2014ApJ...789..154D

Kepler-10b was the first rocky planet detected by the Kepler satellite and confirmed with radial velocity follow-up observations from Keck-HIRES. The mass of the planet was measured with a precision of around 30%, which was insufficient to constrain models of its internal structure and composition in detail. In addition to Kepler-10b, a second planet transiting the same star with a period of 45 days was statistically validated, but the radial velocities were only good enough to set an upper limit of 20 M_⊕ for the mass of Kepler-10c. To improve the precision on the mass for planet b, the HARPS-N Collaboration decided to observe Kepler-10 intensively with the HARPS-N spectrograph on the Telescopio Nazionale Galileo on La Palma. In total, 148 high-quality radial-velocity measurements were obtained over two observing seasons. These new data allow us to improve the precision of the mass determination for Kepler-10b to 15%. With a mass of 3.33±0.49 M_⊕and an updated radius of 1.47_–0.02^+0.03 R_⊕, Kepler-10b has a density of 5.8±0.8 g/cm3, very close to the value predicted by models with the same internal structure and composition as the Earth. We were also able to determine a mass for the 45-day period planet Kepler-10c, with an even better precision of 11%. With a mass of 17.2±1.9 M_⊕ and radius of 2.35_–0.04^+0.09 R_⊕, Kepler-10c has a density of 7.1±1.0 g/cm3. Kepler-10c appears to be the first strong evidence of a class of more massive solid planets with longer orbital periods.