Beyond Pluto

By MICHAEL D. LEMONICK

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.)

The existence of comets provides further evidence for the Oort cloud and the Kuiper belt. Comets are orbiting chunks of dusty ice, whose surfaces evaporate in the warmth of the sun to form halos and tails. In the early 1950s, Dutch astronomer Jan Oort suggested that comets originate in a cloud surrounding the solar system; he based his theory on their highly elongated orbits, which reach into the inner solar system and out beyond Jupiter. Shorter-period comets like Halley's, which returns every 76 years, are believed to originate closer in, hurtling out of the Kuiper belt, a region first proposed by Oort's countryman and contemporary Gerard Kuiper. Because repeated solar heating would boil a comet away after a few million years, the fact that new ones keep appearing suggests that there is a large supply.

"What makes us happy," says Jewitt, "is not just that we may have found the source of the short-period comets, but also that these objects have stayed largely unchanged since the solar system formed." QB1's color probably reflects what little change has happened: carbon compounds on its surface have been bombarded with cosmic rays for eons, turning it reddish.

Jewitt and Luu caution that the object's identity will have to be confirmed, a process that will take a month or two. Agrees Brian Marsden, the Harvard- Smithsonian astronomer who actually calculated its orbit: "All we can say for sure right now is that it's far away, and that it is most likely one of the larger members of the Kuiper belt. But it could be something else." If it is part of the belt, a worldwide search will begin for similar objects.

Proof that the Kuiper belt exists would help demonstrate that another long- sought object almost certainly does not. For nearly a century, astronomers have been looking for a Planet X, a world conjectured to lie far beyond Pluto. But the planet's gravity would have scattered any belt of proto-comets far and wide. Planet X was first dreamed up to explain apparent irregularities in Neptune's orbit. Recent studies have shown those irregularities to be an illusion -- and the sighting of QB1 has probably dashed forever the hope of finding a 10th planet.