Cloudy with a chance of water: Investigating hot Jupiter exoplanet atmospheres through observation and analysis

Since the discovery of the first exoplanet orbiting a sun-like star in 1995, the fundamental questions as to the formation of our Solar System have met a paradigm shift. The presence of hot Jupiter exoplanets, Jupiter sized worlds rapidly orbiting their host stars, was unlike anything previously seen or predicted. The later discovery of these strange new worlds transiting their stars opened up a new realm of studies into their atmospheres using transit spectroscopy to separate the signals between the star and planetary atmosphere. This thesis investigates the transmission spectral properties of hot Jupiter exoplanets through observations and theoretical analysis from the search for H2O in the near-IR to the signatures of cloud condensates in the IR. Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) observations of transiting hot Jupiters were used to investigate the atmospheric composition over water bands in the near infrared. We put forward a new analysis method to treat the varying systematics seen across transit datasets in a consistent and robust way, in which we marginalise over a grid of possible systematic models used to correct the lightcurves, with each model contributing to the extracted spectrum based on its statistical likelihood. We apply this new method to five previously studied hot Jupiter exoplanet transmission spectra and make direct comparisons between the planetary atmospheres. An apparent dichotomy emerges between two possible sub-classes of hot Jupiter atmospheres with clouds and hazes playing a key role. WFC3 appears to cover a critical wavelength range in exoplanet atmospheres where clouds and hazes potentially obscure the expected molecular signatures in systems where they are found to be obscured in the optical. Using analytical models following Mie theory, we explore the potential atmospheric transmission spectral signatures that would be caused by a variety of cloud condensates in hot Jupiter atmospheres. We find that the observed optical slope representing Rayleigh scattering at high altitudes can constrain the cloud condensate particle size and can be used as a diagnostic for potential condensate features in the IR where almost all condensate absorption features occur. We find that the major transmission spectral absorption features are generated by the vibrational modes of the major diatomic bond pair in each condensate species, which is often seen in the IR at 5-25 microns, and explore the potential for future JWST investigations using MIRI.Science and Technology Funding Council (STFC