The TRAPPIST-1 system, consisting of seven small rocky planets orbiting a late-type M8 star, offers an opportunity to search for secondary atmospheres on temperate terrestrial worlds. The 0.8 Earth-radii planet TRAPPIST-1 d lies at the edge of the habitable zone. Researchers have presented the first transmission spectrum of TRAPPIST-1 d from two transits using the James Webb Space Telescope (JWST).
The study found that stellar contamination introduces visit-dependent slopes, but once corrected, the transmission spectrum is flat within a certain range. This suggests no evidence for a haze-like slope or molecular absorption, despite the telescope’s sensitivity to key atmospheric gases such as methane and water vapor.
Analysis of the data excludes clear hydrogen-dominated atmospheres with high confidence. The researchers also leverage their constraints on even trace amounts of methane and water vapor to reject high mean molecular weight compositions often found in other celestial bodies.
The findings indicate that if TRAPPIST-1 d retains an atmosphere, it is likely extremely thin or contains high-altitude aerosols. Alternatively, the planet may be airless. The evolutionary models suggest that TRAPPIST-1 b, c, and d must have formed with less than approximately 4 Earth oceans of water, which would impact atmospheric conditions on other habitable-zone planets.
This study provides new insights into the potential secondary atmosphere on TRAPPIST-1 d and its implications for our understanding of exoplanetary formation.
Source: https://astrobiology.com/2025/08/strict-limits-on-potential-secondary-atmospheres-on-the-temperate-rocky-exo-earth-trappist-1-d.html