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HD 179070, aka Kepler-21, is a V = 8.25 F6IV star and the brightest exoplanethost discovered by Kepler. An early detailed analysis by Howell et al. (2012)of the first thirteen months (Q0 - Q5) of Kepler light curves revealed transitsof a planetary companion, Kepler-21b, with a radius of about 1.60 +/- 0.04R_earth and an orbital period of about 2.7857 days. However, they could notdetermine the mass of the planet from the initial radial velocity observationswith 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 radial velocityobservations of this system obtained with HARPS-N, together with the existing14 HIRES data points. We detect the Doppler signal of Kepler-21b with a radialvelocity semi-amplitude K = 2.00 +/- 0.65 m/s, which corresponds to a planetarymass of 5.1 +/- 1.7 M_earth. We also measure an improved radius for the planetof 1.639 (+0.019, -0.015) R_earth, in agreement with the radius reported byHowell et al. (2012). We conclude that Kepler-21b, with a density of 6.4 +/-2.1 g/cm^3, belongs to the population of terrestrial planets with iron,magnesium silicate interiors, which have lost the majority of their envelopevolatiles via stellar winds or gravitational escape. The radial velocityanalysis presented in this paper serves as example of the type of analysis thatwill be necessary to confirm the masses of TESS small planet candidates.

Kepler-21b: A rocky planet around a V = 8.25 magnitude star