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Turbulence Key to Planet Formation, New Study Suggests

Mason Inman
for National Geographic News
September 4, 2007
 
Swirling eddies and chaotic vortices are crucial to the formation of new planets, suggests a counterintuitive new study.

Such turbulence is vital to helping planets go from "toddler" to "teenage" size by helping rocks and boulders stick together, the computer simulation hints.

This is a turnaround from several years ago, when scientists considered turbulence a destructive bugaboo for newly forming planets.

A few scientists recently suspected that turbulence might help in planet formation, but no one had showed in detail how that might work until now.

"We were the first to model how interacting boulders move around in this turbulence," said Anders Johansen of the Max Planck Institute for Astronomy in Germany, who led the research team that made the new findings. The study appeared last week in the journal Nature.

The research showed that turbulence could create "planetesimals," or planetary precursors, very quickly—in only seven orbits around a star, or around just a hundred years.

Collision Conundrum

New solar systems form from a swirling disk of dust and gas surrounding a central star. (Related: "Planet-Forming Disk Spotted Around Dead Star" [April 5, 2006].)

As the matter swirls around, microscopic bits of dust hit each other and stick together. Gradually they can gather into rocks and boulders, around a yard (a meter) across.

"We have a pretty good grasp of this [process]," Johansen said.

But explaining how matter forms bigger clumps—up to planetesimals about a kilometer across—has eluded scientists.

"That has been known to be a big problem for the last 30 years," Johansen said.

Part of the issue is that when larger boulders collide with each other, "they don't stick to each other very well, but are likely to destroy each other when they collide," Johansen said.

And around this size, the rocks would begin to experience drag from the gas around them.

Eventually the rocks would lose so much energy that they would spiral into the star, crashing to a fiery death.

"Drafting" to Success

In the new computer model, scientists studied what would happen if this disk of orbiting matter does not spin calmly around but instead has turbulence stirring things up.

Although researchers haven't figured out for sure what might cause such turbulence, they're confident that there would be a fair amount of it in the disks surrounding young stars.

The turbulence has high-pressure areas where boulders tend to accumulate, the simulation revealed.

Once a few boulders get stuck together in such locations, the formation can help other boulders stick too, since they shield each other from the gas.

The areas also help the boulders resist the headwind from the gas around them, like "drafting" racers.

Bicyclists in the Tour de France, for instance, ride in packs so that only those in front feel the brunt of the wind, and the rest save energy by drafting along in the low-pressure area behind the leaders.

Similarly, when a pack of boulders are together, they don't lose much energy because of the wind, Johansen said.

Gravity would then pull the boulders closer together, until they gradually collapsed into planetesimals a couple of hundred miles (a few hundred kilometers) across.

Planetesimals that large would attract even more rocks with their gravity, allowing them to grow into full-fledged planets. (Related: "The Search for Other Earths" in National Geographic magazine [December 2004].)

Knowledge Gap

"The new study, for the first time, describes a feasible model of how really big bodies ... could form," said Jürgen Blum of the Technical University of Braunschweig in Germany.

The study shows that "turbulence might be good for growth [of planets], as it concentrates particles in certain areas of the turbulent eddies," he added.

Henry Throop of the Southwest Research Institute in Boulder, Colorado, said that the new study fills a big gap.

"It's kind of ironic," Throop said. "We're used to explaining things on the size of galaxies, and on really small scales the size of light waves.

"In planetesimal formation, however, the tricky part is these medium-sized grains," around a yard (a meter) across, he added.

This new study is "a big step," Throop said, toward figuring out how budding planetesimals pass through their "toddler" stage and grow to full-size planets.

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