--- title: Hot Jupiter canonical_url: https://paxabyssi.com/wiki/Hot_Jupiter markdown_url: https://paxabyssi.com/wiki/Hot_Jupiter.md type: wiki-page revision_id: 153 revision_view: stable last_updated: 2026-09-27 license: CC BY-SA 4.0 license_url: https://creativecommons.org/licenses/by-sa/4.0/ science_status: - observed - sim 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. 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 code: GGH mass: note: "Sim: log-normal about 1.3 M_Jup, clamped 0.31 to 7.9" unit: M_Jup value: about 0.3 to 13 source: observed name: Hot Jupiter image: File:Hot_Jupiter_temperature_sequence_sim.png level: Type radius: unit: R_Jup value: about 0.8 to 2; most above about 1,000 K are inflated source: observed series: G (gas giant) 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)" 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) bond_albedo: unit: dimensionless value: mostly below about 0.1; geometric albedos mostly below 0.25 in the Kepler band source: observed legacy_code: G2-H rings_moons: "None expected: the star's tides and the small Hill sphere forbid them" tidal_state: "Tidally locked: one hemisphere always faces the star" wind_speeds: unit: km/s value: superrotating equatorial jets of kilometres per second (models); hot spots shifted east source: model bulk_density: unit: g/cm³ value: about 0.1 to 2 (inflated planets can be less dense than Saturn) source: observed last_verified: 2026-09-27, writer B 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 typical_orbit: About 0.015 to 0.1 AU; periods of about 1 to 10 days dominant_gases: H2, He; H2O, CO, CO2; Na and K atoms; in ultra-hot planets atomic H, H- ions, Fe, Ti and other metals science_status: observed; sim (subtypes) frequency_in_sim: 53 of the 8,742 planets in the 5,159 generated systems rendered_example: Temperature-class looks HF, HA, HS, HO, HU (sim renders, procedural gas giant generator) 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)" interior_structure: As for other giants, with extra heat deposited in the interior that inflates the radius literature_equivalent: Hot Jupiter (orbital period under about 10 days); ultra-hot Jupiter (dayside above about 2,000 K); Sudarsky classes IV and V cloud_and_haze_species: Sulfides and salts (cooler), silicates and iron (hotter), mostly on the nightside and morning limb; none on ultra-hot daysides equilibrium_temperature: unit: K value: about 800 to 4,000; KELT-9 b's dayside about 4,600 source: observed related: - https://paxabyssi.com/wiki/Gas_giant.md - https://paxabyssi.com/wiki/Temperate_gas_giant.md - https://paxabyssi.com/wiki/Cold_gas_giant.md - https://paxabyssi.com/wiki/Sudarsky_classification.md - https://paxabyssi.com/wiki/Ice_giant.md - https://paxabyssi.com/wiki/Exotic_worlds.md --- # Hot Jupiter > Source: https://paxabyssi.com/wiki/Hot_Jupiter > > Licence: [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/). Text by Pax Abyssi Wiki contributors; history at https://paxabyssi.com/wiki/Hot_Jupiter/history > > Revision 153, 27 September 2026 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 [1]. 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 [2], 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 [3]. ### 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 [4], 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) [5]. 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](https://paxabyssi.com/wiki/Sudarsky_classification.md) predicted for its Class IV planets [6]. TrES-2 b reflects less than a few per cent of the light that falls on it, darker than coal [7], and most hot Jupiters observed by Kepler have geometric albedos below 0.25 [8]. 