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American Astronomical Society, Astronomical Journal, 6(152), p. 160, 2016

DOI: 10.3847/0004-6256/152/6/160

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A 1.9 Earth Radius Rocky Planet and the Discovery of a Non-Transiting Planet in the Kepler-20 System

Journal article published in 2016 by Lars A. Buchhave ORCID, Courtney D. Dressing ORCID, Xavier Dumusque ORCID, Ken Rice, Andrew Michael Vanderburg ORCID, Annelies Mortier, Mercedes Lopez-Morales ORCID, Eric Lopez, Mia S. Lundkvist, Hans Kjeldsen, Laura Affer ORCID, Aldo S. Bonomo, David Charbonneau ORCID, Andrew Collier Cameron ORCID, Rosario Cosentino and other authors.
This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Abstract

Kepler-20 is a solar-type star (V = 12.5) hosting a compact system of five transiting planets, all packed within the orbital distance of Mercury in our own solar system. A transition from rocky to gaseous planets with a planetary transition radius of ~1.6 ${R}_{⊕ }$ has recently been proposed by several articles in the literature. Kepler-20b (${R}_{p}$ ~ 1.9 ${R}_{⊕ }$) has a size beyond this transition radius; however, previous mass measurements were not sufficiently precise to allow definite conclusions to be drawn regarding its composition. We present new mass measurements of three of the planets in the Kepler-20 system that are facilitated by 104 radial velocity measurements from the HARPS-N spectrograph and 30 archival Keck/HIRES observations, as well as an updated photometric analysis of the Kepler data and an asteroseismic analysis of the host star (${M}_{⋆ }$ = $0.948± 0.051$ ${M}_{⊙ }$ and ${R}_{⋆ }$ = $0.964± 0.018$ ${R}_{⊙ }$). Kepler-20b is a ${1.868}_{-0.034}^{+0.066}$ ${R}_{⊕ }$ planet in a 3.7 day period with a mass of ${9.70}_{-1.44}^{+1.41}$ ${M}_{⊕ }$, resulting in a mean density of ${8.2}_{-1.3}^{+1.5}$ ${\rm{g}}\,{\mathrm{cm}}^{-3}$, indicating a rocky composition with an iron-to-silicate ratio consistent with that of the Earth. This makes Kepler-20b the most massive planet with a rocky composition found to date. Furthermore, we report the discovery of an additional non-transiting planet with a minimum mass of ${19.96}_{-3.61}^{+3.08}$ ${M}_{⊕ }$ and an orbital period of ~34 days in the gap between Kepler-20f (P ~ 11 days) and Kepler-20d (P ~ 78 days). ; Astronomy