Roaming giants: did migrating planets shape the solar system?

"A fairy tale of the early solar system." That's how planetary scientist Hal Levison of the Southwest Research Institute in Boulder, Colo., whimsically refers to his team's new computer simulation. In it, the four biggest planets start out bunched together but then break into a planetary version of bowling that violently rearranges the structure of the outer solar system.

Over the past quarter-century, several researchers have suggested that planetary migration shaped the solar system. But "the [new] model reproduces quantitatively many of the characteristics of the outer solar system to an accuracy that has never been achieved before," Levison says.

If his fairy tale turns out to be true, it could explain several long-standing puzzles, including the trajectories of the outer planets, the distribution of asteroids near Jupiter, and the origin of an ancient cataclysmic event that pelted the moon with rubble.

In a trio of articles in the May 26 Nature, Levison and an international team of colleagues propose that Jupiter, Saturn, Neptune, and Uranus all formed within 10 million years of the sun's birth. Originally, their orbits were confined to a much narrower region than they are today. The orbit of Neptune, the outermost of the four planets, was only half its current diameter.

Beyond this gang of giants resided a slowly orbiting band of ice, dust, and gas. As particles leaked from this band, they interacted gravitationally with the big planets. As a result, the researchers propose, the planets began spreading out: Jupiter moved slightly inward, while Saturn, Uranus, and Neptune moved outward.

The migration proceeded at a leisurely pace until about 700 million years after the solar system's birth. That's when Saturn reached a sweet spot, where its orbital period became exactly twice that of Jupiter's.

With the two planets suddenly in synchrony, the effect of their mutual tug strengthened. As a result, Saturn's orbit became slightly elongated, which in turn dramatically altered the orbits of the two lighter-weight giants, Uranus and Neptune. Their paths became highly elongated, even crossing each other.

"That's when all hell breaks loose," says Levison. Within a few million years of this melee, Uranus and Neptune had been kicked so far outward that they penetrated the band of ice, dust, and gas. Like bowling balls scattering pins, the two planets tossed the debris all around the solar system. This gravitational fracas also forced the two planets into their current orbits.

Some of the debris was hurled toward the sun and became trapped around Jupiter. Levison says that this may account for the observed distribution of Trojan asteroids--a group of objects that both lead and trail the planet.

Some of the scattered material journeyed farther inward and could have generated a violent epoch known as the late heavy bombardment. About 3.8 billion years ago, the moon and the inner planets were blasted with debris. Huge impact basins on the moon bear testimony to this cataclysmic era.

Theorist Jack Lissauer of NASA's Ames Research Center in Mountain View, Calif., says that he agrees that Saturn's synchronicity with Jupiter "would have had profound effects on the structure of the outer solar system." But he cautions that the connection of this planetary pas de deux with the lunar bombardment would be tenuous.

Overall, says Lissauer, the study "provides additional evidence that the solar system has been, and remains, a much more active and chaotic place than was envisioned by researchers a quarter-century ago."