(bio)Geochemistry on exoplanets: From desert planets to water worlds

Hilairy Hartnett

Biogeochemistry integrates theory from geology, chemistry, biology and physics to address questions across spatial and temporal scales including the very large and the very long. Exoplanets may provide the ultimate test of our understanding of biogeochemical cycles. Planets around other stars may, in fact, be habitable, but our challenge for detecting life on these planets will be to distinguish 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 provides insight into interactions between organisms and environments at both the individual and the ecosystem scale. However, our knowledge of the ratios of biogeochemically relevant elements available on exoplanets is very limited, and hinders our ability to predict planetary-scale biogeochemical processes. I will compare the ratios of bioessential and rock-forming elements (e.g., C, N, P, S, and Mg, Si, Ca, Fe) for living systems, for our Solar System, and for nearby stars; molar C:P ratios for a Redfield approximation of plankton (e.g., C:P = 106) differ 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 wide range in C:P ratio results from differences in stellar composition, planet formation and differentiation processes, surface processes, and quite possibly the presence of life. I will also explore some thought experiments for understanding systems that might exists on very dry and very wet exoplanets based on results from basic modelling studies of water worlds and analog experiments for desert planets. These thought experiments may prove useful as we work to detect life elsewhere and to engage a fully coordinated effort where biogeochemistry provides a crucial theoretical framework informing future data collection and modelling in astrophysics and planetary science.