A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are sometimes known as failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet", but in the 1990s it became known that Uranus and Neptune are really a distinct class of giant planet, being composed mainly of heavier volatile substances (which are referred to as "ices"). For this reason, Uranus and Neptune are now often classified in the separate category of ice giants.
Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass. They are thought to consist of an outer layer of molecular hydrogen surrounding a layer of liquid metallic hydrogen, with probably a molten rocky core. The outermost portion of their hydrogen atmosphere is characterized by many layers of visible clouds that are mostly composed of water and ammonia. The layer of metallic hydrogen makes up the bulk of each planet, and is referred to as "metallic" because the very large pressure turns hydrogen into an electrical conductor. The gas giants' cores are thought to consist of heavier elements at such high temperatures (20,000 K) and pressures that their properties are poorly understood.
The defining differences between a very low-mass brown dwarf and a gas giant (estimated at about 13 Jupiter masses) are debated. One school of thought is based on formation; the other, on the physics of the interior. Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced nuclear fusion at some point in their history.
The term gas giant was coined in 1952 by the science fiction writer James Blish and was originally used to refer to all giant planets. It is, arguably, something of a misnomer because throughout most of the volume of all giant planets, the pressure is so high that matter is not in gaseous form. Other than solids in the core and the upper layers of the atmosphere, all matter is above the critical point, where there is no distinction between liquids and gases. The term has nevertheless caught on, because planetary scientists typically use "rock", "gas", and "ice" as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what phase the matter may appear in. In the outer Solar System, hydrogen and helium are referred to as "gases"; water, methane, and ammonia as "ices"; and silicates and metals as "rock". Because Uranus and Neptune are primarily composed of, in this terminology, ices, not gas, they are increasingly referred to as ice giants and separated from the gas giants.
Gas giants can, theoretically, be divided into five distinct classes according to their modeled physical atmospheric properties, and hence their appearance: ammonia clouds (I), water clouds (II), cloudless (III), alkali-metal clouds (IV), and silicate clouds (V). Jupiter and Saturn are both class I. Hot Jupiters are class IV or V.
A cold hydrogen-rich gas giant more massive than Jupiter but less than about 500 M⊕ (1.6 MJ) will only be slightly larger in volume than Jupiter. For masses above 500 M⊕, gravity will cause the planet to shrink (see degenerate matter).
Although the words "gas" and "giant" are often combined, hydrogen planets need not be as large as the familiar gas giants from the Solar System. However, smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via hydrodynamic escape than larger planets and planets farther out.
A gas dwarf could be defined as a planet with a rocky core that has accumulated a thick envelope of hydrogen, helium and other volatiles, having as result a total radius between 1.7 and 3.9 Earth-radii.
The smallest known extrasolar planet that is likely a "gas planet" is Kepler-138d, which has the same mass as Earth but is 60% larger and therefore has a density that indicates a thick gas envelope.
A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature.
44P/Reinmuth or Reinmuth 2 is a Jupiter-family comet that is greatly perturbed by the gas giant Jupiter. The diameter of this comet is estimated at 3.22 km and its absolute magnitude at 11.Bespin
Bespin is a fictional planet, a gas giant in Star Wars films and books (in the Bespin System of the Ison Corridor of the Ivax Nebula of the Anoat Sector of Greater Javin of the Outer Rim). The planet was first seen in the 1980 feature film The Empire Strikes Back. Since its introduction, Bespin has gained more specific characteristics in the Star Wars expanded universe.
In The Empire Strikes Back, Bespin's floating city Cloud City hovers suspended by an anti-gravity pod.EPIC 2037 b
EPIC 2037 b also known as EPIC 203771098 b is an exoplanet orbiting the Sun-like star EPIC 203771098 every 21 days. It has about the same density as Saturn, at 0.7103 g/cm3, which indicates that the planet is clearly a gas giant.EPIC 2037 c
EPIC 2037 c also known as EPIC 203771098 c is an exoplanet orbiting the Sun-like star EPIC 203771098 every 42 days. It has a density far lower than that of Saturn, which indicates that the planet is clearly a gas giant.Gas dwarf
A gas dwarf is a gas planet with a rocky core that has accumulated a thick envelope of hydrogen, helium, and other volatiles, having as result a total radius between 1.7 and 3.9 Earth radii (1.7–3.9 R⊕). The term is used in a three-tier, metallicity-based classification regime for short-period exoplanets, which also includes the rocky, terrestrial-like planets with less than 1.7 R⊕ and planets greater than 3.9 R⊕, namely ice giants and gas giants.Smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via hydrodynamic escape than larger planets and planets farther out.The smallest known extrasolar planet that might be a gas dwarf is Kepler-138d, which is less massive than Earth but has a 60% larger volume and therefore has a density (2.1(+2.2/-1.2) grams per cubic centimetre) that indicates either a substantial water content or possibly a thick gas envelope.A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature.Giant planet
A giant planet is any massive planet. They are usually primarily composed of low-boiling-point materials (gases or ices), rather than rock or other solid matter, but massive solid planets can also exist. There are four known giant planets in the Solar System: Jupiter, Saturn, Uranus and Neptune. Many extrasolar giant planets have been identified orbiting other stars.
