Subjects

Abstract

Seven temperate Earth-sized exoplanets readily amenable for atmospheric studies transit the nearby ultracool dwarf star TRAPPIST-1 (refs 1,2). Their atmospheric regime is unknown and could range from extended primordial hydrogen-dominated to depleted atmospheres3,4,5,6. Hydrogen in particular is a powerful greenhouse gas that may prevent the habitability of inner planets while enabling the habitability of outer ones6,7,8. An atmosphere largely dominated by hydrogen, if cloud-free, should yield prominent spectroscopic signatures in the near-infrared detectable during transits. Observations of the innermost planets have ruled out such signatures9. However, the outermost planets are more likely to have sustained such a Neptune-like atmosphere10, 11. Here, we report observations for the four planets within or near the system’s habitable zone, the circumstellar region where liquid water could exist on a planetary surface12,13,14. These planets do not exhibit prominent spectroscopic signatures at near-infrared wavelengths either, which rules out cloud-free hydrogen-dominated atmospheres for TRAPPIST-1 d, e and f, with significance of 8σ, 6σ and 4σ, respectively. Such an atmosphere is instead not excluded for planet g. As high-altitude clouds and hazes are not expected in hydrogen-dominated atmospheres around planets with such insolation15, 16, these observations further support their terrestrial and potentially habitable nature.

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References

  1. 1.

    Gillon, M. et al. Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1. Nature 542, 456–460 (2017).

  2. 2.

    Gillon, M. et al. Temperate Earth-sized planets transiting a nearby ultracool dwarf star. Nature 533, 221–224 (2016).

  3. 3.

    Owen, J. E. & Wu, Y. Kepler planets: a tale of evaporation. Astrophys. J. 775, 105 (2013).

  4. 4.

    Jin, S. et al. Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation. Astrophys. J. 795, 65 (2014).

  5. 5.

    Luger, R. & Barnes, R. Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs. Astrobiology 15, 119–143 (2015).

  6. 6.

    Owen, J. E. & Mohanty, S. Habitability of terrestrial-mass planets in the HZ of M dwarfs—I. H/He-dominated atmospheres. Mon. Not. R. Astron. Soc. 459, 4088–4108 (2016).

  7. 7.

    Sagan, C. Reducing greenhouses and the temperature history of Earth and Mars. Nature 269, 224 (1977).

  8. 8.

    Pierrehumbert, R. & Gaidos, E. Hydrogen greenhouse planets beyond the habitable zone. Astrophys. J. 734, L13 (2011).

  9. 9.

    de Wit, J. et al. A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c. Nature 537, 69–72 (2016).

  10. 10.

    Bolmont, E. et al. Water loss from terrestrial planets orbiting ultracool dwarfs: implications for the planets of TRAPPIST-1. Mon. Not. R. Astron. Soc. 464, 3728 (2016).

  11. 11.

    Bourrier, V. et al. Temporal evolution of the high-energy irradiation and water content of TRAPPIST-1 exoplanets. Astron. J. 154, 121 (2017).

  12. 12.

    Kasting, J. F., Whitmire, D. P. & Reynolds, R. T. Habitable zones around main sequence stars. Icarus 101, 108–128 (1993).

  13. 13.

    Zsom, A., Seager, S., de Wit, J. & Stamenkovic, V. Towards the minimum inner edge distance of the habitable zone. Astrophys. J. 778, 109 (2013).

  14. 14.

    Kopparapu, R. k. et al. The inner edge of the habitable zone for synchronously rotating planets around low-mass stars using general circulation models. Astrophys. J. 819, 84 (2016).

  15. 15.

    Hu, R. et al. Photochemistry in terrestrial exoplanet atmospheres. III. Photochemistry and thermochemistry in thick atmospheres on super Earths and mini Neptunes. Astrophys. J. 784, 63 (2014).

  16. 16.

