Planets with sizes between that of Earth (with radius ) and Neptune (about 4) are now known to be common around Sun-like stars1,2,3. 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 compositions4,5, 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.
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This Letter and another10 were submitted simultaneously and are the result of coordinated, independent radial-velocity observations and analyses of Kepler-78. We thank the HARPS-N team for their collegiality. We also thank E. Chiang, I. Crossfield, R. Kolbl, E. Petigura, and D. Huber for discussions, S. Howard for support, C. Dressing for a convenient packaging of stellar models, and A. Hatzes for a thorough review. This work was based on observations at the W. M. Keck Observatory granted by the University of Hawaii, the University of California, and the California Institute of Technology. We thank the observers who contributed to the measurements reported here and acknowledge the efforts of the Keck Observatory staff. We thank those of Hawaiian ancestry on whose sacred mountain of Mauna Kea we are guests. Kepler was competitively selected as the tenth Discovery mission with funding provided by NASA’s Science Mission Directorate. J.N.W. and R.S.-O. acknowledge support from the Kepler Participating Scientist programme. A.W.H. acknowledges funding from NASA grant NNX12AJ23G.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Wavelength-calibrated spectra of three stars near the age-sensitive Li i line (6,708 Å).
This line is not detected in the Kepler-78 spectrum, suggesting that lithium has been depleted, consistent with an age exceeding half a billion years for this K0 star. The lithium line is also not detected in the 4.6-billion-year-old Sun. (Gyr, billion years; Myr, million years.) It is clearly seen in the rotationally broadened spectrum of [PZ99] J161618.0 − 233947, a star whose spectral type (G8) is similar to that of Kepler-78, but that is much younger (about 11 million years)43. Additional iron and calcium lines are labelled.
Greyscale contours denote confidence levels, with thick black lines highlighting the 1σ, 2σ and 3σ contour levels. The strongest correlations are between transit depth, scaled semi-major axis and impact parameter.
The top panel shows the complete 45-day time series of relative radial velocities (red filled circles). Eight grey boxes highlight nights of intensive observations. The measurements from these nights are shown in the eight subpanels. In each subpanel, the radial velocities (red filled circles) and best-fit offset-slope model (solid black line) are shown. The radial-velocity curves for 100 randomly selected models from the MCMC chain are underplotted in grey, showing the range of variation within the model distribution.
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Howard, A., Sanchis-Ojeda, R., Marcy, G. et al. A rocky composition for an Earth-sized exoplanet. Nature 503, 381–384 (2013). https://doi.org/10.1038/nature12767
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