There’s more to life than O₂: Simulating the detectability of a range of molecules for ground-based high-resolution spectroscopy of transiting terrestrial exoplanets

In the near future, we will be able to search for life on transiting terrestrial exoplanets using extremely large ground-based telescopes (ELTs). Their ability to access the visible and near infrared wavelengths, where the O₂ bands are prominent, will complement the James Webb Space Telescope’s strength in longer wavelength molecule detection. Ground-based telescopes capable of high-resolution spectroscopy are crucial for detecting O₂, but using them to detect other environmental molecules or alternative biosignatures has not been fully explored. Although O₂ is the most readily detectable biosignature on an Earth-like world for high-resolution observations, it must always be interpreted in the context of its planetary environment due to the possibility of abiotic generation. This context may include the presence of other molecules that can point to a habitable environment, or that can strengthen its interpretation as being more likely to be produced by life. To better understand the accessibility of environmental context using ground-based telescopes, we simulated high-resolution observations of Archean and pre-industrial Earth atmospheres transiting a range of M-dwarf hosts, ensuring photochemical consistency with each host star’s spectral type. These simulations included explicit treatment of telescope, instrument, detector, and telluric effects to model realistic signal-to-noise values in high-resolution spectra. Using the cross-correlation technique, we then determined the detectability of O₂, O₃, CH₄, CO₂, CO, and H₂O in these atmospheres. We match the near-infrared O₂ detectability calculations of previous studies, and show that CH₄ and CO₂ are potentially accessible for very nearby systems. However, for an Earth-like atmosphere in the TRAPPIST-1 system, we were unable to detect CH₄ and CO₂ above a three-sigma level within a reasonable number of transits. Molecule detectability is broadly dependent on spectral type, but could also be strongly influenced by other host star properties such as luminosity and size. Our study demonstrates that the upcoming ELTs may play a major role in terrestrial exoplanet characterization beyond O₂ detection, and our results can inform molecule and target prioritization for observers in the near future.