The search for signs of life beyond the Solar System is moving from theory to concrete calculations. A new study estimates whether the future ELT will be able to see biosignature gases in the atmospheres of rocky planets in the habitable zone.
The instrument in question is the ELT (Extremely Large Telescope) and its ANDES spectrograph, which will work in the visible and near-infrared range. The authors modelled observations using transmission spectroscopy — when a planet passes in front of its star, and part of the light travels through its atmosphere, leaving chemical "fingerprints".
The team developed a new signal-processing method based on Bayesian cross-correlation and applied it to 18 known potentially habitable planets. For each, they calculated how many transits are needed for a confident detection of molecules.
Water is the easiest to find. For the TRAPPIST-1 planets, 10–19 transits suffice; for LHS 1140 b, about 30. Other gases are harder: carbon dioxide requires roughly 1.5 times more transits, methane three times, and oxygen four times more than water.
These numbers are a lower limit. They assume ideal observing conditions, cloud-free atmospheres similar to Earth's, and no systematic errors. Even so, they imply very large observing campaigns. That is why the authors note that searching for biosignatures in the reflected light of the nearest planets could be a useful complement to the transit method.