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

@article{deWit2018AtmosphericRO,
  title={Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1},
  author={Julien de Wit and Hannah R. Wakeford and Nikole K. Lewis and Laetitia Delrez and Micha{\"e}l Gillon and Frank Selsis and J{\'e}r{\'e}my Leconte and B-O. Demory and {\'E}meline Bolmont and Vincent Bourrier and Adam J. Burgasser and Simon L. Grimm and Emmanuel Jehin and Susan M. Lederer and James E. Owen and Vlada Stamenkovi{\'c} and Amaury H. M. J. Triaud},
  journal={Nature Astronomy},
  year={2018},
  volume={2},
  pages={214-219}
}
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 atmospheres3–6. Hydrogen in particular is a powerful greenhouse gas that may prevent the habitability of inner planets while enabling the habitability of outer ones6–8. An atmosphere largely dominated by hydrogen, if cloud-free, should yield… 
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TLDR
Observations of the combined transmission spectrum of the two inner planets during their simultaneous transits on 4 May 2016 rule 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 atmospheres.
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