On the Climate and Chemistry of Irradiated Exoplanet Atmospheres

In this thesis, we use numerical simulations to help understand the climate and chemistry of gaseous, irradiated exoplanets. In particular, we aim to uncover the diversity that we can expect within the exoplanet population. In our atmospheric models, we take into account the three-dimensional nature of irradiated exoplanets, along with the heat redistribution, dynamical mixing, and photochemical dissociations in the atmosphere. Furthermore, we investigate the extent to which these effects are observable and identify the main sources of uncertainties in current numerical models. Through a comparison between HD 209458 b and WASP-43 b, two planets with similar temperatures but different rotation rates, we find that the latter shows a tendency for dynamical feedback between the deep atmosphere and the observable layers. This is an important result, because up to recently it was assumed that the deep atmosphere is essentially decoupled from the observable layers. By conducting a grid of 3D climate simulations, we confirm that such deep circulation feedback mainly occurs in fast-rotating exoplanets. Additionally, we derive mixing efficiencies and run pseudo-2D chemical kinetics simulations for each model in the grid. We find that cold exoplanets up to effective temperatures of 1400 K tend to exhibit chemically homogeneous atmospheres because of efficient vertical and horizontal mixing, whereas hotter planets display longitudinal differences. From a sequence of hierarchical models, it appears that changes in the thermal structure of the atmosphere have the most impact on exoplanet transmission spectra, with chemical variations playing a secondary role. Photochemistry is seen to affect the upper atmosphere at the day side, as well as the night side of the planet through horizontal advection. However, the impact of photochemistry on transmission spectra is negligible, so we suggest that photochemistry products are targeted with high-resolution spectroscopy instead. Analogous to the case of WASP-43 b, we find that the deep atmosphere can have a great impact on the chemistry in the observable layers above, in particular in cool exoplanets, where vertical mixing is efficient. We conclude that, although chemical changes in exoplanet atmospheres are mainly driven by the temperature, additional parameters such as the gravity, rotation rate, and UV irradiance can give rise to considerable chemical diversity. Furthermore, taking into account the multidimensional nature and disequilibrium chemistry in exoplanet atmospheres is important, as these effects will likely be observable with the James Webb Space Telescope. We suggest that an additional retrieval analysis of our atmospheric models would be useful to deepen our understanding of how these effects will affect future observations. Finally, we identify the deep atmosphere as a major source of uncertainty in both climate and chemistry models. We aim to conduct a parameter study of the intrinsic heat to examine how the interior can bias atmospheric models and retrievals in a multidimensional context. Ultimately, a 3D coupled interior-atmosphere model would be best equipped to tackle this issue.

Jaar van publicatie:2021

Toegankelijkheid:Open