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'''Sub-Earth''' is a classification of ]s "substantially less massive" than ] and ].<ref name="STEP" /> In the ], this category includes ] and ]. Sub-Earth exoplanets are the most difficult planet type to detect because their small sizes and masses produce the weakest signal. Despite the difficulty, one of the first ]s found was a sub-Earth around a ] ]. ] opened the realm of sub-Earths by discovering them. On January 10, 2012 Kepler discovered the first three sub-Earths around an ordinary star ]. As of April 2014, Kepler has 45 confirmed planets that are smaller than Earth with 17 of them being smaller than 0.8 R<sub>ⴲ</sub>. In addition, there are over 280 planet candidates with <1R<sub>ⴲ</sub> with 119 of them being smaller than 0.8 R<sub>ⴲ</sub>.<ref name="STEP">{{cite arxiv |eprint=1308.6308 |title=Below One Earth Mass: The Detection, Formation, and Properties of Subterrestrial Worlds |date=2013-08-28 |work=] |first2=B. |first3=L. |first4=E. |class=astro-ph.EP |last1=Sinukoff |first1=E. |last2=Fulton |last3=Scuderi |last4=Gaidos}}</ref> <ref>http://exoplanetarchive.ipac.caltech.edu/</ref> |
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'''Sub-Earth''' is a classification of ]s "substantially less massive" than ] and ].<ref name="STEP" /> In the ], this category includes ] and ]. Sub-Earth exoplanets are the most difficult planet type to detect because their small sizes and masses produce the weakest signal. Despite the difficulty, one of the first ]s found was a sub-Earth around a ] ]. ] opened the realm of sub-Earths by discovering them. On January 10, 2012 Kepler discovered the first three sub-Earths around an ordinary star ]. As of June 2014, Kepler has 45 confirmed planets that are smaller than Earth with 17 of them being smaller than 0.8 R<sub>ⴲ</sub>. In addition, there are over 310 planet candidates with the estimated radius of <1R<sub>ⴲ</sub> with 135 of them being smaller than 0.8 R<sub>ⴲ</sub>.<ref name="STEP">{{cite arxiv |eprint=1308.6308 |title=Below One Earth Mass: The Detection, Formation, and Properties of Subterrestrial Worlds |date=2013-08-28 |work=] |first2=B. |first3=L. |first4=E. |class=astro-ph.EP |last1=Sinukoff |first1=E. |last2=Fulton |last3=Scuderi |last4=Gaidos}}</ref> <ref>http://exoplanetarchive.ipac.caltech.edu/</ref> |
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Sub-Earths commonly lack substantial atmospheres because of their low gravity and weak ]s, allowing stellar radiation to wear away the atmospheres.<ref name="STEP" /> Due to their small sizes, and unless there's significant ]s when orbiting close to the parent star, sub-Earths also have short periods of geologic activity. |
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Sub-Earths commonly lack substantial atmospheres because of their low gravity and weak ]s, allowing stellar radiation to wear away the atmospheres.<ref name="STEP" /> Due to their small sizes, and unless there's significant ]s when orbiting close to the parent star, sub-Earths also have short periods of geologic activity. |
