From Cold to Hot Irradiated Gaseous Exoplanets: Fingerprints of Chemical Disequilibrium in Atmospheric Spectra

Abstract

Abstract Almost all planetary atmospheres are affected by disequilibrium chemical processes. In this paper, we introduce our recently developed chemical kinetic model (ChemKM). We show that the results of our HD 189733b model are in good agreement with previously published results, except at the μbar 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 1500 K where above a certain C/O threshold value a greater chance of CH4 detection is expected, still exists after including the vertical mixing. The first robust CH4 detection on an irradiated planet (HD 102195b) places this object within this region, supporting our prediction. We also investigate the detectability of disequilibrium spectral fingerprints by the James Webb Space Telescope and suggest focusing on the targets with T eff between 1000 and 1800 K, orbiting around M dwarfs, and 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 is likely due to the presence of clouds and not disequilibrium processes. However, some cold planets (T eff < 900 K) with very low C/O ratios (<0.25) show significant deviations, making these planets interesting cases for further investigation.