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The planet Venus (P2), photographed by Magellan spacecraft


Our home planet Earth (P3), photographed by Apollo 17

Mid-Earth (M-Class VI, symbol Eb, slogan: low-mass planet), 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.

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.

Abundance Edit

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 only 30 known mid-Earths as of February 2014, two of them in our solar system including Venus (Sol c, P2, 0.82 M) and our home planet, Earth (Sol d, P3, 1.00 M). Extrasolar mid-Earths are extremely hard to detect because they're so small, that's why there are only less than thirty such exoplanets identified out of more than a thousand. The first mid-Earth exoplanet discovered was Moneta (HD 10180 b, P454, 1.75 M) on August 24, 2010. In December 2011 Kepler discovered four mid-Earths: Tvashtri (Kepler-20e, P685, 0.72 M), Shantadurga (Kepler-20f, P686, 1.13 M), Yami (Kepler-70b, P687, 0.56 M), and Hari (Kepler-70c, P688, 0.68 M). Kepler also found two more mid-Earths in August 2012: Hyacinth (Kepler-54c, P805, 1.73 M), and Vili (Kepler-59b, P814, 1.53 M). Two months later, a mid-Earth was discovered in the nearest star system to Earth, Ixionidae (Alpha Centauri Bb, P832, 1.28 M). On June 7, 2013 Evanthes (Gliese 667 Ch, P900, 1.31 M) was discovered in a seven-planet system.

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