are about half as big as Pluto. Scientists estimate there are billions more, both small and large.But they shouldn't all be there.
Not enough stuff
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.
Why Neptune stopped
The new computer model explains another oddity. Astronomers have suspected for about 20 years that Neptune moved outward from its birthplace, based on how it would have reacted to gravitational interactions with small objects.
But theorists have wondered why Neptune stopped where it did.
"Neptune migrated until it hit the edge of the protoplanetary disk, at which point it abruptly stopped," said Morbidelli, of the Observatoire de la Cote d'Azur in France.
The work was detailed in the Nov. 27 issue of the journal Nature.
Rodney Gomes of the National Observatory of Brazil wrote an analysis of the new idea for the journal. In an e-mail interview, Gomes told SPACE.com that Levison and Morbidelli "offer strong arguments that their scenario is correct."
Gomes did not work on the new simulation. But in a separate paper submitted to the journal Icarus, Gomes collaborated with Levison and Morbidelli to show that the area now occupied by the Kuiper Belt might initially have been empty, otherwise Neptune could have migrated outward farther than it did.
More mystery
Gomes said more refined simulations will be needed to determine if the Neptune effect is correctly understood. But even if the mystery of the Kuiper Belt's formation is indeed solved, there is more to figure out.
Previous scenarios have existed in which our solar system would have started with a small, truncated protoplanetary disk, as described in the new simulation. But limited observations of similar disks of gas and dust around newborn stars indicate they are broader, Gomes said.
"Is the solar system then a rare case?" he wonders. "This, I believe, will be a major topic in planetary science for years to come."
This article is part of SPACE.com's weekly Mystery Monday series.
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