Sub-Earth

Sub-Earth, also known as subterran planet or mercurian planet, is a classification of with mass ranging from 0.01 to 0.5 es or 0.8 to 40 es. Below 0.01 M$⊕$, the nonplanetary classification would be 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.

Characteristics
Sub-Earths are since they are too small to hold on considerable amounts of gases like s do. Sub-Earths tend to have short periods of geologic activity owing to their lower mass as these planets don't retain as well as mid-Earths and super-Earths. Because of their typically thinner atmospheres because of their lower gravity, s, s, and s 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 s on the surface. In conclusion, crater planet would be a common property of sub-Earths.

Habitability
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 in caves or aquifers, including Mars. s would only use since there is no sunlight for  to produce foods and free oxygen for s to use for survival.

Abundance
There are an estimated 177 billion sub-Earths in 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
There are only two dozen known sub-Earths as of 2015, two of which are in our --  and. The first extrasolar sub-Earth found was (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, discovered a system of three sub-Earths around  — Moritasgus, Tiresias , and Idunn ; 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 — Bhairav and Hiperborea; meanwhile both of these planets are unconfirmed. In February 2013, two more are found, both orbiting — Cobis  and Corianus. 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 ( (Peklenc),, (Yhi), and , (Kalki).

Related links

 * — article about this planetary class on Wikipedia