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

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 ${R}_{\oplus }$ has recently been proposed by several articles in the literature. Kepler-20b (${R}_{p}$ ~ 1.9 ${R}_{\oplus }$) 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 (${M}_{\star }$ = $0.948\pm 0.051$ ${M}_{\odot }$ and ${R}_{\star }$ = $0.964\pm 0.018$ ${R}_{\odot }$). Kepler-20b is a ${1.868}_{-0.034}^{+0.066}$ ${R}_{\oplus }$ planet in a 3.7 day period with a mass of ${9.70}_{-1.44}^{+1.41}$ ${M}_{\oplus }$, 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 ${19.96}_{-3.61}^{+3.08}$ ${M}_{\oplus }$ and an orbital period of ~34 days in the gap between Kepler-20f (P ~ 11 days) and Kepler-20d (P ~ 78 days).