Toward the analysis of JWST exoplanet spectra: Identifying troublesome model parameters

TL;DR

This paper compares three 1D radiative-convective models for exoplanet atmospheres to assess uncertainties in interpreting JWST spectra, highlighting the importance of molecular line lists and chemical modeling choices.

Contribution

It introduces a benchmarking protocol for exoplanet atmosphere models, compares different models and line lists, and discusses implications for JWST spectral analysis.

Findings

Differences in molecular line lists can exceed JWST observational uncertainties.

Uncertainties in line shapes impact spectral predictions more than expected JWST errors.

Chemical models show minimal differences in spectra, but chemistry assumptions affect interpretations.

Abstract

Given the forthcoming launch of the James Webb Space Telescope (JWST) which will allow observ- ing exoplanet atmospheres with unprecedented signal-over-noise ratio, spectral coverage and spatial resolution, the uncertainties in the atmosphere modelling used to interpret the data need to be as- sessed. As the first step, we compare three independent 1D radiative-convective models: ATMO, Exo-REM and petitCODE. We identify differences in physical and chemical processes taken into ac- count thanks to a benchmark protocol we developed. We study the impact of these differences on the analysis of observable spectra. We show the importance of selecting carefully relevant molecular linelists to compute the atmospheric opacity. Indeed, differences between spectra calculated with Hitran and ExoMol exceed the expected uncertainties of future JWST observations. We also show the limitation in the precision of the models due to uncertainties on alkali and molecule lineshape, which induce spectral effects also larger than the expected JWST uncertainties. We compare two chemical models, Exo-REM and Venot Chemical Code, which do not lead to significant differences in the emission or transmission spectra. We discuss the observational consequences of using equilibrium or out-of- equilibrium chemistry and the major impact of phosphine, detectable with the JWST.Each of the models has benefited from the benchmarking activity and has been updated. The protocol developed in this paper and the online results can constitute a test case for other models.