Like children growing up and moving away from home, the gas giant planets of our solar system took shape twice as close to the sun as they are now and slowly moved outward.
But the new theory of planetary formation, proposed by astrophysicist Steve Desch, only works if Uranus and Neptune swapped orbital positions up to four billion years ago.
Desch's work challenges the established theory that the gas giants formed from planetary materials that moved inward toward the sun.
Existing models have Uranus and Neptune taking billions of years to form. But chemical clues in their atmospheres mean that they didn't have that much time, said Desch, of Arizona State University.
"It's important to form those planets within ten million years, because that's how long hydrogen and helium gas remain in the early solar system," he said.
"Even Uranus and Neptune have a few Earth masses [worth] of these gases," which means they must have taken shape early while the gases were still available.
Proponents of longer-standing theories of solar-system formation say there's never been consensus about where the planets first took shape.
The new work, they say, fills in a piece of the story by accounting for the amount of material it would have taken to create them.
The research appeared in the December 10 issue of the Astrophysical Journal.
Nice Model
Commonly accepted theory has it that the inner rocky planets, such as Earth and Mars, formed from collisions between larger objects. (Explore an interactive solar system.)
But the formation of the gas giants—Jupiter, Saturn, Uranus, and Neptune—has remained puzzling to planetary scientists.
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