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== Terminology ==
== Terminology ==


The term ''gas giant'' was coined in 1952 by the science fiction writer [[James Blish]]. Arguably it is something of a misnomer, since throughout most of the volume of these planets all the components (other than solid materials in the core) are above the [[critical point (thermodynamics)#Pure substances: vapor-liquid critical point|critical point]] and therefore there is no distinction between liquids and gases. ''Fluid planet'' would be a more accurate term. Jupiter is an exceptional case, having [[metallic hydrogen]] near the center, but much of its volume is hydrogen, helium and traces of other gases above their critical points. The observable atmospheres of any of these planets (at less than unit [[optical depth]]) are quite thin compared to the planetary radii, only extending perhaps one percent of the way to the center. Thus the observable portions are gaseous (in contrast to Mars and Earth, which have gaseous atmospheres through which the crust may be seen).
The term ''gas giant'' was coined in 1952 by the science fiction writer [[James Blish]]. Arguably it is something of a misnomer, since throughout most of the volume of these planets all the components (other than solid materials in the core) are above the [[critical point (thermodynamics)#Pure HI

The rather misleading term has 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 of matter|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." When deep planetary interiors are considered, it may not be far off to say that, by "ice" astronomers mean [[oxygen]] and [[carbon]], by "rock" they mean [[silicon]], and by "gas" they mean hydrogen and helium.

The alternative term ''Jovian planet'' refers to the Roman god [[Jupiter (mythology)|Jupiter]]—the genitive form of which is ''Jovis'', hence ''Jovian''—and was intended to indicate that all of these planets were similar to Jupiter. However, the many ways in which Uranus and Neptune differ from Jupiter and Saturn have led some to use the term only for the inner two.{{Who|date=August 2008}}
The alternative term ''Jovian planet'' refers to the Roman god [[Jupiter (mythology)|Jupiter]]—the genitive form of which is ''Jovis'', hence ''Jovian''—and was intended to indicate that all of these planets were similar to Jupiter. However, the many ways in which Uranus and Neptune differ from Jupiter and Saturn have led some to use the term only for the inner two.{{Who|date=August 2008}}


Revision as of 17:54, 29 September 2011

This article is about the type of planet. For the rock band, see Gas Giants.
The Solar System's four gas giants against the Sun, to scale

A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar gas giants have been identified orbiting other stars.

Planets above 10 Earth masses are termed giant planets.[1] Lower-mass gassy planets are sometimes called "gas dwarfs".[2]

Objects large enough to start deuterium fusion (above 13 Jupiter masses for solar composition) are called brown dwarfs and these occupy the mass range between that of large gas giant planets and the lowest mass stars. The 13 Jupiter mass (MJ) cutoff is a rule of thumb rather than something of precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 MJ value is somewhere in between. The amount of deuterium burnt also depends not only on mass but on the composition of the planet, especially on the amount of helium and deuterium present.[3] The Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, and the Exoplanet Data Explorer up to 24 Jupiter masses.

Description

This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen.

A gas giant is a massive planet with a thick atmosphere and a dense molten core. The "traditional" gas giants, Jupiter and Saturn, are composed primarily of hydrogen and helium. Uranus and Neptune are sometimes called ice giants, as they are mostly composed of water, ammonia, and methane molten ices. Among extrasolar planets, Hot Jupiters are gas giants that orbit very close to their stars and thus have a very high surface temperature. Hot Jupiters are currently the most common form of extrasolar planet known, perhaps due to the relative ease of detecting them.

Gas giants are commonly said to lack solid surfaces, but it is closer to the truth to say that they lack surfaces altogether since the gases that make them up simply become thinner and thinner with increasing distance from the planets' centers, eventually becoming indistinguishable from the interstellar medium. Therefore landing on a gas giant may or may not be possible, depending on the size and composition of its core.

Belt-zone circulation

The bands seen in the Jovian atmosphere are due to counter-circulating streams of material called zones and belts, encircling the planet parallel to its equator. The zones are the lighter bands, and are at higher altitudes in the atmosphere. They have an internal updraft, and are high-pressure regions. The belts are the darker bands. They are lower in the atmosphere, and have an internal downdraft. They are low-pressure regions. These structures are somewhat analogous to high- and low-pressure cells in Earth's atmosphere, but they have a very different structure—latitudinal bands that circle the entire planet, as opposed to small confined cells of pressure. This appears to be a result of the rapid rotation and underlying symmetry of the planet. There are no oceans or landmasses to cause local heating, and the rotation speed is much faster than it is on Earth. There are smaller structures as well: spots of different sizes and colors. On Jupiter, the most noticeable of these features is the Great Red Spot, which has been present for at least 300 years. These structures are huge storms. Some such spots are thunderheads as well.

Relative masses of the gas giants of the Solar System

Jupiter and Saturn

Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass.[4] Their structures are thought to consist of an outer layer of molecular hydrogen, surrounding a layer of liquid metallic hydrogen, with a probable molten core with a rocky composition. The outermost portion of the hydrogen atmosphere is characterized by many layers of visible clouds that are mostly composed of water and ammonia. The metallic hydrogen layer makes up the bulk of each planet, and is described as "metallic" because the great pressure turns hydrogen into an electrical conductor. The core is thought to consist of heavier elements at such high temperatures (20,000 K) and pressures that their properties are poorly understood.[4]

Uranus and Neptune

Uranus and Neptune have distinctly different interior compositions from Jupiter and Saturn. Models of their interior begin with a hydrogen-rich atmosphere that extends from the cloud-tops down to about 85% of Neptune's radius and 80% of Uranus'. Below this point is predominantly "icy", composed of water, methane and ammonia. There is also some rock and gas but various proportions of ice/rock/gas could mimic pure ice so the exact proportions are unknown.[5]

Very hazy atmosphere layers with a small amount of methane gives them aquamarine colors such as baby blue and ultramarine colors respectively. Both have magnetic fields that are sharply inclined to their axes of rotation.

