Three Earth-sized exoplanets were recently discovered close to the habitable zone1,2 of the nearby ultracool dwarf star TRAPPIST-1 (ref. 3). The nature of these planets has yet to be determined, as their masses remain unmeasured and no observational constraint is available for the planetary population surrounding ultracool dwarfs, of which the TRAPPIST-1 planets are the first transiting example. Theoretical predictions span the entire atmospheric range, from depleted to extended hydrogen-dominated atmospheres4,5,6,7,8. Here we report observations of the combined transmission spectrum of the two inner planets during their simultaneous transits on 4 May 2016. The lack of features in the combined spectrum rules out cloud-free hydrogen-dominated atmospheres for each planet at ≥10σ levels; TRAPPIST-1 b and c are therefore unlikely to have an extended gas envelope as they occupy a region of parameter space in which high-altitude cloud/haze formation is not expected to be significant for hydrogen-dominated atmospheres9. Many denser atmospheres remain consistent with the featureless transmission spectrum—from a cloud-free water-vapour atmosphere to a Venus-like one.
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This work is based on observations made with the NASA/ESA Hubble Space Telescope that were obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. These observations are associated with program HST-GO-14500 (principal investigator J.d.W.), support for which was provided by NASA through a grant from the Space Telescope Science Institute. The research leading to our results was funded in part by the European Research Council (ERC) under the FP/2007-2013 ERC grant 336480, and through an Action de Recherche Concertée (ARC) grant financed by the Wallonia-Brussels Federation. H.R.W. acknowledges support through an appointment to the NASA Postdoctoral Program at Goddard Space Flight Center, administered by the Universities Space Research Association through a contract with NASA. M.G. is Research Associate at the Belgian Fonds (National) de la Recherche Scientifique (FRS–FNRS). L.D. acknowledges support of the Fund for Research Training in Industry and Agriculture of the FRS–FNRS. We thank D. Taylor, S. Deustua, P. McCullough, and N. Reid for their assistance in planning and executing our observations. We are also grateful for discussions with Z. Berta-Thompson and Pierre Magain about this study and manuscript. We thank the ATLAS and PHOENIX teams for providing stellar models.
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Space Science Reviews (2018)