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Abstract

As long as we have gazed at the night sky we have wondered about the nature of planets both within our Solar System and beyond. The last three decades of observations have revealed a staggering diversity of exoplanets in terms of their size, temperature, and composition. Out of the thousands of known exoplanets, only a small handful can be directly imaged. These rare systems present a unique opportunity to characterise their atmospheres with high precision and broad wavelength coverage. In order to observe these planets, they must retain enough heat from their formation to be observable in thermal emission, using dedicated high-contrast imaging techniques to separate the faint signal of the companion from the nearby star which it orbits. Only young, giant exoplanets are bright enough to be directly observable. Their relative youth enables us to tie their present-day composition back to the mechanism via which they formed.

In this thesis, I present the tools and methods used to observe and characterise such directly imaged exoplanets and apply them to the benchmark system HR 8799. The development of the petitRADTRANS retrieval module allows for the rapid fitting of models to spectroscopic data in a Bayesian framework. This retrieval method is applied to the JWST Early Release Science target WASP-39 b, a hot, Saturn mass exoplanet. WASP-39 b is found to have an atmosphere enriched in metals, and the detections of CO2} and photochemically produced SO2 are confirmed. By comparing three different high-contrast imaging post-processing algorithms, the impacts of data processing techniques used to extract an exoplanet's spectrum on one's ability to infer atmospheric properties are explored. Accounting for the correlation between wavelength channels of spectroscopic measurements is found to be critical to producing unbiased parameter estimates. Having developed the data analysis and modelling framework, the atmospheres of the four HR 8799 planets are systematically characterised. Using new VLTI/GRAVITY observations, together with a broad range of archival data we compile the most complete spectra of these objects to date. The planets are fit using the petitRADTRANS retrieval framework, as well as using self-consistent radiative-convective equilibrium grids. Bulk properties of effective temperature, surface gravity, radius, metallicity, carbon-to-oxygen number ratio, mass, and bolometric luminosity are inferred for all four planets. Their atmospheres are found to be highly metal-rich, with stellar-to-superstellar C/O ratios. Future study will be necessary to link these atmospheric properties to their formation history, but it is clear that these enigmatic planets will remain a target of further observations for years to come.