Not Earth-like Yet Temperate? More Generic Climate Feedback Configurations Still Allow Temperate Climates in Habitable Zone Exo-Earth Candidates

TL;DR

This study explores how adding a fourth climate feedback to models of Earth-like exoplanets can lead to more diverse and potentially less habitable climates, challenging traditional habitable zone assumptions.

Contribution

It introduces a generalized climate feedback into exoplanet habitability models, revealing increased climate variability and implications for the prevalence of temperate exoplanets.

Findings

Fourth feedback causes more diverse climate behaviors.

Positive feedback reduces long-term habitability.

Classical habitable zone boundaries are altered.

Abstract

Earth's climate is influenced by over a dozen feedbacks, but only three dominate its long-term climate behavior. Models of the exoplanet habitable zone (HZ) assume that this is similar for other Earth-like planets. We used dynamical simulations to study Earth-like planets with a fourth, (potentially strong) generalized climate feedback. Across over 20,000 climate simulations, we find that the addition of the fourth feedback produces novel behaviors, including runaway and chaotic climate trajectories, that are more diverse than one would expect based on Earth's climate configuration. Non-negligible fourth feedbacks -- if negative -- would not lessen the probability of planets with temperate climates. However, positive fourth feedbacks decrease the fraction of exo-Earth candidates that are long-term habitable. Therefore, strong fourth feedbacks will alter (and mostly shrink) the boundaries of the classical habitable zone. When combined with occurrence rates of Earth-sized planets around sun-like stars, our results imply that the fraction of stars hosting rocky planets with temperate climates may be substantially lower than classical estimates under Earth-like climate assumptions. Our results are subject to the validity of the model assumptions and not intended to represent conclusive predictions about exoplanet populations but rather to demonstrate the potential climate diversity that emerges from non-Earth-like model configurations. Our conclusions provide context on sample sizes and science questions for next-generation exoplanet surveys.