Migrating Planets May Have Kicked Asteroids Into Orbit

Gas giant planets that migrated early in the history of the solar system could have violently knocked some of the asteroid belt's denizens into their current orbits, according to a new study that aims to solve a number of enduring space rock mysteries.

The research, which uses a theory of the solar system's evolution called the Nice model, explains why the asteroids in the outer part of the belt ? located between the orbits of Mars and Jupiter ? are so different in composition from those in the inner part. Researchers say the model also explains other oddities in the solar system: the far-out Kuiper belt beyond Neptune; the so-called Trojan asteroids of Jupiter; and the Late Heavy Bombardment of the moon billions of years ago.

"It really is the only model we have that can explain the solar system like we see [it]," said Harold Levison, of the Southwest Research Institute in Boulder, Colo., and lead author of the new study, which is detailed in the July 16 issue of the journal Nature.

The Nice model

The Nice model (pronounced like the city in France, where it was formulated), is "a model for the dynamical evolution for the orbits of the giant planets that we believe was a very violent event that happened roughly 700 million years after the solar system formed," when the solar system was in "its teenage years," Levison explained.

Models haven't been able to reproduce the formation of Uranus and Neptune in their current orbits, so Levison and other astronomers think that they formed much closer to Jupiter and Saturn, so that all the gas giants initially sat within 15 AU of the sun. (One AU, or astronomical unit, is the mean distance between Earth and the sun, about 93 million miles. Jupiter currently has a mean distance of 5.2 AU from the sun.)

"We think [the gas giant planets] formed in a much more compact configuration than what we currently see," Levison said.

A protoplanetary disk of planetesimals stretched from just beyond that 15 AU boundary to about 30 AU, the thinking goes.

While this configuration was initially stable, objects leaking out from the disk caused slow changes in the orbits of the gas giants.

According to the model, about 700 million years after the solar system formed, these changes resulted in Jupiter and Saturn hitting a resonance with each other that caused the orbits of Uranus and Neptune to destabilize. The latter two planets gravitationally scattered off each other towards Jupiter and Saturn, which pushed back, sending their smaller siblings out to their current orbits.

Like a bowling ball hitting a set of pins, Uranus and Neptune plowed into the outer protoplanetary disk, whose objects "got scattered all over the solar system," Levison told SPACE.com.

Here's where the asteroid belt comes into the picture.

Inner and outer

The asteroid belt has a "huge diversity of objects," Levison said. The inner edge consists of bodies that have been heated and lack water or other volatile components ? "they're just rocks," Levison said.

"And in the outer part of the asteroid belt, we see things that are much more primitive, meaning less processed," so they posses water and organics, he added.

The original explanation for this diversity was that some rapid change in the original protoplanetary disk of the solar system must have occurred in the vicinity of the asteroid belt. But the Nice model suggests otherwise.

"Essentially what we're saying is that interpretation might not be right, that at least the really primitive objects in the outer asteroid belt probably formed much further away from the sun, and were embedded there during the violent stages we think occurred in the orbits in the planets," Levison said.

So the asteroids seen today in the outer edge of the asteroid belt originally came from much farther out in the solar system and so have retained their water ice and other signatures, while the asteroids in the inner portions were natives to the belt region.

Kuiper belt, Trojans and bombardments

The Nice model also explains some other oddities in the solar system, such as the "Trojan asteroids" of Jupiter.

These asteroids sit in the Lagrange points of Jupiter (points where gravity wells collect and hold on to detritus).

"Right now in the solar system, there's a fence around the Lagrange points; things in them can't get out and things from the outside can't get in," Levison explained. But when Jupiter and Saturn fell into resonance, "that fence, or that wall, goes away" and the planetesimals scattered by the violent planetary changes fell into the wells.

The model can match the number and placement of the Trojan asteroids. "No other model's been able to do that," Levison said.

The Nice model can also explain the structure of the Kuiper belt, which is located beyond the orbit of Neptune, Levison added.

The scattering of planetesimals all over the solar system could also be the source of the proposed Late Heavy Bombardment of the moon and Earth.

Many impact features on the moon are very old and originated much earlier in the history of the solar system when wayward rocks were more common. But as the solar system began to take shape, there were fewer of these migrants flying around. Which is why explaining impact features on the moon that appear younger was difficult ? where would the impactors have come from?

The release of the planetesimals about 700 million years after the formation of the solar system roughly matches the dates given to these lunar basins of about 3.8 billion to 3.9 billion years ago (though some scientists dispute these dates).

"So the Nice model gives us that too," Levison said.

His team is working on looking for more solar system features to see if the Nice model also provides an explanation for them. The model is increasingly gaining acceptance among astronomers, but Levison encourages rigorous testing of other models as the only way to be sure that the Nice model is the best fit for the solar system.

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