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

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.

The Ariel Target List: The Impact of TESS and the Potential for Characterizing Multiple Planets within a System

“…The error bars were computed using ArielRad, and the atmospheric forward models were computed using TauREx 3 (Al-Refaie et al 2021b). The top panel is a free chemistry model, while the TauREx GGChem plug-in is used for the bottom panel (Woitke et al 2018;Al-Refaie et al 2021a). Random Gaussian scatter has been added to each spectrum based on the magnitude of the spectral uncertainties.…”

“…We based these on the modeling in Turrini et al (2018), assuming a ratio of 0.5 and 0.85 for planets b and c, respectively. We then modeled the spectra for these planets assuming equilibrium chemistry using the ACE package (Agúndez et al 2012;Venot & Agúndez 2015), which is a plug-in for the TauREx 3.1 retrieval code (Al-Refaie et al 2021a. We assumed isothermal temperature-pressure profiles and introduced a gray cloud deck at 5e2 Pa (0.005 bar) and 1e3 Pa (0.01 bar) for TOI-1130 b and c, respectively, to produce spectral features in the Hubble Space Telescope WFC3 band that are equivalent in size to those seen in similar planets studied with this instrument (e.g., Guilluy et al 2021).…”