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

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 ^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 ^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 ⁹ . However, the outermost planets are more likely to have sustained such a Neptune-like atmosphere ^{10, 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 surface ^12-14 . 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 insolation ^{15, 16}, these observations further support their terrestrial and potentially habitable nature.