Terrestrial planets

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.
Terrestrial planets possess secondary atmospheres; atmospheres generated through internal vulcanism or comet impacts, as opposed to the gas giants, which possess primary atmospheres, atmospheres captured directly from the original solar nebula.Earth's solar system has four terrestrial planets: Mercury, Venus, Earth and Mars. During the formation of the solar system, there were probably many more (planetesimals), but they have all merged with or been destroyed by the four remaining worlds in the solar nebula. Only one terrestrial planet, Earth, is known to have an active hydrosphere.
== Extrasolar terrestrial planets ==
The majority of planets found outside our solar system to date have been gas giants, simply because gas giants are larger and therefore easier to see or infer from observation. However, a number of extrasolar planets are known or suspected to be terrestrial. The first terrestrial planets ever detected outside our solar system were detected by Aleksander Wolszczan orbiting the pulsar PSR B1257+12. Their masses being 0.02, 4.3, and 3.9 Earth masses. The planets were observed because their transit caused interruptions in the pulsar's radio emissions. Had they not been orbiting around a pulsar, they would never have been found.