Terrestrial planets are rocky bodies that are primarily composed of silicate rocks or metals. They are characterized by a solid surface, and include Mercury, Venus, Earth, and Mars. These planets are located in the inner part of the solar system and have relatively high densities compared to gas giants. The defining features of terrestrial planets include their ability to support geological activity, such as volcanism and tectonics. Unlike gas giants, they have a thin atmosphere, which can vary significantly from one planet to another. For instance, Earth has a rich atmosphere that supports life, while Mars has a thin atmosphere that is mostly carbon dioxide.