Jovian planets are those planets consisting almost entirely of gas, notably hydrogen and helium, with a mixture of other volatile gases like ammonia and methane. Within our own solar system, the Jovian planets are Jupiter, Saturn, Uranus, and Neptune. The vast majority of extrasolar planets detected so far are also presumed to be Jovian planets, because only giant planets have masses large enough to be detectable.

Jovian planets, like all planets, form during the collapse of a gas cloud into a protostar. During collapse, an accretion disk forms around the star due to rotation of the gas cloud and the preservation of its angular momentum. Material within this disk can form clumps, some of which may be massive enough to become self-gravitating. These clumps can then grow due to their stronger gravitational field, and attract more and more material. The most massive planetesimals may rapidly accrete gas from the disk and form Jovian planets.

Prior to the discovery of extrasolar planets, it was believed that Jovian planets could only form at large distances from the star. The rationale was that the temperature of protostellar disks increases closer to the star. This means that volatile elements would not condense in the inner solar system. Farther out in the disk where the temperature is lower, these volatile elements (hydrogen and helium) and compounds (ammonia, methane, etc.) would be easier to accrete. However, extrasolar Jovian planets have been detected at stellar distances of much less than 1 AU from their host star, which suggests that either this theory of planetary formation is wrong, or there are dynamical mechanisms which allow these planets to migrate to the inner solar system.

The Jovian planets are to some extent "failed stars", in that they did not accrete enough mass to trigger nuclear fusion in their cores. Jupiter would have to be 80 times more massive than it is now to be considered even the tiniest of stars. Brown dwarfs are intermediates between true Jovian planets and stars.

See also gas giant.

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