The mass of the Mars-sized exoplanet Kepler-138 b from transit timing

Abstract

Extrasolar planets that pass in front of their host star (transit) cause a temporary decrease in the apparent brightness of the star, providing a direct measure of the planet’s size and orbital period. In some systems with multiple transiting planets, the times of the transits are measurably affected by the gravitational interactions between neighbouring planets (Agol et al. 2005, Holman & Murray 2005). In favourable cases, the departures from Keplerian orbits (that is, unaffected by gravitational effects) implied by the observed transit times permit the planetary masses to be measured, which is key to determining their bulk densities (Holman et al. 2010). Characterizing rocky planets is particularly difficult, because they are generally smaller and less massive than gaseous planets. Therefore, few exoplanets near the size of Earth have had their masses measured. Here we report the sizes and masses of three planets orbiting Kepler-138, a star much fainter and cooler than the Sun. We determine that the mass of the Mars-sized inner planet, Kepler-138 b, is $0.066^{+0.059}_{-0.037}$ Earth masses. The middle and outer planets are both slightly larger than Earth…

Daniel Jontof-Hutter

Faculty