Predictive Rankings of the Probability for Temperate Terrestrial Worlds for the HWO ExEP Mission Star List

Abstract

The Habitable Worlds Observatory (HWO) is NASA's flagship mission design from the Decadal Survey on Astronomy and Astrophysics 2020, meant to observe temperate terrestrial planets via direct imaging and use direct spectroscopy of exoplanet reflected light to investigate their atmospheres for biosignatures. However, there are no known stars in the solar neighborhood conducive to direct imaging observations that are currently known to host rocky planets in their circumstellar habitable zones. Thus, HWO will most likely be running a blind survey; however, prioritizing the rankings of its target stars will help to potentially increase the yield of temperate terrestrial planets observed. Here we use simulated planetary systems with both small and giant planets to test which stellar systems among the HWO Exoplanet Exploration Program (ExEP) Mission Star List are most likely to host a rocky planet with the right temperature to sustain life on its surface. Assuming a simple model of planetary systems with small planets well-ordered in period interior to giant planets based on their respective occurrence rates, we find that some systems are upwards of 50% likely to host a temperate terrestrial planet. We also consider the possibility of a giant planet in or just beyond the circumstellar habitable zone that could host a temperate terrestrial moon capable of hosting life. Additional observations to refine the occurrence rates of small planets at orbital distances $\lesssim$ 1 AU and conditional rates between small and giant planets will refine these analyses and provide updates to these rankings.