A rocky composition for an Earth-sized exoplanet

Doppler spectroscopic measurements of the mass of the Earth-sized planet Kepler-78b reveal that its mean density is similar to Earth’s, suggesting a composition of rock and iron. Like Earth — but a lot hotter A few exoplanets of about the size or mass of Earth have been discovered. Now, for the first time, both size and mass have been determined for one of them. Kepler-78b, first described in August this year, is close-in to its host star, which it orbits every 8.5 hours. Two groups have been able to exploit the closeness of planet and star to make Doppler spectroscopic measurements of the mass of Kepler-78b. The teams, led by Andrew Howard and Francesco Pepe, used different telescopes to arrive at mass estimates of 1.69 ± 0.41 and 1.86 +0.38/−0.245 Earth masses, respectively. They calculate the planet's mean density at 5.3 and 5.57 g cm −3 , very similar to Earth's and consistent with an Earth-like composition of rock and iron. Planets with sizes between that of Earth (with radius ) and Neptune (about 4 ) are now known to be common around Sun-like stars 1 , 2 , 3 . Most such planets have been discovered through the transit technique, by which the planet’s size can be determined from the fraction of starlight blocked by the planet as it passes in front of its star. Measuring the planet’s mass—and hence its density, which is a clue to its composition—is more difficult. Planets of size 2–4 have proved to have a wide range of densities, implying a diversity of compositions 4 , 5 , but these measurements did not extend to planets as small as Earth. Here we report Doppler spectroscopic measurements of the mass of the Earth-sized planet Kepler-78b, which orbits its host star every 8.5 hours (ref. 6 ). Given a radius of 1.20 ± 0.09 and a mass of 1.69 ± 0.41 , the planet’s mean density of 5.3 ± 1.8 g cm −3 is similar to Earth’s, suggesting a composition of rock and iron.