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ABSTRACT 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
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support SIMILAR CONTENT BEING VIEWED BY OTHERS A SUPER-MASSIVE NEPTUNE-SIZED PLANET Article 30 August 2023 A SECONDARY ATMOSPHERE ON THE ROCKY EXOPLANET 55 CANCRI E Article 08 May 2024
EVIDENCE FOR THE VOLATILE-RICH COMPOSITION OF A 1.5-EARTH-RADIUS PLANET Article 15 December 2022 REFERENCES * Howard, A. W. et al. Planet occurrence within 0.25 AU of solar-type stars from
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F-type members of the Scorpius-Centaurus OB association. _Astrophys. J._ 746, 154 (2012) Article ADS Google Scholar Download references ACKNOWLEDGEMENTS 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. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Institute for
Astronomy, University of Hawaii at Manoa, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA, Andrew W. Howard, Benjamin J. Fulton & Evan Sinukoff * Department of Physics, and Kavli
Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, 02139, Massachusetts, USA Roberto Sanchis-Ojeda & Joshua N. Winn * Astronomy Department,
University of California, Berkeley, 94720, California, USA Geoffrey W. Marcy & Howard Isaacson * Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, 02138,
Massachusetts, USA John Asher Johnson * Department of Astronomy, Yale University, New Haven, 06510, Connecticut, USA Debra A. Fischer * Department of Astronomy and Astrophysics, University
of California, Santa Cruz, 95064, California, USA Jonathan J. Fortney Authors * Andrew W. Howard View author publications You can also search for this author inPubMed Google Scholar *
Roberto Sanchis-Ojeda View author publications You can also search for this author inPubMed Google Scholar * Geoffrey W. Marcy View author publications You can also search for this author
inPubMed Google Scholar * John Asher Johnson View author publications You can also search for this author inPubMed Google Scholar * Joshua N. Winn View author publications You can also
search for this author inPubMed Google Scholar * Howard Isaacson View author publications You can also search for this author inPubMed Google Scholar * Debra A. Fischer View author
publications You can also search for this author inPubMed Google Scholar * Benjamin J. Fulton View author publications You can also search for this author inPubMed Google Scholar * Evan
Sinukoff View author publications You can also search for this author inPubMed Google Scholar * Jonathan J. Fortney View author publications You can also search for this author inPubMed
Google Scholar CONTRIBUTIONS This measurement was conceived and planned by A.W.H., G.W.M., J.A.J., J.N.W. and R.S.-O. HIRES observations were conducted by A.W.H., G.W.M., H.I, B.J.F. and
E.S. The HIRES spectra were reduced and Doppler-analysed by A.W.H., G.W.M., H.I. and J.A.J. Data modelling was done primarily by A.W.H. and R.S.-O. A.W.H. was the primary author of the
manuscript, with important contributions from J.N.W., R.S.-O. and J.J.F. Figures were generated by A.W.H., R.S.-O, B.J.F. and E.S. All authors discussed the results, commented on the
manuscript and contributed to the interpretation. CORRESPONDING AUTHORS Correspondence to Andrew W. Howard or Roberto Sanchis-Ojeda. ETHICS DECLARATIONS COMPETING INTERESTS 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. EXTENDED DATA FIGURE 2
CORRELATIONS BETWEEN MODEL PARAMETERS IN THE TRANSIT ANALYSIS. Greyscale contours denote confidence levels, with thick black lines highlighting the 1_σ_, 2_σ_ and 3_σ_ contour levels. The
strongest correlations are between transit depth, scaled semi-major axis and impact parameter. EXTENDED DATA FIGURE 3 APPARENT RADIAL-VELOCITY VARIATIONS OF KEPLER-78 FOR THE OFFSET-SLOPE
MODEL. 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. SUPPLEMENTARY INFORMATION
SUPPLEMENTARY TABLES This file contains Supplementary Table 1, which shows time series radial velocity measurements and associated activity measurements. (PDF 145 kb) POWERPOINT SLIDES
POWERPOINT SLIDE FOR FIG. 1 POWERPOINT SLIDE FOR FIG. 2 RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE 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 Download citation * Received: 25 September 2013 * Accepted: 11 October
2013 * Published: 30 October 2013 * Issue Date: 21 November 2013 * DOI: https://doi.org/10.1038/nature12767 SHARE THIS ARTICLE Anyone you share the following link with will be able to read
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