A new study by researchers at Southwest Research Institute (SwRI) and the Observatoire de la Côte d'Azur provides an explanation for one of the more mysterious aspects of the population of objects beyond Neptune. In doing so, it provides a unique glimpse into the proto-planetary disk from which the Solar System's planets formed. Results will be published in the November 27 issue of Nature.The Kuiper belt is a region of the Solar System that extends outward from Neptune's orbit, containing billions of icy objects from kilometers to thousands of kilometers across. It was discovered in 1992 and, since that time nearly 1,000 objects have been cataloged. Some of these objects are very large - the largest having a diameter of more than 1,000 kilometers.

As astronomers have studied this structure, a mystery has unfolded. Like most of the planets in the Solar System, the large Kuiper belt objects are believed to have been formed from smaller objects that stuck together when they collided. For this process to have worked in the distant regions beyond Neptune, the Kuiper belt would have to contain more than 10 times the amount of material than is in the Earth. However, telescopic surveys of this region show that it currently contains roughly one-tenth the mass of the Earth, or less.

To solve the puzzle, researchers have been searching for several years for a way to remove more than 99 percent of the Kuiper belt's material. However, Dr. Harold Levison (SwRI) and Dr. Alessandro Morbidelli (Observatoire de la Côte d'Azur of Nice, France) describe in their article, "Forming the Kuiper Belt by the Outerward Transport of Objects During Neptune's Migration," that the Kuiper belt may not have lost much mass at all.

"The mass depletion problem has been sticking in our throat for some time," says Levison, a staff scientist in the SwRI Space Studies Department. "It looks like we may finally have a possible answer."


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