Rapid Collapse Gave Giant Planets Gas?

Some of our solar system's "gas giant" planets may have formed when clumps of gas and dust in the early solar system collapsed precipitously, in 1000 years or less, a new computer model suggests. The findings, in the current issue of Science, back a theory of planet formation that fell out of favor decades ago, when it was replaced by a scenario in which the gas giants took shape more gradually.

In 1951, the Dutch-American astronomer G. P. Kuiper had proposed that clumps of material in the nebula around our young sun collapsed rapidly to form the gassy outer planets: Jupiter, Saturn, Uranus, and Neptune. This theory itself quickly collapsed, however, because it failed to explain why all four gas giants seemed to have similarly sized cores. Simulations of "direct collapse," as it was called, also failed, yielding planets that were too big or didn't coalesce at all. "People in the past who have tried the direct-collapse model had trouble making [the planets] collapse in the right mass range," says Jonathan Lunine, an astrophysicist at the University of Arizona.

Instead, most theorists came to believe that the giant planets formed in two stages. First, icy clumps collided to form small planets; after these grew to about 10 Earth masses, they rapidly sucked in gas "like a gravitational vacuum cleaner," explains Lunine. This theory would account for the similar-sized cores, although the process's long time scale--a million years or so--is a sticking point for some astronomers.

But recent theoretical models of the giant planets' interiors suggest that their cores vary in size and, in some cases, may be too small to trigger gas accretion. So Alan Boss, an astrophysicist at the Carnegie Institute of Washington, reevaluated Kuiper's single-step theory using a three-dimensional computer simulation. He found that the gas and dust coalesced into realistic-looking giant planets in about 1000 years. "I'm not saying that the two-phase model is ruled out," says Boss. "But we have to consider both models very seriously."

The one-step process still tends to form planets larger than Uranus and Neptune. But "if the one-step is shown to work [for Saturn or Jupiter], it's very important," says Lunine, because then theorists would have to juggle two models for gas giant growth.