Statistical trends in atmospheric properties of close-in giant exoplanets

Hot Jupiters are a class of gas giant exoplanets that orbit their host stars with an orbital period of a few days and subsequently receive a lot of incoming stellar radiation. Individual studies of hot Jupiter atmospheres have revealed a large diversity in their composition, climate and dynamics. Statistical surveys allow us to generalise the findings of individual studies and reveal trends in the general population of exoplanets. This thesis looks at two different surveys about the atmospheres of gas giant exoplanets, one survey on their mid-transit times, and one in-depth look into the atmosphere of a peculiar exoplanet.
The first survey studies a set of 49 gas giant exoplanets in transmission to examine how the atmospheric chemical properties change as a function of planetary temperature. By comparing the transmission photometry observations to grids of models, we find that the coolest planets show a lack of methane compared to expectations. We can partially explain this finding with models of 30x solar metallicity as higher metallicity lowers the temperature at which the methane is dominant over carbon monoxide in the atmospheric models. Furthermore, we expand our model grid to study different strengths of vertical mixing and we show that the sample of coolest planets (<1000K) rule out 1x solar composition with >3sigma confidence while supporting low vertical mixing (K_zz = 10⁸ cm²/s). On the other hand, we find that the hot planets are best explained by models with 1x solar metallicity and high vertical mixing (K_zz = 10¹² cm²/s). We also discuss some of the biases behind this survey.
The second survey investigates the near-infrared (NIR) emission of 78 hot Jupiter exoplanets. We reveal a clear transition in the observed dayside emission spectra of these hot Jupiters at 1660±100 K in the zero albedo, full redistribution equilibrium temperature. We propose that the measured transition is a result of seeing carbon monoxide in emission due to the formation of temperature inversions in the atmospheres of the hottest planets. Our findings are in remarkable agreement with a new grid of 1D radiative and convective models that contain the relevant physics for temperature inversions to form. We hypothesize that such thermal inversions could be caused by the presence of atomic and molecular species with high opacities in the optical and/or the lack of cooling molecular species.
One planet that displays a strong signature of a thermal inversion is WASP-18b, a particularly massive and bright ultra-hot Jupiter. We observe and analyse ten almost consecutive secondary eclipses at 4.5 μm with Spitzer/IRAC. We find a periodic variability in the eclipse depth in time. The periodic signal has a variability period of 23.12±1.66 days and a peak-to-trough amplitude of 456±71 ppm, corresponding to ~12% variability. We discuss possible causes of this variability, such as stellar variability, variable wind speeds, clouds, changes in atmospheric composition, and magnetic field coupling.
The final chapter of this thesis presents a NIR transit survey of cool gas giant exoplanets. We analyse forty-eight transits of three multi-planet systems (Kepler-9, Kepler-18 and Kepler-32) and one circumbinary system (Kepler-16b) in order to constrain the mid-transit times. We find that our measured mid-transit times of each of the multi-planet systems agree with their respective transit timing variation models and that our observations do not offer any additional constraints on the orbital dynamics. On the other hand, the high-precision lightcurves of Kepler-16b deviate from current photodynamical predictions. We, therefore, use these measurements to update the photodynamical model and report new constraints on the orbital elements of the system.