Biogeosciences as a branch of the Earth and Life Sciences integrates theory from geology, chemistry, biology and physics to address questions across spatial and temporal scales including the very large and the very long. The biogeosciences evolved from studies of the modern Earth, but they are relevant for Earth’s deep past and even for exoplanets. Thus, we can now also consider astrophysics, and the study of exoplanets, to lie within the context of the biogeosciences. Exoplanets may provide the ultimate test of our understanding of biogeochemical cycles, especially those that are rocky and Earth-like. Planets around other stars may 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. As an example, phosphorous (P) and nitrogen (N) on Earth are key limiting nutrients for metabolism. Both elements are both required for DNA and RNA and without them there would be no biological production of oxygen, methane, or any other biogenic gas. This ecological stoichiometry is ultimately driven by the Earth’s geochemistry and the reactions those conditions (e.g. P, T, X) permit (Shock & Boyd 2015).
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. And while it is not yet possible to directly determine the elemental ratios for exoplanetary ecosystems, we generally assume that planets have similar compositions to their host stars (Thiabaud et al. 2015).
Here we compare the ratios of bioessential and rock-forming elements (e.g., C, N, P, Si) for living systems, for our Solar System, and for nearby stars. The range that we observe in the molar ratios of these elements result from differences in stellar composition, planet formation and differentiation processes, and possibly the presence of life. In other words, when going from stars to exoplanets — geophysics, and geobiology are the next step after planet formation. Future work to detect life on exoplanets will require a coordinated effort over the coming years, where the biogeosciences provide a crucial theoretical framework that informs data collection and modelling from astrophysics and planetary science. In addition, the details of these interdisciplinary fields need to be made accessible, such that observations and trends about elements that have been here-to-fore difficult to measure or underappreciated can be preferentially targeted in future observations and missions.