Gas giant

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 "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 ==

[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 () will only be slightly larger in volume than Jupiter. For masses above , gravity will cause the planet to shrink (see degenerate matter). 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. 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. == Precipitation and meteorological phenomena ==

Jovian weather Heat that is funneled upward by local storms is a major driver of the weather on gas giants. Much, if not all, of the deep heat escaping the interior flows up through towering thunderstorms. The GRS is a powerful anticyclone, swirling at about 430 to 680 kilometers per hour counterclockwise around the center. 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. 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. 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. Helium condensation could be responsible for Saturn's excess luminosity as well as the helium depletion in the atmosphere of both Jupiter and Saturn. == See also ==