A Statistical Comparative Planetology Approach to the Hunt for Habitable Exoplanets and Life Beyond the Solar System

Abstract

The search for habitable exoplanets and life beyond the Solar System is one of the most compelling scientific opportunities of our time. Nevertheless, the high cost of building facilities that can address this topic and the keen public interest in the results of such research requires the rigorous development of experiments that can deliver a definitive advance in our understanding. Most work to date in this area has focused on a "systems science" approach of obtaining and interpreting comprehensive data for individual planets to make statements about their habitability and the possibility that they harbor life. This strategy is challenging because of the diversity of exoplanets, both observed and expected, and the limited information that can be obtained with astronomical instruments. Here we propose a complementary approach that is based on performing surveys of key planetary characteristics and using statistical marginalization to answer broader questions than can be addressed with a small sample of objects. The fundamental principle of this comparative planetology approach is maximizing what can be learned from each type of measurement by applying it widely rather than requiring that multiple kinds of observations be brought to bear on a single object. As a proof of concept, we outline a survey of terrestrial exoplanet atmospheric water and carbon dioxide abundances that would test the habitable zone hypothesis and lead to a deeper understanding of the frequency of habitable planets. We also discuss ideas for additional surveys that could be developed to test other foundational hypotheses is this area.

A Methane Extension to the Classical Habitable Zone

“…This parameterization assumes that weathering/volcanic outgassing rates scales with pressure, as predicted for planets that have an operational carbonate-silicate cycle near the outer edge (e.g. Bean et al 2017). Following Ramirez (2017), we assume β = 0.4, which is consistent with experimental measurements for silicate rocks (Lasaga, 1984;Asolekar et al, 1991;Schwartzman and Volk et al 1989).…”

A More Comprehensive Habitable Zone for Finding Life on Other Planets

“…The classical HZ also predicts that atmospheric CO2 pressures on habitable planets should increase at farther distances from the star, with maximum pressures at the outer edge (e.g., [1]). Future observations would be able to infer if such a gradient exists [274]. If it does, this would provide strong support for a long-term carbonate-silicate cycle (or equivalent carbon cycling mechanism) that operates universally on habitable planets [274].…”

“…Future observations would be able to infer if such a gradient exists [274]. If it does, this would provide strong support for a long-term carbonate-silicate cycle (or equivalent carbon cycling mechanism) that operates universally on habitable planets [274]. Knowing the CO2 pressure would allow estimates of the planetary volcanic outgassing rate required to support it.…”

Three Direct Imaging Epochs Could Constrain the Orbit of Earth 2.0 inside the Habitable Zone

“…This conception of the HZ has yet to be empirically validated (but it could through statistical study of exoplanet properties; Bean et al 2017). If a planet does not have exposed surface rock, for example, then the HZs given by a model based on the silicate weathering feedback might not apply (e.g.…”