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 [9]. HD 189733 b reflects blue light and absorbs red, which would make it look deep blue to the eye [10]. ![Artist's impression of a deep blue gas giant next to its bright star](https://media.paxabyssi.com/public/dcd754adba5a52f207c5d6f8d84612e598f28168fed2880e39466ddfe7ba291c/2560.webp "Artist's concept: HD 189733 b, whose deep blue colour was measured by the Hubble Space Telescope in 2013. Credit: NASA, ESA, M. Kornmesser.") *Figure 1.* Artist's concept: HD 189733 b, whose deep blue colour was measured by the Hubble Space Telescope in 2013. Credit: NASA, ESA, M. Kornmesser. Licence: CC BY 4.0. ### 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 [11] [12]. Gases such as titanium oxide, which absorb strongly in the visible, can heat the upper atmosphere into a temperature inversion, a stratosphere [13] [14]. 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 [15]. 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 [16]. ![Artist's impression of a dark planet's night side lit at the edge by its star](https://media.paxabyssi.com/public/e5887fd736300a2f576cd57a7a08f737ddd2b9770d5bc5c8a536bddf1f1b3331/2560.webp "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.") *Figure 2.* 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. Licence: CC BY 4.0. ### 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 [17]. 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 [18]. ## 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 [19]. Two clues point to a violent history for many. Hot Jupiters almost never have small planets on nearby orbits, unlike warm Jupiters [20], and about half have a massive companion farther out, between 1 and 20 AU, that could have done the flinging [21]. ## How common are they? Hot Jupiters orbit about 0.4% of the stars Kepler watched [22] and about 1.2% of nearby Sun-like stars in radial-velocity surveys [23]; 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 [24]. ## 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 [1], and their short orbits make transits frequent: HD 209458 b was the first planet seen to transit, in 1999 [25]. 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 [26], and JWST made the first clear detection of carbon dioxide, in WASP-39 b in 2022 [27], along with sulfur dioxide made by photochemistry high in the same planet's atmosphere [28]. 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 [1] | | HD 209458 b | 3.5 days | First transit (1999) and first atmosphere (2001); escaping hydrogen [25] [26] [17] | | HD 189733 b | 2.2 days | Hot spot 16 degrees east; deep blue in reflected light [3] [10] | | WASP-12 b | 1.09 days | Orbit shrinking by 29 ms a year [18] | | WASP-39 b | 4.1 days | First clear carbon dioxide and sulfur dioxide, with JWST [27] [28] | | WASP-76 b | 1.8 days | Iron condensing on the night side [15] | | WASP-121 b | 1.3 days | Ultra-hot, with a stratosphere [14] | | KELT-9 b | 1.5 days | Dayside near 4,600 K [16] | | TrES-2 b | 2.5 days | Darker than coal [7] | > **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](https://paxabyssi.com/wiki/Gas_giant.md) - [Temperate gas giant](https://paxabyssi.com/wiki/Temperate_gas_giant.md) - [Sudarsky classification](https://paxabyssi.com/wiki/Sudarsky_classification.md) - [Hot Neptune desert](https://paxabyssi.com/wiki/Hot_Neptune_desert.md) - [Atmospheric escape](https://paxabyssi.com/wiki/Atmospheric_escape.md) - [Exotic worlds](https://paxabyssi.com/wiki/Exotic_worlds.md) - [Planet occurrence](https://paxabyssi.com/wiki/Planet_occurrence.md) ## References 1. Mayor, M. and Queloz, D. (1995). A Jupiter-mass companion to a solar-type star. Nature 378, 355-359. 2. Showman, A. P. and Polvani, L. M. (2011). Equatorial Superrotation on Tidally Locked Exoplanets. The Astrophysical Journal 738, 71. 3. Knutson, H. A. et al. (2007). A map of the day-night contrast of the extrasolar planet HD 189733b. Nature 447, 183-186. 