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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.