--- title: Gas giant canonical_url: https://paxabyssi.com/wiki/Gas_giant markdown_url: https://paxabyssi.com/wiki/Gas_giant.md type: wiki-page revision_id: 136 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 - model - sim summary: 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. categories: - Gas giants - Planet classes - Giant planets aliases: - Gas giants - Jovian planet - Jovian planets - Giant planet - Gas planet - Jupiter-like planet - Gas giant (G series) - G series infobox: type: planet_class code: GGC, GGT, GGH mass: note: "about 30 to 4,100 Earth masses; the upper limit is the deuterium-burning convention. Sim: 0.16 to 11 M_Jup (cold, temperate), 0.31 to 7.9 (hot)" unit: M_Jup value: about 0.1 to 13 source: observed name: Gas giant image: File:Gas_giant_GGC-AB-TU_sim.png level: Series (three sim types) radius: unit: R_Jup value: about 0.8 to 1.2 (cool); up to about 2 (inflated hot Jupiters) source: observed series: G (gas giant) caption: "Sim render: a Jupiter-class cold gas giant grown by the sim's procedural generator (GGC-AB-TU-1)" subtypes: "[[Cold gas giant]] (GGC), [[Temperate gas giant]] (GGT), [[Hot Jupiter]] (GGH)" sim_source: Cold, temperate and hot gas giant physics engines and properties modules; the giant planet science references bond_albedo: unit: dimensionless value: Jupiter 0.503; most hot Jupiters below about 0.1 source: observed legacy_code: G1-C, G3-T, G2-H rings_moons: Cold giants hold large moon systems and often rings; hot Jupiters hold neither bulk_density: unit: g/cm³ value: about 0.2 to 2; Saturn 0.687, Jupiter 1.326 source: observed last_verified: 2026-09-27, writer B real_examples: Jupiter, Saturn; 51 Pegasi b, HD 209458 b, KELT-9 b, epsilon Indi Ab, HR 8799 b to e typical_orbit: Most common at 1 to 10 AU; hot Jupiters at a few days' period dominant_gases: H2 about 90%, He about 10% by volume (Jupiter's upper atmosphere); CH4, NH3, H2O and H2S as trace gases science_status: observed escape_velocity: unit: km/s value: Jupiter 59.5, Saturn 35.5 source: observed frequency_in_sim: 1,213 of the 8,742 planets in the 5,159 generated systems (900 cold, 260 temperate, 53 hot) gravity_at_1_bar: unit: m/s² value: Jupiter 23.1, Saturn 9.0 (equatorial) source: observed rendered_example: GGC-AB-TU-1 (sim render, procedural gas giant generator) defining_criteria: "Mostly hydrogen and helium by mass. Sim: three types by equilibrium temperature, cold (below 150 K), temperate (150 to 800 K) and hot (above 800 K)" interior_structure: Molecular hydrogen envelope over metallic hydrogen; heavy elements concentrated toward the centre in a dilute core temperature_at_1_bar: unit: K value: Jupiter 165, Saturn 134 source: observed literature_equivalent: Gas giant, Jovian planet; the hottest are hot Jupiters cloud_and_haze_species: NH3, NH4SH and H2O in cold giants; none in the clear temperate range; silicates, iron and other refractory condensates in hot Jupiters equilibrium_temperature: unit: K value: below 100 to about 4,000 source: observed related: - https://paxabyssi.com/wiki/Cold_gas_giant.md - https://paxabyssi.com/wiki/Hot_Jupiter.md - https://paxabyssi.com/wiki/Temperate_gas_giant.md - https://paxabyssi.com/wiki/Ice_giant.md - https://paxabyssi.com/wiki/Sudarsky_classification.md - https://paxabyssi.com/wiki/Planet_classification.md --- # Gas giant > Source: https://paxabyssi.com/wiki/Gas_giant > > 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/Gas_giant/history > > Revision 136, 27 September 2026 A **gas giant** is a planet made mostly of hydrogen and helium, the two lightest elements, with no solid surface to stand on. Jupiter and Saturn are the Solar System's gas giants; Uranus and Neptune, which are mostly heavier material, are classed separately as [ice giants](https://paxabyssi.com/wiki/Ice_giant.md). Gas giants range from about a tenth of Jupiter's mass up to about 13 Jupiter masses, where objects become massive enough to fuse deuterium and are called [brown dwarfs](https://paxabyssi.com/wiki/Brown_dwarf.md). They were the first planets found around Sun-like stars, and they shape the systems they live in: their gravity steers asteroids and comets, and their migration can clear or scatter everything nearby. ## What makes a gas giant A gas giant has roughly the composition of the