Above the Clouds: Probing the atmospheres of ultra-hot Jupiters at high-resolution

Abstract

One of the most surprising discoveries of the numerous exoplanet surveys conducted over the past two decades is the existence of close-in giant planets orbiting their host stars at orbital separations smaller than that of Mercury's orbit. Although their large numbers are partly due to observational biases, these planets 'namely hot Jupiters and ultra-hot Jupiters'offer astronomers a unique opportunity to study atmospheric physics and chemistry in extreme environments, for which there are no Solar System analogues. The recent significant advancements in ultra-stable, ground-based,high-resolution spectrographs have enabled the detection and characterisation of these objects in unprecedented detail, using techniques that complement similar studies conducted with space-based observatories. This thesis aims to investigate these worlds during transit using high-resolution observations. From these observations, advanced statistical and atmospheric modelling frameworks are employed to infer the physical, chemical, and dynamical properties of their atmospheres. The key results of this thesis are primarily based on data from the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) installed at the Very Large Telescope (VLT) at Paranal Observatory, Chile. Firstly, ESPRESSO was used to observe three transits of the ultra-hot Jupiter WASP-121b. The chemical and dynamical properties of the atmosphere of WASP-121b, such as the vertical temperature structure, chemical abundances, and wind velocity, were monitored using observations taken months/years apart. Similarly, in the second study, three transits of the ultra-hot Jupiter WASP-76b were observed, separated by months/years. A novel rotational broadening kernel was developed to analyse these observations. This kernel allows the contributions from the morning (leading) and evening (trailing) limbs of the planet to be separated in velocity, enabling the extraction of atmospheric properties across the planet's hemispheres. Thus, these properties were monitored with respect to both time and longitude. Finally, two of the most common methods for removing contamination of ground-based spectra from the Earth's atmosphere were compared on simulated exoplanetary signals injected into real ESPRESSO data. These signals were injected on varying orbital architectures, and subsequently reduced using the different telluric removal methods, before recovering them via an atmospheric retrieval. The methods' efficacy, as well as any biases they might impose on the inferred atmospheric parameters, were compared. The key findings are summarised before highlighting future avenues of research which may be extended from the research presented in this thesis to advance the knowledge of the complex, multidimensional nature of these alien worlds.