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.
Your institute does not have access to this article
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Get time limited or full article access on ReadCube.
All prices are NET prices.
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)
Mayor, M. & Queloz, D. A Jupiter-mass companion to a solar-type star. Nature 378, 355–359 (1995)
Batalha, N. M. et al. Kepler’s first rocky planet: Kepler-10b. Astrophys. J. 729, 27 (2011)
Gautier, T. N. et al. A Sun-like star with three sub-Neptune exoplanets and two Earth-size candidates. Astrophys. J. (submitted)
Koch, D. G. et al. Kepler mission design, realized photometric performance, and early science. Astrophys. J. 713, L79–L86 (2010)
Bryson, S. T. et al. The Kepler pixel response function. Astrophys. J. 713, L97–L102 (2010)
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)
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)
Fressin, F. et al. Kepler-10c, a 2.2-Earth radius transiting planet in a multiple system. Astrophys. J. Suppl.197, 5 (2011)
Fressin, F. et al. Spitzer Infrared observations and independent validation of the transiting Super-Earth CoRoT-7 b. Astrophys. J. 669, 1279–1297 (2011)
Queloz, D. et al. The CoRoT-7 planetary system: two orbiting super-Earths. Astron. Astrophys. 506, 303–319 (2009)
Robin, A. C., Derrière, S. & Picaud, S. A synthetic view on structure and evolution of the Milky Way. Astron. Astrophys. 409, 523–540 (2003)
Raghavan, D. et al. A survey of stellar families: multiplicity of solar-type stars. Astrophys. J. Suppl.190, 1–42 (2010)
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)
Schneider, J., Dedieu, C., Le Sidaner, P., Savalle, R. & Zolotukhin, I. Defining and cataloging exoplanets: the exoplanet.eu database. Astron. Astrophys. 532, A79 (2011)
Morton, T. D. & Johnson, J. A. On the low false positive probabilities of Kepler planet candidates. Astrophys. J. 738, 170 (2011)
Lissauer, J. J. et al. Architecture and dynamics of Kepler’s candidate multiple planet systems. Astrophys. J. Suppl.197, 8 (2011)
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)
Selsis, F. et al. Could we identify hot ocean-planets with CoRoT, Kepler and Doppler velocimetry? Icarus 191, 453–468 (2007)
Kuchner, M. J. Volatile-rich Earth-mass planets in the habitable zone. Astrophys. J. 596, L105–L108 (2003)
Valenti, J. A. & Piskunov, N. Spectroscopy made easy: a new tool for fitting observations with synthetic spectra. Astron. Astrophys. 118 (Suppl.). 595–603 (1996)
Yi, S. et al. Toward better age estimates for stellar populations: the Y2 isochrones for solar mixture. Astrophys. J. Suppl.136, 417–437 (2001)
Rogers, L. A., Bodenheimer, P., Lissauer, J. J. & Seager, S. Formation and structure of low-density exo-Neptunes. Astrophys. J. 738, 59 (2011)
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)
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)
Slawson, R. W. et al. Kepler eclipsing binary stars. II. 2165 eclipsing binaries in the second data release. Astrophys. J. 142, 160 (2011)
Seager, S., Kuchner, M., Hier-Majumder, C. A. & Militzer, B. Mass-radius relationships for solid exoplanets. Astrophys. J. 669, 1279–1297 (2007)
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)
Kepler was competitively selected as the tenth Discovery mission. Funding for this mission is provided by NASA’s Science Mission Directorate.
The authors declare no competing financial interests.
About this article
Cite this article
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
Earth, Moon, and Planets (2019)
Astrophysics and Space Science (2017)