Scientists have long puzzled over how gas planets like Jupiter and Saturn got to be so big.
Current theories suggest the cores of these behemoths are comprised of mini-planets, some 62- to 620 miles in diameter, which collided and gradually merged together over time. But computer simulations show this process is more likely to produce hundreds of Earth-sized worlds.
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"Rather than creating a few such cores, (this process) produces a population of hundreds of Earth-mass objects that are inconsistent with the structure of the solar system," astronomer Harold Levison, with the Southwest Research Institute in Boulder, Colo., writes in this week's Nature.
Levison and colleagues say new computer models point to an alternative scenario to form the cores of gas giants, a process they call "slow pebble accretion."
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"If pebbles form slowly enough to allow the planetesimals to gravitationally interact with one another ... the largest planetesimals have time to scatter their smaller siblings out of the disk of pebbles, thereby stifling their growth," Levison writes.
The new models show that slow pebble accretion produces one to four gas giants, orbiting between five and 15 times farther away from the sun than Earth, a close match to the observed structure of the solar system.