Saba, Arianna; (2025) Exploring Exoplanet Atmospheres with Space- and Ground-based Observatories. Doctoral thesis (Ph.D), UCL (University College London).

Text AriannaSaba_PhDThesis.pdf - Accepted Version Access restricted to UCL open access staff until 1 July 2026. Download (100MB)

Abstract

We entered a golden age for exoplanet atmospheric characterisation. Historic observatories like the Spitzer Space Telescope and the Hubble Space Telescope have provided vast spectroscopic data, unravelling the complexities of exoplanet atmospheres. The launch of the James Webb Space Telescope in 2021 has transformed our understanding, delivering unprecedented data across wavelengths never explored before from space. In parallel, high-resolution ground-based facilities offer complementary insights into atmospheric composition and dynamic processes in distant worlds. This thesis presents an analysis of exoplanet spectroscopic transit data collected using various space- and ground-based instruments. I describe a data analysis pipeline for HST/STIS, which, along with established pipelines for HST/WFC3 and Spitzer/IRAC, was employed to characterise the optical-to-infrared spectrum of WASP-17 b. This analysis revealed significant stellar contamination, which was carefully quantified during the retrieval process. Building on this analysis and interpretation framework, and utilising over 25 years of near-ultraviolet, optical, and near-infrared HST data, I present a population study of 20 exoplanets observed in transmission using HST’s STIS and WFC3 instruments. This study highlights the widespread occurrence of stellar contamination across planetary systems, underscoring the need to account for stellar activity in exoplanet transit studies. Additionally, I present my PI proposal for observing WASP-17 b with CHEOPS, which was successfully granted 20 orbits of observing time. Furthermore, I describe the data analysis process of JWST NIRISS transit data of WASP-39 b. The exceptional photometric precision and data quality are highlighted, along with the unique spectral trace morphology and instrumental characteristics that posed challenges during the analysis. From the ground, I employ the HARPS-N high-resolution spectrograph to confirm the presence of the hydrogen Balmer lines in the transmission spectrum of KELT-9 b and the discovery of numerous metal lines. Finally, I present an excerpt of the work I conducted during a 6-months industry placement at Blue Skies Space.

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