Theoretical Underpinnings of the Search for Life on Exoplanets

Transit surveys like Kepler have revealed potentially habitable planets to be common, and upcoming facilities like the James Webb Space Telescope and the Thirty Meter Telescope will at last have the ability to characterize their atmospheres in search of signs of life. However, proceeding from the astrophysical observables of atmospheric composition and bulk planetary/stellar properties to inferences regarding the presence or absence of life will require considerable theoretical intervention. In this talk, I will illustrate this required theoretical investigation using three vignettes. Specifically, I will discuss (1) the UV environment on planets orbiting M-dwarfs and the implications for their habitability, (2) the photochemistry of water vapor and the implications for oxygen as a biosignature gas on planets orbiting Sun-like stars, and (3) efforts to increase the catalog of potential biomarkers of life, with phosphine as a worked example. I will conclude by reviewing the considerable theoretical and laboratory work that remains to be done to prepare for the era of habitable planet characterization.