  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. 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 R$_\oplus$ have proven to have a wide range of densities, implying a
diversity of compositions, but these measurements did not extend down 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. 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$^{-3}$ is
similar to the Earth's, suggesting a composition of rock and iron.
