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].)
"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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