Validation of Habitable Zone Limits with a New Radiative-Convective Equilibrium Climate Model

Abstract

The habitable zone (HZ) boundaries calculated with the Clima one-dimensional radiative-convective equilibrium (RCE) model have served as a standard reference for exoplanet target selection for three decades, but they have never been independently validated with a comparable model. We present ExoColumn, a new one-dimensional RCE model that couples an independently developed atmospheric column physics package to the ExoRT correlated-k radiative transfer core that is also used by the three-dimensional ExoCAM model. We calibrate ExoColumn to present-day conditions, which matches other RCE models to within a few kelvin through the troposphere. Line-by-line benchmarks show that the ExoRT and Clima radiation cores each reproduce the outgoing longwave flux to within a few percent, with the differences due to the spectral line databases used by each model. We follow the same inverse climate modeling method as previous studies have done to find inner edge moist- and runaway-greenhouse limits of 0.970 au and 0.966 au, and an outer-edge maximum-greenhouse limit of 1.611 au at a CO${}_2$ partial pressure of 8.7 bar. We extend these calculations to 12 host stars spanning 2000-7200 K and to planets of 0.1, 1, and 5 Earth mass, and we provide updated HZ boundary coefficients for community use. The ExoColumn limits systematically fall closer to the host star than those of Clima but agree to within a few percent in orbital distance, which validates the robustness of the published Clima HZ boundaries.

arXiv commentThis paper is dedicated to Jim Kasting

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