Gas giant
From Aetilc
A gas giant is a giant planet composed mainly of hydrogen and helium and as a result are sometimes known as a failed star since it contains the same basic elements as a star. 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").
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[edit] Definition
Gas giant 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.[2] 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.
[edit] Characteristics
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.
[edit] 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. For masses above 500 M⊕, gravity will cause the planet to shrink (see degenerate matter). Kelvin–Helmholtz heating can cause a gas giant to radiate more energy than it receives from its host star.
[edit] Gas dwarfs
See also: Gas dwarf
Although the words "gas" and "giant" are often combined, hydrogen planets need not be as large as the familiar gas giants 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 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.
