Metal Oxide Clusters in Gas Giant Exoplanet Atmospheres

Abstract

This study investigates the thermal stability and absorption of metal oxide clusters in exoplanetary atmospheres. Utilizing our thermochemical data, we analyze eight distinct cluster families: magnesium oxide (MgO), silicon monoxide (SiO), titanium monoxide (TiO), vanadium monoxide (VO), titanium dioxide (TiO$_2$), vanadium dioxide (VO$_2$), aluminum oxide (Al$_2$O$_3$), and vanadium pentoxide (V$_2$O$_5$). Equilibrium cluster populations as a function of gas temperature and pressure reveal distinct stability regimes. Under solar elemental abundances, (TiO$_2$)$\rm\rm_N$ and (Al$_2$O$_3$)$\rm_N$ are favored at higher temperatures, while (MgO)$\rm_N$ and (SiO)$\rm_N$ dominate at lower temperatures. Computed absorption spectra exhibit strong size- and composition-dependent absorption features in the mid-infrared (8--50~$\mu$m), many of which fall within the wavelength range accessible to \texttt{JWST/MIRI}. We further coupled cluster thermodynamics with 3D general circulation model (GCM) outputs to investigate the cluster stability across the ultra-hot Jupiters (UHJs) WASP-121 b and WASP-18 b, the hot Jupiter (HJ) WASP-39 b, and the warm Jupiter (WJ) WASP-69 b. In WASP-121 b and WASP-18 b, extreme dayside temperatures suppress large-cluster stability, yielding atmospheres dominated by metal ions at low pressures and neutral metals at depth, with limited cluster survival on the nightside and morning terminator. In WASP-39 b, larger clusters are not thermochemically favoured despite the enhanced metallicity; instead, equilibrium chemistry stabilises smaller species, with only TiO showing a tendency toward stable larger cluster forms, likely due to its open d-orbitals. In contrast, WASP-69 b favors the formation of larger metal oxide clusters across an extended pressure range, highlighting WJs as a favorable environment for metal oxide cluster stability.