Planet Formation in Binary Stars

For many years, planets with twin suns, were objects of science fiction. We now know that not only do they exist, but there are millions of them in our Galaxy. Such planets must survive in complex environments, unusually irradiated and tidally perturbed by two stars. Their discovery was driven by a revolution in astronomy. With telescopes in space, such as Kepler and CoRoT, and on the ground, like the UK's SuperWASP, one thousand extra-solar planets (exoplanets) have been detected in the last decade. Latest estimates suggest there are tens of billions in the Milky Way - that is about ten planets for every person on the Earth! The next few years will see many more discoveries as new telescopes and space missions, for example the European project GAIA and the UK's Next-Generation Transit Survey, probe deeper into the Universe around us.

New discoveries in science lead to breakthroughs in our understanding of the fundamental behaviour of the Universe. The discovery of exoplanets is no exception. Before we found them planet formation theory relied on our Solar System, its eight planets, minor planets and asteroids. However, a good scientific theory of planet formation should reproduce the menagerie of exoplanets in our Galaxy. The properties of planets - masses, radii, compositions, orbital periods (year lengths) and number frequency - provide the clues which help us solve the mystery of planet formation. One thing we know for sure is that planets form in discs of material around stars. In our solar system, and in most stars, these discs are short-lived and so no longer exist except for inert debris outside the orbits of the planets. However there are star systems out there which form discs much later in their lives: these are discs in binary-star systems that form when matter flows from one star to another.

About half the stars in our Galaxy are in binary systems. The two stars in a binary are locked by gravity in orbit around one another. As stars age they become larger and, in many binary systems, they interact by transferring material from one to another. The mass transfer process is complex and some material can end up in discs both around the accreting star or around both the stars. Just as around young stars, planets can form in these discs. Indeed we know that planets form around binary stars because a dozen have already been observed by the Kepler satellite. However, we understand very little about how many planets actually do form this way, why they do, or what influence such planets have on the life of the stars to which they are intimately bound. Discs around binary stars are a unique astrophysical laboratory in which to investigate planetary formation and how it depends on the properties of the discs and the stars, such as their chemical composition. The aim of this project is to predict the numbers and properties of planets around binary stars and compare to what we see. In this way, theories of planet formation and binary stellar evolution will be refined or ruled out and we shall learn much more about the environment in which these planets exist. Our basic understanding of planet formation around stars will be greatly enhanced and we will be one step closer to explaining the mystery of how planets form and manage to survive in close proximity to stars.