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A terrestrial planet is a planet which is primarily composed of silicate rocks. The term is derived from the Latin word for Earth, "Terra", so an alternate definition would be that these are planets which are, in some notable fashion, "Earth-like". Terrestrial planets are substantially different from gas giants, which may not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. Terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but lacks an iron core. Terrestrial planets have canyons, craters, mountains, and volcanoes.
Earth's solar system has four terrestrial planets: Mercury, Venus, Earth and Mars. At one time there were probably many more terrestrials, but most have been ejected from the solar system or otherwise destroyed. Only one terrestrial planet, Earth, is known to have an active hydrosphere.
 The 29 small bodies of the solar system of diameter 400+ km, to scale. First row, left to right: Mercury, Venus, Earth and Moon, Mars and the four large main belt asteroids. Second row: the four galilean moons and Saturn's Mimas. Third row: Saturn's Enceladus, Tethys, Dione, Rhea and Titan. Fourth row: Saturn's Iapetus, Uranus' Miranda, Ariel, Umbriel and Titania. Fifth row: Uranus' Oberon, Neptune's Proteus and Triton, and finally Pluto and Charon. The Sun is in the background, also to scale. NASA is considering a proposed project called the Terrestrial Planet Finder, which will be capable of detecting terrestrial planets outside of our solar system (orbiting other stars). One suspected terrestrial planet has been discovered orbiting Mu Arae. All other known extrasolar planets are extremely large, and are most likely to be gas giants.
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