Four billion years ago, Uranus and Neptune switched places during a gentle ride out to their current orbits.
That's the conclusion of Steve Desch, an astrophysicist at Arizona State University, who thinks that all of the gas-giant planets took shape twice as close to the sun as they are at present.
His work could cut out much of the mystery of how our "impossible" solar system formed.
The solar system is 4.6 billion years old. The formation of rocky planets, from collisions between ever-larger objects, is a fairly rock-solid theory.
But how the outer giants developed remains an open question.
"Models predicted [Jupiter] would take many millions of years for it to form, and billions of years for Uranus and Neptune, but our solar system isn't that old," Desch said. "Having a denser disk of gas bunched up around the sun could explain the two planets' formations, but only if they switched places."
Desch details his work in a recent issue of the Astrophysical Journal.
Neptune is currently the most distant planet from the sun at 2.8 billion miles (4.5 billion kilometers) — sorry Pluto, you no longer count.
At 1.9 billion miles (3 billion kilometers) away, Uranus is the second most-distant planet.
Most theories say planets slowly built up from a disk of gas and dust that once reached out to Neptune's current orbit. Turns out that's too spread-out to explain the formation of our solar system, Desch said.
"By the time Neptune and Uranus would have built up a solid core large enough to drag in helium and hydrogen for their atmospheres, almost all of the gas would have drifted into interstellar space," he told SPACE.com.
To make our solar system work, Desch elaborated on the "Nice" model of planet formation that debuted in 2005.
That theory suggests gassy planets formed about twice as close to the sun as they are now — which means our dusty solar nebula would have been four to 10 times denser than most models predict.
"My colleagues seem pretty shocked by my paper, but they've found nothing wrong with it," Desch said. "Basically, I'm saying we have it all backwards: Planet-forming material had to have drifted outward, not in towards the sun."
Desch said that after an accelerated formation of the gas giants — Jupiter, Saturn, Uranus and Neptune — something pulled them outwards into their current orbits.
Subtle gravitational "tugs" from passing comets, he said, could have done the trick over billions of years.
"It's like when the Voyager spacecraft used Jupiter for a gravity-assisted speed boost," he said. "It slightly pulled on the planet to gain speed, but the fact is that it pulled on the planet."
For Desch's orbital math to jibe, however, Neptune had to have overtaken Uranus about 650 million years into the solar system's evolution.
"And that's something the Nice model anticipated," he said, noting that he added to the work by hashing out the density of planet-forming gas and dust surrounding our infant sun.
"When I graphed out the data, it was almost spooky," he said of the disk density curve. "You hardly ever get data to fall into such a smooth predictive curve like that, but it did."
While Desch cautioned other theorists may find the updated model difficult to swallow, he explained that it's compatible with either of two competing theories of gas giant formation: a sudden collapse of gas or an accretion of it around a rocky core.
"Whatever the case, nobody's ever been able to explain how to form Neptune and Uranus within the window of 10 million years," he said. "I haven't proved anything, but it's strong circumstantial evidence. It would explain a lot of things about our solar system's configuration."
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