<P> Terrestrial planets have a solid planetary surface, making them substantially different from the larger giant planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states . </P> <P> All terrestrial planets in the Solar System have the same basic type of structure, such as a central metallic core, mostly iron, with a surrounding silicate mantle . The Moon is similar, but has a much smaller iron core . Io and Europa are also satellites that have internal structures similar to that of terrestrial planets . Terrestrial planets can have canyons, craters, mountains, volcanoes, and other surface structures, depending on the presence of water and tectonic activity . Terrestrial planets have secondary atmospheres, generated through volcanism or comet impacts, in contrast to the giant planets, whose atmospheres are primary, captured directly from the original solar nebula . </P> <P> The Solar System has four terrestrial planets: Mercury, Venus, Earth, and Mars . Only one terrestrial planet, Earth, is known to have an active hydrosphere . </P> <P> During the formation of the Solar System, there were probably many more terrestrial planetesimals, but most merged with or were ejected by the four terrestrial planets . </P>

A solar system has four earth-size terrestrial planets