Faraway comet spins into the light.

The 6-hour spin cycle of a bluish-gray ball of ice at the fringe of the solar system may hold clues to early planet formation. Called 1996 TO66, the whirling snowball is one of the biggest and brightest residents of the Kuiper Belt, a crowded ring of an estimated 200 million comets just beyond the orbits of Neptune and Pluto.

One of only 70 Kuiper Belt objects identified so far, 1996 TO66 glows brighter than its cohorts, but the faintest stars visible with the naked eye shine 1.5 million times as brightly. For an object so dim and distant, it was a feat just to measure the comet's rotation, comments Alan P. Boss of the Carnegie Institution of Washington (D.C.).

Brian G. Marsden of the Harvard-Smithsonian Center for Astrophysics says that "this may be the best determination" of the rotation of a Kuiper Belt object to date.

Scientists think that Kuiper Belt objects have changed little since the birth of the solar system 4.5 billion years ago, when all the denizens of the solar system were born from a swirling disk of gas and dust that enveloped the young sun.

Changing the rotation speed of an object as big as 1996 TO66--which measures about 600 kilometers across--requires much more energy than changing that of a tiny one.

If 1996 TO66's spin truly hasn't changed since the start, it reveals the "agitation of the stuff from which it formed," says Olivier R. Hainaut, a member of the international team of astronomers who reported the rotation at a workshop held last week at the headquarters of the European Southern Observatory (ESO) in Garching, Germany.

"If there was lots of motion in the outer solar system, this motion would have been preserved as fast rotation," he says. "If it was a very quiet place, the current rotation would be very slow."

Hainaut and his colleagues enlisted the powerful eye of ESO's 3.6-meter New Technology Telescope in La Silla, Chile, to make their observations. For six nights this fall, the scientists tracked the intensity of 1996 TO66's glow through different light filters. Fifty images later, they could detect a clear variation in brightness as dark spots spun by every 6 hours and 15 minutes.

Some researchers consider Pluto to be part of the Kuiper Belt. At least one of these astronomers is less than impressed with the new finding. "Pluto's rotation period has been known for 40 years," says Alan S. Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colo. "I think this is much ado over nothing."

What is important, Hainaut argues, is that the comet joins many other objects in the outer solar system having rotation periods from 4 to 8 hours. For example, Chiron, a minor planet thought to have come from the Kuiper Belt and now in orbit between Saturn and Uranus, also rotates about every 6 hours. Scientists suspect that Earth, too, spun at that rate soon after its formation.

This rapid rotation affirms the idea that as the solar system was forming, planets and comets grew bigger by colliding with and sticking to similar-size bodies that boosted their spin.

Hainaut says that the measured rotation rate of 1996 TO66 will help theorists model the early solar system.