Kepler-102: Masses and Compositions for a Super-Earth and Sub-Neptune Orbiting an Active Star

Radial velocity (RV) measurements of transiting multiplanet systems allow us to understand the densities and compositions of planets unlike those in the solar system. Kepler-102, which consists of five tightly packed transiting planets, is a particularly interesting system since it includes a super-Earth (Kepler-102d) and a sub-Neptune-sized planet (Kepler-102e) for which masses can be measured using RVs. Previous work found a high density for Kepler-102d, suggesting a composition similar to that of Mercury, while Kepler-102e was found to have a density typical of sub-Neptune size planets; however, Kepler-102 is an active star, which can interfere with RV mass measurements. To better measure the mass of these two planets, we obtained 111 new RVs using Keck/HIRES and Telescopio Nazionale Galileo/HARPS-N and modeled Kepler-102's activity using quasiperiodic Gaussian process regression. For Kepler-102d, we report a mass upper limit M ( d ) < 5.3 M (circle plus) (95% confidence), a best-fit mass M ( d ) = 2.5 +/- 1.4 M (circle plus), and a density rho ( d ) = 5.6 +/- 3.2 g cm(-3), which is consistent with a rocky composition similar in density to the Earth. For Kepler-102e we report a mass M ( e ) = 4.7 +/- 1.7 M (circle plus) and a density rho ( e ) = 1.8 +/- 0.7 g cm(-3). These measurements suggest that Kepler-102e has a rocky core with a thick gaseous envelope comprising 2%-4% of the planet mass and 16%-50% of its radius. Our study is yet another demonstration that accounting for stellar activity in stars with clear rotation signals can yield more accurate planet masses, enabling a more realistic interpretation of planet interiors.