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Two Earth-sized planets orbiting Kepler-20

Abstract

Since the discovery of the first extrasolar giant planets around Sun-like stars1,2, evolving observational capabilities have brought us closer to the detection of true Earth analogues. The size of an exoplanet can be determined when it periodically passes in front of (transits) its parent star, causing a decrease in starlight proportional to its radius. The smallest exoplanet hitherto discovered3 has a radius 1.42 times that of the Earth’s radius (R), and hence has 2.9 times its volume. Here we report the discovery of two planets, one Earth-sized (1.03R) and the other smaller than the Earth (0.87R), orbiting the star Kepler-20, which is already known to host three other, larger, transiting planets4. The gravitational pull of the new planets on the parent star is too small to measure with current instrumentation. We apply a statistical method to show that the likelihood of the planetary interpretation of the transit signals is more than three orders of magnitude larger than that of the alternative hypothesis that the signals result from an eclipsing binary star. Theoretical considerations imply that these planets are rocky, with a composition of iron and silicate. The outer planet could have developed a thick water vapour atmosphere.

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Figure 1: Transit light curves.
Figure 2: Density map of stars in the background of Kepler-20.
Figure 3: Mass versus radius relation for small planets.

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References

  1. Latham, D. W., Stefanik, R. P., Mazeh, T., Mayor, M. & Burki, G. The unseen companion of HD114762—a probable brown dwarf. Nature 339, 38–40 (1989)

    Article  ADS  Google Scholar 

  2. Mayor, M. & Queloz, D. A Jupiter-mass companion to a solar-type star. Nature 378, 355–359 (1995)

    Article  ADS  CAS  Google Scholar 

  3. Batalha, N. M. et al. Kepler’s first rocky planet: Kepler-10b. Astrophys. J. 729, 27 (2011)

    Article  ADS  Google Scholar 

  4. Gautier, T. N. et al. A Sun-like star with three sub-Neptune exoplanets and two Earth-size candidates. Astrophys. J. (submitted)

  5. Koch, D. G. et al. Kepler mission design, realized photometric performance, and early science. Astrophys. J. 713, L79–L86 (2010)

    Article  ADS  CAS  Google Scholar 

  6. Bryson, S. T. et al. The Kepler pixel response function. Astrophys. J. 713, L97–L102 (2010)

    Article  ADS  Google Scholar 

  7. Torres, G., Konacki, M., Sasselov, D. D. & Jha, S. Testing blend scenarios for extrasolar transiting planet candidates. I. OGLE-TR-33: a false positive. Astrophys. J. 614, 979–989 (2004)

    Article  ADS  CAS  Google Scholar 

  8. Torres, G. et al. Modeling Kepler transit light curves as false positives: rejection of blend scenarios for Kepler-9, and validation of Kepler-9 d, a super-earth-size planet in a multiple system. Astrophys. J. 727, 24 (2011)

    Article  ADS  Google Scholar 

  9. Fressin, F. et al. Kepler-10c, a 2.2-Earth radius transiting planet in a multiple system. Astrophys. J. Suppl.197, 5 (2011)

    Article  Google Scholar 

  10. Fressin, F. et al. Spitzer Infrared observations and independent validation of the transiting Super-Earth CoRoT-7 b. Astrophys. J. 669, 1279–1297 (2011)

    Google Scholar 

  11. Queloz, D. et al. The CoRoT-7 planetary system: two orbiting super-Earths. Astron. Astrophys. 506, 303–319 (2009)

    Article  ADS  Google Scholar 

  12. Robin, A. C., Derrière, S. & Picaud, S. A synthetic view on structure and evolution of the Milky Way. Astron. Astrophys. 409, 523–540 (2003)

    Article  ADS  Google Scholar 

  13. Raghavan, D. et al. A survey of stellar families: multiplicity of solar-type stars. Astrophys. J. Suppl.190, 1–42 (2010)

    Article  CAS  Google Scholar 

  14. Brown, T. M., Latham, D. W., Everett, M. E. & Esquerdo, G. A. Kepler input catalog: photometric calibration and stellar classification. Astrophys. J. 142, 112 (2011)

    ADS  Google Scholar 

  15. Schneider, J., Dedieu, C., Le Sidaner, P., Savalle, R. & Zolotukhin, I. Defining and cataloging exoplanets: the exoplanet.eu database. Astron. Astrophys. 532, A79 (2011)

    Article  ADS  Google Scholar 

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

    Article  ADS  Google Scholar 

  17. Lissauer, J. J. et al. Architecture and dynamics of Kepler’s candidate multiple planet systems. Astrophys. J. Suppl.197, 8 (2011)

