The thermal structure of exoplanet atmospheres

Sammanfattning: Context. The discovery of hot Jupiters, giant gas exoplanets on tight orbits close to their host star, has proven instrumental in the study of exoplanet atmospheres through transit spectroscopy. Just as a temperature increase with height manifests in Earth’s atmosphere due to ozone absorbing ultraviolet radiation, transit spectra of some hot Jupiters has yielded evidence that suggests the presence of a similar temperature inversion in their stratosphere. A possible mechanism driving such an inversion is the presence of an analogous molecule to ozone. One potential candidate is titanium oxide (TiO) since it is one of the first molecules to form at high temperatures. However, due to the intersection of the vapour pressure curve of TiO and the pressure–temperature curve of some hot Jupiters, further investigation is required to determine if TiO can occur at suciently high abundances, or if it condenses out such as due to the presence of a cold-trap. Aims. The goal of this project is to determine the pressure, density and temperature structure of an exoplanet in thermal and hydrostatic equilibrium and to identify if a temperature inversion will manifest. Methods. A model is developed using the properties of exoplanet HD 209458b and a program is written to numerically and dynamically compute the thermal structure of a hot Jupiter atmosphere by solving the radiative transfer equations. The model takes into account depletion of TiO from destruction at high temperatures and condensation below the vapour pressure curve. Results. The results show that a weak thermal inversion occurs on HD 209458b at TiO mixing fractions greater than solar Ti abundances and when a sucient fraction of the visible waveband interacts with the TiO. The proportion of incident visible radiation that must interact with the TiO in order to drive an inversion decreases as TiO abundance increases. Conclusions. For TiO abundances above solar Ti levels, the model shows a weak thermal inversion can manifest in the upper atmosphere of HD 209458b for pressures below ~ 10^4 bar. This falls within the allowable pressure range as existing literature constrains inversions to pressures below ~ 10^3 bar on HD 209458b. However, inversions at such low pressures are not currently observable.