Understanding Atmospheric Circulation on Tidally Locked Exoplanets

Research by Trinity DPhil student Neil Lewis has investigated the atmospheric circulation of tidally locked exoplanets, which are exotic worlds orbiting stars other than the Sun that have a permanent day side and night side. The research separates the atmospheric circulation of into three distinct components, to understand how the atmosphere keeps heat moving and shapes the climates of these planets.

Lewis and his co-author Mark Hammond ran computer simulations of tidally locked planets using general circulation models (GCMs). GCMs are large computer codes that can be used to solve the equations of fluid dynamics and radiative heating in order to simulate atmospheric motion. They are the tools typically used to study the Earth’s weather and climate but for the study were adapted to study tidally locked planets to produce a simulated atmospheric circulation. 

With the simulated atmospheric circulation, the team then used a mathematical technique called a ‘Helmholtz decomposition’ to separate the circulation into three components: a jet of wind going around the planet at the equator, stationary atmospheric waves, and an overturning circulation that features air rising on the day side and sinking on the night side. This decomposition presents a simple and intuitive description of how the circulation works on these planets, analogous to a description of the Earth’s atmospheric circulation as containing mid-latitude ‘jet streams’ and a Hadley circulation with air rising at the equator and sinking in the subtropics. It will significantly aid future study of tidally locked atmospheres, including determining whether these worlds might be habitable.

The research has been published in Proceedings of the National Academy of Sciences (PNAS). 

Neil Lewis says: ‘This technique will be useful for future research in planetary science, not only because it paints a nice picture of the circulation, but also because it will allow researchers to understand how different parts of the atmospheric circulation contribute to important processes such as the transport of heat, clouds and chemical species around the planet. 

‘I really enjoyed doing this piece of work as it is the first project I have done independently from a senior scientist or faculty member. Mark Hammond (my co-author; a post-doc at the University of Bristol and former Oxford DPhil student) and I came up with the idea, ran the simulations, and wrote the paper together, which was a very enjoyable experience. I’m very grateful for the academic environment that my supervisor Peter Read and the Physics Department have created for me that has allowed me the freedom and confidence to pursue this research. The publication of this paper definitely feels like a significant milestone on my way to becoming a "proper" scientist!’