    Article  Google Scholar 

  18. Cochran, W. D. et al. Kepler 18-b, c, and d: a system of three planets confirmed by transit timing variations, lightcurve validation, Spitzer photometry and radial velocity measurements. Astrophys. J. Suppl.197, 7 (2011)

    Article  Google Scholar 

  19. Selsis, F. et al. Could we identify hot ocean-planets with CoRoT, Kepler and Doppler velocimetry? Icarus 191, 453–468 (2007)

    Article  ADS  Google Scholar 

  20. Kuchner, M. J. Volatile-rich Earth-mass planets in the habitable zone. Astrophys. J. 596, L105–L108 (2003)

    Article  ADS  Google Scholar 

  21. Valenti, J. A. & Piskunov, N. Spectroscopy made easy: a new tool for fitting observations with synthetic spectra. Astron. Astrophys. 118 (Suppl.). 595–603 (1996)

    ADS  Google Scholar 

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

    Article  Google Scholar 

  23. Rogers, L. A., Bodenheimer, P., Lissauer, J. J. & Seager, S. Formation and structure of low-density exo-Neptunes. Astrophys. J. 738, 59 (2011)

    Article  ADS  Google Scholar 

  24. Marcus, R. A., Sasselov, D., Hernquist, L. & Stewart, S. T. Minimum radii of super-Earths: constraints from giant impacts. Astrophys. J. 712, L73–L76 (2010)

    Article  ADS  Google Scholar 

  25. 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 (2011)

    Article  ADS  Google Scholar 

  26. Slawson, R. W. et al. Kepler eclipsing binary stars. II. 2165 eclipsing binaries in the second data release. Astrophys. J. 142, 160 (2011)

    Google Scholar 

  27. Seager, S., Kuchner, M., Hier-Majumder, C. A. & Militzer, B. Mass-radius relationships for solid exoplanets. Astrophys. J. 669, 1279–1297 (2007)

    Article  ADS  CAS  Google Scholar 

  28. Marcus, R. A., Sasselov, D., Stewart, S. T. & Hernquist, L. Water/icy super-Earths: giant impacts and maximum water content. Astrophys. J. 719, L45–L49 (2010)

    Article  ADS  Google Scholar 

Download references

Acknowledgements

Kepler was competitively selected as the tenth Discovery mission. Funding for this mission is provided by NASA’s Science Mission Directorate.

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Authors and Affiliations

Authors

Contributions

F.F. and G.T. developed the ideas and tools to perform the BLENDER analysis, and the statistical interpretation that led to the validation of Kepler-20 e and Kepler-20 f. C.E.H. implemented important modifications to the BLENDER program to improve the mapping of the range of possible blends. T.N.G. and D.C. led the effort to validate the largest planets in the Kepler-20 system. W.J.B. and D.G.K. led the Kepler mission, and supported the BLENDER effort on Kepler-20. N.M.B. led the Kepler science team that identified viable planet candidates coming out of the Kepler pipeline. J.F.R. and S.T.B. performed the light curve analysis to extract the planet characteristics. G.T., G.W.M., A.H., L.A.B., S.N.Q., D.W.L., D.C.F. and J.F.R. established the stellar characteristics from high-resolution spectroscopy and transit constraints. L.A.R., S.S. and D.D.S. worked on modelling the composition of the planets. J.M.J., T.B., F.M., S.E.S., M.S., S.E.T., J.D.T. and K.U. worked on the data collection, processing and review that yielded the time series photometry. D.R.C. provided the constraint on the angular separation from adaptive optics imaging using PHARO at Palomar. S.B.H. carried out speckle interferometry observations. S.T.B. worked on the pixel-level centroid analysis to reduce the sky area in which background binaries can reproduce the observed transits, which contributes to the statistical validation. D.C.F., E.B.F. and M.J.H. worked on the dynamical constraints on the planet properties. D.R. developed and calculated the coplanarity probability boost. C.D.D. worked on the Kepler incompleteness estimates. J.-M.D. analysed the Spitzer observations of Kepler-20. J.J.L. worked on estimating the planet prior. All authors discussed the results and commented on the manuscript. F.F. led the project and wrote the paper.

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Correspondence to Francois Fressin.

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The authors declare no competing financial interests.

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Fressin, F., Torres, G., Rowe, J. et al. Two Earth-sized planets orbiting Kepler-20. Nature 482, 195–198 (2012). https://doi.org/10.1038/nature10780

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