The atmospheric circulation of tidally locked terrestrial exoplanets

Thesis

Exoplanets are planets orbiting stars other than the Sun. Tidally locked exoplanets always present the same face to their host stars, resulting in a global atmospheric circulation unlike anything in the Solar System. This thesis develops theories of the formation of this global circulation, demonstrates the circulation in atmospheric simulations, and shows its effect on observations of the ``lava planet'' 55 Cancri e. I use the the Gierasch-Rossow-Williams mechanism to describe the formation of zonal flow on tidally locked planets. I then use a shallow-water model linearised about an equatorial jet to show how this zonal flow produces the global circulation pattern and hot-spot shift seen in observations and simulations. Next, I use the theory developed in the first part of the thesis alongside atmospheric simulations to interpret the thermal phase curve of 55 Cancri e. I model possible atmospheres with different bulk properties on this planet, and simulate observations for comparison with the original phase curve. Finally, I simulate similar atmospheres using more realistic radiative transfer, and show that the global circulation is similar but that the observed hot-spot shift requires a sufficiently thick atmosphere. I conclude that the meridional circulation is vital to the formation of zonal flow on tidally locked planets, and that the observable hot-spot shift is caused by wave-mean flow interactions between this zonal flow and the stationary wave response to day-night forcing. The case study of 55 Cancri e suggests that the observations are evidence for an atmosphere thicker than 10 bar with a mean molecular weight higher than that of H$_{2}$, with night-side cloud formation. Future work should use this theoretical understanding of the global circulation alongside more realistic modelling, to interpret the more detailed observations that will be made in the coming decades.

Authors: Hammond, M

• DOI: 10.5287/ora-aed7km5y4
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Planetary Physics