  Extrasolar planets that pass in front of their host star (transit) cause a
temporary decrease in the apparent brightness of the star once per orbit,
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. In favorable cases, the departures from Keplerian orbits implied by
the observed transit times permit planetary masses to be measured, which is key
to determining bulk densities. Characterizing rocky planets is particularly
difficult, since they are generally smaller and less massive than gaseous
planets. 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, a
star much fainter and cooler than the Sun. We measure the mass of the
Mars-sized inner planet based on on the transit times of its neighbour and
thereby provide the first density measurement for an exoplanet smaller than
Earth. The middle and outer planets are both slightly larger than Earth. The
middle planet's density is similar to that of Earth, while the outer planet is
less than half as dense, implying that it contains a greater portion of low
density components such as H2O and/or H2.
