Millions of objects in the solar system's main asteroid belt may be icy interlopers from beyond Neptune that were flung into their present orbits after a violent migration of the giant planets, a new simulation suggests.

© NASA/JPL-Caltech
About 20 per cent of the objects in the main asteroid belt may have gotten their start beyond Neptune, a new simulation suggests
The solar system's main asteroid belt is a diverse mix of objects that orbit between Mars and Jupiter. These asteroids are generally thought to have formed close to their present locations, so their compositions should reflect the original distribution of gas and dust that surrounded the sun there and eventually condensed into solid bodies.

"People have just been assuming that what we see there, formed there," says Hal Levison of the Southwest Research Institute in Boulder, Colorado.

But Levison and others suspect some 20 per cent of the asteroids in the belt may be comet-like objects that were born in colder climes, beyond the orbit of Neptune.

Scattered objects

The results come from new simulations using a theory called the Nice model, which suggests the solar system's giant planets were born closer together and were surrounded by a vast disc of leftovers from the planets' formation called planetesimals (see here).

According to the model, Jupiter and Saturn entered a tight orbital dance about 700 million years after the solar system formed. Their gravity then flung Uranus and Neptune out into the planetesimal disc like bowling balls, causing the objects there to scatter like pins.

Past simulations have tracked the trajectories of these planetesimals and showed they wind up forming the Kuiper belt of icy debris where Pluto sits, some of the distant satellites of Jupiter and Saturn, and Trojan asteroids, bodies that share Jupiter's orbit but are centred at two points ahead of and behind the planet.

Outer belt

The new simulation fed planetesimals into the region surrounding Jupiter and Saturn as the planets moved to see how many would be captured as so-called Hilda asteroids, a group outside the main asteroid belt that orbits the sun three times for each two orbits of Jupiter.

While some of the objects became Hildas and Trojans, most of the captured objects wound up in the outer portion of the solar system's main asteroid belt.

Indeed, the outer asteroid belt boasts objects that are thought to have ice, while the inner asteroid belt is dominated by rocky bodies. "The interpretation has been that this represents a change in the nebula or the disc from which the planets formed," Levison told New Scientist.

Partial simulation

But if these icy outer objects are newcomers, it would mean that objects in the asteroid belt did not all form close to their present locations. "It says to the community that the assumptions you've been making don't necessarily have to be true," Levison says.

But Renu Malhotra of the University of Arizona in Tucson says she's not convinced of the estimate that 20 per cent of the asteroids in the main belt could come from the outer solar system.

That's because the new study didn't simulate the process that knocked the planetesimals out of their original orbits - it only sent objects from the outer solar system inwards to find out what fraction would be captured.

"It's not clear how statistically probable this is," Malhotra says. The simplest hypothesis, she adds, is that most of the asteroids in the main belt are original residents.

Journal reference: Nature (vol 460, p 364)