Abstract
Recent analyses of data from the NASA Kepler spacecraft have established that planets with radii within 25 per cent of the Earth's (R Earth symbol) are commonplace throughout the Galaxy, orbiting at least 16.5 per cent of Sun-like stars. Because these studies were sensitive to the sizes of the planets but not their masses, the question remains whether these Earth-sized planets are indeed similar to the Earth in bulk composition. The smallest planets for which masses have been accurately determined are Kepler-10b (1.42 R Earth symbol) and Kepler-36b (1.49 R Earth symbol), which are both significantly larger than the Earth. Recently, the planet Kepler-78b was discovered and found to have a radius of only 1.16 R Earth symbol. Here we report that the mass of this planet is 1.86 Earth masses. The resulting mean density of the planet is 5.57 g cm(-3), which is similar to that of the Earth and implies a composition of iron and rock.
Highlights
Recent analyses[1,2,3,4] of data from the NASA Kepler spacecraft[5] have established that planets with radii within 25 per cent of the Earth’s (R›) are commonplace throughout the Galaxy, orbiting at least 16.5 per cent of Sun-like stars[1]
In addition to its diminutive size, the planet Kepler-78b is interesting because the light curve recorded by the Kepler spacecraft reveals the secondary eclipse of the planet behind the star as well as the variations in the light received from the planet as it orbits the star and presents different hemispheres to the observer
A direct measurement of the mass of Kepler-78b would permit an evaluation of its mean density and, by inference, its composition, and motivated this study
Summary
Recent analyses[1,2,3,4] of data from the NASA Kepler spacecraft[5] have established that planets with radii within 25 per cent of the Earth’s (R›) are commonplace throughout the Galaxy, orbiting at least 16.5 per cent of Sun-like stars[1]. Haywood[2], Keith Horne[2], Mercedes Lopez-Morales[3], Christophe Lovis[1], Luca Malavolta[10,11], Michel Mayor[1], Giusi Micela[12], Fatemeh Motalebi[1], Valerio Nascimbeni[11], David Phillips[3], Giampaolo Piotto[10,11], Don Pollacco[13], Didier Queloz[1,14], Ken Rice[15], Dimitar Sasselov[3], Damien Segransan[1], Alessandro Sozzetti[6], Andrew Szentgyorgyi3 & Christopher A.
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