HARPS-N radial velocities of HD 179070

An early detailed analysis by Howell et al. of the first 13 months (Q0-Q5) of Kepler light curves revealed transits of a planetary companion, Kepler-21b, with a radius of about 1.60+/-0.04R_{Earth}_ and an orbital period of about 2.7857days. However, they could not determine the mass of the planet from the initial radial velocity (RV) observations with Keck-HIRES, and were only able to impose a 2{sigma} upper limit of 10M_{Earth}_.

Here, we present results from the analysis of 82 new RV observations of this system obtained with HARPS-N, together with the existing 14 HIRES data points. We detect the Doppler signal of Kepler-21b with a RV semiamplitude K=2.00+/-0.65ms^-1^, which corresponds to a planetary mass of 5.1+/-1.7M_{Earth}_. We also measure an improved radius for the planet of 1.639 +0.019/-0.015R_{Earth}_, in agreement with the radius reported by Howell et al. We conclude that Kepler-21b, with a density of 6.4+/-2.1gcm^-3^, belongs to the population of small, <~6M_{Earth}_ planets with iron and magnesium silicate interiors, which have lost the majority of their envelope volatiles via stellar winds or gravitational escape.