Abstract

M dwarf stars, which have masses less than 60 per cent that of the Sun, make up 75 per cent of the population of the stars in the Galaxy1. The atmospheres of orbiting Earth-sized planets are observationally accessible via transmission spectroscopy when the planets pass in front of these stars2,3. Statistical results suggest that the nearest transiting Earth-sized planet in the liquid-water, habitable zone of an M dwarf star is probably around 10.5 parsecs away4. A temperate planet has been discovered orbiting Proxima Centauri, the closest M dwarf5, but it probably does not transit and its true mass is unknown. Seven Earth-sized planets transit the very low-mass star TRAPPIST-1, which is 12 parsecs away6,7, but their masses and, particularly, their densities are poorly constrained. Here we report observations of LHS 1140b, a planet with a radius of 1.4 Earth radii transiting a small, cool star (LHS 1140) 12 parsecs away. We measure the mass of the planet to be 6.6 times that of Earth, consistent with a rocky bulk composition. LHS 1140b receives an insolation of 0.46 times that of Earth, placing it within the liquid-water, habitable zone8. With 90 per cent confidence, we place an upper limit on the orbital eccentricity of 0.29. The circular orbit is unlikely to be the result of tides and therefore was probably present at formation. Given its large surface gravity and cool insolation, the planet may have retained its atmosphere despite the greater luminosity (compared to the present-day) of its host star in its youth9,10. Because LHS 1140 is nearby, telescopes currently under construction might be able to search for specific atmospheric gases in the future2,3.

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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. The MEarth team acknowledges funding from the David and Lucille Packard Fellowship for Science and Engineering (awarded to D.C.). This material is based on work supported by the National Science Foundation under grants AST-0807690, AST-1109468, AST-1004488 (Alan T. Waterman Award) and AST-1616624. This publication was made possible through the support of a grant from the John Templeton Foundation and NASA XRP Program #NNX15AC90G. The opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the John Templeton Foundation. HARPS observations were made with European Southern Observatory (ESO) telescopes under observing programs 191.C-0873 and 198.C-0838. This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. E.R.N. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1602597. N.C.S. acknowledges support from Fundação para a Ciência e a Tecnologia (FCT) through national funds and by FEDER through COMPETE2020 by grants UID/FIS/04434/2013&POCI-01-0145-FEDER-007672 and PTDC/FIS-AST/1526/2014&POCI-01-0145-FEDER-016886. N.C.S. was also supported by FCT through Investigador FCT contract reference IF/00169/2012/CP0150/CT0002. X.B., X.D. and T.F. acknowledge the support of the INSU/PNP (Programme national de planétologie) and INSU/PNPS (Programme national de physique stellaire). X.B., J.-M.A. and A.W. acknowledge funding from the European Research Council under ERC Grant Agreement no. 337591-ExTrA. We thank A. Vanderburg for backseat MCMCing. This publication makes use of data products from the Two Micron All Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the National Science Foundation. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the JPL/California Institute of Technology, funded by NASA. This research has made extensive use of the NASA Astrophysics Data System (ADS), and the SIMBAD database, operated at CDS, Strasbourg, France.

Author information

Affiliations

  1. Harvard Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA

    • Jason A. Dittmann
    • , Jonathan M. Irwin
    • , David Charbonneau
    • , Raphaëlle D. Haywood
    • , Joseph E. Rodriguez
    • , Jennifer G. Winters
    • , Gilbert A. Esquerdo
    •  & David W. Latham
  2. CNRS (Centre National de la Recherche Scientifique), IPAG (Institut de Planétologie et d’Astrophysique de Grenoble), F-38000 Grenoble, France

    • Xavier Bonfils
    • , Jose-Manuel Almenara
    • , Xavier Delfosse
    • , Thierry Forveille
    • , Felipe Murgas
    •  & Anaël Wünsche
  3. Université Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France

    • Xavier Bonfils
    • , Jose-Manuel Almenara
    • , Xavier Delfosse
    • , Thierry Forveille
    • , Felipe Murgas
    •  & Anaël Wünsche
  4. Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland

    • Nicola Astudillo-Defru
    • , Jose-Manuel Almenara
    • , Christophe Lovis
    • , Francesco Pepe
    •  & Stephane Udry
  5. University of Colorado, 391 UCB, 2000 Colorado Avenue, Boulder, Colorado 80305, USA

    • Zachory K. Berta-Thompson
  6. Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02138, USA

    • Elisabeth R. Newton
  7. Perth Exoplanet Survey Telescope, Perth, Western Australia, Australia

    • Thiam-Guan Tan
  8. Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France

    • François Bouchy
  9. Instituto de Astrofísica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain

    • Felipe Murgas
  10. Instituto de Astrofísica e Ciéncias do Espaço, Universidade do Porto, CAUP (Centro de Astrofísica da Universidade do Porto), Rua das Estrelas, 4150-762 Porto, Portugal

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

    • Nuno C. Santos
  12. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA

    • Courtney D. Dressing

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Contributions

The MEarth team (J.A.D., D.C., J.M.I., Z.K.B.-T., E.R.N., J.G.W. and J.E.R.) discovered the planet, organized the follow-up observations, and led the analysis and interpretation. J.A.D. analysed the light curve and the radial-velocity data and wrote the manuscript. J.M.I. designed and installed, and maintains and operates the MEarth-South telescope array, and 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 rotational period of the star. R.D.H. conducted the Gaussian process analysis of the radial velocities. J.E.R. and T.-G.T. organized the follow-up effort in Perth. The HARPS team (X.B., N.A.-D., J.-M.A., F.B., X.D., T.F., C.L., F.M., 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. G.A.E. and D.W.L. obtained the reconnaissance spectrum with TRES at FLWO. C.D.D. obtained the infrared spectrum with IRTF/SpeX and determined the stellar metallicity.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Jason A. Dittmann.

Reviewer Information Nature thanks A. Hatzes and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Supplementary information

Text files

  1. 1.

    Supplementary Data

    This file contains all radial velocity data taken with the HARPS spectrograph.

  2. 2.

    Supplementary Data

    This file contains all MEarth and PEST photometric transit data taken of LHS 1140b.

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