Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1

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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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Fig. 1: Hubble/WFC3 white light curves of the four TRAPPIST-1 habitable-zone planets d, e, f and g over four visits.
Fig. 2: Transmission spectra of TRAPPIST-1 d, e, f and g compared with synthetic atmospheres dominated by hydrogen (H2), water (H2O), carbon dioxide (CO2) and nitrogen (N2).

References

  1. 1.

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

    ADS  Article  Google Scholar 

  2. 2.

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

    ADS  Article  Google Scholar 

  3. 3.

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

    ADS  Article  Google Scholar 

  4. 4.

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

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  7. 7.

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

    ADS  Article  Google Scholar 

  8. 8.

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

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  12. 12.

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

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  18. 18.

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

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  23. 23.

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

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  25. 25.

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

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  28. 28.

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

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

  34. 34.

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

    ADS  Article  Google Scholar 

  35. 35.

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

    ADS  Article  Google Scholar 

  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).

    ADS  Article  Google Scholar 

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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.

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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.

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Correspondence to Julien de Wit.

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de Wit, J., Wakeford, H.R., Lewis, N.K. et al. Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1. Nat Astron 2, 214–219 (2018). https://doi.org/10.1038/s41550-017-0374-z

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