  Kepler-78 (KIC 8435766) was identified by Sanchis-Ojeda et al. (2013) as
harbouring a transiting planet of 1.16 times the size of the Earth and an
orbital period of only 8.5 hours. While the exquisite Kepler photometry was
able to determine its radius and period, the mass of the planet (and thus its
mean density) remained unconstrained in the absence of precise radial-velocity
measurements. Here we present an accurate mass measurement of Kepler-78b using
the HARPS-N spectrograph, recently installed on the Telescopio Nazionale
Galileo (INAF) at the Roque de los Muchachos Observatory, La Palma, Spain.
These new data yield a mass of 1.86 Earth masses. The resulting mean density of
the planet is 5.57 grams per cubic centimetre, which is similar to that of the
Earth and implies a composition of iron and rock. Kepler-78b, which orbits a
Sun-like star called Kepler 78 located in the Cygnus constellation at a
distance of about 400 light years from us, is now the smallest exoplanet for
which both the mass and radius are known accurately.
