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

The Kepler-138 planetary system. Credit: Danielle Fulselaar, SETI Institute


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…


Animated Zoom-In of How Kepler is Revolutionizing the Understanding of the Relationship between Planet’s Masses & Radii: The animation shows the mass-radius diagram based on measurements of 127 exoplanets. The video begins by shows planets with masses similar to Jupiter and slowly zooms towards small masses and radii to display a comparison of the physical properties of the Kepler-138 planets relative to Earth, Venus, Mars and Mercury. Credit: Jason Rowe.

Discussion of Kepler-138 Results: Penn Stat’s Dr. Daniel Jontof-Hutter joins SETI Institute research scientists Jawson Rowe, Nathalie Cabrol, and Jill Tarter for a discussion about Kepler-138 system and the future of planet hunting.

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