Planet Formation Group
Astrophysics Department, School of Physics, University of Bristol

Planet Formation Group: Stefan Lines

There is great difficulty in explaining the presence of circumbinary planets (planets that orbit two stars - think Star Wars’ Tatooine and Doctor Who’s Gallifrey…!), recently observed by the well known exoplanet hunting Kepler mission.

Planets must form out of the disk of dust and gas (known as the protoplanetary disk) that surrounds the stars. Initially, interacting dust begins to stick together forming larger solid bodies. These must then continue to grow through collisions. However, not all collisions are preferable to growth - high energy interactions can cause these small ‘planetesimals’ to erode and fragment rather than merge and grow.

Circumbinary Planetesimal Disk

PKDGRAV Visualization of Kepler-34 planetesimal disk response to binary forcing.

Around a single star, conditions are favourable to planet formation, and planets can form in abundance. However, intense gravitational perturbations from binary stars on the disk can drive up planetesimal velocities and eccentricities which lead to explosive, erosive collisions which grind down any aspiring planets into small rock and dust.

Yet to date we observe 8 of these mysterious planets. How did they form?

Kepler-16 Fluid Disk

FARGO Visualisation of Kepler-16 fluid disk response to binary forcing.

Our work involves performing high-resolution N-body simulations using the Bristol Bluecrystal High-Performance Computers. In our fully 3D simulations, we integrate the orbits of one million planetesimals which feel both the stars and each other. We use a state-of-the-art collision model to accurately determine the outcome of a collision, allowing for perfect merging, partial accretion, bouncing (hit-and-runs) and erosion.

We find that all but one of the planets we observe must have formed far away from the binary, where conditions are less hostile, and then migrated inwards. Our work has supported the notion that planetary migration is an essential part of planet formation. It could also help future exoplanet missions more accurately determine the location of circumbinary planets.