4. Demory, B. O. and Seager, S. (2011). Lack of Inflated Radii for Kepler Giant Planet Candidates Receiving Modest Stellar Irradiation. The Astrophysical Journal Supplement Series 197, 12. 5. Thorngren, D. P. and Fortney, J. J. (2018). Bayesian Analysis of Hot-Jupiter Radius Anomalies: Evidence for Ohmic Dissipation?. The Astronomical Journal 155, 214. 6. Sudarsky, D., Burrows, A. and Pinto, P. (2000). Albedo and Reflection Spectra of Extrasolar Giant Planets. The Astrophysical Journal 538, 885-903. 7. Kipping, D. M. and Spiegel, D. S. (2011). Detection of visible light from the darkest world. Monthly Notices of the Royal Astronomical Society Letters 417, L88-L92. 8. Esteves, L. J., De Mooij, E. J. W. and Jayawardhana, R. (2015). Changing Phases of Alien Worlds: Probing Atmospheres of Kepler Planets with High-Precision Photometry. The Astrophysical Journal 804, 150. 9. Demory, B. O. et al. (2013). Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere. The Astrophysical Journal Letters 776, L25. 10. Evans, T. M. et al. (2013). The Deep Blue Color of HD 189733b: Albedo Measurements with Hubble Space Telescope/Space Telescope Imaging Spectrograph at Visible Wavelengths. The Astrophysical Journal Letters 772, L16. 11. Arcangeli, J. et al. (2018). H- Opacity and Water Dissociation in the Dayside Atmosphere of the Very Hot Gas Giant WASP-18b. The Astrophysical Journal Letters 855, L30. 12. Parmentier, V. et al. (2018). From thermal dissociation to condensation in the atmospheres of ultra hot Jupiters: WASP-121b in context. Astronomy & Astrophysics 617, A110. 13. Fortney, J. J. et al. (2008). A Unified Theory for the Atmospheres of the Hot and Very Hot Jupiters: Two Classes of Irradiated Atmospheres. The Astrophysical Journal 678, 1419-1435. 14. Evans, T. M. et al. (2017). An ultrahot gas-giant exoplanet with a stratosphere. Nature 548, 58-61. 15. Ehrenreich, D. et al. (2020). Nightside condensation of iron in an ultrahot giant exoplanet. Nature 580, 597-601. 16. Gaudi, B. S. et al. (2017). A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star host. Nature 546, 514-518. 17. Vidal-Madjar, A. et al. (2003). An extended upper atmosphere around the extrasolar planet HD209458b. Nature 422, 143-146. 18. Yee, S. W. et al. (2020). The Orbit of WASP-12b Is Decaying. The Astrophysical Journal Letters 888, L5. 19. Dawson, R. I. and Johnson, J. A. (2018). Origins of Hot Jupiters. Annual Review of Astronomy and Astrophysics 56, 175-221. 20. Huang, C., Wu, Y. and Triaud, A. H. M. J. (2016). Warm Jupiters Are Less Lonely than Hot Jupiters: Close Neighbors. The Astrophysical Journal 825, 98. 21. Knutson, H. A. et al. (2014). Friends of Hot Jupiters. I. A Radial Velocity Search for Massive, Long-period Companions to Close-in Gas Giant Planets. The Astrophysical Journal 785, 126. 22. Fressin, F. et al. (2013). The False Positive Rate of Kepler and the Occurrence of Planets. The Astrophysical Journal 766, 81. 23. Wright, J. T. et al. (2012). The Frequency of Hot Jupiters Orbiting Nearby Solar-type Stars. The Astrophysical Journal 753, 160. 24. Fischer, D. A. and Valenti, J. (2005). The Planet-Metallicity Correlation. The Astrophysical Journal 622, 1102-1117. 25. Charbonneau, D. et al. (2000). Detection of Planetary Transits Across a Sun-like Star. The Astrophysical Journal 529, L45-L48. 26. Charbonneau, D. et al. (2002). Detection of an Extrasolar Planet Atmosphere. The Astrophysical Journal 568, 377-384. 27. JWST Transiting Exoplanet Community Early Release Science Team et al. (2023). Identification of carbon dioxide in an exoplanet atmosphere. Nature 614, 649-652. 