Two Earth-sized planets orbiting Kepler-20


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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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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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).

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