    Morley, C. V. et al. Thermal emission and reflected light spectra of super Earths with flat transmission spectra. Astrophys. J. 815, 110 (2015).

  17. 17.

    Deming, D. et al. Infrared transmission spectroscopy of the exoplanets HD 209458b and XO-1b using the Wide Field Camera-3 on the Hubble Space Telescope. Astrophys. J. 774, 95 (2013).

  18. 18.

    Kreidberg, L. et al. Clouds in the atmosphere of the super-Earth exoplanet GJ1214b. Nature 505, 69–72 (2014).

  19. 19.

    Wakeford, H. R., Sing, D. K., Evans, T., Deming, D. & Mandell, A. Marginalizing instrument systematics in HST WFC3 transit light curves. Astrophys. J. 819, 10 (2016).

  20. 20.

    Wakeford, H. R. et al. HST PanCET program: a cloudy atmosphere for the promising JWST target WASP-101b. Astrophys. J. 835, L12 (2017).

  21. 21.

    Gillon, M. et al. The TRAPPIST survey of southern transiting planets. I. Thirty eclipses of the ultra-short period planet WASP-43 b. Astron. Astrophys. 542, A4 (2012).

  22. 22.

    Demory, B.-O. et al. Hubble Space Telescope search for the transit of the Earth-mass exoplanet α Centauri B b. Mon. Not. R. Astron. Soc. 450, 2043–2051 (2015).

  23. 23.

    Leconte, J., Forget, F. & Lammer, H. On the (anticipated) diversity of terrestrial planet atmospheres. Exp. Astron. 40, 449–467 (2015).

  24. 24.

    Sing, D. K. et al. A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion. Nature 529, 59–62 (2016).

  25. 25.

    Wakeford, H. R. & Sing, D. K. Transmission spectral properties of clouds for hot Jupiter exoplanets. Astron. Astrophys. 573, A122 (2015).

  26. 26.

    Barstow, J. K. & Irwin, P. G. J. Habitable worlds with JWST: transit spectroscopy of the TRAPPIST-1 system? Mon. Not. R. Astron. Soc. 461, L92–L96 (2016).

  27. 27.

    Morley, C. V., Kreidberg, L., Rustamkulov, Z., Robinson, T. & Fortney, J. J. Observing the atmospheres of known temperate Earth-sized planets with JWST. Astrophys. J. 850, 121 (2017).

  28. 28.

    Zsom, A. A population-based habitable zone perspective. Astrophys. J. 813, 9 (2015).

  29. 29.

    McCullough, P. & MacKenty, J. Considerations for Using Spatial Scans with WFC3 Instrument Science Report WFC3 2012-08 (Space Telescope Science Institute, Baltimore, 2012).

  30. 30.

    Knutson, H. A. et al. Hubble Space Telescope near-IR transmission spectroscopy of the super-Earth HD 97658b. Astrophys. J. 794, 155 (2014).

  31. 31.

    Berta, Z. K. et al. The flat transmission spectrum of the super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope. Astrophys. J. 747, 35 (2012).

  32. 32.

    Line, M. R. et al. No thermal inversion and a solar water abundance for the hot Jupiter HD 209458b from HST/WFC3 spectroscopy. Astron. J. 152, 203 (2016).

  33. 33.

    Zhou, Y. et al. A physical model-based correction for charge traps in the Hubble Space Telescope’s Wide Field Camera 3 near-IR detector and its applications to transiting exoplanets and brown dwarfs. Astron. J. 153, 243 (2017).

  34. 34.

    de Wit, J. & Seager, S. Constraining exoplanet mass from transmission spectroscopy. Science 342, 1473–1477 (2013).

  35. 35.

    Benneke, B. & Seager, S. Atmospheric retrieval for super-Earths: uniquely constraining the atmospheric composition with transmission spectroscopy. Astrophys. J. 753, 100 (2012).

  36. 36.

