOI: 10.1086/428383 - id: huang2016 type: article-journal author: [{family: Huang, given: Chelsea}, {family: Wu, given: Yanqin}, {family: Triaud, given: Amaury H. M. 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A Radial Velocity Search for Massive, Long-period Companions to Close-in Gas Giant Planets" container-title: The Astrophysical Journal volume: 785 page: 126 issued: 2014 DOI: 10.1088/0004-637X/785/2/126 - id: knutson2007 type: article-journal author: [{family: Knutson, given: Heather A.}, {family: Charbonneau, given: David}, {family: Allen, given: Lori E.}, {literal: "et al."}] title: "A map of the day-night contrast of the extrasolar planet HD 189733b" container-title: Nature volume: 447 page: 183-186 issued: 2007 DOI: 10.1038/nature05782 - id: showman2011 type: article-journal author: [{family: Showman, given: Adam P.}, {family: Polvani, given: Lorenzo M.}] title: "Equatorial Superrotation on Tidally Locked Exoplanets" container-title: The Astrophysical Journal volume: 738 page: 71 issued: 2011 DOI: 10.1088/0004-637X/738/1/71 - id: demory2011 type: article-journal author: [{family: Demory, given: Brice-Olivier}, {family: Seager, given: Sara}] title: "Lack of Inflated Radii for Kepler Giant Planet Candidates Receiving Modest Stellar Irradiation" container-title: The Astrophysical Journal Supplement Series volume: 197 page: 12 issued: 2011 DOI: 10.1088/0067-0049/197/1/12 - id: thorngren2018 type: article-journal author: [{family: Thorngren, given: Daniel P.}, {family: Fortney, given: Jonathan J.}] title: "Bayesian Analysis of Hot-Jupiter Radius Anomalies: Evidence for Ohmic Dissipation?" container-title: The Astronomical Journal volume: 155 page: 214 issued: 2018 DOI: 10.3847/1538-3881/aaba13 - id: charbonneau2000 type: article-journal author: [{family: Charbonneau, given: David}, {family: Brown, given: Timothy M.}, {family: Latham, given: David W.}, {family: Mayor, given: Michel}] title: "Detection of Planetary Transits Across a Sun-like Star" container-title: The Astrophysical Journal volume: 529 page: L45-L48 issued: 2000 DOI: 10.1086/312457 - id: charbonneau2002 type: article-journal author: [{family: Charbonneau, given: David}, {family: Brown, given: Timothy M.}, {family: Noyes, given: Robert W.}, {family: Gilliland, given: Ronald L.}] title: "Detection of an Extrasolar Planet Atmosphere" container-title: The Astrophysical Journal volume: 568 page: 377-384 issued: 2002 DOI: 10.1086/338770 - 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: ers2023 type: article-journal author: [{literal: "JWST Transiting Exoplanet Community Early Release Science Team"}, {family: Ahrer, given: Eva-Maria}, {family: Alderson, given: Lili}, {literal: "et al."}] title: "Identification of carbon dioxide in an exoplanet atmosphere" container-title: Nature volume: 614 page: 649-652 issued: 2023 DOI: 10.1038/s41586-022-05269-w - id: tsai2023 type: article-journal author: [{family: Tsai, given: Shang-Min}, {family: Lee, given: Elspeth K. H.}, {family: Powell, given: Diana}, {literal: "et al."}] title: "Photochemically produced SO2 in the atmosphere of WASP-39b" container-title: Nature volume: 617 page: 483-487 issued: 2023 DOI: 10.1038/s41586-023-05902-2 - id: ehrenreich2020 type: article-journal author: [{family: Ehrenreich, given: David}, {family: Lovis, given: Christophe}, {family: Allart, given: Romain}, {literal: "et al."}] title: "Nightside condensation of iron in an ultrahot giant exoplanet" container-title: Nature volume: 580 page: 597-601 issued: 2020 DOI: 10.1038/s41586-020-2107-1 - id: fortney2008 type: article-journal author: [{family: Fortney, given: J. J.}, {family: Lodders, given: K.}, {family: Marley, given: M. S.}, {family: Freedman, given: R. S.