Sustained and comparative habitability beyond Earth

Although we have a reasonable understanding of the physical and chemical conditions required to support the growth and reproduction of organisms, we still have only a rudimentary grasp of the geophysical conditions required to sustain those conditions over geological timescales. We propose that a strengthening of the interface between geophysics and biology is required to quantify sustained habitability and ultimately to mature the science of comparative habitability. Comparative habitability will inform our understanding of the common principles that allow habitability to be sustained on different planetary bodies, and whether habitability is predictable or contingent for a given set of planetary body characteristics. These developments are enabled by missions in our Solar System, including those to icy bodies such as Europa, Enceladus and Titan, in combination with telescopes allowing us to study habitability on exoplanets, thus providing essential insights into processes that can enable sustained habitability. Comparative habitability will help to determine whether Earth is a rare outpost of conditions suitable for a multi-billion-year biosphere, or whether the conditions that allowed for sustained habitability here are common in the Universe. The habitability of a planet is defined at a fixed time. A bigger challenge is to understand how that habitability is sustained over geological timescales, and how the underlying processes compare across different planetary bodies.