A gas giant (sometimes also known as a Jovian planet or jovial 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', as they are mostly composed of water, ammonia, and methane, unlike the "traditional" gas giants Jupiter or Saturn. While all four planets lack a solid surface, the hydrogen and helium in Uranus and Neptune is mostly in the outermost region. A gas giant may have a rocky or metallic core—in fact, such a core is thought to be required for a gas giant to form. The majority of the mass of Jupiter and Saturn is hydrogen and helium. Although familiar to us as gases on Earth, these constituents are thought to be compressed into liquids or solids deep in a gas giant's interior. The majority of Uranus and Neptune is icy, though again the extreme heat and pressure of these planets' interiors put the ices into less familiar physical states.
Unlike rocky planets gas giants do not have a clearly defined surface. The atmospheres of gas giants simply become denser toward the core, perhaps with liquid or liquid-like states in between. Therefore one cannot "land on" such planets in the traditional sense. Terms such as diameter, surface area, volume, surface temperature, and surface density may refer only to the outermost layer visible from space.
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. On Jupiter and Saturn there is no clear boundary between atmosphere and body, but on Uranus and Neptune some models show that the boundary could indeed be sharp. In this regard, our four gas giants exemplify the classic "matter phase-gradient" in the materials sciences. They have very hot interiors, ranging from about 7,000 kelvin (K) for Uranus and 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. Rather, what is meant is a region in which the concentration of heavier elements such as iron and nickel 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.
All four planets 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 2008, Jupiter is known to have the most moons with sixty-three.
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 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. These structures are somewhat analogous to high- and low-pressure cells in Earth's atmosphere, but they have a much 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 mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass. Their structures are thought to consist of an outer layer of molecular hydrogen, surrounding a layer of liquid metallic hydrogen, with a probable rocky core. 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, if it exists, consists of heavier elements at such high temperatures (20000K) and pressures that their properties are poorly understood.
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.
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
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.
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 all the components (other than solid materials in the core) are above the 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 centre, 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 centre. 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 and metals 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. Another term used is "jovial" or "jovial planet". Though more often this meaning represents Jupiter itself, it is used as a antonymic word for terrestrial planet (which can mean the Earth or a rocky planet).
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.