Extrasolar Planets

Extrasolar planets

Group members working in this area: Adrian Barker, Chris Jones

Since 1995 astronomers have discovered and partially characterised several thousand extrasolar planets. These planets have a diverse range of masses and radii, consisting of some that are as small as Earth, and some that are much bigger than Jupiter. Some planets orbit their stars very closely, much more closely than Mercury orbits our Sun, and some orbit much more distantly, with wider orbits than that of the Earth in the Solar System.

Many extrasolar planets have masses similar to Jupiter, and orbit their host stars in only a few Earth days – a fascinating class of planets that we refer to as hot Jupiters. The surface temperatures of these worlds can be as high as 2,500 Kelvin. Since these planets orbit their stars very closely, gravitational tidal interactions between the planet and star can determine the fates of these planets, and the spins (axial rotations) of the planet and its host star.

A familiar example of tides, much closer to home, is the interaction between the Earth and the Moon, which results in a gradual lengthening of both the day and month. 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)! This is thought to be caused mainly by dissipation of the kinetic energy of tidal flows in the Earth’s oceans. Similar processes are thought to be important for extrasolar planets. In particular, it is thought that hot Jupiters will be tidally locked, so that they will rotate about their own axis of rotation at the same rate as they orbit the star. This means that the same part of the planet would always face the star much like what occurs for the Moon in orbit about the Earth.

EVSPEvidence 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 primarily thought to be the result of the dissipation of tidal flows inside these planets.

These observations motivate theoretical work to understand tidal flows inside planets and stars, to determine and elucidate the mechanisms by which they may dissipate. This requires understanding the fluid dynamics and magnetohydrodynamics of tidal flows inside planets and stars, including the effects of rotation, stratification, convection and magnetic fields. Our research in this area uses a combination of computer simulations and analytical calculations to understand tidal flows in stars and in the fluid layers of planets, with the ultimate goal to explain the  observations and make predictions to be tested by future observations.