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# The Extrasolar Planets Kepler-20 e and Kepler 20 f

The exoplanets Kepler 20 e and Kepler-20 f were made famous by being advertized as the first rocky Earth-sized planets (not necessarily Earth-like) confirmed in the sample of exoplanet canidates observed by the Kepler mission. (The announcement paper is Fressin et al. 2011.) Up to that point, suspected rocky planets were larger than Earth. The radii given in the paper are 87% and 103% that of Earth for Kepler-20 e and Kepler-20 f respectively. (Note that the Extrasolar Planets Encyclopedia records slightly different radii for planets.) The two proposed rocky planets are part of a 5-planet system.

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However, since the exoplanets were discovered by the transit method and no other follow-ups to measure mass had yet been done, the masses had to be guessed and this meant guessing the composition. This means that the claim that the planets are rocky is rather circular. You can read more details about this in an artcile at exoplanetsdigest.com.

The star Kepler-20 is about 945 light years from Earth, which is more than 90 times further away than the nearest confirmed exoplanet. The star's mass is similar to our Sun ($M=0.912\pm0.035 \ M_{\odot}$), and its size is also similar to our Sun ($R=0.944\pm0.095 \ R_{\odot}$).

Both planets orbit very close to the host star so will be very hot and unlikely to be habitable (at least by any life-form that we know of on Earth). The maximum star-planet distances (semimajor axes) are only 5% and 11% of the Earth-Sun distance for Kepler-20 e and Kepler-20 f respectively. (More precisely, 0.051 and 0.11 astronomical units (AU) for Kepler-20 e and Kepler-20 f respectively.) The orbitals periods are correspondingly short: 6.1 and 19.6 Earth days for Kepler-20 e and Kepler-20 f respectively.

File under: Kepler-20 e and Kepler 20 f rocky terrestrial planets in the Kepler candidates sample; Kepler-20 5-planet system.