The creation and use of a chemical kinetics code to model and understand the atmospheres of hot Jupiters

Change log
Hobbs, Richard 

Chemical compositions of exoplanets can provide key insights into their physical processes, and formation and evolutionary histories. Atmospheric spectroscopy provides a direct avenue to probe exoplanetary compositions. However, whether obtained in transit or thermal emission, spectroscopic observations probe limited pressure windows of planetary atmospheres and are directly sensitive to only a limited set of spectroscopically active species. It is therefore critical to have chemical models that can relate retrieved atmospheric compositions to an atmosphere's bulk physical and chemical state. To this end we present Levi, a chemical kinetics code for modelling exoplanetary atmospheres.

Levi is constructed as a eulerian solver of a series of coupled 1-D continuity equations for the evolution of molecular species. Levi is able to calculate the gas phase hydrogen, oxygen, carbon, and nitrogen chemistry in hot Jupiters to produce abundance profiles of the planets’ atmospheres. We perform extensive testing of Levi to ensure its accuracy, including investigations of how both the boundary and initial conditions of the model could affect the final result, as well as comparing the thermochemical processes to the diffusional and photochemical processes. Levi underwent benchmarking by testing it against already existing codes of the same type, to ensure the code was consistent with previously produced models.

We use Levi to run a suite of models across a range of metallicities to produce the abundance of a number of molecules in any hot Jupiter's atmosphere. Our parameter sweep covers metallicities between 0.1x and 10x solar values for the C/H, O/H and N/H ratios, and equilibrium temperatures of hot Jupiters between 1000K and 2000K. We link this parameter sweep to hot Jupiter formation and migration models from previous works to produce predictions of the link between molecular abundance and planet formation pathways, for the spectrally active molecules H2O, CO, CH4, CO2, HCN and NH3. We investigate the detections of numerous molecules in the atmosphere of HD 209458b, and find that within the framework of our model, the abundance of these molecules best matches with a planet that formed by gravitational instability between the CO2 and CO snowlines and underwent disk-free migration to reach its current location.

We next extend the underlying chemical network used in Levi. We present and validate a new network of atmospheric thermo-chemical and photo-chemical sulfur reactions. Sulfur was chosen as the element to add due to its importance on planets such as Venus and the existence of previous studies that have shown that sulfur may be significant in hot Jupiter atmospheres. We use Levi to investigate these reactions as part of a broader HCNO chemical network in a series of hot and warm Jupiters. We also investigate how the inclusion of sulfur can manifest in a hot Jupiter's atmosphere indirectly. Sulfur chemistry can result in the depletion of many non-sulfur-bearing species, including CH4, NH3 and HCN, by several orders of magnitude.

In summary, we create a 1-D photochemical-kinetic model and show that it can be used to constrain the dynamics of exoplanet atmospheres and their origins. Some of the key directions for future development include expanding the sophistication of the underlying chemical networks, work begun by our introduction of sulfur, and exploring the possibility of 2-D and 3-D dynamical models, to account for zonal redistribution. More developed chemical networks allow us to better constrain the atmospheric chemistry, and thus the overall atmospheric composition, breaking existing degeneracies we highlight, while improving the treatment of the dynamics ensures our modelling gives a more accurate depiction of the disequilibrium chemistry taking place.

Shorttle, Oliver
Madhusudhan, Nikku
Astronomy, Exoplanets, Hot Jupiter, Atmospheres, Atmospheric composition, metallicity, Atmospheric model
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
STFC (1945173)
Science and Technology Facilities Council (1945173)