Degenerate interpretations of O$_3$ spectral features in exoplanet atmosphere observations due to stellar UV uncertainties: a 3D case study with TRAPPIST-1e

TL;DR

This study demonstrates that uncertainties in stellar UV flux significantly impact the predicted atmospheric ozone features of exoplanet TRAPPIST-1e, complicating spectral interpretation and emphasizing the need for accurate stellar characterization.

Contribution

It provides a comprehensive 3D modeling analysis showing how stellar UV uncertainties affect ozone predictions in exoplanet atmospheres, highlighting the importance of stellar UV measurements.

Findings

Ozone column differences can be up to a factor of 26 due to UV flux uncertainties.

Spectral features of ozone vary significantly between different stellar UV assumptions.

Characterization of host star UV spectra is crucial for accurate atmospheric interpretation.

Abstract

TRAPPIST-1e is a potentially habitable terrestrial exoplanet orbiting an ultra-cool M Dwarf star and is a key target for observations with the James Webb Space Telescope (JWST). One-dimensional photochemical modelling of terrestrial planetary atmospheres has shown the importance of the incoming stellar UV flux in modulating the concentration of chemical species, such as O$_3$ and H$_2$O. In addition, three-dimensional (3D) modelling has demonstrated anisotropy in chemical abundances due to transport in tidally locked exoplanet simulations. We use the Whole Atmosphere Community Climate Model Version 6 (WACCM6), a 3D Earth System Model, to investigate how uncertainties in the incident UV flux, combined with transport, affect observational predictions for TRAPPIST-1e (assuming an initial Earth-like atmospheric composition). We use two semi-empirical stellar spectra for TRAPPIST-1 from the literature. The UV flux ratio between them can be as large as a factor of 5000 in some wavelength bins. Consequently, the photochemically-produced total O$_3$ columns differ by a factor of 26. Spectral features of O$_3$ in both transmission and emission spectra vary between these simulations (e.g. differences of 19 km in transmission spectra effective altitude for O$_3$ at 0.6 $\mu$m). This leads to potential ambiguities when interpreting observations, including overlap with scenarios that assume alternative O$_2$ concentrations. Hence, to achieve robust interpretations of terrestrial exoplanetary spectra, characterisation of the UV spectra of their host stars is critical. In the absence of such stellar measurements, atmospheric context can still be gained from other spectral features (e.g. H$_2$O), or by comparing direct imaging and transmission spectra in conjunction.