Nightside Clouds on Tidally-locked Terrestrial Planets Mimic Atmosphere-Free Scenarios

TL;DR

Nightside cloud formation on tidally-locked terrestrial planets can mimic atmosphere-free conditions in observations, complicating atmospheric detection and characterization efforts, especially for planets with small day-night temperature contrasts.

Contribution

This work demonstrates that nightside clouds can cause observational signatures similar to atmosphereless planets, highlighting the need for careful interpretation of exoplanet atmospheric data.

Findings

Nightside clouds can obscure atmospheric signals in observations.

Small day-night temperature contrasts can lead to false negatives for atmospheres.

Additional observations are necessary for accurate planetary atmosphere characterization.

Abstract

We investigate the impact of nightside cloud formation on the observable night-day contrast of tidally-locked terrestrial planet atmospheres. We demonstrate that, in the case where the planetary dayside is only 10s of Kelvin hotter than the planetary nightside, the presence of optically thick nightside clouds can lead to observations that mimic a planet without an atmosphere, despite the planet actually hosting a significant (10 bar) atmosphere. The scenario presented in this work requires a level of intrinsic atmospheric day/night temperature contrast such that the nightside can form clouds while the dayside is too hot for cloud formation to occur. This scenario is most likely for hotter terrestrials and terrestrials with low volatile inventories. We thus note that a substantial dayside/nightside temperature difference alone does not robustly indicate that a planet does not host an atmosphere and additional observations and modeling are essential for characterization. We further discuss several avenues for future study to improve our understanding of the terrestrial planets and how best to characterize them with JWST.