It has long been thought that the rocky cores of all the planets in the solar system formed by a slow process of dust aggregation followed by mutual gravitation and collisions. This theory suggests that the cores formed at roughly the same time, and that the giant planets attracted their gas exteriors later. But it does not account for the strange orbits of many of the planets discovered beyond our solar system.

Astronomers recently adapted this model, suggesting that huge clouds of gas could have merged in the early solar system to form the gas planets long before the terrestrial planets took shape. This means that the giant planets would have disturbed the orbits of the young planets as they circled the central star. As these planetesimals gained mass, this effect would have intensified. Gas drag would also have slowed down the planetesimals, and these effects could have led to unusual orbits.

Kortenkamp's team developed a computer simulation that accounted for these effects, but ignored the weak gravity between the planetesimals, which was central to the original theory. "We were very surprised to find that the influence of the giant planet led to runaway growth of planetesimals bigger than a large asteroid", Kortenkamp told PhysicsWeb. The group has dubbed the phenomenon type-II runaway growth. Type I occurs in the earlier stages of aggregation.

The adapted model describes both the solar system and systems in which planets orbit more than one star, because the orbital disturbances can be caused by any massive object - giant planets, brown dwarfs or other stars. "Our earlier work focused on the solar system, and we made the connection with many-star systems later", Kortenkamp says.

The new results suggest that the process of planet formation is even more robust than astronomers previously thought, at least for terrestrial planets. 'To use a phrase from Jurassic Park', says Kortenkamp, 'give planet formation a chance and it will find a way'.