Characterisation of exoplanetary atmospheres and planetary systems

The discovery of over three thousand exoplanets in the past two decades has unveiled a large and diverse population, far exceeding the diversity seen in our own Solar System. Today, research efforts need to shift from the discovery to the characterisation of exoplanetary systems, and this thesis aims to be a further step in this direction. Two different techniques are investigated to chemically characterise exoplanetary systems: atmospheric retrievals and metal-polluted white dwarfs. The study of exoplanetary atmospheres through their spectra offers a very promising way to understand not only the chemistry of exoplanets, but also their atmospheric dynam- ics, formation and evolution history. As part of this thesis, a novel retrieval tool, called TauREx, was developed to interpret exoplanetary spectra. Spectral models were created and benchmarked with existing models, and a state-of-the-art database of absorption cross sections was also developed. The uncertainties in these models, and their propagation in the retrieval stage, were analysed in detail. These methods were used to investigate the retrievability of the carbon-to-oxygen ratio in simulated exoplanet spectra, and to interpret the atmospheres of two exoplanets, HD209458 b and 55 Cnc e. Lastly, these models were used to study the effects of stellar flares on the chemistry and spectra of typical exoplanets. Complementary to the observations of exoplanetary atmospheres, metal polluted white dwarfs are today a unique laboratory to infer the chemical composition of terrestrial exo- planets, and to study evolved planetary systems. It has become clear that the metals seen at a fraction of white dwarfs result from accreted circumstellar dust, originating from the tidal disruption of rocky planetesimals. Through the analysis of these stars, it is possible to infer the composition of terrestrial planetesimals, as their photospheres, in principle, mirror the composition of the accreted material, in turn providing clues on the nature of rocky plane- tary bodies. In this thesis I will discuss this technique, and present a recent survey that has unambiguously determined the fraction of detectable planetary debris at white dwarfs.