E.W.Thommes / Science / AAAS
This graphic shows the final stages in a simulation of planet formation for a system with "hot Jupiters" (left), a solar system like our own (middle) and a system without gas giants like Jupiter or Saturn (right). The simulation indicates that solar systems "just right" for stable Earthlike planets are  rare.
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updated 8/7/2008 5:44:50 PM ET 2008-08-07T21:44:50

Our solar system emerged in surprisingly good order from the violence of planetary creation, according to a new simulation.

Researchers found that planetary formation in the first few million years often resembles a violent wrestling match among hungry siblings, with planets fighting to feed on gas and dust while pulling at each other with gravitational arms.

"There's massive bodies competing with each other and flinging each other around," said Edward Thommes, a physicist at the University of Guelph in Canada, lead author on the new research published in Friday's issue of the journal Science.

The simulation traces the creation of a planetary system from almost beginning to end, for the first time, and suggests that our solar system started with just the right mass to become a relatively orderly place in the universe.

"You have to have the conditions just right," Thommes told Space.com. "They have to be in a fairly narrow range."

Thommes and his former colleagues at Northwestern University ran the simulation through over 100 scenarios to see how gas giants formed from the gas disks that surrounded young stars. Newborn planets typically seemed to get pushed toward the central star by the gas disk remnant surrounding them.

"The same disk from which they're born is also trying to kill them," Thommes said.

Too much starting mass in the disk results in a swarm of gas giants crowding into the central star. However, too little mass produces nothing bigger than Neptunelike ice giants.

The tussle among gas giants can typically lead to loopy elliptical orbits. Sometimes a gas giant even acts as a slingshot to throw a sibling into deep space.

By comparison, our solar system's gas giants — Saturn and Jupiter — have nearly circular orbits that suggest less violent interaction. The two planets also appear to have stayed close to where they grew up, instead of migrating into the sun.

"They never really got into each other's face, so to speak," Thommes said. "They kept their personal space."

Slideshow: Month in Space The simulated planetary systems mostly line up with observations of more than 300 exoplanets discovered so far. But Thommes cautioned that the observed exoplanets represent those that are relatively easiest to find, or "a filtered sample" of what astronomers can see.

The researchers chose to sacrifice some detail in their simulation in order to model planetary systems from start to finish. They hope to extend their hybrid model approach so that they can eventually model planets spiraling all the way into the central star. Currently, the simulation cannot track such planets beyond a certain point.

An outside essay accompanying the Science paper and written by John Papaloizou, an astrophysicist at the University of Cambridge in Britain, calls the new simulation "compelling" and says that it achieves "reasonable success" in modeling what astronomers have observed.

Papaloizou pointed out that some simulated planetary orbits do not match up with the usual equatorial plane of star systems, something that astronomers have not seen in exoplanetary systems so far. However, he also adds that current knowledge of exoplanetary systems remains limited.

For his part, Thommes remained confident that our solar system was uniquely quiet, especially considering the birthing process.

"What our solar system seems to represent in all of this is a peaceful and quiet form of this process," Thommes said.

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