THE MASS OF CoRoT-7b

The mass of CoRoT-7b, the first transiting super-Earth exoplanet, is still a subject of debate. A wide range of masses have been reported in the literature ranging from as high as 8 M⊕ to as low as 2.3 M⊕. This range in mass is largely due to the activity level of the star that contributes a significant amount of radial velocity (RV) 'jitter' and how the various methods correct this jitter. Although most mass determinations give a density consistent with a rocky planet, the lower value permits a bulk composition that can be up to 50% water. We present an analysis of the CoRoT-7b RV measurements that uses very few and simple assumptions in treating the activity signal. By analyzing those RV data for which multiple measurements were made in a given night, we remove the activity related RV contribution without any a priori model. We argue that the contribution of activity to the final RV curve is negligible and that the K-amplitude due to the planet is well constrained. This yields a mass of 7.42 ± 1.21 M⊕ and a mean density of ρ = 10.4 ± 1.8 gm cm–3. CoRoT-7b is similar in mass and radius to the second rocky planet to be discovered, Kepler-10b, and within the errors they have identical bulk densities—they are virtual twins. These bulk densities lie close to the density-radius relationship for terrestrial planets similar to what is seen for Mercury. CoRoT-7b and Kepler-10b may have an internal structure more like Mercury than the Earth.