The Planet Family
Once formed, planets come in a variety of classes that astronomers have distinguished over a period of several centuries. We are partial to the class of terrestrial planets because one of them is our home. In general, a terrestrial planet is one whose constituents are rocky and therefore similar to the Earth in composition, by comparison with the gas giant or ice giant planets (P 3 and P 4). By anyone’s definition, Mercury, Venus, Earth, and Mars are classified as the terrestrial planets in our Solar System. It is notable that these are the four inner planets of the system, certainly a clue to their formation mechanism.