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.
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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.
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Supplementary information
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 (PDF 404 kb)
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Buchhave, L., Latham, D., Johansen, A. et al. An abundance of small exoplanets around stars with a wide range of metallicities. Nature 486, 375–377 (2012). https://doi.org/10.1038/nature11121
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DOI: https://doi.org/10.1038/nature11121
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