New transit surveys such as TESS, NGTS and PLATO have the potential to discover small, temperate planets in orbit around nearby stars. Planets orbiting bright stars provide excellent opportunities for spectroscopic follow-up, and some may provide the necessary mix of environmental conditions to be habitable or even inhabited. During the transit of an exoplanet, it is possible to detect absorption signatures of specific gases in their atmospheres by measuring the increase in the transit depth at wavelengths of enhanced absorption. The presence of certain gases, such as water vapour, may be indicators that a planet is habitable; others, such as oxygen and ozone, may go further and suggest that the planet is inhabited. More broadly, evidence of chemical disequilibrium in a planet’s atmosphere could be a sign that life is present.

It is theoretically possible to identify these ‘biosignature’ gases from transit spectra; however, small, rocky, cool planets usually have compact atmospheres, meaning that the absorption features are tiny and we would require a very sensitive telescope in order to detect them. In addition, the presence of clouds can complicate the identification of gaseous absorption features.

We consider the potential of the James Webb Space Telescope and hypothetical future large space platforms to characterize potentially habitable atmospheres using transit spectroscopy. We use the techniques described in Barstow et al. 2013 and Barstow et al. 2015 to determine the retrievability of gas abundances from transit spectra of Earth- and Venus-like planets, and consider whether or not we can distinguish between these two extreme terrestrial planet climates, one inhabited and one uninhabitable.