gas its star formed from. Jupiter's upper atmosphere is about 90% molecular hydrogen and 10% helium by volume, with methane, ammonia, water and hydrogen sulfide as trace gases [1]. The heavier elements are enriched relative to the Sun: the Galileo probe, which fell into Jupiter in 1995, measured carbon, nitrogen and sulfur at about three times the solar proportion [2]. That enrichment is a clue to how the planet formed, since it means Jupiter swallowed solids as well as gas. The name is slightly misleading. Only the outer few per cent of a gas giant's radius behaves like a gas. Deeper, the pressure squeezes hydrogen into a dense fluid, and beyond about a million times the pressure at Earth's surface the hydrogen's electrons break free and it conducts electricity like a liquid metal. Currents in this metallic hydrogen generate the planet's magnetic field [3]. Gas giants are also oddly uniform in size. Adding mass to a giant squeezes its interior harder, and above about half a Jupiter mass the extra compression nearly cancels the extra material. An old, cool giant of 0.5 Jupiter masses and one of 10 both have radii close to Jupiter's own, and the curve stays nearly flat into the brown dwarf range [4] [5]. Saturn, at 0.30 Jupiter masses, is on the rising part of the curve at 0.84 Jupiter radii, and its mean density of 0.687 g/cm³ is lower than that of water [6]. The main exceptions are the [hot Jupiters](https://paxabyssi.com/wiki/Hot_Jupiter.md), many of which are inflated well beyond Jupiter's size by their stars' heat. ## Inside a gas giant NASA's Juno spacecraft, orbiting Jupiter since 2016, has mapped the planet's gravity field closely enough to test models of its interior. The result was unexpected: Jupiter's heavy elements are not packed into a small, dense core but spread through a large "dilute core" that blends into the hydrogen envelope around it [7] [8]. Saturn has a similar diffuse core, revealed by the way oscillations inside the planet disturb the waves in its rings [9]. Helium is separating out of both planets. At the pressures and temperatures deep in a cooling giant, helium stops mixing with metallic hydrogen and condenses into droplets that sink, a process called helium rain. The Galileo probe found Jupiter's upper atmosphere depleted in helium, with a mass fraction of 0.234 against about 0.27 in the gas the Sun formed from [10]. Neon, which dissolves into the helium droplets and is carried down with them, is depleted about tenfold, the signature helium rain predicts [11]. Saturn, smaller and colder, has taken the process further [12]. (Image pending: Cutaway of Jupiter and Saturn showing cloud decks, a molecular hydrogen envelope, metallic hydrogen, falling helium droplets and a dilute core that blends outward) *Figure 1.* Diagram: inside Jupiter and Saturn, as Juno's gravity data and Saturn's ring seismology now picture them. Gas giants also shine by their own heat. Jupiter radiates 7.5 watts per square metre from its interior on top of the sunlight it re-emits, so it gives off slightly more than twice the energy it absorbs from the Sun [13]. The heat is left over from the planet's formation and is released as the planet slowly cools and contracts; helium rain adds to it. Internal heat drives the convection that stirs the atmosphere and powers the magnetic field. ## Clouds, colours and temperature Rapid rotation (Jupiter turns in 9.93 hours [1]) organises a gas giant's weather into bands of alternating east and west winds: the familiar belts and zones, with long-lived storms like the Great Red Spot riding between them. Which clouds form, and so what colour the bands are, depends on temperature. A cold giant like Jupiter has ammonia ice clouds on top; a warmer one would have water clouds; warmer still, no clouds form high in the atmosphere at all; and the hottest have clouds of rock and metal. The [Sudarsky classification](https://paxabyssi.com/wiki/Sudarsky_classification.md) turns that sequence into five classes. Astronomers usually sort giant exoplanets by orbital period or equilibrium temperature. **Hot Jupiters** have periods of a few days; **warm Jupiters** orbit in roughly 10 to 200 days; **cold Jupiters** orbit beyond about 1 AU, the region where giants are most common. The sim uses equilibrium temperature for the same split, with three types: [cold](https://paxabyssi.com/wiki/Cold_gas_giant.md) (below 150 K), [temperate](https://paxabyssi.com/wiki/Temperate_gas_giant.md) (150 to 800 K) and [hot](https://paxabyssi.com/wiki/Hot_Jupiter.md) (above 800 K). ![Jupiter in true colour, cream zones and brown belts, the Great Red Spot in the southern hemisphere](https://media.paxabyssi.com/public/bcd8549b7bbcc0041309d0a35673a2329ee78dcecc4f06bfc86488434132f3cd/1920.webp "Observation: Jupiter in true colour, assembled from Cassini images taken on 29 December 2000. Credit: NASA/JPL/Space Science Institute.") *Figure 2.