A comparison of chemical models of exoplanet atmospheres enabled by TauREx 3.1

TL;DR

This study compares three chemical equilibrium codes used in exoplanet atmosphere spectral retrievals, highlighting the importance of correct assumptions to avoid biased interpretations in upcoming JWST and Ariel observations.

Contribution

It provides a cross-comparison of ACE, FastChem, and GGchem codes within TauREx 3.1, revealing their accuracy and potential biases when assumptions about elemental composition are mismatched.

Findings

All codes accurately retrieve parameters when assumptions match truth.

Incorrect element assumptions lead to significant biases in parameters like metallicity.

Self-consistent chemical models must include proper assumptions to avoid false confidence.

Abstract

Thermochemical equilibrium is one of the most commonly used assumptions in current exoplanet retrievals. As the James Webb Space Telescope (JWST) and Ariel launch draw near, assessing the underlying biases and assumptions made when applying self-consistent chemistry into spectral retrievals is crucial. Here we use the flexibility of TauREx 3.1 to cross-compare three state of the art chemical equilibrium codes: ACE, FastChem and GGchem. We simulate JWST spectra for ACE, FastChem, GGchem and GGchem+condensation containing only C, H, O, N elements and spectra for FastChem, GGchem and GGchem+condensation with a more extensive range of elements giving seven simulated JWST spectra in total and then cross-retrieve giving a total of 49 retrievals. Our analysis demonstrates that like-for-like, all chemical codes retrieve the correct parameters to <1% of the truth. However, retrievals, where the contained elements do not match the truth, parameters such as metallicity deviate by 20% while maintaining extremely low uncertainties <1% giving false confidence. This point is of major importance for future analyses on JWST and Ariel, highlighting that self-consistent chemical scheme that do not employ the proper assumptions (missing species, fixed elemental ratios, condensation) are at risk of confidently biasing interpretations. Free chemistry retrievals employing parametric descriptions of the chemical profiles can provide alternative unbiased explorations.