Atmospheric Thermal Tides of Exoplanets

Gravitational interactions between a planet and its host star may have important effects on the orbit and rotation rates of the two bodies. Gravitational tides, through tidal friction, change the rotation period of a planet, ultimately synchronizing it with its orbital period and tidally locking it when the orbit is nearly circular. While the theory of gravitational tidal friction (Darwin 1898) has been invoked for some time now to understand circularization of orbits and synchronization of spins, the possibility of additional physical effects which may compete with tidal friction is relatively less explored. In this thesis the mechanism of “thermal tides” will be explored through hydrodynamic simulations, and applied to understand the expected rotation rates of planets orbiting very close to their host star. Thermal tides describe fluid flows in planetary atmospheres driven by time-dependent stellar irradiation as experienced on asynchronously rotating planets. This time-dependent heating may also generate mass quadrupoles which could then be torqued by the stellar tidal force. These “thermal tide torques” may either reinforce or counteract torques from gravitational tidal friction, either trying to accelerate or reverse a planet’s rotation rate. A balance of opposing torques could occur at a rotation rate distinct from synchronous rotation, and therefore the addition of thermal tide effects may allow a theoretical understanding of exoplanet rotation rates, which are as yet unmeasured. We conducted four simulations varying the orbital period of a hypothetical planet. We found that the thermal tide torque increases toward shorter orbital periods. Additionally, we found that the time scale for torque equilibrium increases with orbital period. We recommend the analysis of the density, velocity, and temperature profiles in the atmosphere in order to exhibit the fluid motions for future studies. These results can contribute to further understanding of the effects of stellar irradiation on atmospheric heating and planetary rotation rates, and can shed light on expected rotation rates of planets orbiting very near their star, where tidal effects are important.