* Astronomy

By using a computer to play a complex game of "what if,"
scientists have developed a tale of chaos in the early solar system
that they say explains several mysteries about why our cosmic neighbourhood turned out the way it did.

"We get so many of the answers right. I never dreamed it. Would be so spectacularly successful." - Harold Levison of the Southwest Research institute in Boulder, Colo.

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

- What set off an intense asteroid bombardment some 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? And how did their orbits became so tilted compared to other planets?
- Why does Jupiter share its orbit around the sun with a swarm of asteroids?

The work is presented in Thursday's issue of the journal Nature by Alessandro Morbidelli of the Cote d'Azur Observatory in Nice, France, along with Levison and others. They used computer simulations to study various scenarios about how the outer solar system may have developed.
Their favourite scenario follows the generally accepted idea that some 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 Levison called the controversial 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, chunks measuring up to hundreds of miles across. As the planets and chunks of rubble exerted their gravitational tugs on each other, what would change?

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.

Then all hell broke loose.
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," Levison said. It sent the two planets outward and into the ring of planetary rubble which, as Levison put it, went "kaplooey."
It's as if Saturn was a bowler, Uranus or Neptune the ball, and the rubble chunks the pins, he 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, which also produced the observed tilts of the orbits of all four planets in the story.

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 crucial 2-to-1 ratio in orbit times.
Renu Malhotra, a professor of planetary sciences at the University of Arizona, called the work "very interesting and provocative," but said it's probably not the last word on the subject.

---

Planetary billiards answer Solar System riddle

Scattering rocks moved planets and battered the Moon.

Why does the Solar System look like it does? The question has teased
astronomers for centuries, but researchers have now come up with a single theory that they hope can explain three of the most mysterious features of our corner of the Universe.
The giant planets Jupiter and Saturn have unusually elliptical orbits that are significantly tilted out of the plane occupied by the smaller, rocky planets close to the Sun. And Jupiter itself is accompanied by small asteroids called Trojans, which fan out for millions of kilometres ahead of and behind the planet, speeding along the same orbital path.
Most perplexing of all is the 'late heavy bombardment' about 3.8 billion years ago, which peppered the Moon with chunks of rubble left behind from the planets' formation some 700 million years earlier. No convincing explanation has so far been put forward for why this sudden battering happened so long after the Solar System's violent early years.

But according to Hal Levison of the Southwest Research Institute in Boulder, Colorado, all of these quirks can be explained by a game of planetary billiards that began when Jupiter and Saturn fell into a specific orbital pattern just a few million years after they formed.
"We have really explained a lot of the structure of the Solar System with this model," says Levison. He and his colleagues present their computer simulations of the Solar System's history in this week's Nature.

Junk theory

Levison singles out his colleague Alessandro Morbidelli, of the Observatory of Nice in France, as the prime architect of their idea.
They argue that the tiny gravitational effects of junk left over from the formation of Saturn and Jupiter gently nudged the planets around until Saturn fell into an orbit that sent it around the Sun exactly once for every two orbits completed by Jupiter.
This resonance meant that twice every saturnine year, at almost exactly the same two points in space, Jupiter's gravity tugged at its ringed colleague and forced it into an ever more elliptical and tilted orbit.
The migration had a knock-on effect for Saturn's outer neighbour, Neptune, flinging it beyond Uranus into the outer Solar System where it now resides. Neptune's headlong crash into the rubble that had accumulated in the system's outer reaches sent thousands of these planetesimals spinning towards the Sun.

Some were trapped around Jupiter, falling into line with its orbit to form the trail of Trojans. And the rest battered the inner planets and their satellites, including the Moon, leaving its face scarred by impact craters. "This is the first fully self-consistent model of the late heavy bombardment," says Levison.

Hot topic

The theory has had a warm reception at recent presentations to astronomers, says Levison. But some remain wary.
"The fact that a simulation of planet formation produces an end-state in good agreement with the observed Solar System does not prove that the simulated events actually happened." - Joe Hahn, an astronomer at St Mary’s University in Halifax, Canada, who also studies the Solar System's evolution.
Hahn is most sceptical about the explanation for the late heavy bombardment. Planetesimals from the outer Solar System should have been scattered towards the Moon very soon after the planets began their wanderings, rather than taking 700 million years to get there.

Levison argues that they were slowed down by the gas and dust left over from planet formation. But Hahn is doubtful that they could have been delayed for so long. "These models are all a bunch of fairy tales anyway. I won't be convinced by any of them until astronomers can observe planet formation around other stars." - Joe Hahn.

Further support for Levison's theory could come from the surface of Jupiter's Trojans, concedes Hahn. If they came from the outskirts of the Solar System, they should look like the icy Kuiper belt objects found there today.
Levison himself thinks that establishing the orbits of Kuiper belt objects should help to refine his theory, and plans to spend the next couple of years checking their effects on his model.


Extract: