  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 where
liquid water could exist on a planetary surface (refs 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 8, 6 and 4 sigma, 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 (refs 15,16), these observations further support
their terrestrial and potentially habitable nature.
