Billions of years ago, the four biggest planets--Jupiter, Saturn, Uranus, and Neptune--lurched through the solar system, tearing out large chunks of the main asteroid belt, according to new computer simulations. The findings explain why the belt contains several gaps, and they support the increasingly popular hypothesis that planets tend to migrate in their orbits for a long time after a solar system is formed.
Our planetary neighborhood might seem calm and orderly, but evidence of its often-violent past is easy to spot. Earth's moon and the planet Mercury, for instance, are scarred with impact craters. Pluto's orbit is elliptical, meaning that something probably knocked it out of sync with the circular paths of the planets. And the main asteroid belt, located between the orbits of Mars and Jupiter, contains at least half a dozen places where there are far fewer space rocks than expected.
What caused those gaps? To find out, planetary scientists David Minton and Renu Malhotra of the University of Arizona, Tucson, ran a computer simulation of the motions of an asteroid belt through 4 billion years of history. They report tomorrow in Nature that the number of asteroids remaining at the end of the simulation was far larger than what is observed in our solar system. Moreover, Minton says, when they compared their simulation with the actual asteroid distribution, "there was a very distinct pattern in where the asteroids were missing."
So the researchers tweaked the original simulation to include migration by the outer planets and their gravitational interactions with the asteroids. The result, Minton says, was that the combination "seems to reproduce the observed abundance of asteroids quite well across the entire main belt."
The study also suggests that Jupiter has drifted inward toward the sun, whereas Saturn and the other gas giants drifted outward. However, Minton says the findings can't predict "exactly when the migration took place" or the precise paths of the planets. On the other hand, he says, "we hoped to see an unambiguous record of the migration, and that's what we think we've found."
The findings add to the growing evidence that "something big and dramatic happened to the orbits of Jupiter, Saturn, Uranus, and Neptune long after they formed," says space scientist William Bottke of the Southwest Research Institute in Boulder, Colorado. He says such studies of the asteroid belt can provide "critical clues that, if properly interpreted, can be used to decipher many of the mysteries that still surround solar system formation."
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