Where is the edge of our solar system? In recent years, astronomers have peered farther and farther beyond Neptune to discover a rapidly increasing number of bodies littering the outer reaches of the solar system. Now the end may be in sight. Two reports presented today at the annual Division for Planetary Sciences (DPS) meeting in Pasadena, California, place the limit just beyond little Pluto's farthest wanderings, about 50 times Earth's distance from the sun (50 astronomical units or AU).
Putting a close-in edge on the solar system would have more than merely territorial implications. The extent of the Kuiper belt, a disk of bodies left over from the primordial gas, ice, and dust that agglomerated to form the planets, can provide hints of how the planets came together. An edge at 50 or 55 AU would mean the original preplanetary disk was far smaller than those of young stars seen today, or that something--perhaps a too-close encounter with another newly formed star--tore away much of the disk.
Proponents of an edge say the solar system ends at the apparent outer limit of today's Kuiper belt. But just how far Kuiper belt objects (KBOs) extend beyond Neptune has long been a mystery. In the last decade, however, improved techniques have enabled astronomers to detect fainter and fainter objects while surveying larger parts of the sky.
At the DPS meeting, astronomers Lynne Allen and Gary Bernstein of the University of Michigan, Ann Arbor, and planetary dynamicist Renu Malhotra of the University of Arizona in Tucson reported that their state-of-the-art survey has discovered 24 new KBOs, but none was found beyond 53 AU even though their search was sensitive enough to find 160-kilometer objects out to 65 AU. And Chadwick Trujillo of the California Institute of Technology in Pasadena, David Jewitt of the University of Hawaii, Manoa, and Jane Luu of Leiden University in the Netherlands reported that their search over a much larger area for less-faint objects than Allen and colleagues could detect turned up 57 KBOs, none of which is beyond 50 AU. "It seems like there's an edge," says Trujillo.
But astronomer Brett Gladman of the Observatory of Nice isn't convinced. "I don't think the evidence is strong," he says about the new results. "It's very hard, because these objects become faint very fast with increasing distance. And the results depend on the assumptions used to model the data." Those assumptions may be spelled out in the next year, says Gladman, leaving astronomers in a better position to judge whether the solar system ends abruptly.