Searching for Signs of Habitability and Life in the Era of Extremely Large Telescopes

Abstract

We are entering an exciting era for astrobiology, with terrestrial exoplanet characterization studies now underway with the JWST and the next generation of ground-based Extremely Large Telescopes (ELTs) expected to be online by the end of the decade. However, the prospects for searching for signs of habitability and life with the upcoming 30 m class ELTs are not yet thoroughly explored. Specifically, it is not well understood the extent to which the high resolution spectroscopy and high contrast imaging capabilities of the ELTs can be leveraged to characterize terrestrial exoplanet atmospheres. Previous theoretical studies on ELT capabilities have focused on the detectability of the biosignature gas O2 in Earth-twin atmospheres, but have not rigorously considered the environmental context gained by detecting other molecules. Models that produce atmospheres that are photochemically self-consistent with their host stars can be used to show how additional molecules reveal more about the planet and its processes, either strengthening the interpretation of O2 as a biosignature, or ruling in or out biosignature false positive mechanisms that can generate abiotic O2. Furthermore, our ability to use future ELT observations to observationally constrain the composition of nearby transiting and non-transiting terrestrial exoplanets is uncertain, and could provide independent avenues for determining the origin of O2 in particular. The goal of this dissertation is to explore and define the terrestrial exoplanet characterization capabilities of the upcoming ELTs, which can complement and support JWST and the future Habitable Worlds Observatory (HWO) NASA flagship mission. To that end, this work provides recommendations and observational protocols that will enhance and maximize the science of the ELTs to search for signs of habitability and life in terrestrial exoplanet atmospheres, laying the foundation for ground-based terrestrial exoplanet science in the near term, and space-based characterization studies in the future. In this work, we develop and apply techniques for analyzing simulated high-resolution ground-based spectra and retrieving molecular abundances in simulated ELT data. Our approach extends well beyond current approaches for characterizing terrestrial exoplanet atmospheres with the ELTs by considering the detectability of a suite of molecular species that can help constrain the origin of atmospheric O2, and provide further environmental context for a potential biosignature detection. We simulated ELT detectability of atmospheric molecules for a range of different inhabited and uninhabited terrestrial atmosphere types, as well as a sub-Neptune atmosphere, for planets orbiting M dwarf host stars. We found that CH4, CO2, H2O, and CO are all potentially detectable for both transiting and non-transiting terrestrial exoplanets, and that two biosignature pairs (O2/CH4 and CO2/CH4) may be detectable for nearby Earth-like worlds in ~10 hours of observing for Proxima Centauri b, the nearest non-transiting target. Furthermore, we may be able to discriminate biosignature false positive environments using the direct imaging capabilities of the ELTs by detecting CO, an indicator gas for several false positive cases, in as little as 10 hours for nearby targets. We could also identify false positives by searching for signs of significant abiotic O2 buildup via H2O photolysis, which may be possible in <100 hours of observing time. Discriminating the atmospheres of non-transiting planets as either terrestrial- or sub-Neptune-like may also be possible via spectral characterization in ~1 hour of observing with the ELTs to detect absorption from hydrogen-bearing species such as NH3. We also explore our ability to measure the abundance of O2 in habitable Proxima Centauri b atmospheres using atmospheric retrieval methods for high-resolution cross-correlation spectroscopy for the first time, and we find that we may be able to measure Earth-like O2 abundances or lower (<=21% O2) in 100 hours of observing the O2 A-band; however, retrieving high O2 abundances in post-ocean loss scenarios with thick atmospheres may be challenging due to significant saturation of the O2 A-band. Considering other O2 bands that are less prone to saturation or searching for the spectral features of O2--O2 collisionally-induced absorption may instead be used to identify this scenario.This dissertation proposes future ELT observing protocols and observational strategies for characterizing terrestrial exoplanets. The tools we have developed will continue to be relevant in the preparation for the analysis of the first ELT transit transmission and reflected light observations of nearby terrestrial exoplanets, as well as for the development of future ground- and space-based instrumentation and science strategies. The code and methodology developed in this work are available to the exoplanet and astrobiology communities. The goal of this dissertation is to explore and define the terrestrial exoplanet characterization capabilities of the upcoming ELTs, which can complement and support JWST and the future Habitable Worlds Observatory (HWO) NASA flagship mission. To that end, this work provides recommendations and observational protocols that will enhance and maximize the science of the ELTs to search for signs of habitability and life in terrestrial exoplanet atmospheres, laying the foundation for ground-based terrestrial exoplanet science in the near term, and space-based characterization studies in the future. In this work, we develop and apply techniques for analyzing simulated high-resolution ground-based spectra and retrieving molecular abundances in simulated ELT data. Our approach extends well beyond current approaches for characterizing terrestrial exoplanet atmospheres with the ELTs by considering a suite of molecular species that can help constrain the origin of atmospheric O2, or provide further environmental context for a potential biosignature detection, for a range of different inhabited and uninhabited terrestrial atmosphere types, as well as a sub-Neptune atmosphere, for planets orbiting M dwarf host stars. We found that CH4, CO2, H2O, and CO are all potentially detectable for both transiting and non-transiting terrestrial exoplanets, and that two biosignature pairs (O2/CH4 and CO2/CH4) may be detectable for nearby Earth-like worlds in ~10 hours of observing for nearest target Proxima Centauri b. Furthermore, we may be able to discriminate biosignature false positive environments using the direct imaging capabilities of the ELTs by detecting CO, a false positive indicator gas for several cases, in as little as 10 hours for nearby targets, or by searching for signs of significant abiotic O2 buildup via H2O photolysis, which may be possible in <100 hours of observing time. Discriminating the atmospheres of non-transiting planets as either terrestrial- or sub-Neptune-like may also be possible via spectral characterization in ~1 hour of observing with the ELTs. We also explore our ability to measure the abundance of O2 in habitable Proxima Centauri b atmospheres using atmospheric retrieval methods for high-resolution cross-correlation spectroscopy for the first time, and we find that we may be able to measure Earth-like O2 abundances or lower (<=21% O2) in 100 hours of observing the O2 A-band; however, retrieving high O2 abundances in post-ocean loss scenarios with thick atmospheres may be challenging due to significant saturation of the O2 A-band--- considering other O2 bands that are less prone to saturation or searching for the spectral features of O2--O2 collisionally-induced absorption may instead be used to identify this scenario.This dissertation proposes future ELT observing protocols and observational strategies for characterizing terrestrial exoplanets. The tools we have developed will continue to be relevant in the preparation for the analysis of the first ELT transit transmission and reflected light observations of nearby terrestrial exoplanets, as well as for the development of future ground- and space-based instrumentation and science strategies. The code and methodology developed in this work are available to the exoplanet and astrobiology communities.