Clouds can enhance direct imaging detection of O2 and O3 on terrestrial exoplanets

TL;DR

Clouds can significantly influence the detectability of biosignatures like O2 and O3 on terrestrial exoplanets in direct imaging, with low clouds enhancing and high clouds diminishing signals, depending on cloud properties.

Contribution

This study quantifies how different cloud types and properties affect the detectability of biosignatures in direct imaging of exoplanets, providing new constraints for future observations.

Findings

Low clouds enhance O2 and O3 detectability.

High clouds diminish biosignature signals.

Clouds generally increase SNR for biosignature detection.

Abstract

Clouds are often considered a highly uncertain barrier for detecting biosignatures on exoplanets, especially given intuition gained from transit surveys. However, for direct imaging reflected light observations, clouds could increase the observational signal by increasing reflected light. Here we constrain the impact of clouds on the detection of O2 and O3 by a direct imaging telescope such as the Habitable Worlds Observatory (HWO) using observations simulated with the Planetary Spectrum Generator (PSG). We first perform sensitivity tests to show that low clouds enhance O2 and O3 detectability while high clouds diminish it, and the effect is greater when cloud particles are smaller. We next apply clouds produced by the cloud microphysics model CARMA with varied planetary parameters and clouds drawn from observations of different types of clouds on Earth to PSG. We find that clouds are likely to increase the SNR of O2 and O3 for terrestrial exoplanets under a wide range of scenarios. This work provides important constraints on the impact of clouds on observations by telescopes including HWO.