There are three types of planets in our solar system: terrestrial planets, gas giants, and ice giants.

The terrestrial planets (including Earth!) are solid, and made of metal and rock. The gas giants, for example Jupiter, are mostly made of hydrogen and helium--the same helium that goes into balloons! The ice giants such as Neptune are made of some hydrogen and helium, but much less than the gas giants, and also include oxygen, carbon, and sulfur. Some of the oxygen and hydrogen is combined in the ice giants to form frozen water, or ice!