Stratospheric Clouds Do Not Impede JWST Transit Spectroscopy for Exoplanets with Earth-Like Atmospheres

TL;DR

This study shows that JWST can effectively observe Earth-like exoplanet atmospheres despite the presence of stratospheric clouds, which are unlikely to hinder the detection of biosignatures or atmospheric features.

Contribution

It provides a detailed analysis of the impact of stratospheric clouds on JWST transit spectroscopy using Earth analogs and synthetic spectra, considering refraction and noise effects.

Findings

Stratospheric clouds cause at most 8.5 km difference in atmospheric thickness.

JWST cannot detect Earth-like stratospheric clouds in the TRAPPIST-1 system.

Clouds are detectable around white dwarf habitable zones after 4 transits.

Abstract

The James Webb Space Telescope (JWST) will provide an opportunity to investigate the atmospheres of potentially habitable planets. Aerosols, significantly mute molecular features in transit spectra because they prevent light from probing the deeper layers of the atmosphere. Earth occasionally has stratospheric/high tropospheric clouds at 15-20 km that could substantially limit the observable depth of the underlying atmosphere. We use solar occultations of Earth's atmosphere to create synthetic JWST transit spectra of Earth analogs orbiting dwarf stars. Unlike previous investigations, we consider both clear and cloudy sightlines from the SCISAT satellite. We find that the maximum difference in effective thickness of the atmosphere between a clear and globally cloudy atmosphere is 8.5 km at 2.28 microns with a resolution of 0.02 microns. After incorporating the effects of refraction and Pandexo's noise modeling, we find that JWST would not be able to detect Earth like stratospheric clouds if an exo-Earth was present in the TRAPPIST-1 system, as the cloud spectrum differs from the clear spectrum by a maximum of 10 ppm. These stratospheric clouds are also not robustly detected by TauREx when performing spectral retrieval for a cloudy TRAPPIST-1 planet. However, if an Earth size planet were to orbit in a white dwarf's habitable zone, then, we predict that JWST's NIRSpec would be able to detect its stratospheric clouds after only 4 transits. We conclude that stratospheric clouds would not impede JWST transit spectroscopy or the detection of biosignatures for Earth-like atmospheres.