From cold to hot irradiated gaseous exoplanets: Fingerprints of chemical disequilibrium in atmospheric spectra

TL;DR

This paper introduces the ChemKM model to study disequilibrium chemistry in exoplanet atmospheres, analyzing its effects on spectral features, quenching, and detectability with JWST, highlighting the importance of various atmospheric parameters.

Contribution

The paper presents a new chemical kinetic model, ChemKM, and provides insights into disequilibrium effects, quenching levels, and spectral fingerprints relevant for exoplanet characterization.

Findings

Quenching pressure decreases with effective temperature.

The 'Methane Valley' region persists after vertical mixing inclusion.

Cold planets with low C/O ratios show significant spectral deviations.

Abstract

Almost all planetary atmospheres are affected by disequilibrium chemical processes. In this paper we introduce our recently developed Chemical Kinetic Model (\texttt{ChemKM}). We show that the results of our HD189733b model are in good agreement with previously published results, except at $\mu$bar regime, where molecular diffusion and photochemistry are the dominant processes. We thus recommend careful consideration of these processes when abundances at the top of the atmosphere are desired. We also propose a new metric for a quantitative measure of quenching levels. By applying this metric, we find that quenching pressure decreases with the effective temperature of planets, but it also varies significantly with other atmospheric parameters such as [Fe/H], log(g), and C/O. In addition, we find that the "Methane Valley", a region between 800 and 1500K where above a certain C/O threshold value a greater chance of CH$_4$ detection is expected, still exists after including the vertical mixing. The first robust CH$_4$ detection on an irradiated planet (HD102195b) places this object within this region; supporting our prediction. We also investigate the detectability of disequilibrium spectral fingerprints by JWST, and suggest focusing on the targets with T$_{eff}$ between 1000 and 1800K, orbiting around M-dwarfs, having low surface gravity but high metallicity and a C/O ratio value around unity. Finally, constructing Spitzer color-maps suggests that the main two color-populations are largely insensitive to the vertical mixing. Therefore any deviation of observational points from these populations are likely due to the presence of clouds and not disequilibrium processes. However, some cold planets (T$_{eff}<$900K) with very low C/O ratios ($<$0.25) show significant deviations; making these planets interesting cases for further investigation.