Jupiter has the most moons. As of 2009, Jupiter has 63 known satellites or moons.

Jupiter is the largest planet in our solar system. It is the fifth planet from the sun and was named after the Roman god Jupiter. It is considered one of the gas planets with the others being Uranus, Neptune and Saturn. Jupiter is mainly made up of hydrogen, and is about one quarter helium. Scientists speculate that it may have a rock core. It rotates in about ten hours on its own axis and take about 12 years to make one orbit around the sun.

Jupiter’s largest moons were discovered by Galileo Galilei. Galileo, was an Italian astronomer, philosopher, mathematician and physicist. He was born on Feb. 15th 1564 and died on the 8th of Jan. in 1642. He discovered the moons of Jupiter and published his findings in 1610. Another astronomer, Simon Marius claimed to have observed the moons at the same time, however since he did not publish his findings the credit is given to Galileo.
The moons which Galileo discovered were named IO, Europa,, Ganymede and Callistro. Gannymede is actually larger than the planet Mercury. It is the largest known satellite in our solar system. Io, is the most volcanic satellite. Europa is believed to have more water than land.

In 2002, 11 new moons were discovered. They were first observed and recorded by David Jewitt and Scott shepherd both of whom were working at the University of Hawaii.  Eight more moons, were discovered in April of 2003.