Giant planets are also sometimes called jovian planets, after Jupiter ("Jove" being another name for the Roman god "Jupiter"). They are also sometimes known as gas giants. However, many astronomers now apply the latter term only to Jupiter and Saturn, classifying Uranus and Neptune, which have different compositions, as ice giants. Both names are potentially misleading: all of the giant planets consist primarily of fluids above their critical points, where distinct gas and liquid phases do not exist. The principal components are hydrogen and helium in the case of Jupiter and Saturn, and water, ammonia and methane in the case of Uranus and Neptune.
The defining differences between a very low-mass brown dwarf and a gas giant (~13 MJ) are debated. One school of thought is based on formation; the other, on the physics of the interior. Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced nuclear fusion at some point in their history.HD 103197
HD 103197 is a 9th magnitude K-type main sequence star located approximately 170 light years away in the constellation Centaurus. The star is smaller, cooler, dimmer, and less massive than our Sun. Its metal content is thirteen-eighths as much as the Sun. In 2009, a gas giant planet was found in orbit around the star.HD 125595
HD 125595 is a 9th magnitude K-type main sequence star located approximately 89 light years away in the constellation Centaurus. This star is smaller, cooler, dimmer, and less massive than our Sun. Also its metal content is 4% more than the Sun. In 2009, a gas giant planet was found in orbit around the star.HD 155233 b
HD 155233 b is a confirmed exoplanet orbiting around the K Giant star HD 155233 every 885 days some 244.94 light-years away. It has a mass of 636 Earth masses or 2 Jupiter masses and is likely a gas giant similar of that to Jupiter just double the mass. It was discovered by Wittenmyer et al. on October 22nd 2015.HD 164604
HD 164604 is a K dwarf star in the Sagittarius constellation. It has a single gas giant planet detected by the Magellan Planet Search Program in 2010.HD 181720
HD 181720 is an 8th magnitude G-type main sequence star located approximately 190 light years away in the constellation Sagittarius. This star is larger, hotter, brighter, and less massive than our Sun. Also its metal content is three-tenths as much as the Sun. In 2009, a gas giant planet was found in orbit around the star.HD 219134 h
HD 219134 h, also known as HR 8832 h, is an exoplanet orbiting around the K-type star HR 8832 in the constellation of Cassiopeia. It has a mass of 108 Earth Masses, which indicates that the planet is likely a gas giant. Unlike HD 219134 b it has not been observed by the Spitzer telescope and thus its radius and density are unknown. It is in the ammonia habitable zone, so if it has a large moon with an atmosphere, liquid ammonia could flow on the moon's surface.HD 24496
HD 24496 is a 7th magnitude G-type wide binary star and M-type located approximately 67 light years away in the constellation Taurus. This star is smaller, cooler, fainter, and less massive than our Sun. Also its metal content is just 2% greater than the Sun. In 2009, a gas giant planet was discovered of HD 24496 A.Kepler-1625
Kepler-1625 is a 14th magnitude Sun-like star located in the constellation of Cygnus approximately 8,000 light years away. It is just under 80% larger in radius than the Sun and has about 8% more mass. In 2016, it was discovered that the star is orbited by an exoplanet, Kepler-1625b, which is a gas giant orbiting within the system's habitable zone. In 2018, it was reported that this exoplanet might have an exomoon orbiting it. This possibility will be verified on October 29, during the planet's next transit.Kepler-68d
Kepler-68d is a gas giant with the minimum mass about the same as Jupiter. It is at least a jovian-mass planet orbiting 1.4 astronomical units from its parent star, Kepler-68, well within habitable zone of the star. It was detected by radial velocity.After planets Kepler-68b and c were detected by observing planetary transits in front of its star, doppler spectroscopy measurements were used to make follow-up observations of the star. Kepler-68d was discovered using that method.Kepler-76b
Kepler-76b is a gas giant with the mass about two times of Jupiter.It is a Hot Jupiter that orbits its star every 1.5 days. Confirmed with Trans-Atlantic Exoplanet Survey and SOPHIE échelle spectrograph.Kepler-8
Kepler-8 is a star located in the constellation Lyra in the field of view of the Kepler Mission, a NASA-led operation tasked with discovering terrestrial planets. The star, which is slightly hotter, larger, and more massive than the Sun, has one gas giant in its orbit, Kepler-8b. This gas giant is larger than Jupiter, but is less massive, and thus more diffuse. The planet's discovery was announced to the public on January 4, 2010 along with four other planets. As the fifth confirmed planetary system verified by Kepler, it helped demonstrate the capabilities of the Kepler spacecraft.Sudarsky's gas giant classification
Sudarsky's classification of gas giants for the purpose of predicting their appearance based on their temperature was outlined by David Sudarsky and colleagues in the paper Albedo and Reflection Spectra of Extrasolar Giant Planets and expanded on in Theoretical Spectra and Atmospheres of Extrasolar Giant Planets, published before any successful direct or indirect observation of an extrasolar planet atmosphere was made. It is a broad classification system with the goal of bringing some order to the likely rich variety of extrasolar gas-giant atmospheres.
Gas giants are split into five classes (numbered using Roman numerals) according to their modeled physical atmospheric properties. In the Solar System, only Jupiter and Saturn are within the Sudarsky classification, and both are Class I.
The appearance of planets that are not gas giants cannot be predicted by the Sudarsky system, for example terrestrial planets such as Earth and Venus, HD 85512 b (3.6 Earth masses) and OGLE-2005-BLG-390Lb (5.5 Earth masses), or ice giants such as Uranus (14 Earth masses) and Neptune (17 Earth masses).