A gas giant is a giant planet mainly composed of hydrogen and helium. There are two gas giants in the Solar System: Jupiter and Saturn. While it originally was synonymous with "giant planet" and also included Uranus and Neptune, starting in 1970s-1980s it began common to separate them into another group called ice giants, with gas giants being, as mentioned, mostly made of hydrogen and helium, and ice giants being mostly made of elements heavier than hydrogen and helium, to be specific volatiles like water, ammonia or methane.
Elements heavier than hydrogen and helium make up less than 13% of the mass of Jupiter and Saturn. They are thought to be made of a very deep atmosphere of molecular hydrogen, with its outermost layers having many layers of water-ammonia clouds, surrounding a layer of liquid metallic hydrogen, inside of which is a molten rocky core with such high temperatures (20000+ K) and pressures its properties are not fully understood.
The border between gas giants and brown dwarves is blurry and debated, it can depend on the way of formation or on physics of its interior, or on if brown dwarves must experience nuclear fusion in their core atleast once in history.
The term gas giant was coined by the science fiction writer James Blish in 1952, originally, as mentioned, meaning all giant planets. The name "gas giants" is not entirely accurate, most of the matter in gas giants is not really gas. Between the gaseous upper atmosphere and the solid core everything is in a critical state where there is no difference between liquids and gases, however, it works as a simplification and was caught on by scientists.
Common features[]
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 5000 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 granite, or even, at 20,000 K, liquid granite. 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.
Both gas giants in our system rotate relatively rapidly, which causes wind patterns to break up into east-west bands or stripes. These bands are prominent on Jupiter and muted on Saturn.
Finally, all 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.
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 90% 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 70 times that of Jupiter (around 0.08 times the mass of the Sun). Above this point, the intense heat and pressure at the planet's core begin to induce nuclear 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.