Cloud formation in exoplanet atmospheres

: nucleation of small molecular clusters

Abstract

It has been inferred that many exoplanets host clouds in their atmospheres. Understanding the processes through which these clouds form, and being able to model them is essential for our understanding of their climate and to explain our observations. The first step of cloud formation in gaseous exoplanet atmospheres is the formation of cloud condensation nuclei (CCN). These CCN form from the gas phase through a process called nucleation. This work aims to investigate the nucleation process, analysing it through the nucleation species TiO₂ and its small molecular clusters. We present a method to find energetically favourable isomers for small (TiO₂)N nano clusters of size N “ 3´15. These isomers are analysed using high-level quantum chemical density functional theory (DFT) methods, to calculate their thermochemical properties. Two different nucleation descriptions are used to calculate the nucleation rate in an example exoplanet atmosphere. We find that using the updated, accurate cluster data changes the physical regime where nucleation is viable, possibly changing the lower boundaries of cloud formation in these atmospheres. We also analyse the harmonic vibrational spectra of the TiO₂ clusters. I find visible differences in the spectra for clusters of different sizes and for different isomers of clusters of the same size. I compute their total absorbance in two example hot Jupiter atmospheres and find significant differences in the abundances of different cluster sizes between morning and evening terminators of the same planet, between different types of planets and between different heights in the atmosphere. At some location the most abundant cluster is not the largest one in the dataset, hinting at possible limits to nucleation at those locations.

Date of Award	12 Jun 2023
Original language	English
Awarding Institution	University of St Andrews KU Leuven
Supervisor	Christiane Helling (Supervisor) & Leen Decin (Supervisor)