Mercury, Venus, Earth, and Mars are the inner planets. Because they all have rocky surfaces, they are sometimes called the terrestrial planets Jupiter, Saturn, Uranus, and Neptune are the outer planets. They are much larger than the inner planets. Because they are made mostly of gas, they are called the gas giant planets. Small rocky objects called asteroids also orbit the sun. Many asteroids orbit the sun in the asteroid belt located between the orbits of Mars and Jupiter.