Exploring TRAPPIST-1 Climate States with an Energy Balance Model

TL;DR

This study adapts an energy balance model to explore climate states of habitable planets around low-mass stars, focusing on TRAPPIST-1 e and f, revealing potential ice cover conditions based on atmospheric CO2 levels.

Contribution

It calibrates and validates a simplified climate model for tidally-locked planets, enabling efficient exploration of their climate states and guiding future detailed studies.

Findings

TRAPPIST-1 e may have partial ice cover under certain conditions.

TRAPPIST-1 f could be fully ice-covered unless CO2 levels are high.

The model effectively captures climate bistability and can inform more complex simulations.

Abstract

This paper presents a version of the HEXTOR energy balance model that has been configured for the study of habitable terrestrial planets orbiting low-mass stars. The model is validated for rapidly-rotating Earth-like planets using latitudinal coordinates, which shows expected patterns of bistability. A tidally-locked coordinate transformation is then applied to the model, which is calibrated to match mean values of the minimum, average, and maximum surface temperatures from a general circulation model ensemble of TRAPPIST-1 e. This calibrated energy balance model is used to characterize the possible climate states of such a synchronously rotating planet across a parameter space of instellation and carbon dioxide partial pressure. These calculations suggest a state of partial ice cover for TRAPPIST-1 e and complete ice cover for TRAPPIST-1 f, unless carbon dioxide partial pressure is ~1 bar or greater. This approach demonstrates the capability of a simplified one-dimensional model to study the climates of terrestrial planets in synchronous rotation, which can help guide more complex models and observations toward the most promising targets of interest.