Searching for signs of habitability and life on M-dwarf planets with next-generation ELTs

The extremely large telescopes (ELTs) coming online within the next decade will enable us to characterize terrestrial exoplanets in M dwarf systems for the first time from the ground. Using high-resolution cross-correlation spectroscopy, and either transmission or direct imaging techniques, it is possible to detect molecular features in spectra of planetary atmospheres using the ELTs. These detections can be used to infer characteristics that can constrain a variety of evolutionary outcomes for planetary environments, including the possibility of searching for signs of habitability and life. To that end, ELTs may be the best near-term tools for detecting oxygen in nearby M dwarf exoplanet atmospheres; however additional aspects of the planetary environment are needed to determine the likelihood that the O₂ has a biological or abiotic source. One way to gain a greater understanding of the planetary context is to search for a wide suite of atmospheric gases. This can enhance the science return from the ELTs while simultaneously expanding our ability to search for signs of life by e.g. including alternative biosignature gas pairs. In this work, we have developed a novel detectability pipeline to simulate ELT observations of a range of gases in terrestrial exoplanet atmospheres orbiting M dwarf stars. In addition to O₂, we predict the detectability of CH₄, CO₂, CO, O₃, and H₂O for photochemically self-consistent simulations of M dwarf planets with modern/Archean Earth-like atmospheres, and for abiotic O₂ buildup due to photochemical generation and ocean loss processes. For transiting targets, we find that CO₂ and CH₄ are the most accessible molecular features, and may be detectable for TRAPPIST-1 e in about 10 transits with the E-ELT. For non-transiting planets, targets will be most accessible within ~5pc, and we find that O₂, H₂O, CO₂, and CH₄ may all be accessible. Finally, we will present observing protocols for both transiting and non-transiting terrestrial planets orbiting M dwarf stars to maximize the science yield of ELT observations, and inform instrument development beyond first light capabilities. This Virtual Planetary Laboratory work was funded via NASA Astrobiology Program Grant No. 80NSSC18K0829.