Investigating the atmospheres of understudied exoplanet populations through space-based transmission spectroscopy

Booth, Luke 2025. Investigating the atmospheres of understudied exoplanet populations through space-based transmission spectroscopy. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

"With over 6000 exoplanets presently confirmed, the onus of exoplanet science has begun to shift from the identification and classification of new worlds into an unprecedented era of characterision, hoping to unveil answers to key questions such as "how do planets form and evolve?". Chemical characterisation, most readily achievable through spectroscopic observations of planetary atmospheres, is a promising place to start, with early work started by historic space-based facilities including the Hubble and Spitzer Space Telescopes. New observatories e.g. the James Webb Space Telescope now provide unparalleled spectral coverage, and upcoming survey satellites such as the Twinkle Space Telescope and Ariel will augment current capabilities by adding spectral data for 100's-1000's of planets to the detailed spectra currently available for a small selection. In this thesis, I explore spectroscopically under-studied planetary populations, and additionally search for an observationally-uncommon atmospheric phenomenon in the hot-Jupiter HD~209458~b. I present HST/WFC3 transmission spectra of two planets: an independent reduction and analysis of the rare, hot-Neptune desert planet LTT~9779~b and an original spectrum extracted from the transit portion of a phase-curve observation of the hot-Jupiter HD~209458~b, observed in 2022. I present evidence suggesting that HD~209458~b does not exhibit temporal variation in its atmosphere at a detectable level between the two epochs studied, and I find strong evidence confirming that LTT~9779~b possesses an atmosphere, which my preferred model suggests may contain a high abundance of CO_2. This thesis also presents two theoretical transmission spectroscopy studies, focussing on cool gaseous planets. I explore the feasibility of conducting an homogeneous spectroscopic survey on a statistically significant number of planets in this population with the upcoming Twinkle space telescope, and later seek to verify that analysis of simulated Twinkle and Ariel spectra yield inferences consistent with one-another for the same planet under their respectively wavelength grids and spectral resolutions. I present a pathway to explore the understudied cool gaseous planets population with Twinkle, and illustrate the potential scientific return of such a survey.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Physics and Astronomy
Subjects:	Q Science > QC Physics
Uncontrolled Keywords:	Exoplanets, Transmission Spectroscopy, Atmospheres
Funders:	STFC
Date of First Compliant Deposit:	13 March 2026
Last Modified:	27 Mar 2026 09:31
URI:	https://orca.cardiff.ac.uk/id/eprint/185721

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