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Kepler-20 is a solar-type star (V = 12.5) hosting a compact system of fivetransiting planets, all packed within the orbital distance of Mercury in ourown Solar System. A transition from rocky to gaseous planets with a planetarytransition radius of ~1.6 REarth has recently been proposed by severalpublications in the literature (Rogers 2015;Weiss & Marcy 2014). Kepler-20b (Rp~ 1.9 REarth) has a size beyond this transition radius, however previous massmeasurements were not sufficiently precise to allow definite conclusions to bedrawn regarding its composition. We present new mass measurements of three ofthe planets in the Kepler-20 system facilitated by 104 radial velocitymeasurements from the HARPS-N spectrograph and 30 archival Keck/HIRESobservations, as well as an updated photometric analysis of the Kepler data andan asteroseismic analysis of the host star (MStar = 0.948+-0.051 Msun and Rstar= 0.964+-0.018 Rsun). Kepler-20b is a 1.868+0.066-0.034 REarth planet in a 3.7day period with a mass of 9.70+1.41-1.44 MEarth resulting in a mean density of8.2+1.5-1.3 g/cc indicating a rocky composition with an iron to silicate ratioconsistent with that of the Earth. This makes Kepler-20b the most massiveplanet with a rocky composition found to date. Furthermore, we report thediscovery of an additional non-transiting planet with a minimum mass of19.96+3.08-3.61 MEarth and an orbital period of ~34 days in the gap betweenKepler-20f (P ~ 11 days) and Kepler-20d (P ~ 78 days).

A 1.9 Earth radius rocky planet and the discovery of a non-transiting planet in the Kepler-20 system