The (bio)geochemistry of exoplanets: What do we know and what do we need to know?
Exoplanets around other stars may provide the ultimate test of our understanding of biogeochemical cycles. These planets may be habitable, but our challenge for detecting life on these planets will be to distinguish (from first principles) the BIOgeochemical rates and fluxes of a living planet, from the strictly geochemical and physical processes of an abiotic planet. Ecosystem stoichiometry is a powerful theory based on the conservation of matter and energy that may provide insight into both organisms and environments at the individual and the ecosystem scale. However, our knowledge of the ratios of biogeochemically relevant elements available on exoplanets is very limited. I will compare the ratios of bioessential and rock-forming elements (e.g., C, N, P, S, and Mg, Si, Ca, Fe) for living organisms, for our Solar System, and for nearby stars. The molar C:P ratios for generic plankton (e.g., C:P = 106) differs markedly from the C:P ratios for Earth’s crust (2) and for our Sun (~2200). The very limited stellar abundance data for P reveals that our Sun might be comparatively P-depleted relative to nearby stars. This range in C:P and in other elemental ratios results from differences in stellar composition, planet formation and differentiation processes, surface processes, and possibly the presence of life. I will also explore some of the types of exoplanets we might encounter (water worlds, desert planets) and their biogeochemistry. Finally, I will discuss how future work to detect life on exoplanets will require a coordinated effort where biogeochemistry provides a crucial theoretical framework that informs data collection and modelling from astrophysics and planetary science.