Where does the solar system end? At Pluto, most folks would reply. Or at Neptune, the cognoscenti might say, because thanks to Pluto's odd, egg-shaped orbit, the eighth planet has been outermost since 1979 and will be through 1998. But astronomers suspect that the sun's family actually extends far beyond either of these two planets. Out there in the frigid darkness beyond any known planet, they believe, lies the Kuiper belt, a ring of dusty ice chunks that surrounds the solar system. Beyond that, astronomers say, is the similarly composed Oort cloud, which forms a vast sphere around our planetary system. The cloud stretches two light-years from the sun, halfway to Alpha Centauri, the next nearest star. Occasionally one of the icy lumps in these outer regions is nudged toward the sun by a passing star or gas cloud. As it falls toward our world, it flares into view as a streaking comet.
Ever since these vast comet nurseries were first proposed four decades ago, the only evidence for them has been indirect and theoretical. At last there is something concrete. A tiny reddish spot of light recorded on a sensitive electronic detector in Hawaii last month appears to be the first component of the Kuiper belt ever observed. The body, known for now as 1992 QB1, is about 200 km (120 miles) across, and a preliminary calculation puts it at more than 5.1 billion km (3.2 billion miles) away. That doesn't necessarily make it the most remote object in the solar system, since Pluto retreats to more than 7 billion km from the sun. But it does imply that the Kuiper belt and the Oort cloud really exist and that the solar system's boundary may lie 10,000 times as far away as Pluto ever ventures.
The discovery was no accident. David Jewitt, a University of Hawaii astronomer, and Harvard's Jane Luu, now a postdoctoral fellow at the University of California, Berkeley, had been searching for just such an object for five years. Says Jewitt: "We were trying to understand why the outer solar system is so empty." Is it because there is really nothing out there or because things are just hard to see?
There were already several reasons to think the latter is true. For one thing, the existence of the belt and cloud are natural consequences of established theories about the birth of the solar system. According to such theories, the early sun, formed from a cloud of gas and dust, was surrounded by a disk-shaped nimbus made up of the leftovers. The newborn star's heat drove smaller particles and gases, including water vapor, out from the center. The heavier, metal-rich rock left behind condensed into asteroids and the inner planets: Mercury, Venus, Earth and Mars. Much of the gas and light dust , farther out was gathered up into the so-called gas giants: Jupiter, Saturn, Uranus and Neptune. The rest was blown by solar heat and wind to the outskirts, where it presumably congealed into chunks of ice and dust. (Rocky Pluto is an anomaly, and many astronomers believe it isn't a planet at all but a giant comet or asteroid flung into its present position when it had a close gravitational encounter with one of the outer planets.)