In 2011, most experts came to agree that Jupiter has a total of 67 satellites. Its four largest moons are known as the Galilean satellites: Europa, lo, Callisto and Ganymede. Around 50 of its satellites are less than 6.2 miles in diameter.
Jupiter's moons have orbital spans that range from 7 hours to approximately 3 years. Some moons orbit in a circular fashion, while the ones farthest out maintain more unusual orbits. The outermost 33 moons have orbits that are opposite Jupiter's orbit, indicating that these satellites were originally asteroids that became trapped by Jupiter's gravity.
The Galilean satellites orbit closer to the planet. Ganymede is the largest moon in the solar system, and has its own magnetic field. Io was the first moon discovered by Galileo, and it is around the same age as Jupiter.