Mystery solved?: The asteroid belt is a ring of rocky debris between Jupiter and Mars, thought to have been created when Jupiter's mass made the area too unstable for planet formation.

Credit: NASA/JPL-Caltch/T.Pyle

BRISBANE: Mysterious gaps in the asteroid belt may have been caused by a shift in the orbits of Jupiter and Saturn four billion years ago, researchers say.

A computer simulation of the gravitational influence of these migrating planets matches previously unexplained gaps in the belt, according to a report published in the British journal Nature today.

"The pattern in the missing asteroids confirms other lines of evidence that the giant planets went through a brief episode of migration some time in the Solar System's early history," said David Minton study co-author and astronomer at the University of Arizona in Tucson, USA.

Ring of rocky debris

The asteroid belt is a ring of rocky debris between Jupiter and Mars, thought to have been created when Jupiter's mass made the area too unstable for planet formation. The debris is not evenly distributed, and the belt has zones where there are far fewer asteroids than expected, said Minton.

Some of those gaps, called Kirkwood gaps, are in zones where Jupiter or Saturn's gravitational influence destabilises the asteroids so much that they are ejected from the belt, but many are in areas that are currently stable.

Minton and University of Arizona colleague Renu Malhotra suspected that the answer lay in the migration of the giant planets, which are thought to have formed elsewhere in the Solar System and moved to their present location around four billion years ago.

Jupiter is thought to have formed slightly further away from the Sun than it is today, and Saturn, Uranus and Neptune were once closer, Minton said.

The planets were subsequently dragged into their present positions by the gravity of large objects ejected from the Kuiper belt, a ring of icy debris lying beyond the planets. Once the Kuiper belt was depleted of large objects, the planets settled into their current orbits.

Bombardment of the planets

The researchers designed a computer simulation that described the changing gravitational influence of the planets on the asteroid belt, and found that many of the unexplained gaps matched areas that would have been destabilised by Saturn or Jupiter's gravity during the planetary relocation.

The destabilised asteroids would have been ejected from the belt, and many would have become projectiles that bombarded Earth and the other inner planets. "The effects of all those impacting asteroids would have been quite dramatic and violent," said Minton.

Paul Francis, an astronomer at the Australian National University's Mt Stromlo Observatory, near Canberra, said that astronomers used to think that planets stayed where they had formed, until the first planets were discovered near other stars.

Several of these planets were too close to their stars to have formed there, leading to the theory of planetary migration, Francis said. "[This study] is giving us the hard evidence that migration also happened in our own Solar System," he said.