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Planets with sizes between that of Earth (with radius R$_\oplus$) and Neptune(about 4 R$_\oplus$) are now known to be common around Sun-like stars. Mostsuch planets have been discovered through the transit technique, by which theplanet's size can be determined from the fraction of starlight blocked by theplanet as it passes in front of its star. Measuring the planet's mass-and henceits density, which is a clue to its composition-is more difficult. Planets ofsize 2-4 R$_\oplus$ have proven to have a wide range of densities, implying adiversity of compositions, but these measurements did not extend down toplanets as small as Earth. Here we report Doppler spectroscopic measurements ofthe mass of the Earth-sized planet Kepler-78b, which orbits its host star every8.5 hours. Given a radius of 1.20 $\pm$ 0.09 R$_\oplus$ and mass of 1.69 $\pm$0.41 M$_\oplus$, the planet's mean density of 5.3 $\pm$ 1.8 g cm$^{-3}$ issimilar to the Earth's, suggesting a composition of rock and iron.

A Rocky Composition for an Earth-sized Exoplanet