Overview

• NIRSpec/PRISM observations of two transits yield a flat transmission spectrum within ±100–150 ppm and detect no water, methane or carbon dioxide features.
• Clear hydrogen-dominated atmospheres are excluded at over 3σ confidence, ruling out analogues to early Earth, cloud-free Venus or Titan‐like compositions.
• Researchers identified and corrected 500–1,000 ppm visit-dependent slopes caused by unocculted bright heterogeneities on the active red dwarf host.
• Only an extremely thin atmosphere, high-altitude aerosols or a barren surface remain as viable explanations for the absence of molecular absorption.
• Efforts now shift toward probing the colder outer TRAPPIST-1 planets and employing secondary-eclipse and thermal-emission techniques to overcome transmission spectroscopy limits.