 Mars is small, about half the diameter of Earth. It's made of rock and metal. Jupiter, on the other hand, is the largest planet in the solar system. A giant ball of gas that's 20 times wider than Mars and about 3,000 times as massive. In fact, Jupiter is more massive than all the other planets and moons in the solar system combined. Mars and Jupiter are the fourth and fifth planets out from the sun, so you might expect them to be more alike.