A
terrestrial planet is a
planet which is mostly composed of
silicate rocks. The term is derived from the Greek 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.
Luna 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.
NASA is considering a proposed project called the Terrestrial Planet Finder, which will be capable of detecting terrestrial planets in other solar systems. All currently-known extrasolar planets are extremely large, and are most likely to be gas giants.
See also:
All Wikipedia text
is available under the
terms of the GNU Free Documentation License