Abstract

M-dwarf stars—hydrogen-burning stars that are smaller than 60 per cent of the size of the Sun—are the most common class of star in our Galaxy and outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M dwarfs host Earth-sized planets in great numbers1,2: the average number of M-dwarf planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per star3. The nearest such planets known to transit their star are 39 parsecs away4, too distant for detailed follow-up observations to measure the planetary masses or to study their atmospheres. Here we report observations of GJ 1132b, a planet with a size of 1.2 Earth radii that is transiting a small star 12 parsecs away. Our Doppler mass measurement of GJ 1132b yields a density consistent with an Earth-like bulk composition, similar to the compositions of the six known exoplanets with masses less than six times that of the Earth and precisely measured densities5,6,7,8,9,10,11. Receiving 19 times more stellar radiation than the Earth, the planet is too hot to be habitable but is cool enough to support a substantial atmosphere, one that has probably been considerably depleted of hydrogen. Because the host star is nearby and only 21 per cent the radius of the Sun, existing and upcoming telescopes will be able to observe the composition and dynamics of the planetary atmosphere.

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Acknowledgements

We thank the staff at the Cerro Tololo Inter-American Observatory for assistance in the construction and operation of MEarth–South; J. Winn and J. Berta-Thompson for comments on the manuscript; S. Seager and A. Zsom for conversations that improved the work; L. Delrez for her independent analysis of the TRAPPIST data; and J. Eastman, D. Dragomir and R. Siverd for their efforts to observe additional transits. The MEarth Project acknowledges funding from the David and Lucile Packard Fellowship for Science and Engineering, and the National Science Foundation, and a grant from the John Templeton Foundation. The opinions expressed here are those of the authors and do not necessarily reflect the views of the John Templeton Foundation. The development of the PISCO imager was supported by the National Science Foundation. HARPS observations were made with European Southern Observatory (ESO) Telescopes at the La Silla Paranal Observatory. TRAPPIST is a project funded by the Belgian Fund for Scientific Research, with the participation of the Swiss National Science Foundation. Z.K.B.-T. is funded by the MIT Torres Fellowship for Exoplanet Research. X.B., X.D., T.F. and A.W. acknowledge the support of the French Agence Nationale de la Recherche and the European Research Council. M.G. and E.J. are FNRS Research Associates. V.N. acknowledges a CNPq/BJT Post-Doctorate fellowship and partial financial support from the INCT INEspaço. N.C.S. acknowledges the support from the Portuguese National Science Foundation (FCT) as well as the COMPETE program.

Author information

Affiliations

  1. Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA

    • Zachory K. Berta-Thompson
  2. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA

    • Zachory K. Berta-Thompson
    • , Jonathan Irwin
    • , David Charbonneau
    • , Elisabeth R. Newton
    • , Jason A. Dittmann
    •  & Antony A. Stark
  3. Observatoire de Genève, Université de Genéve, 51 chemin des Maillettes, 1290 Sauverny, Switzerland

    • Nicola Astudillo-Defru
    • , Francois Bouchy
    • , Christophe Lovis
    • , Michel Mayor
    • , Francesco Pepe
    •  & Stéphane Udry
  4. Université Grenoble Alpes, IPAG, F-38000 Grenoble, France

    • Xavier Bonfils
    • , Xavier Delfosse
    • , Thierry Forveille
    •  & Anaël Wünsche
  5. CNRS, IPAG, F-38000 Grenoble, France

    • Xavier Bonfils
    • , Xavier Delfosse
    • , Thierry Forveille
    •  & Anaël Wünsche
  6. Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bâtiment B5C, 4000 Liège, Belgium

    • Michaël Gillon
    •  & Emmanuël Jehin
  7. Institute for Astronomy, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA

    • Brian Stalder
  8. Laboratoire d’Astrophysique de Marseille, UMR 6110 CNRS, Université de Provence, 38 rue Frédéric Joliot-Curie, 13388, Marseille Cedex 13, France

    • Francois Bouchy
  9. Departamento de Física, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, Rio Grande do Norte, Brazil

    • Vasco Neves
  10. Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal

    • Nuno C. Santos
  11. Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal

    • Nuno C. Santos

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Contributions

The MEarth team (D.C., J.I., Z.K.B.-T., E.R.N. and J.A.D.) discovered the planet, organized the follow-up observations, and led the analysis and interpretation. Z.K.B.-T. analysed the light curve and radial velocity data, and wrote the manuscript. J.I. designed, installed, maintains, and operates the MEarth–South telescope array, identified the first triggered transit event, and substantially contributed to the analysis and interpretation. D.C. leads the MEarth Project, and assisted in analysis and writing the manuscript. E.R.N. determined the metallicity, kinematics, and rotation period of the star. J.A.D. confirmed the star’s trigonometric parallax and helped install the MEarth–South telescopes. The HARPS team (N.A.-D., X.B., F.B., X.D., T.F., C.L., M.M., V.N., F.P., N.C.S., S.U. and A.W.) obtained spectra for Doppler velocimetry, with N.A.-D. and X.B. leading the analysis of those data. M.G. and E.J. gathered photometric observations with TRAPPIST. A.A.S. and B.S. gathered photometric observations with PISCO. All authors read and discussed the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Zachory K. Berta-Thompson.

Extended data

Supplementary information

Text files

  1. 1.

    Supplementary Data 1

    This text file contains photometric observations of transits of GJ 1132b from the spring of 2015. Observations with different telescopes or on different nights are presented as separate light curves.

  2. 2.

    Supplementary Data 2

    This text file contains radial velocity observations of GJ 1132, collected with HARPS in the spring and summer of 2015, which appear in Figure 2.

  3. 3.

    Supplementary Data 3

    This text file contains the curated compilation of transiting exoplanets that appears in Figure 3. Data were downloaded from the NASA Exoplanet Archive on 25 August 2015, and minor modifications were made to correct egregious errors in the stellar/planetary parameters of some systems.

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https://doi.org/10.1038/nature15762

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