The General Circulation Of Tidally Locked And Non-Tidally Locked Exoplanets

Abstract

The advent of space telescopes has begun a new age of discovery – there are trillions of exoplanets orbiting stars other than the Sun waiting to be found, each of which is its own world with its own climate. The majority of dynamical modelling of exoplanets to date has focussed on tidally locked planets, due to the assumed likelihood that most planets we have discovered will be in this state. However, there is a growing body of evidence that suggests that close-in planets need not be tidally locked, and a survey of our Solar System shows that there are at least 8 planets that are not. We address the question of how relaxing the assumption of tidal locking changes the climate of a terrestrial planet. We show that the atmospheric circulation is sensitive to the speed of the diurnal cycle, a key ratio being the speed compared with the internal wave speed of the atmosphere. Equatorial superrotation is shown to be modulated by the presence of a diurnal cycle, and in some circumstances completely eliminated. The results are explored in the context of a remote observer looking at the planet through a telescope and recording a thermal phase curve. Observations of exoplanets have shown both eastward and westward offsets of the hottest face of an exoplanet from the brightest, implying distinct heat redistribution patterns. We investigate the relationship between asynchronous rotation and the thermal phase curve offset and in particular consider the inverse problem: can the rotation rate of a planet be inferred from its phase curve? We demonstrate with an idealised model that hot spot offsets to both the east and west are possible when the planet is allowed to spin independently of its orbital rate. Lastly, the idea of a moving heat source of the diurnal cycle is generalised and its application to an unsolved peculiarity in the climate of Earth is investigated. The Madden-Julian Oscillation (MJO), a quasi-periodic system of deep cloud convection and increased rainfall in the tropics on Earth, is modelled as an equatorial moving forcing, akin to the diurnal cycle of a slowly rotating exoplanet. We propose a simple dynamical model that has characteristics of the MJO and investigate its properties.