Impact of Clouds and Hazes on the Simulated JWST Transmission Spectra of Habitable Zone Planets in the TRAPPIST-1 System

TL;DR

This study uses 3-D climate and photochemistry models to assess how clouds and hazes affect JWST's ability to detect atmospheric molecules on TRAPPIST-1 planets, highlighting the importance of self-consistent modeling for future observations.

Contribution

It introduces a comprehensive 3-D GCM and photochemistry approach to evaluate cloud and haze impacts on atmospheric detection prospects with JWST for habitable zone exoplanets.

Findings

CO2 can be detected in less than 15 transits at 3 sigma.

Water vapor detection is very challenging unless in a moist greenhouse state.

Clouds and hazes significantly influence molecular detectability in transmission spectra.

Abstract

The TRAPPIST-1 system, consisting of an ultra-cool host star having seven known Earth-size planets will be a prime target for atmospheric characterization with JWST. However, the detectability of atmospheric molecular species may be severely impacted by the presence of clouds and/or hazes. In this work, we perform 3-D General Circulation Model (GCM) simulations with the LMD Generic model supplemented by 1-D photochemistry simulations at the terminator with the Atmos model to simulate several possible atmospheres for TRAPPIST-1e, 1f and 1g: 1) modern Earth, 2) Archean Earth, and 3) CO2-rich atmospheres. JWST synthetic transit spectra were computed using the GSFC Planetary Spectrum Generator (PSG). We find that TRAPPIST-1e, 1f and 1g atmospheres, with clouds and/or hazes, could be detected using JWST's NIRSpec prism from the CO2 absorption line at 4.3 um in less than 15 transits at 3 sigma or less than 35 transits at 5 sigma. However, our analysis suggests that other gases would require hundreds (or thousands) of transits to be detectable. We also find that H2O, mostly confined in the lower atmosphere, is very challenging to detect for these planets or similar systems if the planets' atmospheres are not in a moist greenhouse state. This result demonstrates that the use of GCMs, self-consistently taking into account the effect of clouds and sub-saturation, is crucial to evaluate the detectability of atmospheric molecules of interest as well as for interpreting future detections in a more global (and thus robust and relevant) approach.