- The Washington Times - Wednesday, May 25, 2005

NEW YORK (AP) — By using a computer to play a complex game of “what if,” scientists have developed a tale of chaos in the early solar system they say explains several mysteries about why our cosmic neighborhood turned out the way it did.

“We get so many of the answers right,” said Harold Levison of the Southwest Research Institute in Boulder, Colo. “I never dreamed it would be so spectacularly successful.”

In a single stroke, he and his colleagues say, the tale answers such questions as:

• What set off an intense asteroid bombardment about 3.9 billion years ago that created huge lava-filled basins on the moon and may have set back the development of life on Earth?

• Why did Jupiter and Saturn leave their circular orbits and take on the more oval paths seen today?

• Why does Jupiter share its orbit around the sun with a swarm of asteroids?

The work is presented in today’s issue of the journal Nature by Alessandro Morbidelli of the Cote d’Azur Observatory in Nice, France, along with Mr. Levison and others. They used computer simulations to study various scenarios about how the outer solar system may have developed.

Their favorite scenario follows the generally accepted idea that about 4.6 billion years ago, the sun and planets formed from the gravitational collapse of a cloud of gas, dust and ice. But it adopts what Mr. Levison called the disputed position that the solar system started out as quite compact. In this scenario, for example, Neptune starts out less than 15 times as far away from the sun as Earth is now, rather than the 23 times other scientists propose.

So the question was what would happen over eons as the planets followed circular orbits, surrounded by a huge ring of planetary rubble.

Here’s what the scenario suggests:

As the planets tugged on the rubble, the rubble tugged back and that nudged Uranus, Neptune and Saturn outward from the sun and Jupiter inward, as previous research has suggested. That in turn affected how long each planet took to complete an orbit of the sun, since a wider orbit takes longer. At some point, Saturn started taking exactly twice as long as Jupiter to complete a lap.

Because of their tugs on each other, Jupiter and Saturn began to leave circular orbits and follow more oval-shaped paths, similar to what’s observed today. That wreaked gravitational havoc on the much less massive Uranus and Neptune, making their orbits “totally nuts,” Mr. Levison said.

The gravity of the intruding planets scattered the rubble, and one result was the bombardment of Earth and the moon.

The rubble’s gravitational tugs on Uranus and Neptune eventually nudged those planets into the orbits seen today, according to the scenario.

The work also explains the presence and highly tilted orbits of Jupiter’s “Trojans,” asteroids that share essentially the same orbit as Jupiter. While some scientists have suggested they formed near Jupiter, the scenario suggests they developed far away and were captured in Jupiter’s orbital path just after Saturn and Jupiter hit their 2-to-1 ratio in orbit times.

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