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English: 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 accepted by the IAU are the inner planets closest to the Sun: Mercury, Venus, Earth and Mars. Among astronomers who use the geophysical definition of a planet, two or three planetary-mass satellites– Earth's Moon, Io, and sometimes Europa – may also be considered terrestrial planets; and so may be the rocky protoplanet-asteroids Pallas and Vesta. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth (Terra and Tellus), as these planets are, in terms ofstructure, Earth-like. Terrestrial planets are generally studied by geologists, astronomers, and geophysicists.Terrestrial planets have a solid planetary surface, making them substantially different from the larger gaseous planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.Most of the planets discovered outside the Solar System are giant planets, because they are more easily detectable. But since 2005, hundreds of potentially terrestrial extrasolar planets have also 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.It is likely that most known super-Earths are in fact gas planets similar to Neptune, as examination of the relationship between mass and radius of exoplanets (and thus density trends) shows a transition point at about two Earth masses. This suggests that this is the point at which significant gas envelopes accumulate. In particular, Earth and Venus may already be close to the largest possible size at which a planet can usually remain rocky. Exceptions to this are very close to their stars (and thus would have had their volatile atmospheres boiled away).In 2005, the first planets orbiting a main-sequence star and which show signs of being terrestrial planets were found: Gliese 876 d and OGLE-2005-BLG-390Lb. From 2007 to 2010, three potential terrestrial planets were found orbiting within the Gliese 581 planetary system. Two of them, Gliese 581c and Gliese 581d, as well as a disputed planet, Gliese 581g, are 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 possibly terrestrial planet, HD 85512 b, was discovered in 2011; it has at least 3.6 times the mass of Earth. The radius and composition of all these planets are unknown.The first confirmed terrestrial exoplanet, Kepler-10b, was found in 2011 by the Kepler Mission, specifically designed to discover Earth-size planets around other stars using the transit method. 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" and in the habitable zone of their star. Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range.Image created by Pablo Carlos Budassi in 2023 (pablocarlosbudassi.com)