Until recently our solar system appeared to have no end in sight. Astronomers had discovered that the space between Neptune and Pluto is full of Kuiper belt objects— chunks of rock and ice left over from the formation of the solar system— that seemed to continue as far as telescopes could explore. But graduate student Lynne Allen of the University of Michigan may have spotted the end of the vastness. Along with her adviser, Gary Bernstein, and Renu Malhotra of the University of Arizona, Allen surveyed the outer fringes with the powerful four-meter telescope at Chile's Cerro Tololo Observatory. The team easily detected Kuiper belt objects out to a distance of about 5 billion miles. But beyond there, where scientists expected the icy bodies to be even more common, they found nothing.
"It could be that the solar system has an edge to it," Allen says. Perhaps early on, a rogue protoplanet broke free and took a bunch of the Kuiper belt objects with it, or a passing star stripped away our sun's outer kingdom. On the other hand, the edge might denote the original boundary of the primordial solar system. If so, astronomers have greatly overestimated the size of the gassy disk from which the planets coalesced. "That would be strange, because when we look at other stars we generally see much larger clouds of gas and dust," Allen says. She and her collaborators are now double-checking to make sure there's really no there there.