Study: Small rocks helped build Jupiter, Saturn

BOULDER, Colo., Aug. 19 (UPI) -- The solar system's biggest planets, gas giants, got their start as balls of pebbles.

According to a new study in the journal Nature, Jupiter and Saturn began their planetary lives as growing accumulations of centimeter-sized rocks, clumps of dust and ice that coalesced as they orbited the newborn sun.

The new research perfects a previously postulated idea called "pebble accretion theory," put forth by two Swedish scientists in 2012. Prior to the pebble-accretion concept, scientists posited that ice and dust slowly consolidated into large balls that eventually combined to form the nuclei of gas giants.

But the timeline for such a process didn't mesh with how long scientists estimated the sun's early circumstellar disk would last before it ran out of planet-forming materials. The more aggressive pebble-accretion model sped up the process, but when scientists tested the theory, their computer models spit out hundreds of Earth-sized planets -- as opposed to a handful of much larger protoplanets.

"For planet formation, such a democratic outcome is a bad result," Chris Ormel, an astronomer at the University of Amsterdam, told Nature.

The new model solves this problem by allowing these early accumulations of pebbles to interact with and influence each other. In doing so, researchers showed that the first pebble conglomerates likely began to push slightly smaller protoplanets out of the way, making room for them to grow to larger sizes.

"It's kind of a runt of the litter thing," explained lead study author Harold Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colo. "The runt is pushed aside by its bigger siblings so that they grow, and it doesn't."

When they tested the new model, researchers got between one and four young gas giants. Scientists say the new results play nicely with another theory, the Nice model, which explains the behavior and interaction of maturing gas giants after they left the protoplanetary disk and drifted to the outer reaches of the solar system.

"We basically produce the start of the Nice model," Levison said.

Now researchers are testing whether the pebble accretion model, or some variation of it, can explain the formation of terrestrial planets like Mars and Earth.

"Things are falling together in a way that haven't happened for previous models," said co-author Katherine Kretke.