We examine the properties of exoplanetary atmospheres using state-of-the-art retrieval methods and theoretical models. The examinations include investigations of the chemical compositions and abundances, the cloud and haze properties, and the temperature structures of exoplanetary atmospheres.

The detailed theoretical models we have developed study the chemistry of exoplanetary atmospheres across the gas and solid phases of matter. The first theoretical model investigates the extent to which solids evaporate in the envelopes of giant planets and how they contribute to enriching the atmosphere with metallic elements. The results of this study illustrate that the observable chemistry in the atmospheres of giant planets may indeed be used as a tool for tracing their formation conditions and histories. The second model considers the observable signatures of clouds in the spectra of transiting planets. We explore three metrics that may provide constraints on cloud properties: the slope in the optical wavelength region, the uniformity of this slope, and features in the infrared.

We then present several state-of-the-art retrieval studies of the chemical and thermal properties of transiting and directly-imaged planetary atmospheres. First, we apply our transmission retrieval method to the spectra of ten hot giant exoplanets and, for the first time, provide detailed statistical estimates of atmospheric properties for a size-able exoplanet sample with broadband data. Our analysis reveals a trend of low abundances of water vapour in the hot Jupiter atmospheres compared to their stars and the Sun, suggesting that a majority of hot Jupiters in our galaxy may harbour atmospheres depleted in water vapour. Importantly, the low water and oxygen abundances suggest that the majority of hot Jupiters undergo disk-free migration to their present locations. Second, we introduce a novel retrieval framework, AURA, for the joint study of planetary and stellar properties imprinted in a transmission spectrum. This method is the first in the literature to provide combined constraints on stellar and planetary properties. Our study finds that the spectra of four hot Jupiters in the previous sample show potential evidence of stellar contamination due to heterogeneity features in the photospheres of their stars. Such a joint framework lays a first foundation to disentangling spectral features which originate in the atmospheres of exoplanets from those present in the stellar photospheres.

We also present a new retrieval method for directly-imaged exoplanets and brown dwarfs. The retrieval structure is applied to the most iconic imaged system, HR 8799, composed of four giant companions. We find the presence of water vapour at high confidence (>5-sigma) in all four atmospheres and strong evidence for carbon monoxide at high abundances in the atmospheres of the two outer companions. The O/H ratios in the companion atmospheres are enhanced by over 6 times the solar value, and the C/O ratios of HR 8799b and HR 8799c are distinctly super-solar.

We finally present two additional applications of our retrieval methods to atmospheric observations with collaborators outside of Cambridge. We interpret the day-side atmosphere of the highly irradiated hot Jupiter WASP-18b and find evidence of a strong thermal inversion due to carbon monoxide and an atmosphere with a super-solar metallicity and C/O ratio. Finally, we characterise the most complete transmission spectrum of the giant planet XO-1b spanning the optical to infrared and find an atmosphere composed of inhomogeneous clouds and a low water abundance compared to the solar expectation.