Abstract
Recent analyses1,2,3,4 of data from the NASA Kepler spacecraft5 have established that planets with radii within 25 per cent of the Earth’s () are commonplace throughout the Galaxy, orbiting at least 16.5 per cent of Sun-like stars1. 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 determined6,7 are Kepler-10b (1.42) and Kepler-36b (1.49), which are both significantly larger than the Earth. Recently, the planet Kepler-78b was discovered8 and found to have a radius of only 1.16. 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.
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Acknowledgements
This Letter was submitted simultaneously with the paper by Howard et al.17. Both papers are the result of a coordinated effort to carry out independent radial-velocity observations and studies of Kepler-78. Our team greatly appreciates the spirit of this collaboration, and we sincerely thank A. Howard and his team for the collegial work. We wish to thank the technical personnel of the Geneva Observatory, the Astronomical Technology Centre, the Smithsonian Astrophysical Observatory and the Telescopio Nazionale Galileo for their enthusiasm and competence, which made the HARPS-N project possible. The HARPS-N project was funded by the Prodex Program of the Swiss Space Office, the Harvard University Origins of Life Initiative, the Scottish Universities Physics Alliance, the University of Geneva, the Smithsonian Astrophysical Observatory, the Italian National Astrophysical Institute, the University of St Andrews, Queen’s University Belfast and the University of Edinburgh. P.F. acknowledges support from the European Research Council/European Community through the European Union Seventh Framework Programme, Starting Grant agreement number 239953, and from the Fundação para a Ciência e a Tecnologia through grants PTDC/CTE-AST/098528/2008 and PTDC/CTE-AST/098604/2008. The research leading to these results received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement number 313014 (ETAEARTH).
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Contributions
The underlying observation programme was conceived and organized by F.P., A.C.C., D.W.L., C.L., D. Ségransan, S.U. and E.M. Observations with HARPS-N were carried out by A.C.C., A.S.B., D.C., R.C., C.D.D., X.D., P.F., A.F.M.F., S.G., A.H., R.D.H., M.L.-M., V.N., D. Pollacco, D.Q., K.R., A. Sozzetti, A. Szentgyorgyi and C.A.W. The data-reduction pipeline was adapted and updated by C.L., who also implemented the correction for charge-transfer-efficiency errors and the automatic computation of the activity indicator log(R′HK). M.L.-M. and S.G. independently computed the S-index and log(R′HK) values. A.C.C., D. Ségransan, A.S.B. and X.D. analysed the data using the offset-correction method. A.C.C. and D. Ségransan re-analysed the data for the determination of the stellar rotational period based on the Kepler light curve. P.F. investigated for possible correlations between the radial velocities and the line bisector. L.A.B. conducted the stellar parameter classification analysis for the re-determination of the stellar parameters based on HARPS-N spectra. An independent determination of the stellar parameters was conducted by L.M. by analysis of the cross-correlation function. D. Ségransan compared many different models to fit the observed data and selected the most appropriate by using the Bayesian information criterion. D. Ségransan performed a detailed MCMC analysis for the determination of the planetary parameters, with contributions also from A.S.B. and A. Sozzetti. F.P. was the primary author of the manuscript, with important contributions by D.C., D. Ségransan, D. Sasselov, C.A.W., K.R. and C.L. All authors are members of the HARPS-N Science Team and have contributed to the interpretation of the data and the results.
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Extended data figures and tables
Extended Data Figure 1 Generalized Lomb–Scargle periodogram of several parameters measured by HARPS-N.
The panels show, from top to bottom, the periodogram of the radial velocities (RV) of Kepler-78, the line bisector (CCF-BIS), the activity indicator (log(R′HK)) and the full-width at half maximum (CCF-FWHM) of Kepler-78. The dotted and dashed horizontal lines represent the 10% and 1% false-alarm probabilities, respectively. The vertical lines show the stellar rotational period (solid) and its two first harmonics (dashed). All the indicators show excess energy at periods of around 6 d and above, indicating that the peak observed in the radial-velocity data at a period of about 10 d is most likely to have a stellar origin. The additional power in the line bisector periodogram at periods longer than 1 d is most probably induced by stellar spots.
Extended Data Figure 2 Kepler light curve of Kepler-78d.
The data have been de-trended using the PDC-MAP algorithm. Different colours represent different quarters of observation.
Extended Data Figure 3 Spectral analysis of the Kepler light curve.
Left panel, ACF of the Kepler light curve showing correlation peaks every 12.6 d and a decay on an e-folding timescale of ∼50 d. Right panel, the power spectral distribution of the Kepler light curve. Peaks are well identified at the stellar rotational period of 12.6 d and its two first harmonics. At shorter periods, the signal and several harmonics of the transiting planet Kepler-78b can be identified.
Extended Data Figure 4 Periodogram of the radial-velocity residuals after subtraction of the 4.2-d and 10.0-d stellar components.
The dotted and dashed horizontal lines represent the 10% and 1% false-alarm probabilities, respectively. The signature of Kepler-78b (and its aliases) can now clearly be identified with a false-alarm probability significantly lower than 1%.
Extended Data Figure 5 Probability density functions derived from the MCMC analysis.
Probability density function of the planetary mass (left) and probability density function of the planetary density (right).
Supplementary information
Supplementary Data
This file contains HARPS-N Data for Kepler-78b. From left to right are given: Julian Date, Radial Velocity RV, the estimated error σRV on the radial velocity, the Full-Width Half Maximum (FWHM) and the line-bisector (BIS) of the Cross-Correlation Function (CCF), the CaII activity indicator log(R’HK) and its error σlog(R′HK. (TXT 6 kb)
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Pepe, F., Cameron, A., Latham, D. et al. An Earth-sized planet with an Earth-like density. Nature 503, 377–380 (2013). https://doi.org/10.1038/nature12768
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DOI: https://doi.org/10.1038/nature12768
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