(c) SCIENCE

Current models of solar system evolution posit that a planet of Jupiter's size and scope would need more than a million years to form. But scientists have discovered close to 100 similar giant planets orbiting nearby stars, which suggests that they are more plentiful than would be expected if they had such a long gestation period. New findings may help account for the relative abundance of these large gas giants. According to a study published in the current issue of the journal Science, gas giants may take shape much more quickly than previously thought.

Lucio Mayer of the University of Washington and his colleagues spent two years refining a mathematical model that describes how planets form from protoplanetary disks, those spinning disks of matter that orbit young stars. The prevailing theory holds that material from a disk slowly congeals into masses that make up the centers of planets. Gas giants then slowly amass their atmospheres over millions of years. But the new work indicates that the protoplanetary disc breaks up quickly--after just a few spins around its star--and that the cores of gas giants begin to draw in their gas shrouds soon thereafter. The whole process could take as little as a few hundred years, the researchers report.

"If a gas giant planet can't form quickly," study co-author Thomas R. Quinn of the University of Washington notes, "it probably won't form at all." That's because the radiation emitted from the central star heats the gases circling a gas giant's center and, over time, scatters them away from the nascent planet, the scientists say. This effect--caused by neighboring stars that were present when Uranus and Neptune were forming but have since moved away--may explain why these planets don't have gas envelopes like Jupiter and Saturn do.