-- This 'gas-giant' planet is larger than all of the other 'gas-giant' planets combined.

Jupiter is the fifth planet from the Sun. It is the largest planet in our solar system. In fact, it is larger than all of the other planets combined. Jupiter is 1,400 times larger than Earth. It is the most massive planet; its mass is 318 times greater than the Earth's, its diameter is more than 11 times greater than the Earth's, and its volume is 1,400 times greater than the Earth's. Despite all of this, Jupiter is only one-fourth as dense as the Earth. Jupiter was named after the ruler of the gods in Roman mythology.

Jupiter has no ground to land a spacecraft on. It is a giant ball of gas; like a cloud squeezed tight. It is the largest of the "gas giant" planets. The other "gas giant" planets are Saturn, Neptune, and Uranus. Hydrogen and helium are the main components of Jupiter.

Even though Jupiter is the largest planet, it's 1,000 times smaller than the Sun. Interestingly enough, scientists believe Jupiter and its 16 known satellites probably formed as a miniature solar system; the planet was a large rotating ball of gas. A planetary nebula surrounded it and developed into a planet and its moons.

Jupiter's magnetic field, which extends to 10 times the planet's radius, is the strongest of any planet. It causes intense bursts of radio noise. Layers of dense clouds surround Jupiter, making it impossible to see the planet's surface from Earth; the planet's atmosphere is hundreds of miles deep. Jupiter probably has a rocky core with a mass that's 10 times greater than that of our planet. A mixture of liquid hydrogen and helium surrounds the planet's core. The enormous pressure of Jupiter's upper layers has squeezed the mixture into metallic form.

If you look at Jupiter through a telescope, it's highest clouds look like dark belts and bright zones that surround the planet. The clouds are probably made of frozen crystals of ammonia and methane. Even though Jupiter radiates almost two times as much heat as it gets from the Sun, it is a very cold planet. The temperature at the tops of its clouds is estimated be around -130 degrees Centigrade. Like Saturn, Jupiter has rings composed of tiny rocks and dust particles. However, Jupiter's ring system is narrow.

Look at Jupiter and you'll probably see a Giant Red Spot. Scientists think the spot is a hurricane that lasts hundreds of years. The spot is so big that it's large enough to cover three Earths.

Jupiter rotates on its axis faster than any other planet. Although a year on Jupiter would be equivalent to 11.9 years on our planet, its day is only 9.9 hours long. The planet's rapid rotation causes a noticeable bulge at its equator.

Jupiter's four largest moons were the first heavenly bodies in our solar system to be discovered using a telescope. They are called the Galilean moons because Galileo discovered them in 1610. They are Io, Europa, Ganymede, and Callisto. Io is covered with active volcanoes, while some of the plant's other moons are covered with ice.

Several spacecraft have been sent through space to study Jupiter. Pioneer 10 and 11 visited the planet in 1973 and 1974, and Voyagers 1 and 2 visited in 1979. In October 1989, the Galileo spacecraft started its trip to Jupiter. By 1995, the Galileo was close enough to Jupiter to release a probe into the planet's atmosphere. It takes approximately six years for a spacecraft launched from Earth to reach Jupiter.

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