The Hotspot Offset on Asynchronously Rotating Exoplanets: A Shallow Water Model

If there is no heat transport present in the atmosphere of a tidally-locked planet, the hottest point (the hotspot) will be at the substellar point: the location on the planet receiving the most radiation from the host star. Phase curve observations of transiting exoplanets show that the hotspot can be offset from the substellar point by up to 50º degrees in longitude.

Most small exoplanets discovered to date have been close-in to their host star and as a result most often assumed to be tidally-locked. However, recent modelling studies demonstrated that a modest atmosphere (as little as 1 bar) is sufficient to induce drag forces strong enough to prevent tidal-locking and leave planets in a range of possible spin states.

We use a numerical shallow water model to investigate how the hotspot offset from substellar point changes when a planet is not tidally-locked. The offset is found to be sensitive to the direction and velocity of the substellar point, as well as the absolute rotation rate of the planet. Slowly rotating planets exhibit thermally direct heat transfer from day to night sides. Results from fast rotating planets are discussed in the context of a global-scale, moving Matsuno-Gill problem.