Gas giant


May 25, 2022

Gas giant (also called Jupiter planet) is a generic astronomical term, invented by the science fiction writer James Blish and now in common use, to describe a large planet that is not mainly composed of rock. Gas giants may actually have a rocky core, and indeed it is suspected that such a core is necessary for their formation. However, most of their mass is present in the form of gas (or gas compressed in a liquid state). Unlike rocky planets, gas giants do not have a well-defined surface. The solar system features four gas giants: Jupiter, Saturn, Uranus and Neptune.


Planets with mass greater than 10 Earth masses are usually defined as gas giants. An object with a mass greater than 70 times that of Jupiter (i.e. 0.08 times the mass of the Sun) has such heat and pressure inside it to be able to trigger a nuclear fusion reaction, which transforms the celestial body into a small star. There are also objects of smaller mass but large enough to be able to trigger the fusion of deuterium, but they are not considered planets but brown dwarfs. A limit of 13 Jovian masses has been assumed beyond which a body is no longer defined as a planet but a brown dwarf. This is not a limit with a precise physical meaning but a convention adopted by the International Astronomical Union, as large objects will burn most of their deuterium and smaller ones will burn only a small part of it. The amount of deuterium burned depends not only on the mass but also on the composition of the planet, in particular on the amount of helium and deuterium present. For example, the Encyclopedia of Extrasolar Planets includes objects up to 25 Jupiter masses, and the Exoplanet Data Explorer up to 24 Jupiter masses.

Jupiter and Saturn

Jupiter and Saturn are made up primarily of hydrogen and helium, and most of their mass is made up of liquid or metallic hydrogen, possibly with a rocky core or made up of nickel and iron. The outer layer is made up of molecular hydrogen, which surrounds a layer of liquid metallic hydrogen, with a molten core probably rocky and, in the case of Jupiter, 12,000 km in diameter. The outermost layers of the hydrogen atmosphere are characterized by visible clouds generally composed of water and ammonia. Inside, hydrogen is called "metallic" because the great pressure turns it into an electrical conductor.

Uranus and Neptune

The composition of the other gas giants is similar, but Uranus and Neptune contain much less hydrogen and greater quantities of water, ammonia, and methane, which is why they have also been defined as ice giants. Although the internal composition is not well known, rocks and gases may also be present, but to a much lesser extent. Since it was thought plausible that methane could dissociate at the very high pressures reached in the depths of the two planets and that carbon could crystallize directly there as a diamond, some astronomers believed that the nuclei of Uranus and Neptune were composed of diamonds; however, subsequent research has ruled out this possibility. In the lower layers, the liquid hydrogen inside the gas giants is so compressed that it becomes metallic in nature. Metallic hydrogen is only stable under such enormous pressures.

Extrasolar gas giants

Due to available techniques, many of the known exoplanets possess masses comparable to and often much greater than Jupiter, and on this basis it has been suggested that they may be gas giants. Their composition and structure, however, is unknown; many of these planets orbit very close to their parent star, such as the so-called hot Jupiters, and it is debatable whether they