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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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.