A new preprint study has assessed how effectively the Extremely Large Telescope could search for atmospheric signs of habitability and life on nearby rocky exoplanets. Researchers simulated observations with ANDES, the telescope’s planned high-resolution visible and near-infrared spectrograph, focusing on water vapor, carbon dioxide, methane, and oxygen.

The team modeled cloud-free, modern Earth-like atmospheres for 18 known transiting planets in or near their stars’ habitable zones. They created synthetic transmission spectra, added expected instrumental noise, and analyzed the signals using a new Bayesian cross-correlation framework designed to account for correlations within the data. Water vapor emerged as the easiest molecule to detect. The simulations suggest that ANDES could identify H2O in the atmospheres of the studied TRAPPIST-1 planets after approximately 10–19 transits, and around LHS 1140 b after about 30. Carbon dioxide, methane, and oxygen would require roughly 1.5, three, and four times more transits, respectively.

Overall, eight planets showed at least one detectable molecule within 100 transits. TRAPPIST-1 d ranked as the strongest target, with all four gases potentially detectable in fewer than 36 transits. Water and carbon dioxide were generally the most accessible, while oxygen remained the most challenging.

The estimates are optimistic. The models assume cloud-free atmospheres, Earth-like compositions, ideal removal of terrestrial and stellar contamination, and predictable noise reduction when multiple observations are combined. Real campaigns may therefore require substantially more telescope time.

For more details, read the original paper.