Degenerate Interpretations of O₃ Spectral Features in Exoplanet Atmosphere Observations Due to Stellar UV Uncertainties: A 3D Case Study with TRAPPIST-1 e

Abstract

TRAPPIST-1 e is a potentially habitable terrestrial exoplanet orbiting an ultracool M dwarf star and is a key target for observations with the James Webb Space Telescope. One-dimensional photochemical modeling of terrestrial planetary atmospheres has shown the importance of the incoming stellar UV flux in modulating the concentration of chemical species, such as O3 and H2O. In addition, three-dimensional (3D) modeling 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-1 e (assuming an initial Earth-like atmospheric composition). We use two semiempirical 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 O3 columns differ by a factor of 26. Spectral features of O3 in both transmission and emission spectra vary between these simulations (e.g., differences of 20 km in the transmission spectrum effective altitude for O3 at 0.6 μm). This leads to potential ambiguities when interpreting observations, including overlap with scenarios that assume alternative O2 concentrations. Hence, to achieve robust interpretations of terrestrial exoplanetary spectra, characterization 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., H2O), or by comparing direct imaging and transmission spectra in conjunction.

Artificial Greenhouse Gases as Exoplanet Technosignatures

“…To generate synthetic transmission spectra and quantify the detectability of artificial greenhouse gases in our simulated transmission spectra, we use the Planetary Spectrum Generator (PSG; Villanueva et al 2018Villanueva et al , 2022. PSG is a public and versatile radiative transfer tool that can be employed to simulate a wide variety of planetary environments and has been used extensively for simulating terrestrial planets, particularly the TRAPPIST-1 planetary system (e.g., Fauchez et al 2019Fauchez et al , 2020Pidhorodetska et al 2020;Suissa et al 2020;Cooke et al 2023;Ostberg et al 2023). In our proof of concept transit cases, we use PSG to calculate the number of transits necessary to identify each molecule or combination of molecules investigated for a TRAPPIST-1 f test case, assuming stellar and planetary parameters sourced from NASA'S Exoplanet Archive and shown in Table 1.…”