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

L. A. Buchhave, C. D. Dressing, X. Dumusque, K. Rice, A. Vanderburg, A. Mortier, M. López-Morales, E. Lopez, M. Lundkvist, H. Kjeldsen, L. Affer, A. S. Bonomo, D. Charbonneau, A. C. Cameron, R. Cosentino, P. Figueira, A. F. M. Fiorenzano, A. Harutyunyan, R. D. Haywood, J. A. Johnson, D. W. Latham, C. Lovis, L. Malavolta, M. Mayor, G. Micela, E. Molinari, F. Motalebi, V. Nascimbeni, F. Pepe, D. F. Phillips, G. Piotto, D. Pollacco, D. Queloz, D. Sasselov, D. Segransan, A. Sozzetti, S. Udry, C. A. Watson

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 (Rp ~ 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 g 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).

Keywords
planetary systems, planets and satellites: composition, stars: individual, techniques: radial velocities

Notes
Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofísica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.

The Astronomical Journal
Volume 152, Number 6
2016 December

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia