Jupiter is the largest planet in the solar system, the 5th from the
Sun. It is not quite as bright as
Venus when seen from
Earth.
Measured across, Jupiter is ten times the size of the Earth and one-tenth the size of the Sun. Like the Sun and
Saturn, it is mostly
hydrogen and
helium. There may be a small
core of rock and perhaps
ice at the center. This is surrounded by an ocean of liquid hydrogen and helium in the form of a metal. Above this is a layer of liquid hydrogen and helium, and above this a thick atmosphere.
Jupiter is radiating energy, possibly as a result of radioactivity. The most famous feature is the 
Great Red Spot, which may be a huge storm in the atmosphere. If so it must be quite a storm, as it has been recorded for 300 years!
Jupiter also has a faint ring made of dust which was only recently discovered.
Far more easy to see are the moons. The four largest of these were seen by Galileo when he made one of the first telescopes. The way they moved round the planet convinced him that the old idea, that everything moves round the Earth, must be wrong.
The following was kindly contributed by Russell Odell
The sun contains 99.9 percent of the mass in the solar system. The remaining 0.1 percent make up the other planets and their moons, and Jupiter took most of that mass. If Jupiter were a shell, all the other planets and their moons could fit inside with room to spare. Jupiter could hold 317 of our earth's.
Jupiter's diameter is 88,000 miles, thirty times the width of the United States. It orbits the sun at 8.1 miles per second (mps), or 19,160 miles per hour (mph). It is 485 million miles from the sun making it a very cold planet in the range of minus 240 degrees F. However, due to the enormous pressure of its mass, the center is estimated to be 54,000 degrees F., or five times hotter than the surface of the sun. (The interior of the sun is 40 million ºF.)
The earth rotates on its axis in 24 hours establishing day and night. Jupiter rotates in 9 hours, 54 minutes, giving it the shortest day and night of any of the planets. The surface speed of the earth at its equator is a little over 1,000 mph. Jupiter's surface speed at its equator is 30,000 mph.
Jupiter is in full view six months of the year. It is very bright. Only the planet Venus is brighter. If you compare the brightness of Jupiter to the stars, only one star, Sirius, is brighter.
Jupiter, like Saturn, Uranus and Neptune is a ball of gas while the other four planets are solid (Mercury, Venus, Earth & Mars).
Galileo discovered Jupiter and three of its moons on January 7, 1610. Six nights later, January 13th , he discovered the fourth moon. He named them, J1, J2, J3, and J4.
A few nights later the Dutch astronomer Simon Marius, quite independently, saw the moons through his telescope. Marius gave them names from mythology after the sons and daughters of the Greek gods naming them Io, Europa, Gannymede and Callisto. Later, when Marius announced his discovery he was ridiculed by the followers of the great Galileo who was a patron of the Grand Duke of Tuscany. Marius was ostracized and no one would honor the names he had given to the moons. As other moons were discovered Galileo's method of numbering prevailed over giving names to the moons. These first four moons of Jupiter are known as the Galilean Moons. However, later, to recognize the work of Marius, the names he gave the moons are the names we know them by today.
The 14th moon was discovered in October, 1975, by the American astronomer Charles T. Kowal. Its distance from the planet has not yet been determined.
Jupiter's 12th moon is the incredible distance of 14, 880,000 miles from the planet. With such a distance, it takes the moon two years to orbit its planet.
The 16 moons of Jupiter is a story by itself. For example, its four outer most moons are 15 million miles from the planet and orbit in the opposite direction of Jupiter’s other moons. .
One of the mysteries of Jupiter is its big red spot. The Space Probe Pioneer Ten flew by Jupiter on December 3, 1973. It was followed a year later by Pioneer Eleven. In March of 1979 Voyager One sent back pictures of the big Red Spot. Voyager Two sent back additional pictures in July (1974). The Red Spot was first reported by Robert Hooke in 1664. Several years later it was also noted by Cassini, the Italian-French astronomer. Drawings of it appeared in 1831 by William Dawes and later by other astronomers. We have known about the great Red Spot of Jupiter for more than 300 years.
We do not know why this Red Spot grows and diminishes in size from 7000 miles wide and 13,000 to 30,000 miles long. We know it rises and sinks in the gaseous clouds but don't know why. Neither do we know why it drifts east and west but never north or south.
Did you ever wonder how or why the planet Jupiter came into existence? What or who determined its size? What keeps it rotating, where does the energy come from to keep it zooming around the sun at 19,160 mph? Why does it rotate on its axis anti-clockwise while the planet Venus rotates clockwise? Why does Jupiter have 16 moons but Mercury and Venus have none? Science says this all happened purely by chance over a period of billions of years.
However, we have many theories that explain its creation from many different viewpoints, but, which theory is right?
The gravitational fields of Jupiter are so strong they disintegrated the Shoemaker-Levy 9 Comet into 20 fragments before it hit the planet.