* Observation: Jupiter in true colour, assembled from Cassini images taken on 29 December 2000. Credit: NASA/JPL/Space Science Institute. Licence: Public domain (NASA). ## Formation Two ideas compete to explain how gas giants form. In **core accretion**, the favoured model for most giants, a core of rock and ice grows in the disc around a young star until, at about ten Earth masses, its gravity can pull in the surrounding gas faster and faster. The planet must finish before the disc disperses, within a few million years, which favours the region just beyond the snow line, where ice adds to the solid material available [14]. In **disc instability**, a massive, cold disc fragments directly into giant planets under its own gravity, far faster than a core can grow [15]; this route may explain some massive giants on very wide orbits. Giants need not stay where they form. Interactions with the gas disc, with other planets or with a distant companion star can move them inward, and a giant pushed close to its star becomes a hot Jupiter [16]. Core accretion predicts that stars richer in heavy elements should form giants more easily, and they do. The chance that a Sun-like star has a giant planet rises roughly with the square of its iron abundance, so a star with twice the Sun's iron is about four times as likely to host one [17]. ## How common are they? Radial-velocity surveys, which detect the wobble a planet induces in its star, find about 14 giant planets per 100 Sun-like stars with orbits of 2 to 8 AU, and about 9 per 100 at 8 to 32 AU. Giants are about four times more common beyond 1 AU than inside it [18]. Hot Jupiters are rare: they orbit about 0.4% of the stars Kepler watched [19] and about 1.2% of nearby Sun-like stars in radial-velocity surveys [20]. They were found first because they are the easiest planets to detect. ## How we know - **Spacecraft** have visited all four giants of the Solar System. The Galileo probe sampled Jupiter's atmosphere directly in 1995; Cassini measured Saturn's gravity, rings and heat balance and refined Jupiter's [13]; Juno is mapping Jupiter's gravity and magnetic field. - **Radial velocities** found the first gas giant around a Sun-like star, 51 Pegasi b, in 1995 [21]. - **Transits** give radii and, combined with radial velocities, densities; HD 209458 b in 1999 was the first planet seen to transit [22]. - **Spectroscopy** during transits and eclipses reads the atmospheres. JWST made the first clear detection of carbon dioxide in an exoplanet, the Saturn-mass WASP-39 b, in 2022 [23]. - **Direct imaging** separates the light of young or wide-orbit giants from their stars' glare [24] [25]. ## Notable examples | Planet | Mass | Radius | Orbit | Notes | | ------------------ | ------------------------------ | --------------- | ------------------- | --------------------------------------------------------------------------------------------- | | Jupiter | 1 M_Jup (317.8 Earth masses) | 71,492 km | 5.20 AU, 11.9 years | Largest planet in the Solar System [1] | | Saturn | 0.30 M_Jup (95.2 Earth masses) | 60,268 km | 9.6 AU, 29.4 years | Mean density below water's [6] | | 51 Pegasi b | at least about 0.5 M_Jup | unknown | 4.23 days | First gas giant found around a Sun-like star [21] | | HD 209458 b | 0.69 M_Jup | 1.27 R_Jup | 3.5 days | First planet seen to transit [26] [22] | | KELT-9 b | about 2.9 M_Jup | about 1.9 R_Jup | 1.5 days | Dayside near 4,600 K, one of the hottest planets known [27] | | epsilon Indi Ab | about 6 M_Jup | | about 200 years | Cold giant 12 light years away, imaged by JWST; effective temperature about 275 K [25] [28] | | HR 8799 b, c, d, e | several M_Jup each | | about 