}] title: "A Unified Theory for the Atmospheres of the Hot and Very Hot Jupiters: Two Classes of Irradiated Atmospheres" container-title: The Astrophysical Journal volume: 678 page: 1419-1435 issued: 2008 DOI: 10.1086/528370 - id: evans2017 type: article-journal author: [{family: Evans, given: Thomas M.}, {family: Sing, given: David K.}, {family: Kataria, given: Tiffany}, {literal: "et al."}] title: "An ultrahot gas-giant exoplanet with a stratosphere" container-title: Nature volume: 548 page: 58-61 issued: 2017 DOI: 10.1038/nature23266 - id: parmentier2018 type: article-journal author: [{family: Parmentier, given: Vivien}, {family: Line, given: Mike R.}, {family: Bean, given: Jacob L.}, {literal: "et al."}] title: "From thermal dissociation to condensation in the atmospheres of ultra hot Jupiters: WASP-121b in context" container-title: Astronomy & Astrophysics volume: 617 page: A110 issued: 2018 DOI: 10.1051/0004-6361/201833059 - id: arcangeli2018 type: article-journal author: [{family: Arcangeli, given: Jacob}, {family: Désert, given: Jean-Michel}, {family: Line, given: Michael R.}, {literal: "et al."}] title: "H- Opacity and Water Dissociation in the Dayside Atmosphere of the Very Hot Gas Giant WASP-18b" container-title: The Astrophysical Journal Letters volume: 855 page: L30 issued: 2018 DOI: 10.3847/2041-8213/aab272 - id: gaudi2017 type: article-journal author: [{family: Gaudi, given: B. Scott}, {family: Stassun, given: Keivan G.}, {family: Collins, given: Karen A.}, {literal: "et al."}] title: "A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star host" container-title: Nature volume: 546 page: 514-518 issued: 2017 DOI: 10.1038/nature22392 - id: yee2020 type: article-journal author: [{family: Yee, given: Samuel W.}, {family: Winn, given: Joshua N.}, {family: Knutson, given: Heather A.}, {literal: "et al."}] title: "The Orbit of WASP-12b Is Decaying" container-title: The Astrophysical Journal Letters volume: 888 page: L5 issued: 2020 DOI: 10.3847/2041-8213/ab5c16 - id: kipping2011 type: article-journal author: [{family: Kipping, given: David M.}, {family: Spiegel, given: David S.}] title: "Detection of visible light from the darkest world" container-title: Monthly Notices of the Royal Astronomical Society Letters volume: 417 page: L88-L92 issued: 2011 DOI: 10.1111/j.1745-3933.2011.01127.x - id: esteves2015 type: article-journal author: [{family: Esteves, given: Lisa J.}, {family: De Mooij, given: Ernst J. W.}, {family: Jayawardhana, given: Ray}] title: "Changing Phases of Alien Worlds: Probing Atmospheres of Kepler Planets with High-Precision Photometry" container-title: The Astrophysical Journal volume: 804 page: 150 issued: 2015 DOI: 10.1088/0004-637X/804/2/150 - id: demory2013 type: article-journal author: [{family: Demory, given: Brice-Olivier}, {family: de Wit, given: Julien}, {family: Lewis, given: Nikole}, {literal: "et al."}] title: "Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere" container-title: The Astrophysical Journal Letters volume: 776 page: L25 issued: 2013 DOI: 10.1088/2041-8205/776/2/L25 - id: evans2013 type: article-journal author: [{family: Evans, given: Thomas M.}, {family: Pont, given: Frédéric}, {family: Sing, given: David K.}, {literal: "et al."}] title: "The Deep Blue Color of HD 189733b: Albedo Measurements with Hubble Space Telescope/Space Telescope Imaging Spectrograph at Visible Wavelengths" container-title: The Astrophysical Journal Letters volume: 772 page: L16 issued: 2013 DOI: 10.1088/2041-8205/772/2/L16 - id: sudarsky2000 type: article-journal author: [{family: Sudarsky, given: David}, {family: Burrows, given: Adam}, {family: Pinto, given: Philip}] title: "Albedo and Reflection Spectra of Extrasolar Giant Planets" container-title: The Astrophysical Journal volume: 538 page: 885-903 issued: 2000 DOI: 