Jupiter is the fifth planet away from the Sun in our solar system. It is also the largest planet in the system. Jupiter is called a "gas giant" because most of it is made of the gas hydrogen. Jupiter has a giant storm called the "Great Red Spot", which is three times bigger than the planet Earth, and has probably existed since the 1600's.
Jupiter has lots of moons. The four largest are called Europa, Ganymede, Callisto and Io. They were first seen by Gallileo in the 1600s. In the 1600s, everybody thought that everything went around the planet Earth. When Gallileo saw Jupiter's moons, he showed that not everything went around the Earth.