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Mid-Earth, also known as terran planet, is a classification of planet with mass ranging from 0.5 to 2 Earth masses or 0.0016 to 0.0063 Jupiter masses. Hence it is the sixth most massive mass class of planet, mid-Earth is rated M-Class VI and has the symbol Eb.
Characteristics and habitability Edit
Mid-Earths typically have rocky surface since they don't usually have enough mass to have predominant fluid composition with gaseous envelope. Many mid-Earths have similar bulk properties to Earth. Those planets tend to have right amount of heat and convection in their interiors to initiate plate tectonics and volcanism. Mid-Earths also tend to have thick atmospheres as they have gravity strong enough to keep gases from escaping. On average, those planets have atmospheres thicker than sub-Earths but thinner than super-Earths, because their gravitational pull of mid-Earths are stronger than sub-Earths but weaker than super-Earths. That amount of atmosphere with abundant supply of oxygen and carbon dioxide would be conducive for multicellular life.
There are an estimated 185 billion mid-Earths in our galaxy alone, making it the most abundant mass class of planet. This corresponds that 226‰ of all 820 billion planets in our galaxy are mid-Earths.
Known mid-Earths Edit
There are over 40 known mid-Earths as of 2015, including two in our solar system — Venus and Earth. Extrasolar mid-Earths are extremely hard to detect because they're so small, that's why there are only less than fifth such exoplanets identified out of nearly 2000 total. The first mid-Earth exoplanet discovered was HD 10180 b (Moneta) on August 24, 2010. In December 2011 Kepler discovered four mid-Earths: Kepler-20e (Tvashtri), Kepler-20f (Shantadurga), Kepler-70b (Yami), and Kepler-70c (Hari). Kepler also found two more mid-Earths in August 2012 — Kepler-54c (Hyacinth) and Kepler-59b (Vili). Two months later, a mid-Earth was discovered in the nearest star system to Earth, Alpha Centauri Bb (Ixionidae), but the planet's existence has not been confirmed. On June 7, 2013 a mid-Earth was discovered around Gliese 667 C, but subsequent observations showed that this and signals of two other planets in the system were artifacts of noise and stellar activity.