10.1086/309160 images_wanted: - file: File:Hot_Jupiter_temperature_sequence_sim.png subject: "Five hot giants in a row, cool to hot, each near full phase above and as a crescent below: pinkish-brown, ochre, grey, pale grey and a glowing rust-red ultra-hot giant" source: sim source_ref: "GGQA:ggh_temperature_sequence_final_strip/strip.png" credit: "Pax Abyssi (sim renders, procedural gas giant generator)" note: "2250 x 1000 desk-oracle strip; crop off the white labels or relabel them in the site's type (700 to 1,000 K; 1,000 to 1,300; 1,300 to 1,600; 1,600 to 1,900; above 1,900 K). These are the generator's temperature-class looks, drawable but not yet assigned by the sim's generator; say so in the caption." - file: File:WASP-76b_nightside_ESO.jpg subject: "Artist's impression of the night side of the ultra-hot Jupiter WASP-76 b, where iron vapour condenses" source: eso page_url: https://www.eso.org/public/images/eso2005a/ credit: "ESO/M. Kornmesser/L. Calçada" licence: "CC BY 4.0 (ESO usage terms)" - file: File:HD_189733b_deep_blue_artist_impression.jpg subject: "Artist's impression of the deep blue hot Jupiter HD 189733b beside its star" source: esa-hubble page_url: https://esahubble.org/images/heic1312a/ credit: "NASA, ESA, M. Kornmesser" licence: "CC BY 4.0 (ESA/Hubble usage terms)" --- A **hot Jupiter** is a gas giant that orbits its star in less than about ten days, typically at a twentieth of the Earth-Sun distance or less. At that range the star fills a large part of the planet's sky, the planet's dayside is heated to 1,000 to more than 4,000 K, and tides have locked it with one face permanently towards the star. The first planet found around a Sun-like star, 51 Pegasi b in 1995, was a hot Jupiter, and nobody had expected giants so close :cite[mayor1995]. They are rare, orbiting fewer than one Sun-like star in a hundred, but they are large, hot and frequently transit their stars, so they are the exoplanets whose atmospheres are best known. ## Characteristics ### Locked, lopsided and windy A hot Jupiter's orbit is so tight that the star's tides have long since slowed its spin to match its orbit: its day equals its year, and one hemisphere never sees night. That fixed pattern of heating drives the atmosphere hard. Models predict a broad jet that blows eastward around the equator at kilometres per second, a phenomenon called superrotation :cite[showman2011], and the observations agree. The Spitzer Space Telescope followed HD 189733 b through half an orbit and found its brightest point 16 ± 6 degrees east of the point facing the star, the heat carried downwind by the jet, with infrared brightness temperatures ranging from about 970 K on the cooler side to 1,210 K on the hotter :cite[knutson2007]. ### Puffed-up planets Many hot Jupiters are larger than any model of a cooling giant allows. The extra size appears only above an orbit-averaged stellar flux of about 2 × 10⁸ erg s⁻¹ cm⁻², roughly an equilibrium temperature of 1,000 K :cite[demory2011], so something must be carrying a small share of the starlight deep into the interior. An analysis of 281 giants found that the share needed rises to about 2.5% of the incoming energy near 1,500 K and falls again at higher temperatures, the pattern expected if currents induced in the partly ionised atmosphere dissipate heat inside the planet (Ohmic heating) :cite[thorngren2018]. The most inflated hot Jupiters reach about twice Jupiter's radius with less than Jupiter's mass, lower in density than Saturn. ### Dark worlds Hot Jupiters are, for the most part, very dark. Sodium and potassium atoms and molecules such as water absorb most of the visible and near-infrared light that reaches them, as the [[Sudarsky classification]] predicted for its Class IV planets :cite[sudarsky2000]. TrES-2 b reflects less than a few per cent of the light that falls on it, darker than coal :cite[kipping2011], and most hot Jupiters observed by Kepler have geometric albedos below 0.25 :cite[esteves2015]. Clouds make a few brighter. Kepler-7 b has a geometric albedo of 0.35, with its brightest point on the western, morning side, where cooler air lets reflective clouds condense :cite[demory2013]. HD 189733 b reflects blue light and absorbs red, which would make it look deep blue to the eye :cite[evans2013]. ::figure{src="File:HD_189733b_deep_blue_artist_impression.jpg" size=medium alt="Artist's impression of a deep blue gas giant next to its bright star" caption="Artist's concept: HD 189733 b, whose deep blue colour was measured by the Hubble Space Telescope in 2013. Credit: NASA, ESA, M. Kornmesser."} ### Chemistry from warm to ultra-hot The atmosphere changes with temperature. In the cooler hot Jupiters, below about 1,000 K, methane still holds some of the carbon and clouds of sulfides and salts can form; hotter, carbon monoxide takes over and clouds of silicate rock and iron condense at depth and on the cooler nightside. Above an equilibrium temperature of about 2,000 K, the **ultra-hot Jupiters** have daysides too hot for any cloud: water molecules are broken apart, hydrogen partly dissociates, and the negative hydrogen ion becomes a major source of opacity :cite[arcangeli2018] :cite[parmentier2018]. Gases such as titanium oxide, which absorb strongly in the visible, can heat the upper atmosphere into a temperature inversion, a stratosphere :cite[fortney2008] :cite[evans2017]. On the ultra-hot WASP-76 b, iron vapour from the dayside condenses as it crosses to the night, so iron probably rains out on the planet's night side :cite[ehrenreich2020]. KELT-9 b is the extreme case: its dayside is near 4,600 K, hotter than many stars, and its host star's intense ultraviolet light is thought to be driving gas off the planet :cite[gaudi2017]. ::figure{src="File:WASP-76b_nightside_ESO.jpg" size=medium alt="Artist's impression of a dark planet's night side lit at the edge by its star" caption="Artist's concept: the night side of WASP-76 b, where iron vapour from the dayside is thought to condense and fall as rain. Credit: ESO/M. Kornmesser/L. Calçada."} ### Losing mass and falling in Starlight also heats the uppermost atmosphere enough for gas to escape. Hydrogen streaming away from HD 209458 b was detected in 2003 as an absorption signal far larger than the planet itself :cite[vidalmadjar2003]. The planet's own pull on its star raises tides that slowly drain its orbit. WASP-12 b, on a 1.09-day orbit, is the first planet whose orbit is known to be shrinking: the time between its transits is decreasing by 29 milliseconds a year, and it will spiral into its star in a few million years :cite[yee2020]. ## Formation Nobody thinks hot Jupiters form as giants where they are now, and the question of how they got there is one of the oldest in exoplanet science. Three routes are discussed: formation in place from a large core, migration inward through the gas disc while the planet was young, and **high-eccentricity migration**, in which another body flings the giant onto an elongated orbit that tides raised at each close pass then shrink and circularise. No single route explains everything; disc migration and high-eccentricity migration together probably account for most :cite[dawson2018]. Two clues point to a violent history for many. Hot Jupiters almost never have small planets on nearby orbits, unlike warm Jupiters :cite[huang2016], and about half have a massive companion farther out, between 1 and 20 AU, that could have done the flinging :cite[knutson2014]. ## How common are they? Hot Jupiters orbit about 0.4% of