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'''Sub-Earth''' (also known as '''subterrestrial planet''' ('''STEP''')) 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 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-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. |
Sub-Earths commonly lack substantial atmospheres because of their low gravity and weak magnetic fields, allowing stellar radiation to wear away the atmospheres. Due to their small sizes, and unless there's significant tidal forces when orbiting close to the parent star, sub-Earths also have short periods of geologic activity.