Tidal flows in extrasolar planets and stars

Since 1995 astronomers have discovered and partially characterised several thousand extrasolar planets. Many of these planets have masses similar to Jupiter’s, and orbit their host stars in only a few Earth days -- a fascinating class of planets that we refer to as hot Jupiters. Since these planets orbit their stars very closely, gravitational tidal interactions between the planet and star can determine the orbits of these planets, and the spins (axial rotations) of the planet and its host star.

This means that the Earth is gradually spinning down (by a few milliseconds per century) as the Moon is receding from the Earth (at a rate of a few centimetres per year). Evidence for the influence of gravitational tides in extrasolar planetary systems is most clearly seen in the orbits of the closest planets. Tides can change the shape of the orbit: how circular or elliptical (eccentric) it is. Indeed, the closest planets, that orbit their stars in less than 10 Earth days, preferentially have circular orbits, while those that orbit their stars more distantly tend to have eccentric orbits. This is thought to primarily result from the dissipation of tidal flows inside these planets. These (and related) observations motivate theoretical work to understand tidal flows inside planets and stars, to determine and elucidate the mechanisms by which they may dissipate.