15 to 70 AU | Four young giants imaged around one star [24] [29] | > **In Pax Abyssi** > > Gas giants are the sim's G series: cold (GGC), temperate (GGT) and hot (GGH), assigned by equilibrium temperature when a planetary system's architecture places a giant in an orbital slot. Real exoplanets from the catalogue are typed by mass first: 50 to 4,000 Earth masses makes a gas giant, split hot, temperate or cold by temperature. Each type has its own physics engine for atmosphere, interior heat, magnetosphere, rings and moons. Cold and temperate giants are drawn from a log-normal mass distribution centred near one Jupiter mass. Of the 8,742 planets in the 5,159 generated systems of the naked-eye sky, 1,213 are gas giants: 900 cold, 260 temperate and 53 hot. > > The game draws its giants with its own procedural generator (BUILT, awaiting the owner's verdict), which grows each planet's cloud bands by tracing a simulated wind field of zonal jets, eddies and vortices backwards in time, so belts scallop and storms spiral for physical reasons, and lights them with limb darkening under a thin haze. Every giant in the generated systems can wear it with its own seed, and the render at the top of this page is one of its looks. ## See also - [Cold gas giant](https://paxabyssi.com/wiki/Cold_gas_giant.md) - [Temperate gas giant](https://paxabyssi.com/wiki/Temperate_gas_giant.md) - [Hot Jupiter](https://paxabyssi.com/wiki/Hot_Jupiter.md) - [Sudarsky classification](https://paxabyssi.com/wiki/Sudarsky_classification.md) - [Ice giant](https://paxabyssi.com/wiki/Ice_giant.md) - [Brown dwarf](https://paxabyssi.com/wiki/Brown_dwarf.md) - [Planet classification](https://paxabyssi.com/wiki/Planet_classification.md) - [Sol](https://paxabyssi.com/wiki/Sol.md) ## References 1. NASA Space Science Data Coordinated Archive. Jupiter Fact Sheet. 2. Atreya, S. K. et al. (1999). A comparison of the atmospheres of Jupiter and Saturn: deep atmospheric composition, cloud structure, vertical mixing, and origin. Planetary and Space Science 47, 1243-1262. 3. Stevenson, D. J. (2020). Jupiter's Interior as Revealed by Juno. Annual Review of Earth and Planetary Sciences 48, 465-489. 4. Fortney, J. J., Marley, M. S. and Barnes, J. W. (2007). Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits. The Astrophysical Journal 659, 1661-1672. 5. Chen, J. and Kipping, D. (2017). Probabilistic Forecasting of the Masses and Radii of Other Worlds. The Astrophysical Journal 834, 17. 6. NASA Space Science Data Coordinated Archive. Saturn Fact Sheet. 7. Wahl, S. M. et al. (2017). Comparing Jupiter interior structure models to Juno gravity measurements and the role of a dilute core. Geophysical Research Letters 44, 4649-4659. 8. Militzer, B. et al. (2022). Juno Spacecraft Measurements of Jupiter's Gravity Imply a Dilute Core. The Planetary Science Journal 3, 185. 9. Mankovich, C. R. and Fuller, J. (2021). A diffuse core in Saturn revealed by ring seismology. Nature Astronomy 5, 1103-1109. 10. von Zahn, U., Hunten, D. M. and Lehmacher, G. (1998). Helium in Jupiter's atmosphere: Results from the Galileo probe Helium Interferometer Experiment. Journal of Geophysical Research: Planets 103, 22815-22829. 11. Wilson, H. F. and Militzer, B. (2010). Sequestration of Noble Gases in Giant Planet Interiors. Physical Review Letters 104, 121101. 12. Mankovich, C. R. and Fortney, J. J. (2020). Evidence for a Dichotomy in the Interior Structures of Jupiter and Saturn from Helium Phase Separation. The Astrophysical Journal 889, 51. 13. Li, L. et al. (2018). Less absorbed solar energy and more internal heat for Jupiter. Nature Communications 9, 3709. 14. Pollack, J. B. et al. (1996). Formation of the Giant Planets by Concurrent Accretion of Solids and Gas. Icarus 124, 62-85. 15. Boss, A. P. (1997). Giant Planet Formation by Gravitational Instability. Science 276, 1836-1839. 16. Dawson, R. I. and Johnson, J. A. (2018). Origins of Hot Jupiters. Annual Review of Astronomy and Astrophysics 56, 175-221. 17. Fischer, D. A. and Valenti, J. (2005). The Planet-Metallicity Correlation. The Astrophysical Journal 622, 1102-1117. 18. Fulton, B. J. et al. (2021). California Legacy Survey. II. Occurrence of Giant Planets beyond the Ice Line. The Astrophysical Journal Supplement Series 255, 14. 