High-Resolution Transmission Spectroscopy of Gas Giant Atmospheres

The discovery of exoplanets has revealed an intriguing class of planets unlike anything in the Solar System: ultra-hot Jupiters. These gas giants share similarities with Jupiter in size and in their hydrogen- and helium-dominated atmospheres, yet their proximity to their host stars results in vastly different atmospheric conditions. Intense stellar radiation heats them to temperatures exceeding 2000 K, allowing metals such as iron and titanium to exist in the gas phase rather than condensing out. Among these species, titanium oxide (TiO) is of particular interest, as it has long been hypothesised to drive thermal inversions—temperature increases with altitude—in highly irradiated atmospheres. This thesis investigates the atmospheric composition of the ultra-hot Jupiter WASP-189 b using high-resolution transmission spectroscopy, which probes a planet’s atmosphere by analysing starlight filtered through it during transit. A key result is the first unambiguous detection of TiO in the transmission spectrum of an ultra-hot Jupiter, confirmed through observations with multiple high-resolution spectrographs. Given the high signal-to-noise observations of the system, WASP-189 b serves as an ideal benchmark for atmospheric characterisation. This thesis also presents a wide chemical inventory of its optical transmission spectrum using both the cross-correlation technique and narrow-band spectroscopy, based on observations taken with HARPS, HARPS-N, ESPRESSO, and MAROON-X. Additionally, time-resolved absorption signals from these methods offer insights into the planet’s atmospheric variations over the course of the transit. Beyond ultra-hot Jupiters, this work explores the limitations of high-resolution transmission spectroscopy for warm Jupiters—gas giants on longer orbits where slower orbital velocities complicate atmospheric signal extraction. This analysis highlights the role of orbital configuration in the success of these techniques and identifies systems where observations remain feasible with current and future instruments. As exoplanet characterisation advances toward cooler planets in search of molecules such as carbon monoxide and water, overcoming these methodological challenges will be crucial for constraining elemental abundance ratios and linking atmospheric composition to planetary formation and migration histories.