    Howe, A. R., Burrows, A. & Verne, W. Mass-radius relations and core-envelope decompositions of super-Earths and sub-Neptunes. Astrophys. J. 787, 173 (2014).

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Acknowledgements

This work is based on observations made with the NASA (National Aeronautics and Space Administration)/European Space Agency HST that were obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy. These observations are associated with programme GO-14873 (principal investigator J.d.W.), support for which was provided by NASA through a grant from the Space Telescope Science Institute. H.R.W. acknowledges funding from the European Research Council (ERC) under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement no. 336792, and funding under the Space Telescope Science Institute Giacconi Fellowship. This work was partially conducted while on appointment to the NASA Postdoctoral Program at Goddard Space Flight Center, administered by the Universities Space Research Association through a contract with NASA. L.D. acknowledges support from the Gruber Foundation Fellowship. E.J. and M.G. are Research Associates at the Belgian Fonds (National) de la Recherche Scientifique (FRS-FNRS). The research leading to these results has received funding from the ERC under the FP/2007–2013 ERC grant agreement no. 336480, and from a grant from the Concerted Research Actions, financed by the Wallonia-Brussels Federation. B.-O.D. acknowledges support from the Swiss National Science Foundation (PP00P2–163967). This work was also partially supported by a grant from the Simons Foundation (PI Queloz, grant no. 327127). This project has received funding from the ERC under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 679030/WHIPLASH). V.B. acknowledges the financial support of the Swiss National Science Foundation. We thank D. Taylor, K. Stevenson, N. Reid and K. Sembach for their assistance in the planning, execution and/or analysis of our observations. J.d.W., H.R.W. and N.K.L. thank also the Howards-Lewis Team and F. Dory for their support and contributions during the data-processing phase of this work.

Author information

Author notes

  1. Julien de Wit, Hannah R. Wakeford and Nikole K. Lewis contributed equally to this work.

Affiliations

  1. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA

    • Julien de Wit
  2. Astrophysics Group, University of Exeter, Devon, UK

    • Hannah R. Wakeford
  3. Space Telescope Science Institute, Baltimore, MD, USA

    • Hannah R. Wakeford
    •  & Nikole K. Lewis
  4. Astrophysics Group, Cavendish Laboratory, Cambridge, UK

    • Laetitia Delrez
  5. Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Liege, Belgium

    • Michaël Gillon
    •  & Emmanuël Jehin
  6. Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, Pessac, France

    • Frank Selsis
    •  & Jérémy Leconte
  7. University of Bern, Center for Space and Habitability, Bern, Switzerland

    • Brice-Olivier Demory
    •  & Simon Grimm
  8. Astrophysics Division of CEA de Saclay, Gif-sur-Yvette, France

    • Emeline Bolmont
  9. Observatoire de l’Université de Genève, Sauverny, Switzerland

    • Vincent Bourrier
  10. Center for Astrophysics and Space Science, University of California, San Diego, La Jolla, CA, USA

    • Adam J. Burgasser
  11. NASA Johnson Space Center, Houston, TX, USA

    • Susan M. Lederer
  12. Astrophysics Group, Imperial College London, Blackett Laboratory, London, UK

    • James E. Owen
  13. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

    • Vlada Stamenković
  14. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA

    • Vlada Stamenković
  15. School of Physics and Astronomy, University of Birmingham, Birmingham, UK

    • Amaury H. M. J. Triaud

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Contributions

J.d.W. and N.K.L. led the management of the survey. J.d.W. planned the observations. J.d.W and H.R.W. led the data reduction and analysis with the support of N.K.L., L.D., M.G. and B.-O.D. J.d.W. led the data interpretation with the support of H.R.W., N.K.L., V.S., J.L., J.E.O. and F.S. Every author contributed to the writing of the manuscript and/or the HST proposal behind these observations.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Julien de Wit.

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https://doi.org/10.1038/s41550-017-0374-z

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