Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

An abundance of small exoplanets around stars with a wide range of metallicities

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.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.

$32.00

All prices are NET prices.

Figure 1: Average host-star metallicities.
Figure 2: Comparison of host-star metallicities for small and large planets.
Figure 3: Individual host-star metallicity as a function of planet radius.

References

  1. Santos, N. C., Israelian, G. & Mayor, M. Spectroscopic [Fe/H] for 98 extra-solar planet-host stars. Exploring the probability of planet formation. Astron. Astrophys. 415, 1153–1166 (2004)

    Article  ADS  CAS  Google Scholar 

  2. Fischer, D. A. & Valenti, J. The planet-metallicity correlation. Astrophys. J. 622, 1102–1117 (2005)

    Article  ADS  CAS  Google Scholar 

  3. Johnson, J. A., Aller, K. M., Howard, A. W. & Crepp, J. R. Giant planet occurrence in the stellar mass-metallicity plane. Publ. Astron. Soc. Pacif. 122, 905–915 (2010)

    Article  ADS  Google Scholar 

  4. Sousa, S. G., Santos, N. C., Israelian, G., Mayor, M. & Udry, S. Spectroscopic stellar parameters for 582 FGK stars in the HARPS volume-limited sample. Revising the metallicity-planet correlation. Astron. Astrophys. 533, A141 (2011)

    Article  ADS  Google Scholar 

  5. Pollack, J. B. et al. Formation of the giant planets by concurrent accretion of solids and gas. Icarus 124, 62–85 (1996)

    Article  ADS  Google Scholar 

  6. Udry, S. et al. The HARPS search for southern extra-solar planets. V. A 14 Earth-masses planet orbiting HD 4308. Astron. Astrophys. 447, 361–367 (2006)

    Article  ADS  Google Scholar 

  7. Sousa, S. G. et al. Spectroscopic parameters for 451 stars in the HARPS GTO planet search program. Stellar [Fe/H] and the frequency of exo-Neptunes. Astron. Astrophys. 487, 373–381 (2008)

    Article  ADS  CAS  Google Scholar 

  8. Ghezzi, L. et al. Stellar parameters and metallicities of stars hosting Jovian and Neptunian mass planets: a possible dependence of planetary mass on metallicity. Astrophys. J. 720, 1290–1302 (2010)

    Article  ADS  CAS  Google Scholar 

  9. Mayor, M. et al. The HARPS search for southern extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets. Preprint at http://arxiv.org/abs/1109.2497 (2011)

  10. Borucki, W. J. et al. Kepler planet-detection mission: introduction and first results. Science 327, 977–980 (2010)

    Article  ADS  CAS  Google Scholar 

  11. Borucki, W. J. et al. Characteristics of planetary candidates observed by Kepler. II. Analysis of the first four months of data. Astrophys. J. 736, 19–41 (2011)

    Article  ADS  Google Scholar 

  12. Yi, S. et al. Toward better age estimates for stellar populations: the Y2 isochrones for solar mixture. Astrophys. J. Suppl. Ser. 136, 417–437 (2001)

    Article  ADS  Google Scholar 

  13. Schlaufman, K. C. & Laughlin, G. Kepler exoplanet candidate host stars are preferentially metal rich. Astrophys. J. 738, 177–186 (2011)

    Article  ADS  Google Scholar 

  14. Lissauer, J. J. et al. A closely packed system of low-mass, low-density planets transiting Kepler-11. Nature 470, 53–58 (2011)

    Article  ADS  CAS  Google Scholar 

  15. Haisch, K. E., Lada, E. A. & Lada, C. J. Disk frequencies and lifetimes in young clusters. Astrophys. J. 553, L153–L156 (2001)

    Article  ADS  CAS  Google Scholar 

  16. Ida, S. & Lin, D. N. C. Toward a deterministic model of planetary formation. II. The formation and retention of gas giant planets around stars with a range of metallicities. Astrophys. J. 616, 567–572 (2004)

    Article  ADS  Google Scholar 

  17. Throop, H. B. & Bally, J. Can photoevaporation trigger planetesimal formation? Astrophys. J. 623, L149–L152 (2005)

    Article  ADS  CAS  Google Scholar 

  18. Johansen, A., Youdin, A. & Mac Low, M.-M. Particle clumping and planetesimal formation depend strongly on metallicity. Astrophys. J. 704, L75–L79 (2009)

    Article  ADS  Google Scholar 

  19. Ercolano, B. & Clarke, C. J. Metallicity, planet formation and disc lifetimes. Mon. Not. R. Astron. Soc. 402, 2735–2743 (2010)

    Article  ADS  CAS  Google Scholar 

  20. Yasui, C., Kobayashi, N., Tokunaga, A. T., Saito, M. & Tokoku, C. Short lifetime of protoplanetary disks in low-metallicity environments. Astrophys. J. 723, L113–L116 (2010)

    Article  ADS  Google Scholar 

  21. Lin, D. N. C., Bodenheimer, P. & Richardson, D. C. Orbital migration of the planetary companion of 51 Pegasi to its present location. Nature 380, 606–607 (1996)

    Article  ADS  CAS  Google Scholar 

  22. Gorti, U. & Hollenbach, D. Photoevaporation of circumstellar disks by far-ultraviolet, extreme-ultraviolet and X-ray radiation from the central star. Astrophys. J. 690, 1539–1552 (2009)

    Article  ADS  CAS  Google Scholar 

  23. Hester, J. J., Desch, S. J., Healy, K. R. & Leshin, L. A. The cradle of the Solar System. Science 304, 1116–1117 (2004)

    Article  ADS  Google Scholar 

  24. Schiller, M. et al. Rapid timescales for magma ocean crystallization on the howardite-eucrite-diogenite parent body. Astrophys. J. 740, L22–L28 (2011)

    Article  ADS  Google Scholar 

  25. Gonzalez, G., Brownlee, D. & Ward, P. The galactic habitable zone: galactic chemical evolution. Icarus 152, 185–200 (2001)

    Article  ADS  CAS  Google Scholar 

  26. Morton, T. D. & Johnson, J. A. On the low false positive probabilities of Kepler planet candidates. Astrophys. J. 738, 170–182 (2011)

    Article  ADS  Google Scholar 

  27. Howard, A. W. et al. Planet occurrence within 0.25 AU of Solar-type stars from Kepler. Preprint at http://arxiv.org/abs/1103.2541 (2011)

Download references

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

Authors and Affiliations

Authors

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.

Corresponding author

Correspondence to Lars A. Buchhave.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

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)

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

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

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nature11121

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing