Planets come in different sizes, compositions and colors. The four planets closer to the Sun are called "rocky" planets. They are small in size and similar to Earth in composition. They have no rings and only two of them (Earth and Mars) have moons.

The four outer planets, also called "gas giants," are much larger than the rocky planets. They all have rings and have many moons. The gas giants are made up mostly of hydrogen, helium, frozen water, ammonia, methane and carbon monoxide.

Pluto, the most remote planet, might be little more than a giant comet. Its composition is similar to that of comets, and its orbit is quite different from that of the other planets. In fact, some scientists do not even consider it a planet because it has much more in common with comets than planets. Pluto has one moon: Charon.