Cat amongst the pigeons: Neptune's violent migration sent rocks hurtling towards the Sun.© NASA

Why does the Solar System look like it does? The question has teased astronomers for centuries, but researchers have now come up with a single theory that they hope can explain three of the most mysterious features of our corner of the Universe.

The giant planets Jupiter and Saturn have unusually elliptical orbits that are significantly tilted out of the plane occupied by the smaller, rocky planets close to the Sun. And Jupiter itself is accompanied by small asteroids called Trojans, which fan out for millions of kilometres ahead of and behind the planet, speeding along the same orbital path.

Most perplexing of all is the 'late heavy bombardment' about 3.8 billion years ago, which peppered the Moon with chunks of rubble left behind from the planets' formation some 700 million years earlier. No convincing explanation has so far been put forward for why this sudden battering happened so long after the Solar System's violent early years.

But according to Hal Levison of the Southwest Research Institute in Boulder, Colorado, all of these quirks can be explained by a game of planetary billiards that began when Jupiter and Saturn fell into a specific orbital pattern just a few million years after they formed. "We have really explained a lot of the structure of the Solar System with this model," says Levison. He and his colleagues present their computer simulations of the Solar System's history in this week's Nature ¹-³

Junk theory

Levison singles out his colleague Alessandro Morbidelli, of the Observatory of Nice in France, as the prime architect of their idea. They argue that the tiny gravitational effects of junk left over from the formation of Saturn and Jupiter gently nudged the planets around until Saturn fell into an orbit that sent it around the Sun exactly once for every two orbits completed by Jupiter.

This resonance meant that twice every saturnian year, at almost exactly the same two points in space, Jupiter's gravity tugged at its ringed colleague and forced it into an ever more elliptical and tilted orbit.

The migration had a knock-on effect for Saturn's outer neighbour, Neptune, flinging it beyond Uranus into the outer Solar System where it now resides. Neptune's headlong crash into the rubble that had accumulated in the system's outer reaches sent thousands of these planetesimals spinning towards the Sun.

Some were trapped around Jupiter, falling into line with its orbit to form the trail of Trojans. And the rest battered the inner planets and their satellites, including the Moon, leaving its face scarred by impact craters. "This is the first fully self-consistent model of the late heavy bombardment," says Levison.

Hot topic

The theory has had a warm reception at recent presentations to astronomers, says Levison. But some remain wary. "The fact that a simulation of planet formation produces an end-state in good agreement with the observed Solar System does not prove that the simulated events actually happened," says Joe Hahn, an astronomer at St Mary's University in Halifax, Canada, who also studies the Solar System's evolution.

Hahn is most sceptical about the explanation for the late heavy bombardment. Planetesimals from the outer Solar System should have been scattered towards the Moon very soon after the planets began their wanderings, rather than taking 700 million years to get there.

Levison argues that they were slowed down by the gas and dust left over from planet formation. But Hahn is doubtful that they could have been delayed for so long. "These models are all a bunch of fairy tales anyway," he says. "I won't be convinced by any of them until astronomers can observe planet formation around other stars."

Further support for Levison's theory could come from the surface of Jupiter's Trojans, concedes Hahn. If they came from the outskirts of the Solar System, they should look like the icy Kuiper belt objects found there today.

Levison himself thinks that establishing the orbits of Kuiper belt objects should help to refine his theory, and plans to spend the next couple of years checking their effects on his model. 

• References

  1. Tsiganis, K., Gomes, R., Morbidelli, A. & Levison, H. F. Nature 435, 459–461 (2005). | Article | PubMed | ISI | ChemPort |
  2. Morbidelli, A., Levison, H. F., Tsiganis, K. & Gomes, R. Nature 435, 462–465 (2005). | Article | PubMed | ISI | ChemPort |
  3. Gomes, R., Levison, H. F., Tsiganis, K. & Morbidelli, A. Nature 435, 462–465 (2005). | Article | PubMed | ISI | ChemPort |