28. Tsai, S. M. et al. (2023). Photochemically produced SO2 in the atmosphere of WASP-39b. Nature 617, 483-487. ## Infobox (planet class) | Field | Value | | --- | --- | | Code | GGH | | Mass | about 0.3 to 13 M_Jup | | Name | Hot Jupiter | | Image | File:Hot_Jupiter_temperature_sequence_sim.png | | Level | Type | | Radius | about 0.8 to 2; most above about 1,000 K are inflated R_Jup | | Series | G (gas giant) | | 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) | | 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) | | Bond albedo | mostly below about 0.1; geometric albedos mostly below 0.25 in the Kepler band dimensionless | | Legacy code | G2-H | | Rings moons | None expected: the star's tides and the small Hill sphere forbid them | | Tidal state | Tidally locked: one hemisphere always faces the star | | Wind speeds | superrotating equatorial jets of kilometres per second (models); hot spots shifted east km/s | | Bulk density | about 0.1 to 2 (inflated planets can be less dense than Saturn) g/cm³ | | Last verified | 2026-09-27, writer B | | 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 | | Typical orbit | About 0.015 to 0.1 AU; periods of about 1 to 10 days | | Dominant gases | H2, He; H2O, CO, CO2; Na and K atoms; in ultra-hot planets atomic H, H- ions, Fe, Ti and other metals | | Science status | observed; sim (subtypes) | | Frequency in sim | 53 of the 8,742 planets in the 5,159 generated systems | | Rendered example | Temperature-class looks HF, HA, HS, HO, HU (sim renders, procedural gas giant generator) | | 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) | | Interior structure | As for other giants, with extra heat deposited in the interior that inflates the radius | | Literature equivalent | Hot Jupiter (orbital period under about 10 days); ultra-hot Jupiter (dayside above about 2,000 K); Sudarsky classes IV and V | | Cloud and haze species | Sulfides and salts (cooler), silicates and iron (hotter), mostly on the nightside and morning limb; none on ultra-hot daysides | | Equilibrium temperature | about 800 to 4,000; KELT-9 b's dayside about 4,600 K | ## Related pages - [Gas giant](https://paxabyssi.com/wiki/Gas_giant.md): A planet made mostly of hydrogen and helium, with no solid surface, from about a tenth of Jupiter's mass up to the brown dwarf boundary near 13 Jupiter masses. Jupiter and Saturn are the Solar System's two; thousands more are known around other stars. - [Temperate gas giant](https://paxabyssi.com/wiki/Temperate_gas_giant.md): A gas giant warmer than Jupiter but cooler than a hot Jupiter, roughly 150 to 800 K, where ammonia clouds give way to water clouds and then to clear air. It spans the widest range of looks of any giant, from brilliant white to dark blue. - [Cold gas giant](https://paxabyssi.com/wiki/Cold_gas_giant.md): A gas giant far enough from its star that ammonia freezes into clouds high in its atmosphere, like Jupiter and Saturn. Its own internal heat rivals the sunlight it absorbs, and it can keep large families of moons and rings. - [Sudarsky classification](https://paxabyssi.com/wiki/Sudarsky_classification.md): A theoretical scheme from 2000 that sorts giant planets into five classes by the temperature of their upper atmospheres, which decides what clouds can form and so how much light, and what colour, the planet reflects. - [Ice giant](https://paxabyssi.com/wiki/Ice_giant.md): A giant planet made mostly of water, methane and ammonia rather than hydrogen and helium, like Uranus and Neptune. Their interiors are hot, dense fluids, their magnetic fields are lopsided, and planets of about their mass may be among the commonest in the Galaxy. - [Exotic worlds](https://paxabyssi.com/wiki/Exotic_worlds.md): Four kinds of planet that physics allows but the Solar System lacks. Iron-rich super-Mercuries are observed; carbon planets, helium-atmosphere planets and chthonian planets (the stripped cores of giants) remain hypotheses with candidates but no confirmed member. Categories: [Gas giants](https://paxabyssi.com/wiki/Category:Gas_giants.md), [Planet classes](https://paxabyssi.com/wiki/Category:Planet_classes.md), [Giant planets](https://paxabyssi.com/wiki/Category:Giant_planets.md)