Subjects

Abstract

The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a ‘fossil’ record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets1,2,3,4, supporting the model that planets form by accumulation of dust and ice particles5. Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets4,6,7,8,9. However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA’s Kepler mission10, including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation.

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Acknowledgements

The Kepler mission was competitively selected as the tenth NASA Discovery mission. Funding for this mission is provided by NASA’s Science Mission Directorate. The Centre for Star and Planet Formation is funded by the Danish National Research Foundation. L.A.B. was funded by the Carlsberg Foundation. A.J. was partially funded by the European Research Council under ERC Starting Grant agreement 278675-PEBBLE2PLANET.

Author information

Affiliations

  1. Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark

    • Lars A. Buchhave
    •  & Terese Hansen
  2. Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen, Denmark

    • Lars A. Buchhave
    •  & Martin Bizzarro
  3. Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA

    • David W. Latham
    • , Guillermo Torres
    • , Gilbert A. Esquerdo
    • , John C. Geary
    • , Robert P. Stefanik
    •  & Samuel N. Quinn
  4. Lund Observatory, Lund University, Box 43, 221 00 Lund, Sweden

    • Anders Johansen
  5. SETI Institute/NASA Ames Research Center, Moffett Field, California 94035, USA

    • Jason F. Rowe
  6. San Jose State University, San Jose, California 95192, USA

    • Natalie M. Batalha
  7. NASA Ames Research Center, Moffett Field, California 94035, USA

    • William J. Borucki
    •  & Stephen T. Bryson
  8. University of Texas, Austin, Texas 78712, USA

    • Erik Brugamyer
    • , Caroline Caldwell
    • , William D. Cochran
    • , Michael Endl
    •  & Paul Robertson
  9. NASA Exoplanet Science Institute/California Institute of Technology, Pasadena, California 91109, USA

    • David R. Ciardi
  10. University of Florida, 211 Bryant Space Sciences Center, Gainesville, Florida 32611, USA

    • Eric B. Ford
  11. Space Telescope Science Institute, Baltimore, Maryland 21218, USA

    • Ronald L. Gilliland
  12. University of California, Berkeley, California 94720, USA

    • Howard Isaacson
    •  & Geoffrey W. Marcy
  13. Bowling Green State University, Bowling Green, Ohio 43403, USA

    • John B. Laird
  14. Centre for Astrophysics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK

    • Philip W. Lucas
  15. Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, CII 9015, 110 8th Street, Troy, New York 12180, USA

    • Jon A. Morse
  16. Las Cumbres Observatory, Global Telescope Network, Santa Barbara, California 93117, USA

    • Avi Shporer
  17. Department of Physics, University of California, Santa Barbara, California 93106, USA

    • Avi Shporer
  18. Bay Area Environmental Research Institute/NASA Ames Research Center, Moffett Field, California 94035, USA

    • Martin Still

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Contributions

L.A.B. led the project and developed the classification tools for the metallicity analysis. D.W.L., A.J. and M.B. contributed to the discussion of the theoretical implications of the data. G.T. supplied the isochrone fitting tools. J.F.R. provided the planet radii from the Kepler photometry. D.W.L., C.C., W.D.C., G.A.E., E.B., M.E., J.C.G., T.H., G.W.M., P.R., R.P.S. and S.N.Q. worked on gathering the spectroscopic observations. D.W.L., W.J.B., S.T.B., N.M.B., D.R.C., W.D.C., R.L.G., P.W.L., G.W.M., A.S., M.S., H.I., E.B.F. and S.N.Q. worked on identifying the Kepler planets and eliminating false positives. W.J.B. led the Kepler mission. J.B.L., J.A.M. and D.W.L. worked on the synthetic stellar model library. All authors discussed the results and commented on the manuscript. L.A.B. wrote the paper with equal input from D.W.L., A.J. and M.B.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Lars A. Buchhave.

Supplementary information

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  1. 1.

    This file contains Supplementary Figures 1-7 and Supplementary Tables 1-4.

    This file contains Supplementary Figures 1-7 and Supplementary Tables 1-4. This file contains Supplementary Text 1-5, Supplementary Figures 1-3, Supplementary References and Supplementary Table 1. Please note that the file, which provides the ASCII text version of the extracted wavelength and calibrated spectra used in the analysis, is available at the following link: http://exoplanets.dk/data/Nature/Kepler_metal_spectra_coadd.txt

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https://doi.org/10.1038/nature11121

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