Jupiter is the fifth planet from the Sun and the largest in the solar system, more than 10 times the diameter of Earth and more than 300 times its mass. In fact the mass of Jupiter is almost 2.5 times that of all the other planets combined. Being composed largely of the light elements hydrogen and helium, its mean density is only 1.314 times that of water. The mean density of Earth is 5.245 times that of water. The pull of gravity on Jupiter at the top of the clouds at the equator is 2.4 times as great as gravity's pull at the surface of Earth at the equator. Jupiter rotates at a dizzying pace -- once every 9 hours 55 minutes 30 seconds, although the period determined by watching cloud features differs by up to five minutes due to intrinsic cloud motions. The massive planet takes almost 12 Earth years to complete a journey around the Sun. From the Earth, Jupiter can be seen to show a disc with polar flattening, this is due to a very rapid rotation.
Jupiter has a Great Red Spot, an enormous anti-cyclonic system which has lasted for hundreds of years. This hurricane-like storm in Jupiter's atmosphere is more than twice the size of the Earth. As a high- pressure region, the Great Red Spot spins in a direction opposite to that of low-pressure storms on Jupiter; it is surrounded by swirling currents that rotate around the spot and are sometimes consumed by it. Across the disc several bands of dark and light clouds can be seen and the GRS is visible during each rotation. Pictures returned by the Voyager probes have shown the complexity of the structures within these bands. It is thought that the brighter zones are cloud-covered regions of upward moving atmosphere, while the belts are the regions of descending gases. An elongated yellow cloud within the GRS is swirling around the spot's interior boundary in a counter-clockwise direction with a period of a little less than six days, confirming the whirlpool-like circulation that astronomers have suspected from ground-based photographs.
Jupiter has 16 known satellites, the four large Galilean moons and 12 small ones. Jupiter is very gradually slowing down due to the tidal drag produced by the Galilean satellites. Also, the same tidal forces are changing the orbits of the moons, very slowly forcing them farther from Jupiter.
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