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Lack of Objects Beyond Pluto Hints at Solar System's Edge
By Robert Roy Britt
Senior Science Writer
posted: 07:00 am ET
08 November 2000
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SciAm: Pluto's is eccentric: during one complete revolution, the planet's distance from the sun varies from 29 A recent hunt for the most distant objects orbiting our Sun turned up 24 frozen space rocks, some as big as New Jersey. But none of them was much beyond the orbit of Pluto, indicating that there may be an edge to the solar system, scientists said.Pinning down a possible limit to the solar system will help researchers understand how it formed. The early years Shortly after the Sun was born out of a swirling cloud of matter, roughly 5 billion years ago, leftover rocks and dust raced around the new star. They collided with each other, stuck together in some cases, and formed nine planets and more than 60 moons. The developing planets swept wide swaths of space clean of debris. But some stuff remained. Thousands of small rocks came to dominate an area between Mars and Jupiter, a region now called the Asteroid Belt. The frigid outer reaches of the solar system, beyond Neptune, became home to another collection of rocks and icy bodies.Scientists call the better part of this distant region of the solar system the Kuiper Belt. Kuiper Belt Objects, called KBOs, were only presumed to exist until 1992. Since then, more than 300 have been found. All of them are within 55 astronomical units (AU) of the Sun (1 AU is the distance from the Earth to the Sun). Neptune is 30 AU from the Sun. Pluto, with an oblong orbit that is more like a KBO than a planet, ranges from 30 to 50 AU.Looking past Pluto To help determine if Pluto marks the edge of the solar system, a group of scientists looked for objects beyond the tiny planet. Lynne Allen and Gary Bernstein, of the University of Michigan and Renu Malhotra of the University of Arizona Lunar and Planetary Laboratory, searched six patches of sky, each about the size of the full Moon. Using the Cerro Tololo observatory in the Chile, the scientists were prepared to spot any 100-mile- (160 kilometer-) wide objects up to 65 AU away. The team did find 24 new Kuiper Belt Objects. All, however, were inside 55 AU. 
| The circled dots are KBOs found in the survey. Each is about the size of New Jersey. The telescope followed the KBOs that move across the sky as they orbit the Sun, so stars and galaxies appear streaked. |
While the findings do not prove there is an edge to the solar system, they do suggest that there is not much happening beyond 55 AU. The researchers, who presented their results at the recent annual meeting American Astronomical Society's Division for Planetary Sciences, said some KBOs have orbits that will carry them well beyond the orbit of Pluto. But these are all believed to have formed inside Pluto's orbit, then pushed outward by a gravitational encounter with Neptune or another planet. But, they say, there are no objects known to have formed beyond the orbit of Pluto. Why an edge? While theorists cannot say why there might be such an abrupt end to the solar system, they are beginning to look into the question, Bernstein told SPACE.com. It could be that the original solar system was simply not any bigger. Or perhaps a passing star, long ago, stripped away the debris that was orbiting the Sun beyond Pluto. David Jewitt of the University of Hawaii, who co-discovered the first KBO, thinks there will be another Pluto or two found in the next few years. Other scientists agree that this is likely, and expect the potential object or objects to be farther out than Pluto. Bernstein said that because the new study looked at only six patches of sky, the possibility of another Pluto can't be ruled out. But he suspects that any such objects would still be within 50 AU. Others scientists have kept alive the possibility that a large 10th planet might lurk well beyond this range, though no conclusive proof of this has ever been found. If there was a large planet that had long ago been booted out beyond 50 AU, some have said it could be what swept the area clean, creating the apparent edge. Bernstein said something the size of Neptune, however, would have to be hundreds of AU away to have avoided detection over the past 400 years.
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