A terrestrial planet, telluric planet or rocky planet is a planet that is primarily compose of silicate rocks. They are also the closest planets to the Sun. The terms are derived from Latin words for Earth (Terra and Tellus), so an alternative definition would be that these are planets which are, in some notable fashion, "Earth-like". Terrestrial planets are substantially different from gas giants, which might not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.