Modern telescopes can measure the light filtered through an exoplanet’s atmosphere during a transit. That spectrum can reveal gases like water vapor, carbon dioxide, methane, and ozone.
None of these are proof of life on their own. The key is context: a planet’s temperature, stellar radiation, and geochemistry can produce “false positives.” Researchers look for combinations of gases that are difficult to maintain without a continuous source.
Upcoming observatories and improved retrieval models aim to reduce uncertainty by comparing multiple wavelengths and atmospheric scenarios.