A planet is a large space object which revolves around a star. It also reflects that star's light. Nine planets have been discovered in our solar system. Mercury, Venus, Earth, and Mars are the planets closest to the Sun. They are called the inner planets. The inner planets are made up mostly of rock. The outer planets are Jupiter, Saturn, Uranus, Neptune, and Pluto. Jupiter, Saturn, Uranus, and Neptune are large balls of gases with rings around them. Pluto is the farthest planet from the Sun. It is coated with ice. All nine planets travel around the Sun in a different orbit.