Jupiter is the fifth planet from the Sun about 5.2 Astronomical Units. Jupiter is a gas-giant with a composite structure that is radically different from the four terrestrial planets. It has a diameter of 89,400 miles (143,800 km) and weighs more than all the other planets put together. Its immense gravity directs the fate of many comets and asteroids. Jupiter has 63 moons as of May 4, 2005. The four moons that were discovered by Galileo are the largest. They are Io, Europa, Ganymede, and Callisto. Together these are known as the Galilean moons, and each has its own geological character. Ganymede is the largest moon in our Solar System.
Jupiter's make-up is similar to the Sun's. Hydrogen accounts for 80 percent and helium 19 percent, the rest of the planet consists mainly of water vapor, methane, and ammonia. The surface of Jupiter consists of layers of cloud standing near the top of an immense atmosphere thousands of miles deep. The colors and striping result from chemical reactions in the ammonia and methane. Below the surface of the clouds the atmosphere remains gaseous to a depth of perhaps a few hundred miles. At its center lies a core of molten rock, perhaps several times Earth's mass. Sometimes called "a failed star," Jupiter would need about 100 times more mass to become even the smallest type of star.
Jupiter has very faint rings. These rings were discovered during the Voyager flybys in 1979. They are composed of particles of rock that reflect very little light. This makes it difficult to observe the rings from Earth.
Jupiter spins very rapidly on its axis. Its day is less than 10 hours long. This rapid rotation and powerful winds force that atmosphere and clouds in dark and light-colored stripe patterns. The turbulence in the clouds often causes storms. One such storm is known to us as "The Great Red Spot" and is nearly four times as large as the Earth and has raged for at least 150 years.