Overview

• Stellar contamination from unocculted bright regions introduced 500–1,000 ppm visit-dependent slopes that, once modeled, revealed a featureless transmission spectrum within ±100–150 ppm across 0.6–5.2 µm.
• Observations exclude clear, hydrogen-dominated atmospheres at greater than 3σ confidence and rule out high mean-molecular-weight compositions analogous to Titan, Venus, early Mars, Archean Earth and a cloud-free modern Earth at over 95% confidence.
• The non-detection of water, methane, carbon dioxide and other common molecules suggests TRAPPIST-1 d either hosts an extremely thin envelope, is shrouded by high-altitude aerosols or is effectively airless.
• Detailed correction for stellar heterogeneities highlights the challenges of transmission spectroscopy for small planets orbiting active red-dwarf stars.
• Follow-up JWST observations of the cooler outer planets e through h are under way to explore whether these more distant worlds retain detectable atmospheres.