There’s more to life than O₂: Assessing the detectability of biosignatures and environmental context for high-resolution spectroscopy of terrestrial exoplanet

Major science drivers for the upcoming class of ground-based extremely large telescopes (ELTs) include the prospects for characterizing the atmospheres of habitable zone terrestrial exoplanets orbiting M dwarf stars. Combining high contrast imaging with high-resolution cross-correlation spectrometry (HRCCS), the ELTs may present an opportunity to search for signs of habitability and life, and may be the only near-term opportunity to detect molecular oxygen in these atmospheres. However, determining the likelihood that O₂ has a biological origin requires contextual information from the planetary environment to support the identification and rule out scenarios that generate O₂ abiotically. In this work, we simulate ELT observations of self-consistent Earth atmospheres through time orbiting M dwarf host stars, in addition to atmospheres with detectable levels of abiotically-generated O₂. We estimate the detectability of gases that can help give context to ELT O₂ detections in terrestrial exoplanet atmospheres, including CO₂, CH₄, H₂O, CO, and O₃. We find that CO₂ and CH₄ are the most detectable gases in M dwarf planetary atmospheres, and may be detectable on TRAPPIST-1 e in less than 35 transits. However, for closer targets, the ELTs alone may be capable of discriminating an inhabited world from one without life with tens of hours of observation time under ideal conditions. Additionally, we find the detectability of all gases is strongly dependent on host star type and absorption band wavelength—NIR absorption bands may require less overall observation time for planets orbiting late-type M dwarfs. All simulations assume perfectly removed telluric and stellar lines; we find that the standard methods of removing tellurics from high-resolution spectra may be difficult to apply to some habitable zone terrestrial exoplanet targets whose change in Doppler shift over the course of the observation is not sufficiently large. Finally, we develop an observing protocol for characterizing terrestrial planets with the ELTs, prioritizing the most detectable gas absorption bands to maximize the science output of ELT observations and inform instrument development beyond the first light capabilities.