Life on the edge: Where is the outer edge of the habitable zone?

The conventional habitable zone is generally defined as the range of orbits around a planet’s host star where surface liquid water can exist, given a suitable atmosphere. Determining what kind of atmospheres lead to surface liquid water, and the circumstances under which such atmospheres can be accreted or generated, and then maintained, is a key part of the habitability problem. In this lecture I will focus on the outer edge of the habitable zone. I will begin with a re-examination of the concept of “maximum greenhouse effect,” and an explanation why carbon dioxide yields a max-GH limit but hydrogen does not, leading to the concept of “hycean worlds” in which liquid water oceans can be maintained by H2-dominated atmospheres. I will then turn to the more familiar case of CO2-supported habitability, and present results on the way the deep carbon cycle, which determines the amount of CO2 in the atmosphere, limits the range of actual habitability in space and time. It is argued that the deep carbon cycle is the main limit on habitability lifetime for planets orbiting M stars, which would otherwise remain habitable for a trillion years or more, provided they were in a habitable state at the time the host star enters the main sequence.

About this series: The PAOC Colloquium is a weekly interdisciplinary seminar series that brings together the whole PAOC community. Seminar topics include all research concerning the physics, chemistry, and biology of the atmospheres, oceans and climate, but also talks about e.g. societal impacts of climatic processes. The seminars take place on Monday from 12-1pm. Contact paoc-colloquium-comm@mit.edu for more information and Zoom password.