A gas giant (sometimes also known as a Jovian planet after the planet Jupiter) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our solar system: Jupiter, Saturn, Uranus, and Neptune. Uranus and Neptune may be considered a separate subclass of giant planets, 'ice giants', or 'Uranian planets', as they are mostly composed of ice, rock as well as gases of water, ammonia and methane, unlike the "traditional" gas giants Jupiter or Saturn. However, they share the same qualities of the lack of the solid surface; their differences stem from the fact that their proportion of hydrogen and helium is lower, due to their greater distance from the Sun.

Gas giants may have a rocky or metallic core—in fact, such a core is thought to be required for a gas giant to form—but the majority of its mass is in the form of the gaseous hydrogen and helium, with traces of water, methane, ammonia, and other hydrogen compounds. (Although familiar to us as gases on Earth, these constituents are expected to be compressed into liquids or solids deep in a gas giant's atmosphere.)

Unlike rocky planets, which have a clearly defined difference between atmosphere and surface, gas giants do not have a well-defined surface; their atmospheres simply become gradually denser toward the core, perhaps with liquid or liquid-like states in between. One cannot "land on" such planets in the traditional sense. Thus, terms such as diameter, surface area, volume, surface temperature and surface density may refer only to the outermost layer visible from space.

Common features

The four solar system gas giants share a number of features. All have atmospheres that are mostly hydrogen and helium and that blend into the liquid interior at pressures greater than the critical pressure, so that there is no clear boundary between atmosphere and body. They have very hot interiors, ranging from about 5,000 kelvins (K) for Neptune to over 20,000 K for Jupiter. This great heat means that, beneath their atmospheres, the planets are most likely entirely liquid. Thus, when discussions refer to a "rocky core", one should not picture a ball of solid rock, or even, at 20,000 K, liquid rock. Rather, what is meant is a region in which the concentration of heavier elements such as iron and silicon is greater than that in the rest of the planet.

All four planets rotate relatively rapidly, which causes wind patterns to break up into east-west bands or stripes. These bands are prominent on Jupiter, muted on Saturn and Neptune, and barely detectable on Uranus. Uranus has an extreme tilt unlike the other gas giants that causes extreme seasons.

Finally, all four are accompanied by elaborate systems of rings and moons. Saturn's rings are the most spectacular, and were the only ones known before the 1970s. As of 2006, Jupiter is known to have the most moons, with sixty-three.

Belt-Zone Circulation

The bands we see in the Jovian atmosphere are due to counter-circulating streams of material called zones and belts. Dark belts and bright zones encircle the planet parallel to its equator.

The zones are the lighter bands, and are at higher altitudes in the atmosphere. They have internal updraft, and are high-pressure regions. The belts are the darker bands. They are lower in the atmosphere, and have internal downdraft. They are low-pressure regions. So these structures are analogous to high- and low-pressure cells in Earth's atmosphere. But they have such a 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. Astronomers have observed lightning from a number of them.

Jupiter and Saturn

Jupiter and Saturn consist almost entirely of hydrogen and helium, and they are so large that this is true even though both are thought to have several Earth masses of heavier elements. Their interiors most likely consist of (liquid) metallic hydrogen, a form of hydrogen distinguished by the fact that it conducts electricity. Both planets have magnetic fields oriented fairly close to their axes of rotation.

Uranus and Neptune

Uranus and Neptune have distinctly different interior compositions, with the bulk of their interiors thought to consist of a mixture (or layered assortment) of rock, water, methane, and ammonia. Both have magnetic fields that are sharply inclined to their axes of rotation.

Terminology

The term gas giant was coined in 1952 by the science fiction writer James Blish. Arguably it is somewhat of a misnomer, since throughout most of the volume of these planets, there is no distinction between liquids and gases, since all the components (other than solid materials in the core) are above the critical point, so that the transition between gas and liquid is smooth. Jupiter is an exceptional case, having metallic hydrogen near the center, as explained above, 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 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 they appear in. In the outer solar system, hydrogen and helium are "gases"; water, methane, and ammonia are "ices"; and silicates are 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—a 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 latter two.

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

Extrasolar gas giants

Because of the limited techniques currently available to detect extrasolar planets, most 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 75% hydrogen), 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.

The upper mass limit of a gas giant planet is approximately eighty times that of Jupiter (around 0.08 times the mass of the Sun), if brown dwarfs fusing deuterium are considered as gas giants. Above this point, the intense heat and pressure at the planet's core begin to induce proton-proton chain reaction fusion, and the planet ignites to become a red dwarf. Interestingly, there appears to be a mass gap between the heaviest gas giant planets detected (about 10 times the mass of Jupiter) and the lightest red dwarfs. This has led to suggestions that the formation process for planets and binary stars may be fundamentally different.

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