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Extrasolar planets that pass in front of their host star (transit) cause atemporary decrease in the apparent brightness of the star once per orbit,providing a direct measure of the planet's size and orbital period. In somesystems with multiple transiting planets, the times of the transits aremeasurably affected by the gravitational interactions between neighbouringplanets. In favorable cases, the departures from Keplerian orbits implied bythe observed transit times permit planetary masses to be measured, which is keyto determining bulk densities. Characterizing rocky planets is particularlydifficult, since they are generally smaller and less massive than gaseousplanets. Thus, few exoplanets near Earth's size have had their masses measured.Here we report the sizes and masses of three planets orbiting Kepler-138, astar much fainter and cooler than the Sun. We measure the mass of theMars-sized inner planet based on on the transit times of its neighbour andthereby provide the first density measurement for an exoplanet smaller thanEarth. The middle and outer planets are both slightly larger than Earth. Themiddle planet's density is similar to that of Earth, while the outer planet isless than half as dense, implying that it contains a greater portion of lowdensity components such as H2O and/or H2.

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