INVESTIGATING EXOPLANETARY ATMOSPHERES THROUGH TRANSMISSION SPECTROSCOPY

Over thirty years since the first exoplanet was identified, the tally of recognized exoplanets has surged dramatically to exceed 6000. This swift increase in discoveries has largely resulted from the deployment of the highly productive Kepler and Transiting Exoplanet Survey Satellite (TESS) mis ions, dedicated to extensively surveying exoplanets from space. The identification of initial exoplanets spurred interest in exploring their atmosphericproperties. The primary challenge in directly observing exoplanet atmospheres stems from the significant contrast ratio between the planet and itshost star. Fortunately, not long after, scientists identified a highly beneficial group of exoplanets, known as transiting planets, which enabled the initialdetection of an exoplanetary atmosphere. This group primarily consists of planets orbiting closely with short periods, which increases the likelihoodof transit events occurring within their orbits. An exoplanet transit occurs when the planet moves across the face of its star from the observer’s view-point. When an exoplanet is in transit, some stellar light travels through its atmosphere, and based on the atmospheric composition, subtle spectralfeatures are imprinted on the observed stellar spectrum. This technique is known as transmission spectroscopy. This thesis is focused on the search and characterisation of atmospheric signals from the exoplanet atmosphere, the transmission spectrum, using both high- and low-resolution transmission spectroscopy observations of several exoplanets. The majority of the research presented in this thesis pertains to applying themethod I developed for deriving the transmission spectrum of individual lines while thoroughly removing Earth and stellar contaminants. I will discuss various studies focusing on KELT-9 b, the most scorching exoplanet discovered. Among these, I will detail the planet’s atmospheric analysisthrough single line examination as outlined in D’Arpa et al. (2024b), introducing my method for extracting and fitting transmission spectra of individual lines. I will also compare the findings from D’Arpa et al. (2024b) with other research conducted by the Global Architecture of PlanetarySystem (GAPS) collaboration concerning the single line analysis of KELT-9 b. Furthermore, I will demonstrate how this method, initially devised forKELT-9 b, can be applied to other subjects, including several hot Jupiters. This includes work in progress utilizing Echelle SPectrograph for RockyExoplanets and Stable Spectroscopic Observations (ESPRESSO) and other studies. A notable application of the framework developed for single line analysis is using both the Hα line and the He I triplet as markers of two distinct star-planet interactions: stellar activity and photo-evaporationas discussed in Guilluy et al. (2024) and D’Arpa et al. (2024a). Regarding the low-resolution spectroscopy I will introduce the preliminaryresults of one ongoing project regarding the comparison between different pipelines used to extract the transmission spectra from Hubble Space Telescope (HST) Wield Field Camera 3 (WFC3) data. I will show how slightly different results obtained by these pipelines would lead to differences in the chemical composition retrieved.