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] Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranus and Neptune are really a distinct class of giant planets, 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]

Contents

Jupiter and Saturn consist mostly of elements such as hydrogen and helium, with heavier elements making up between 3 and 13 percent of their mass. [3] They are thought to consist of an outer layer of compressed molecular hydrogen surrounding a layer of liquid metallic hydrogen, with probably a molten rocky core inside. The outermost portion of their hydrogen atmosphere contains many layers of visible clouds that are mostly composed of water (despite earlier consensus that there was no water anywhere in the Solar System besides Earth) and ammonia. The layer of metallic hydrogen located in the mid-interior makes up the bulk of every gas giant and is referred to as "metallic" because the very large atmospheric 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 [19,700  °C ; 35,500  °F ]) and pressures that their properties are not yet completely understood. The placement of the solar system's gas giants can be explained by the Grand tack hypothesis. [3]

The defining differences between a very low-mass brown dwarf (which can have a mass as low as roughly 13 times that of Jupiter [4] ) and a gas giant are debated. [5] One school of thought is based on formation; the other, on the physics of the interior. [5] Part of the debate concerns whether brown dwarfs must, by definition, have experienced nuclear fusion at some point in their history.

Terminology

The term gas giant was coined in 1952 by the science fiction writer James Blish [6] 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. [7] 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. [8] 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 "rocks". In this terminology, since Uranus and Neptune are primarily composed of ices, not gas, they are more commonly called ice giants and distinct from the gas giants.

Classification

Theoretically, gas giants can 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.

Extrasolar

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
A gas giant exoplanet [right] with the density of a marshmallow has been detected in orbit around a cool red dwarf star [left] by the NASA-funded NEID radial-velocity instrument on the 3.5-meter WIYN Telescope at Kitt Peak National Observatory. Artist impression of ultra fluffy gas giant planet orbiting a red dwarf star.jpg
A gas giant exoplanet [right] with the density of a marshmallow has been detected in orbit around a cool red dwarf star [left] by the NASA-funded NEID radial-velocity instrument on the 3.5-meter WIYN Telescope at Kitt Peak National Observatory.

Cold gas giants

A cold hydrogen-rich gas giant more massive than Jupiter but less than about 500  ME (1.6  MJ) will only be slightly larger in volume than Jupiter. [9] For masses above 500 ME, gravity will cause the planet to shrink (see degenerate matter). [9]

Kelvin–Helmholtz heating can cause a gas giant to radiate more energy than it receives from its host star. [10] [11]

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. [12] [13]

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. [14] [15]

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. [16]

A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature. [17]

Precipitation and meteorological phenomena

Jovian weather

Heat that is funneled upward by local storms is a major driver of the weather on gas giants. [18] Much, if not all, of the deep heat escaping the interior flows up through towering thunderstorms. [18] These disturbances develop into small eddies that eventually form storms such as the Great Red Spot on Jupiter. [18] On Earth and Jupiter, lightning and the hydrologic cycle are intimately linked together to create intense thunderstorms. [18] During a terrestrial thunderstorm, condensation releases heat that pushes rising air upward. [18] This "moist convection" engine can segregate electrical charges into different parts of a cloud; the reuniting of those charges is lightning. [18] Therefore, we can use lightning to signal to us where convection is happening. [18] Although Jupiter has no ocean or wet ground, moist convection seems to function similarly compared to Earth. [18]

Jupiter's Red Spot

The Great Red Spot (GRS) is a high-pressure system located in Jupiter's southern hemisphere. [19] The GRS is a powerful anticyclone, swirling at about 430 to 680 kilometers per hour counterclockwise around the center. [19] The Spot has become known for its ferocity, even feeding on smaller Jovian storms. [19] Tholins are brown organic compounds found within the surface of various planets that are formed by exposure to UV irradiation. The tholins that exist on Jupiter's surface get sucked up into the atmosphere by storms and circulation; it is hypothesized that those tholins that become ejected from the regolith get stuck in Jupiter's GRS, causing it to be red.

Helium rain on Saturn and Jupiter

Condensation of helium creates liquid helium rain on gas giants. On Saturn, this helium condensation occurs at certain pressures and temperatures when helium does not mix in with the liquid metallic hydrogen present on the planet. [20] Regions on Saturn where helium is insoluble allow the denser helium to form droplets and act as a source of energy, both through the release of latent heat and by descending deeper into the center of the planet. [21] This phase separation leads to helium droplets that fall as rain through the liquid metallic hydrogen until they reach a warmer region where they dissolve in the hydrogen. [20] Since Jupiter and Saturn have different total masses, the thermodynamic conditions in the planetary interior could be such that this condensation process is more prevalent in Saturn than in Jupiter. [21] Helium condensation could be responsible for Saturn's excess luminosity as well as the helium depletion in the atmosphere of both Jupiter and Saturn. [21]

See also

Related Research Articles

<span class="mw-page-title-main">Exoplanet</span> Planet outside 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 then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. As of 7 November 2024, there are 5,787 confirmed exoplanets in 4,320 planetary systems, with 969 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition, environmental conditions, and potential for life.

