Mass and Mass Scalings of Super-Earths

Abstract

Abstract The majority of the transiting planets discovered by the Kepler mission (called super-Earths here, includes the so-called “sub-Neptunes”) orbit close to their stars. As such, photoevaporation of their hydrogen envelopes etches sharp features in an otherwise bland space spanned by planet radius and orbital period. This, in turn, can be exploited to reveal the mass of these planets, in addition to techniques such as radial velocity and transit-timing-variation. Here, using updated radii for Kepler planet hosts from Gaia DR2, I show that the photoevaporation features shift systematically to larger radii for planets around more massive stars (ranging from M-dwarfs to F-dwarfs), corresponding to a nearly linear scaling between planet mass and its host mass. By modeling planet evolution under photoevaporation, one further deduces that the masses of super-Earths peak narrowly around 8 M ⊕(M */M ⊙). When such a stellar mass dependence is scaled out, Kepler planets appear to be a homogeneous population surprisingly uniform in mass, in core composition (likely terrestrial), and in initial mass fraction of their H/He envelope (a couple percent). The masses of these planets do not appear to depend on the metallicity values of their host stars, while they may weakly depend on the orbital separation. Taken together, the simplest interpretation of our results is that super-Earths are at the so-called “thermal mass”, where the planet’s Hill radius is equal to the vertical scale height of the gas disk.