New Theory on Uranus, Neptune Births

— Scientists say they may have solved a far-out mystery: how Uranus and Neptune came to exist at the very edges of the solar system.

A new study says the two icy planets may have been born much closer to the sun than previously thought, and ended up in their current orbits after gravitational forces from Jupiter violently hurled them away.

That would explain how the two planetary giants — each more than 10 times the mass of the Earth — could exist at the far edge of the solar system, where there was not enough gas and dust to make a planet eons ago.

The study is based on computer simulations conducted by Martin Duncan of Queen’s University in Kingston, Ontario, and colleagues. It was published in Thursday’s issue of the journal Nature.

Bits and Pieces

All of the planets in the solar system are believed to have evolved through the accumulation of a large number of small bodies that circulated in a huge disk around the sun.

The researchers theorize that Uranus and Neptune formed their cores near the orbits of Jupiter and Saturn, within a ring of about five to 10 astronomical units from the sun. One astronomical unit equals the distance from the sun to the Earth. (Earth, however, is thought to have formed much later than the big planets.)

Previous estimates of 10 to 20 AU have been given for the birthplaces of Uranus and Neptune, which now orbit at 19 and 30 AUs, respectively.

Duncan said Jupiter grew fastest because it was closest to the sun, where the planet-forming disk was the most dense, and then exerted gravitational forces on its smaller planetary siblings. Saturn may have helped eject Uranus and Neptune.

Gas Missing from Explanation

Renu Malhotra, a scientist at the Lunar and Planetary Institute in Houston, said the study does not explain why Uranus and Neptune did not accumulate gas like Jupiter and Saturn, since the four planets formed at roughly the same time and in roughly the same place.

Malhotra said the evidence shows that Uranus and Neptune were formed perhaps 30 percent closer to the sun than their present locations — but not as close as Duncan proposes. The planets then may have gently migrated out to their current locations, she said.

Malhotra also said that gravity and friction from gas that surrounded Uranus and Neptune could have prevented them from being hurled out.

Alan Boss, an astrophysicist at the Carnegie Institution in Washington, said more research is needed on Duncan’s theory.

“It’s a radical idea,” he said, “but since we’re in a stalemate on Neptune and Uranus, maybe we need a radical idea.”