Triggered planet formation in massive protoplanetary discs
Astronomers have so far discovered around 30 extrasolar planets orbiting nearby, solar type stars. Current observational techniques are only sensitive to rather massive planets (typically with masses similar to that of Jupiter), but already these surveys demonstrate that extrasolar planetary systems are abundant in the solar neighbourhood. The fact that extrasolar planets are common came as no surprise, since the protoplanetary discs of gas and dust that form planets are observed around most young stars. What was surprising is the shape of these new planetary systems, which in most cases are very different from our own Solar System. Giant planets have been found both very close to their parent star (much closer than the orbit of Mercury in our system), and further out but with significantly eccentric orbits. These surprising properties have led theorists to believe that interactions between giant planets, and between planets and the protoplanetary disc, are of crucial importance in determining the final structure of planetary systems.
At MPA we are studying the planet-disc interaction in the case where the disc is massive. This is appropriate if planet formation occurs early, before gas in the disc has had time to drain onto the young star. Starting with a stable, quiescent disc (upper left panel), the introduction of a Jupiter mass planet first acts to clear a partial gap in the gaseous disc (upper right panel). This process occurs rapidly -- on the dynamical timescale at the radius of the planet. However, accretion onto the planet continues along the prominent spiral arms, which become increasingly unstable as the planet mass grows. Eventually, when the planet mass is around 5 times that of Jupiter, the disc becomes unstable to fragmentation into additional planetary mass objects (lower panels). Subsequent gravitational interactions between multiple planets formed in this way provide a possible route to the formation of extrasolar planetary systems containing massive planets on eccentric orbits.