Jupiter is the fifth planet from our Sun. The planet Jupiter is named for the king of ancient Roman gods. It is the biggest planet in our solar system. More than 1,300 Earths could fit inside.

Jupiter is the most massive planet in our solar system, with four planet-sized moons and many smaller moons. Jupiter and it's moons form a kind of miniature solar system. The planet Jupiter resembles a star in composition. In fact, if it had been about eighty times more massive, it would have become a star rather than a planet.

On January 7, 1610, using his primitive telescope, astronomer Galileo Galilei saw four small 'stars' near Jupiter. He had discovered Jupiter's four largest moons, now called Io, Europa, Ganymede, and Callisto. Collectively, these four moons are known today as the Galilean satellites.

The planet Jupiter's four largest moons are called the Galilean satellites, after Italian astronomer Galileo Galilei, who observed them in 1610. The German astronomer Simon Marius claimed to have seen the moons around the same time, but he did not publish his observations and so Galileo is given the credit for their discovery. These large moons, named Io, Europa, Ganymede, and Callisto, are each distinctive worlds.

Jupiter's ring was discovered by Voyager 1 in a single image that was targeted specifically to search for a faint ring system. Subsequently, Voyager 2 was reprogrammed to take a more complete set of images. The ring is now known to be composed of three major components. The main ring is about 7000 km wide and has an abrupt outer boundary 129, 130 km from the center of the planet. The main ring encompasses the orbits of two small moons, Adrastea and Metis, which may act as the source for the dust that makes up most of the ring. At its inner edge the main ring merges gradually into the halo. The halo is a broad, faint torus of material about 20,000 km thick and extending halfway from the main ring down to the planet's cloudtops. Just outside the main ring is the broad and exceedingly faint gossamer ring, which extends out beyond the orbit of the moon Amalthea.