Unlike the other gas giants, Uranus has an extreme tilt that causes its seasons to be severely pronounced.

Extrasolar gas giants

An artist's conception of 79 Ceti b, the first extrasolar gas giant found with a minimum mass less than Saturn.
See also: Hot Jupiter

Because of the limited techniques currently available to detect extrasolar planets, many of those found to date have been of a size associated, in our solar system, with gas giants. Because these large planets are inferred to share more in common with Jupiter than with the other gas giant planets, some have claimed that "Jovian planet" is a more accurate term for them. Many of the extrasolar planets are much closer to their parent stars and hence much hotter than gas giants in the solar system, making it possible that some of those planets are a type not observed in our solar system. Considering the relative abundances of the elements in the universe (approximately 98% hydrogen and helium) it would be surprising to find a predominantly rocky planet more massive than Jupiter. On the other hand, previous models of planetary system formation suggested that gas giants would be inhibited from forming as close to their stars as have many of the new planets that have been observed.

Cold gas giants

A cold hydrogen-rich gas giant more massive than Jupiter but less than about 500 ME (1.6 MJ), will be just slightly larger in volume than Jupiter.[6] For masses above 500 ME, degenerate pressure will cause the planet to shrink.[6] Kelvin-Helmholtz heating can cause a gas giant, such as Jupiter, to radiate more energy than it receives from its host star.[7][8]

Smaller gas planets

Comparison of sizes of planets with different compositions

Although the words "gas" and "giant" are often combined, hydrogen planets need not be as large as the familiar gas giants from our solar system. However, smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via hydrodynamic escape than larger planets or planets farther out.[9][10]

The smallest known extrasolar planet that is likely a "gas planet" is Kepler-11f, at about 2.3 Earth masses.

Terminology

The term gas giant was coined in 1952 by the science fiction writer James Blish. Arguably it is something of a misnomer, since throughout most of the volume of these planets all the components (other than solid materials in the core) are above the [[critical point (thermodynamics)#Pure HI The alternative term Jovian planet refers to the Roman god Jupiter—the genitive form of which is Jovis, hence Jovian—and was intended to indicate that all of these planets were similar to Jupiter. However, the many ways in which Uranus and Neptune differ from Jupiter and Saturn have led some to use the term only for the inner two.[who?]

With this terminology in mind, some astronomers are starting to refer to Uranus and Neptune as "ice giants" to indicate the apparent predominance of the "ices" (in liquid form) in their interior composition.[11]

See also

Notes

  1. ^ Page 20 of The quest for very low-mass planets, M Mayor, S Udry - Physica Scripta, 2008[dead link]
  2. ^ StarGen - Solar System Generator, 2003
  3. ^ The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets, David S. Spiegel, Adam Burrows, John A. Milsom
  4. ^ a b The Interior of Jupiter, Guillot et al., in Jupiter: The Planet, Satellites and Magnetosphere, Bagenal et al., editors, Cambridge University Press, 2004
  5. ^ L. McFadden, P. Weissman, T. Johnson (2007). Encyclopedia of the Solar System (2nd ed.). Academic Press. ISBN 978-0-12-088589-3.{{cite book}}: CS1 maint: multiple names: authors list (link)
  6. ^ a b Seager, S. (2007). "Mass-Radius Relationships for Solid Exoplanets". The Astrophysical Journal. 669 (2): 1279–1297. arXiv:0707.2895. Bibcode:2007ApJ...669.1279S. doi:10.1086/521346. {{cite journal}}: |access-date= requires |url= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ Patrick G. J. Irwin (2003). Giant Planets of Our Solar System: Atmospheres, Composition, and Structure. Springer. ISBN 3540006818.
  8. ^ "Class 12 - Giant Planets - Heat and Formation". 3750 - Planets, Moons & Rings. Colorado University, Boulder. 2004. Retrieved 2008-03-13.
  9. ^ TRANSONIC HYDRODYNAMIC ESCAPE OF HYDROGEN FROM EXTRASOLAR PLANETARY ATMOSPHERES, Feng Tian, Owen B. Toon, Alexander A. Pavlov, H. De Sterck, The Astrophysical Journal, 621:1049–1060, 2005 March 10
  10. ^ Mass-radius relationships for exoplanets, Damian C. Swift, Jon Eggert, Damien G. Hicks, Sebastien Hamel, Kyle Caspersen, Eric Schwegler, and Gilbert W. Collins
  11. ^ Jack J. Lissauer, David J. Stevenson (2006). "Formation of Giant Planets" (PDF). NASA Ames Research Center; California Institute of Technology. Retrieved 2006-01-16.

References

  • Episode "Giants" on The Science Channel TV show Planets
  • SPACE.com: Q&A: The IAU's Proposed Planet Definition 16 August 2006 2:00 am ET
  • BBC News: Q&A New planets proposal Wednesday, 16 August 2006, 13:36 GMT 14:36 UK
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