  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 REarth has recently been proposed by several
publications in the literature (Rogers 2015;Weiss & Marcy 2014). Kepler-20b (Rp
~ 1.9 REarth) 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 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 (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 3.7
day period with a mass of 9.70+1.41-1.44 MEarth resulting in a mean density of
8.2+1.5-1.3 g/cc 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+3.08-3.61 MEarth and an orbital period of ~34 days in the gap between
Kepler-20f (P ~ 11 days) and Kepler-20d (P ~ 78 days).
