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Sub-Earth, also known as subterran planet or mercurian planet, is a classification of planet with mass ranging from 0.01 to 0.5 Earth masses or 0.8 to 40 Lunar masses. Below 0.01 M⊕, the nonplanetary classification would be dwarf planet or hypo-Earth. Hence it is the seventh most massive mass (least massive) class of planet, sub-Earth is rated M-Class VII and has the symbol Ec.
Sub-Earths are rocky since they are too small to hold on considerable amounts of gases like gas giants do. Sub-Earths tend to have short periods of geologic activity owing to their lower mass as these planets don't retain heat in their interiors as well as mid-Earths and super-Earths. Because of their typically thinner atmospheres because of their lower gravity, meteors, comets, and asteroids bombard the surface of sub-Earths more frequent than their larger cousins despite their smaller targets. That's because thinner atmospheres don't burn up the object as quickly as thicker atmospheres do. So sub-Earths tend to have a lot of impact craters on the surface. In conclusion, crater planet would be a common property of sub-Earths.
Because of the thinner atmospheres because of their small sizes, sub-Earths are not very conducive for life. So it is believed that underground habitats would be common on sub-Earths in the habitable zone in caves or aquifers, including Mars. Producers would only use chemosynthesis since there is no sunlight for photosynthesis to produce foods and free oxygen for consumers to use for survival.
There are an estimated 177 billion sub-Earths in our galaxy alone, making it the second most abundant mass class of planet after mid-Earth. This corresponds that 216‰ of all 820 billion planets in our galaxy are sub-Earths.
Known sub-Earths Edit
There are only two dozen known sub-Earths as of 2015, two of which are in our solar system -- Mercury and Mars. The first extrasolar sub-Earth found was PSR B1257+12 b (Sisyphus), which was one of the first exoplanets discovered on April 22, 1994. On January 10, 2012, nearly 18 years since the last discovery of a sub-Earth exoplanet, Kepler discovered a system of three sub-Earths around Kepler-42 — Moritasgus (Kepler-42b), Tiresias (Kepler-42c), and Idunn (Kepler-42d); these are the first sub-Earths known around a main sequence star besides the Sun. Later in 2012, three more sub-Earths were discovered, two around Gliese 436 — Bhairav and Hiperborea; meanwhile both of these planets are unconfirmed. In February 2013, two more are found, both orbiting Kepler-37 — Cobis (Kepler-37b) and Corianus (Kepler-37c). Cobis is the least massive exoplanet known around an ordinary star, massing 40 times less than Earth's. In December 2013, three more are found: Peklenc (Kepler-102b (Peklenc), Kepler-106b, (Yhi), and Kepler-408b, (Kalki).
- Sub-Earth — article about this planetary class on Wikipedia