A terrestrial planet, also known as a rocky planet or telluric planet, is a planet that is composed primarily of silicate rocks or metals and has a solid surface. The term "terrestrial" comes from the Latin word "Terra," meaning Earth, as these planets are Earth-like in terms of structure. Within our solar system, the terrestrial planets are the four innermost planets closest to the Sun: Mercury, Venus, Earth, and Mars. Earth is the largest of the terrestrial planets and the only one with extensive regions of liquid water, which is necessary for life as we know it. Terrestrial planets have a solid planetary surface, making them substantially different from larger gaseous planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. Terrestrial planets can have surface structures such as canyons, craters, mountains, volcanoes, and others, depending on the presence at any time of an erosive liquid or tectonic activity or both.