Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1
J. de Wit, H. R. Wakeford, N. Lewis, L. Delrez, M. Gillon, F. Selsis,, J. Leconte, B.-O. Demory, E. Bolmont, V. Bourrier, A. J. Burgasser, S. Grimm,, E. Jehin, S. M. Lederer, J. Owen, V. Stamenkovic, A. H. M. J. Triaud

TL;DR
This study uses atmospheric observations of TRAPPIST-1's habitable-zone planets to rule out cloud-free hydrogen-dominated atmospheres, supporting their terrestrial and potentially habitable characteristics.
Contribution
First atmospheric constraints on four TRAPPIST-1 planets within the habitable zone, ruling out cloud-free hydrogen atmospheres and supporting their terrestrial nature.
Findings
No prominent hydrogen spectral signatures detected
Hydrogen-dominated atmospheres ruled out for planets d, e, and f
Supports terrestrial and potentially habitable nature of these planets
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 atmospheres (refs 3-6). Hydrogen in particular is a powerful greenhouse gas that may prevent the habitability of inner planets while enabling the habitability of outer ones (refs 6-8). 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 signatures (ref 9). However, the outermost planets are more likely to have sustained such a Neptune-like atmosphere (refs 10,11). Here, we report observations for the four planets within or near the system's habitable zone, the circumstellar region…
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