The largest planet in the Solar System and the fifth in order from the Sun. Jupiter is a gas giant, with 11 times the diameter of Earth, and two and half times the mass of all the other planets and satellites combined. It has an extensive family of satellites and a faint ring system. Jupiter's immense atmosphere consists of about 75% hydrogen and 25% helium by mass (90% hydrogen and 10% helium by number of atoms), with trace amounts of methane, ammonia, and other light substances. Jupiter spins around on its axis once every 9.8 hours (Jupiter's "day"). This is so fast, given Jupiter's size and the fact that it mostly a fluid, that Jupiter is noticeably squashed at its poles. Jupiter radiates about 2½ times more heat than it receives from the Sun, indicating a substantial source of internal heat, almost certainly gravitational contraction, to account for the surface temperature. The Jupiter system contains 63 moon discovered to date. In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results.

(For further details see Jupiter »)

(Source: The Internet Encyclopedia of Science »)