Predicting quantitative biosignature patterns from populations of exoplanets

As we advance towards the observation and comparative analysis of exoplanets atmospheres, the search for life beyond our Solar System will be enhanced with a new statistical dimension, enabling population-level studies of potentially habitable worlds. Here we ask, what trends in surface properties can we predict to emerge across multiple exoplanets, given the probability that simple ecosystems emerge on some of them, and what observational effort is needed to differentiate life-bearing and lifeless habitable planets based on such trends? To answer these questions, we focus on Earth-like exoplanets, where hydrogenotrophic methanogenesis would be among the simplest, most primitive candidate metabolisms. We build on our astro-eco (astrophysics + ecology) quantitative framework in which models of atmosphere and surface processes (ocean chemistry, carbon cycle, and methanogenesis) are explicitly coupled (Sauterey et al. 2020 Nature Comm.). Given values for that determine basic characteristics of broadly Earth-like exoplanets (stellar luminosity, orbital parameters, ocean surface fraction, volcanic outgassing), our model predicts steady-state atmospheric composition and temperature, under different scenarios (methanogenesis, if the models habitability criterion is met; stagnant lid or plate tectonics interior convective regime). By coupling our astro-eco framework to exoplanet population and biosignature survey simulator (Bioverse, Bixel & Apai 2021) we quantify the observational effort required to detect such correlations. Our results emphasize the importance of investigating habitability and biosignatures together, grounded in the same ecological principles, and allow us to re-assess fundamental concepts such as the Habitable Zone. The predicted trends and our assessment of potential for observationally testing such trends via population-level studies of habitable worlds can guide the design of future space telescopes that focus on biosignature-surveys.