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
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
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.