The planet Jupiter is 5.2 A.U. from the Sun on average, which corresponds to an orbital period of 11.9 Earth years. It is the most massive planet, being 318 times as massive as the Earth, and about 2 1/2 times the mass of all other planets combined.

Jupiter is massive because it has a large diameter (11.2 times that of Earth). Its density is only 1.33 g/cc, which is 1/4 that of Earth, and only slightly more than the 1 g/cc characteristic of water. This very low density compared with the terrestrial planets is because it is composed mostly of hydrogen and helium, in ratios similar to that found in stars (approximately 82% hydrogen and 17% helium by mass).

Furthermore, Jupiter is almost entirely gas and liquid. Therefore, Jupiter and the related planets Saturn, Uranus, and Neptune are sometimes called Gas Giants (these are also called the Jovian Planets, since Jove was another name for Jupiter). We guess that Jupiter probably has a small rocky core comparable in size to terrestrial planets and perhaps with a similar composition, but we have no direct evidence of this.

Because Jupiter is not solid its parts do not rotate at exactly the same velocity. However, measurements of the rotation of the magnetic field give a rotation period of 9 hours and 55 minutes. This rapid rotation and the gas and liquid structure give rise to a pronounced equatorial bulge that is evident in images such as the adjacent one.

As also indicated in the adjacent computer-enhanced image (Ref), the atmosphere has clouds with a colorful and complex structure. These features are partially associated with the effect of very high velocity winds in the Jovian atmosphere (as much as 900 km/hr).

It also has an large, complex, and very intense magnetic field, and has a system of 16 moons that is a kind of miniature Solar System unto itself. It has an internal heat source, since it emits more radiation than it receives. This is thought to be associated with residual heat left over from the formation of the planet. Finally, it even has a faint ring.