Gas giant

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Jupiter New Horizons.jpg
Jupiter photographed by New Horizons in January 2007
Saturn Equinox 09212014.jpg
Saturn at equinox, photographed by Cassini in August 2009

A gas giant is a giant planet composed mainly of hydrogen and helium. [1] 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. [2]


Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass. [3] 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. [3]

The defining differences between a very low-mass brown dwarf and a gas giant (estimated at about 13 Jupiter masses) are debated. [4] One school of thought is based on formation; the other, on the physics of the interior. [4] 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 [5] 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. [6] 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.


Artist's impression of the formation of a gas giant around the star HD 100546 Artist's impression of a gas giant planet forming in the disc around the young star HD 100546.jpg
Artist's impression of the formation of a gas giant around the star HD 100546

Cold gas giants

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. [7] For masses above 500 M, gravity will cause the planet to shrink (see degenerate matter). [7]

Kelvin–Helmholtz heating can cause a gas giant to radiate more energy than it receives from its host star. [8] [9]

Gas dwarfs

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. [10] [11]

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. [12] [13]

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. [14]

A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature. [15]

See also

Related Research Articles

Exoplanet Any planet beyond the Solar System

An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such. The first confirmation of detection occurred in 1992. This was followed by the confirmation of a different planet, originally detected in 1988. As of 1 October 2020, there are 4,354 confirmed exoplanets in 3,218 systems, with 712 systems having more than one planet.

Giant planet Planet much larger than the Earth

A giant planet is any planet much larger than Earth. They are usually primarily composed of low-boiling-point materials, 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.

Planet Class of astronomical body directly orbiting a star or stellar remnant

A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.

Solar System The planets and their moons that orbit around the Sun

The Solar System is the gravitationally bound system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, the dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury.

Jupiter Fifth planet from the Sun and largest planet in the Solar System

Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass one-thousandth that of the Sun, but two-and-a-half times that of all the other planets in the Solar System combined. Jupiter is one of the brightest objects visible to the naked eye in the night sky, and has been known to ancient civilizations since before recorded history. It is named after the Roman god Jupiter. When viewed from Earth, Jupiter can be bright enough for its reflected light to cast visible shadows, and is on average the third-brightest natural object in the night sky after the Moon and Venus.

Chthonian planet Hypothetical class of celestial objects resulting from the stripping away of a gas giants hydrogen and helium atmosphere and outer layers

Chthonian planets are a hypothetical class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a star. The remaining rocky or metallic core would resemble a terrestrial planet in many respects.

HD 149026 b extrasolar planet

HD 149026 b, formally named Smertrios, is an extrasolar planet approximately 250 light-years from the Sun in the constellation of Hercules.

Ice giant giant planet primarily composed of elements heavier than hydrogen and helium

An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune.

Sudarskys 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.

Gliese 876 d exoplanet

Gliese 876 d is an exoplanet approximately 15 light-years away in the constellation of Aquarius. The planet was the third planet discovered orbiting the red dwarf Gliese 876. It was the lowest-mass extrasolar planet apart from the pulsar planets orbiting PSR B1257+12 at the time of its discovery. Due to this low mass, it can be categorized as a super-Earth.

Gliese 436 b extrasolar planet

Gliese 436 b is a Neptune-sized exoplanet orbiting the red dwarf Gliese 436. It was the first hot Neptune discovered with certainty and was among the smallest-known transiting planets in mass and radius, until the much smaller Kepler exoplanet discoveries began circa 2010.

Super-Earth Type of planet

A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability. The alternative term "gas dwarfs" may be more accurate for those at the higher end of the mass scale, although "mini-Neptunes" is a more common term.

Extraterrestrial atmosphere atmosphere of an object other than the Earth

The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earth's atmosphere. In addition to Earth, many of the other astronomical objects in the Solar System have atmospheres. These include all the gas giants, as well as Mars, Venus, and Pluto. Several moons and other bodies also have atmospheres, as do comets and the Sun. There is evidence that extrasolar planets can have an atmosphere. Comparisons of these atmospheres to one another and to Earth's atmosphere broaden our basic understanding of atmospheric processes such as the greenhouse effect, aerosol and cloud physics, and atmospheric chemistry and dynamics.

Exoplanetology, or exoplanetary science, is an integrated field of astronomical science dedicated to the search for and study of exoplanets. It employs an interdisciplinary approach which includes astrobiology, astrophysics, astronomy, astrochemistry, astrogeology, geochemistry, and planetary science.

A hot Neptune or Hoptune is a type of giant planet with a mass similar to that of Uranus or Neptune orbiting close to its star, normally within less than 1 AU. The first hot Neptune to be discovered with certainty was Gliese 436 b in 2007, an exoplanet about 33 light years away. Recent observations have revealed a larger potential population of hot Neptunes in the Milky Way than was previously thought. Hot Neptunes may have formed either in situ or ex situ.

Gliese 1214 b Extrasolar planet

Gliese 1214 b is an exoplanet that orbits the star Gliese 1214, and was discovered in December 2009. Its parent star is 48 light-years from the Sun, in the constellation Ophiuchus. As of 2017, GJ 1214 b is the most likely known candidate for being an ocean planet. For that reason, scientists have nicknamed the planet "the waterworld".

Helium planet A planet with a helium-dominated atmosphere

A helium planet is a planet with a helium-dominated atmosphere. This contrasts with ordinary gas giants such as Jupiter and Saturn, whose atmospheres consist primarily of hydrogen, with helium as a secondary component only. Helium planets might form in a variety of ways. Gliese 436 b is a possible helium planet.

A mini-Neptune is a planet less massive than Neptune but resembles Neptune in that it has a thick hydrogen–helium atmosphere, probably with deep layers of ice, rock or liquid oceans.

Gliese 3470 b Extrasolar planet

Gliese 3470 b, abbreviated as GJ3470b, is an exoplanet orbiting the star Gliese 3470, both of which are located in the constellation Cancer. With a mass of just under 14 Earth-masses and a radius approximately 4.3 times that of Earth's, it is likely something akin to a mini-Neptune despite the initially strong belief that the planet was not covered in clouds like the gas giants we are familiar with in our solar system. The planet belongs to the rare category of super-puffs.

The following outline is provided as an overview of and topical guide to Uranus:


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