<span class="mw-page-title-main">Giant planet</span> Planet much larger than the Earth

A giant planet, sometimes referred to as a jovian planet, is a diverse type of planet much larger than Earth. Giant planets are usually primarily composed of low-boiling point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. There are four such planets in the Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar giant planets have been identified.

<span class="mw-page-title-main">Planet</span> Large, round non-stellar astronomical object

A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets by the most restrictive definition of the term: the terrestrial planets Mercury, Venus, Earth, and Mars, and the giant planets Jupiter, Saturn, Uranus, and Neptune. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion.

<span class="mw-page-title-main">Jupiter</span> Fifth planet from the Sun

Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than 2.5 times that of all the other planets in the Solar System combined and slightly less than one-thousandth the mass of the Sun. Its diameter is eleven times that of Earth, and a tenth that of the Sun. Jupiter orbits the Sun at a distance of 5.20 AU (778.5 Gm), with an orbital period of 11.86 years. It is the third brightest natural object in the Earth's night sky, after the Moon and Venus, and has been observed since prehistoric times. Its name derives from that of Jupiter, the chief deity of ancient Roman religion.

<span class="mw-page-title-main">Nebular hypothesis</span> Astronomical theory about the Solar System

The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System. It suggests the Solar System is formed from gas and dust orbiting the Sun which clumped up together to form the planets. The theory was developed by Immanuel Kant and published in his Universal Natural History and Theory of the Heavens (1755) and then modified in 1796 by Pierre Laplace. Originally applied to the Solar System, the process of planetary system formation is now thought to be at work throughout the universe. The widely accepted modern variant of the nebular theory is the solar nebular disk model (SNDM) or solar nebular model. It offered explanations for a variety of properties of the Solar System, including the nearly circular and coplanar orbits of the planets, and their motion in the same direction as the Sun's rotation. Some elements of the original nebular theory are echoed in modern theories of planetary formation, but most elements have been superseded.

<span class="mw-page-title-main">Hot Jupiter</span> Class of high mass planets orbiting close to a star

Hot Jupiters are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods. The close proximity to their stars and high surface-atmosphere temperatures resulted in their informal name "hot Jupiters".

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<span class="mw-page-title-main">Ice giant</span> Giant planet primarily consisting of compounds with freezing points exceeding 100 K

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<span class="mw-page-title-main">Formation and evolution of the Solar System</span>

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<span class="mw-page-title-main">Extraterrestrial atmosphere</span> Area of astronomical research

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 giant planets, as well as Mars, Venus and Titan. 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.

This page describes exoplanet orbital and physical parameters.

<span class="mw-page-title-main">Hot Neptune</span> Planet with a mass similar to Uranus or Neptune orbiting close to its star

A hot Neptune is a type of giant planet with a mass similar to that of Neptune or Uranus orbiting close to its star, normally within less than 1 AU. The first hot Neptune to be discovered with certainty was Gliese 436 b (Awohali) 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.

<span class="mw-page-title-main">Helium planet</span> Planet with helium-dominated atmosphere

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<span class="mw-page-title-main">Kepler-11d</span> Extrasolar planet

Kepler-11d is an exoplanet discovered in the orbit of the sun-like star Kepler-11. It is named for the telescope that discovered it, a NASA spacecraft named Kepler that is designed to detect Earth-like planets by measuring small dips in the brightness of their host stars as the planets cross in front. This process, known as the transit method, was used to note the presence of six planets in orbit around Kepler-11, of which Kepler-11d is the third from its star. Kepler-11d orbits Kepler-11 well within the orbit of Mercury approximately every 23 days. The planet is approximately six times more massive than the Earth, and has a radius that is three and a half times larger than that of Earth's. It is, however, far hotter than Earth is. Its low density, comparable to that of Saturn, suggests that Kepler-11d has a large hydrogen–helium atmosphere. Kepler-11d was announced with its five sister planets on February 2, 2011 after extensive follow-up studies.

<span class="mw-page-title-main">Kepler-11e</span> Extrasolar planet orbiting Kepler-11

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References

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