DeFord Lecture: Planetary interior controls on the habitability of rocky exoplanets
Speaker: Brad Foley
Host: Demian Saffer
Title: Planetary interior controls on the habitability of rocky exoplanets
Abstract: An essential factor for the habitability of rocky exoplanets is climate regulation via the carbonate-silicate cycle. Without such regulation, uninhabitably hot or cold climates can form, even for planets lying within their host star’s habitable zone. The planetary interior plays a critical role in driving the carbonate-silicate cycle, as it controls how volatiles are cycled between surface and interior over time. I will explore these links between interior dynamics and climate evolution considering the role of a planet’s tectonic state, thermal evolution, and area of exposed land. While it has long been thought that plate tectonics might be essential for the carbonate-silicate cycle to regulate climate, I show that the same stabilizing feedbacks can also operate on planets lacking plate tectonics. However, these results point to the importance of active volcanism, which is in large part controlled by the budget of radiogenic elements (U, Th, and K) a planet acquires during formation. Radiogenic heat producing element (HPE) abundances are not directly measurable in other planets, but estimates can be made based on the composition of stars planets orbit. I therefore use measured HPE abundances in stars to determine the likely range of heat budgets for rocky exoplanets and estimate how long they can stay volcanically active in a stagnant-lid regime. The results show many known exoplanets, including the Trappist system, may be too old to still be volcanically active today. Finally, I discuss the role of exposed land in sustaining habitable surface conditions. Whether sea floor weathering provides a climate regulating feedback, like weathering of exposed land, has been long debated. Based on recent estimates of the seafloor weathering rate and its dependence on temperature, ocean pH, and spreading rate, I show that planets lacking exposed land can potentially regulate their climates better than planets with land. This result opens up the possibility of habitable climates on waterworld planets.