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The solar system that Neptune built

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The solar system used to be much smaller. According to a new theory, Neptune long ago migrated away from the Sun and forced a vast field of giant boulders out with it.

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The Voyager 2 spacecraft took this snapshot of Neptune, with the planet's "Great Dark Spot" at the center.

THE IDEA FOR Neptune’s big construction project is based on a computer simulation that attempts to solve a mystery that’s been nagging astronomers in recent years. In 1992, researchers discovered the first object besides Pluto that is beyond Neptune. (Pluto is sometimes inside Neptune’s orbit and sometimes outside.)

The region is now known as the Kuiper Belt, and nearly 1,000 Kuiper Belt Objects (KBOs) have been found. Some of them are about half as big as Pluto. Scientists estimate there are billions more, both small and large.

But they shouldn’t all be there.


The leading theory of solar system formation holds that KBOs, like asteroids, comets and the rocky planets, formed shortly after the Sun’s birth 4.6 billion years ago. Leftover gas and dust, in what’s called a protoplanetary disk, collided and stuck together.

For the hit-and-stick scenario to have worked in the present-day Kuiper Belt, the region would have had to contain 10 times the amount of material that’s in the Earth. That much stuff would be needed to allow the chance collisions that created so many large objects.

Observations so far suggest, however, that the Kuiper Belt contains no more than one-tenth the mass of Earth.

Researchers have tried to figure out where the other material might have gone. Perhaps collisions between KBOs reduced much of it to dust, and it was blown out of the solar system. That explanation is not widely accepted, though. The new theory simply concludes the stuff wasn’t out there in the first place.

“We really didn’t solve the mass-depletion problem,” says Harold Levison of the Southwest Research Institute. “We circumvented it.”

Levison and his colleague, Alessandro Morbidelli, conclude that the Kuiper Belt’s outer boundary was initially where Neptune is now, at about 30 astronomical units (AU). One AU is the distance from Earth to the Sun.

Within that space, there would have been enough material for the KBOs to develop. Then the gravitation of Neptune, also newly formed, would have forced some of the objects outward, where they settled into relatively stable orbits around the Sun as far as 48 AU away.

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