Terrestrial planets, as their name suggests, have a size and structure similar to Earth's. Within our Solar System, Mercury, Venus, Earth, and Mars fit in this category. Orbiting in the inner part of the Solar System, close to the Sun, these rocky worlds are too small and too warm to have captured massive hydrogen envelopes such as those that cloak the outer planets. Nor have they the array of moons possessed by their cold, giant brethren. In fact, of the terrestrial planets, only Earth has a large moon, although Mars is orbited by two tiny captured asteroids.