Lynne Allen and Gary Bernstein, of the University of Michigan, and Renu Malhotra of the University of Arizona Lunar and Planetary Laboratory presented the evidence today at the Division of Planetary Sciences meeting in Pasadena.

It has long been thought that some comets must originate from a collection of small icy bodies orbiting beyond Neptune. These so-called "Kuiper Belt Objects" would be left over from the formation of the large planets 5 billion years ago. The Kuiper Belt Objects were purely hypothetical until 1992, when David Jewitt and Jane Luu of the University of Hawaii discovered the first one. Since that time, over 300 Kuiper Belt Objects have been discovered -- but none of them are more than about 55 times as far from the sun as Earth, or 55 AU.

Astronomers talk about solar system distances in terms of "astronomical units." An astronomical unit, or one AU, is the distance from Earth to the sun. By comparison, Neptune is 30 AU from the sun, and Pluto ranges from between 30 to 50 AU.

Does the solar system really end beyond Pluto's orbit? Or are the more distant objects just too faint to have been found so far? To address this question, Allen, Bernstein, and Malhotra searched 6 patches of sky, each about the size of the full moon, using a state-of-the-art electronic camera at the Cerro Tololo Inter-American Observatory in the Chilean Andes.

These observations, in 1998 and 1999, were sensitive enough to see a 160-kilometer (100-mile) Kuiper Belt Object to at least 65 AU. They discovered 24 new Kuiper Belt Objects, 9 of which are 160 kilometers or bigger, but again the most distant is near the outer limit of Pluto's orbit. This is the strongest evidence yet that more distant objects are missing.

Some of the known Kuiper Belt Objects as well as many comets are on trajectories that will carry them well beyond the orbit of Pluto. But these are all believed to have formed inside Pluto's orbit and then been pushed outward by an encounter with Neptune or another planet. There are still no known objects which appear to have been created outside Pluto's orbit.

So astronomers are left to wonder what explains this apparent edge: was the primordial solar system originally "small"? Or were there once more distant objects that were pulled away by the gravity of a passing star? Astronomers at telescopes around the world are currently conducting further surveys in an effort to learn more about the history of our solar system.

This Kuiper Belt survey was funded by grants from NASA and the National Science Foundation.

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