19. Fressin, F. et al. (2013). The False Positive Rate of Kepler and the Occurrence of Planets. The Astrophysical Journal 766, 81. 20. Wright, J. T. et al. (2012). The Frequency of Hot Jupiters Orbiting Nearby Solar-type Stars. The Astrophysical Journal 753, 160. 21. Mayor, M. and Queloz, D. (1995). A Jupiter-mass companion to a solar-type star. Nature 378, 355-359. 22. Charbonneau, D. et al. (2000). Detection of Planetary Transits Across a Sun-like Star. The Astrophysical Journal 529, L45-L48. 23. JWST Transiting Exoplanet Community Early Release Science Team et al. (2023). Identification of carbon dioxide in an exoplanet atmosphere. Nature 614, 649-652. 24. Marois, C. et al. (2008). Direct Imaging of Multiple Planets Orbiting the Star HR 8799. Science 322, 1348-1352. 25. Matthews, E. C. et al. (2024). A temperate super-Jupiter imaged with JWST in the mid-infrared. Nature 633, 789-792. 26. Mazeh, T. et al. (2000). The Spectroscopic Orbit of the Planetary Companion Transiting HD 209458. The Astrophysical Journal 532, L55-L58. 27. Gaudi, B. S. et al. (2017). A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star host. Nature 546, 514-518. 28. Max Planck Society (2024). Webb images nearest super-Jupiter, opening a new window to exoplanet research. 29. Marois, C. et al. (2010). Images of a fourth planet orbiting HR 8799. Nature 468, 1080-1083. ## Infobox (planet class) | Field | Value | | --- | --- | | Code | GGC, GGT, GGH | | Mass | about 0.1 to 13 M_Jup | | Name | Gas giant | | Image | File:Gas_giant_GGC-AB-TU_sim.png | | Level | Series (three sim types) | | Radius | about 0.8 to 1.2 (cool); up to about 2 (inflated hot Jupiters) R_Jup | | Series | G (gas giant) | | Caption | Sim render: a Jupiter-class cold gas giant grown by the sim's procedural generator (GGC-AB-TU-1) | | Subtypes | [[Cold gas giant]] (GGC), [[Temperate gas giant]] (GGT), [[Hot Jupiter]] (GGH) | | Sim source | Cold, temperate and hot gas giant physics engines and properties modules; the giant planet science references | | Bond albedo | Jupiter 0.503; most hot Jupiters below about 0.1 dimensionless | | Legacy code | G1-C, G3-T, G2-H | | Rings moons | Cold giants hold large moon systems and often rings; hot Jupiters hold neither | | Bulk density | about 0.2 to 2; Saturn 0.687, Jupiter 1.326 g/cm³ | | Last verified | 2026-09-27, writer B | | Real examples | Jupiter, Saturn; 51 Pegasi b, HD 209458 b, KELT-9 b, epsilon Indi Ab, HR 8799 b to e | | Typical orbit | Most common at 1 to 10 AU; hot Jupiters at a few days' period | | Dominant gases | H2 about 90%, He about 10% by volume (Jupiter's upper atmosphere); CH4, NH3, H2O and H2S as trace gases | | Science status | observed | | Escape velocity | Jupiter 59.5, Saturn 35.5 km/s | | Frequency in sim | 1,213 of the 8,742 planets in the 5,159 generated systems (900 cold, 260 temperate, 53 hot) | | Gravity at 1 bar | Jupiter 23.1, Saturn 9.0 (equatorial) m/s² | | Rendered example | GGC-AB-TU-1 (sim render, procedural gas giant generator) | | Defining criteria | Mostly hydrogen and helium by mass. Sim: three types by equilibrium temperature, cold (below 150 K), temperate (150 to 800 K) and hot (above 800 K) | | Interior structure | Molecular hydrogen envelope over metallic hydrogen; heavy elements concentrated toward the centre in a dilute core | | Temperature at 1 bar | Jupiter 165, Saturn 134 K | | Literature equivalent | Gas giant, Jovian planet; the hottest are hot Jupiters | | Cloud and haze species | NH3, NH4SH and H2O in cold giants; none in the clear temperate range; silicates, iron and other refractory condensates in hot Jupiters | | Equilibrium temperature | below 100 to about 4,000 K | ## Related pages - [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. - [Hot Jupiter](https://paxabyssi.com/wiki/Hot_Jupiter.md): 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. - [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. - [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. - [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. - [Planet classification](https://paxabyssi.com/wiki/Planet_classification.md): How astronomers sort planets by size, mass, temperature and composition, and how Pax Abyssi files every world it generates under one of 37 coded types. 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)