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A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth (Terra and Tellus), as these planets are, in terms of composition, "Earth-like".
Terrestrial planets have a solid planetary surface, making them substantially different from the usually larger gas giants, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.
All terrestrial planets have approximately the same type of structure: 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 possess secondary atmospheres, generated through internal volcanism or comet impacts, in contrast to the gas giants, whose atmospheres are primary, captured directly from the original solar nebula.
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.
Dwarf planets, like Ceres and Pluto, and other large asteroids are similar to terrestrial planets in the fact that they do have a solid surface, but are, on average, composed of more icy materials (Ceres and Pluto have a density of 2.1 g cm−3, and Haumea's density is similar to Pallas's 2.8 g cm−3).
The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure. A greater uncompressed density indicates greater metal content. Uncompressed density differs from the true average density because compression within planet cores increases their density; the average density depends on planet size as well as composition.
|Object||Density (g cm−3)||Semi-major axis (AU)|
The density of terrestrial planets trends towards lower values as the distance from the Sun increases. The rocky minor planet Vesta orbiting outside of Mars is less dense than Mars still, at 3.4 g cm−3.
It is unknown whether extrasolar terrestrial planets in general will also follow this trend.
Most of the planets found outside the Solar System are gas giants, because they are more easily detectable. But since 2005, hundreds of potentially terrestrial extrasolar planets have been found, with several being confirmed as terrestrial. Most of these are super-Earths, i.e. planets with masses between Earth's and Neptune's; super-Earths may be gas planets or terrestrial, depending on their mass and other parameters.
When 51 Pegasi b, the first planet found around a star still undergoing fusion, was discovered, many astronomers assumed it to be a gigantic terrestrial, because it was assumed no gas giant could exist as close to its star (0.052 AU) as 51 Pegasi b did. It was later found to be a gas giant.
In 2005, the first planets around main-sequence stars that may be terrestrial were found: Gliese 876 d, has a mass 7 to 9 times that of Earth and an orbital period of just two Earth days. It orbits the red dwarf Gliese 876, 15 light years from Earth. OGLE-2005-BLG-390Lb, about 5.5 times the mass of Earth, orbits a star about 21,000 light years away in the constellation Scorpius. From 2007 to 2010, three (possibly four) potential terrestrial planets were found orbiting the red dwarf Gliese 581. The smallest, Gliese 581 e, is only about 1.9 Earth mass, but orbits very close to the star. An ideal terrestrial planet would be 2 Earth masses with a 25-day orbital period around a red dwarf. Two others, Gliese 581 c and Gliese 581 d, as well as a disputed planet, Gliese 581 g, are more-massive super-Earths orbiting in or close to the habitable zone of the star, so they could potentially be habitable, with Earth-like temperatures.
Another potentially habitable and terrestrial planet, HD 85512 b, was discovered in 2011; it has at least 3.6 times the mass of Earth. But the radius and composition of all these planets are unknown.
In the same year, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including six that are "Earth-size" or "super-Earth-size" (i.e. they have a radius less than 2 Earth radii) and in the habitable zone. Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range (see image).
A number of other telescopes capable of directly imaging extrasolar terrestrial planets are also being designed. These include the Terrestrial Planet Finder, Space Interferometry Mission, Darwin, New Worlds Mission, and Overwhelmingly Large Telescope.
The following exoplanets have a density of at least 5 g/cm3 and a mass below Neptune's and are thus very likely terrestrial:
The Neptune-mass planet Kepler-10c also has a density >5 g/cm3 and is thus very likely terrestrial.
In 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth- and super-Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting Sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. However, this does not give estimates for the number of extrasolar terrestrial planets, because there are planets as small as Earth that have been shown to be gas planets (see KOI-314c).
Several possible classifications for terrestrial planets have been proposed: