Heavy Metals, Global Winds and Evaporating Worlds: High-Resolution Spectroscopy of Ultra-Hot Jupiters

Ultra-hot Jupiters are compelling case studies for exoplanetary science because their atmospheres exhibit properties not found in other worlds. Their extreme proximity to their host stars drives molecular dissociation, producing upper layers dominated by atomic species, while their brightness and deep transits make them especially amenable to observation. These qualities transform them into natural laboratories for testing models of atmospheric chemistry and dynamics, and provide the ideal setting for refining the tools required to probe exoplanet atmospheres. This thesis brings together five papers that apply high-resolution transmission spectroscopy to ultra-hot Jupiters, unified by the aim of improving how atmospheric signals are extracted and interpreted, asking whether we can approach familiar problems in better or different ways, and what insights become possible when we do.The work focuses primarily on the archetypal ultra-hot Jupiter KELT-9b, whose extreme temperature and brightness make it well suited for atmospheric studies. Most of the methodological developments and novel results of this thesis have focused on this system, while complementary observations of WASP-189b have also been used. Central to the analysis is the cross-correlation technique, which selectively combines flux values to amplify the absorption patterns of planetary atmospheres. Across multiple case studies, I demonstrate both its power and its limitations, and I develop strategies to overcome these challenges. A major advance, developed in the later stages of this thesis, is a bootstrapping framework that mitigates collinearity in cross-correlation functions, a long-standing obstacle that has hindered quantitative detections of atmospheric species. Applying this framework across diverse datasets not only strengthens the robustness of detections but also enables new discoveries.The thesis also demonstrates that observing exoplanet atmospheres is, at its core, a photon-collection exercise: if enough photons are gathered, meaningful measurements are possible regardless of telescope size. Using two modest-sized facilities, the Wendelstein telescope with FOCES and the Anglo-Australian Telescope with Veloce, I show that atmospheric signals can be resolved with a single-transit observation, challenging the notion that such studies require only the largest facilities or extensive observing campaigns. This reframes whether such studies can be done to what can realistically be achieved with the resources already available.The most novel result is the robust identification of an asymmetric evaporative tail in the atmosphere of KELT-9b, a signal previously hinted at but not established. This discovery invites the re-examination of archival data and has important implications for our understanding of atmospheric escape and the compositional evolution of exoplanets.The findings of this thesis demonstrate that heavy species are ubiquitous in these extreme worlds, that even modest facilities can perform pioneering science, and that understanding the field’s current state and questioning assumptions can reveal unexpected atmospheric phenomena. By broadening the range of tools and facilities accessible to atmospheric studies, this thesis provides clarity what can be learned about planetary formation, escape processes, and even aspects of planetary habitability.