the stars Kepler watched :cite[fressin2013] and about 1.2% of nearby Sun-like stars in radial-velocity surveys :cite[wright2012]; the difference may reflect the metal-richer stars of the solar neighbourhood. Like other giants they are much more common around metal-rich stars: the chance that a star hosts a giant rises roughly with the square of its iron abundance :cite[fischer2005]. ## How we know Hot Jupiters are the easiest planets to detect. Their mass pulls their stars into a fast, large wobble, found by radial velocities :cite[mayor1995], and their short orbits make transits frequent: HD 209458 b was the first planet seen to transit, in 1999 :cite[charbonneau2000]. Transits made hot Jupiters the first planets with measured atmospheres. Sodium in HD 209458 b in 2001 was the first detection of an exoplanet atmosphere :cite[charbonneau2002], and JWST made the first clear detection of carbon dioxide, in WASP-39 b in 2022 :cite[ers2023], along with sulfur dioxide made by photochemistry high in the same planet's atmosphere :cite[tsai2023]. Phase curves, which follow the planet's brightness round its orbit, map its day-night contrast and hot spot. ## Notable examples | Planet | Period | Notes | |---|---|---| | 51 Pegasi b | 4.23 days | First planet found around a Sun-like star, 1995 :cite[mayor1995] | | HD 209458 b | 3.5 days | First transit (1999) and first atmosphere (2001); escaping hydrogen :cite[charbonneau2000] :cite[charbonneau2002] :cite[vidalmadjar2003] | | HD 189733 b | 2.2 days | Hot spot 16 degrees east; deep blue in reflected light :cite[knutson2007] :cite[evans2013] | | WASP-12 b | 1.09 days | Orbit shrinking by 29 ms a year :cite[yee2020] | | WASP-39 b | 4.1 days | First clear carbon dioxide and sulfur dioxide, with JWST :cite[ers2023] :cite[tsai2023] | | WASP-76 b | 1.8 days | Iron condensing on the night side :cite[ehrenreich2020] | | WASP-121 b | 1.3 days | Ultra-hot, with a stratosphere :cite[evans2017] | | KELT-9 b | 1.5 days | Dayside near 4,600 K :cite[gaudi2017] | | TrES-2 b | 2.5 days | Darker than coal :cite[kipping2011] | :::callout{type=sim title="In Pax Abyssi"} The sim's hot gas giants (GGH) are placed by the hot-Jupiter system architecture, which puts one giant at 0.02 to 0.055 AU scaled by the square root of the star's luminosity, and occasionally by the warm-Jupiter architecture. The implemented classifier sorts them by dayside temperature into six subtypes of the sim's own naming (MJ, SS, FC, OP, AS and UD, from below 950 K to above 2,200 K), which do not map onto Sudarsky's classes. The sim's hot giant science set also proposes a temperature-class scheme closer to the literature, HF, HA, HS, HO and HU in 300 K steps from 700 K upward (faint and cool, alkali-warm, standard, opaline and ultra-hot), with tags for thermal inversions, inflation, mass loss and Roche-lobe filling; it is not yet assigned by the generator. There are 53 hot giants among the 8,742 planets of the 5,159 generated systems. The procedural gas giant generator (BUILT, awaiting the owner's verdict) can already draw all five temperature classes (the render at the top of this page) and a tidally locked layout, in which the day side is cleared and hot, the night side clouded and the brightest point carried east of the substellar point by the equatorial jet, following HD 189733 b. A glow map for the planet's own thermal light, strongest at the hot spot, works in the generator's desk tools and is being carried into the game. Giving every generated hot giant a synchronous spin, so that the pattern stays facing its star, is in progress. ::: ## See also - [[Gas giant]] - [[Temperate gas giant]] - [[Sudarsky classification]] - [[Hot Neptune desert]] - [[Atmospheric escape]] - [[Exotic worlds]] - [[Planet occurrence]]