That means one year on Jupiter is 12 years on Earth. It is very cold on Jupiter. Gravity is different, too. There is more gravity on Jupiter than on Earth. Someone who weighs 100 pounds on Earth would weigh about 240 pounds on Jupiter. How Many Moons Does Jupiter Have? Jupiter's four largest moons (Io, Europa, Ganymede and Callisto) are known as the Galilean satellites because they were discovered by Galileo Galilei in 1610. NASA's Juno mission will study how Jupiter, the largest planet in the solar system, formed and became the dynamic world it is today.