Planet hunters bay at another clue: a belt of cosmic dust

Astronomers have long reasoned that if solar systems anchored by sunlike stars are anything like ours, they'll have comets and asteroids, as well as planets.

Now, researchers appear to be confirming that hunch. A team of scientists announced Wednesday that it has uncovered the first evidence outside our solar system for an asteroid belt around a sunlike star.

In addition, a second team reported earlier this month that it had discovered signs of Kuiper Belt-like structures around six stars with planets. In our solar system, the Kuiper Belt is the cache for comets that swing past the sun every 20 to 200 years.

Together the finds represent the first discoveries of debris disks around mature planetary systems beyond ours. Like the sun's asteroid and Kuiper belts, these disks are thought to be the jumbled leftovers from planet formation. They also could serve as "look here" flags for planet hunters.

The two teams' work is part of a broad 20-year search for examples of major planetary components beyond the sun's. Humans emerged in what by then was a ready-made solar system. To fully understand how that system came together and how it compares with others, researchers have to peer into the wider stellar neighborhood for clues.

"We're trying to find all the constituents of planetary systems - the comet belts, the asteroid belts, big gas-giant planets, and ultimately Earth-like planets," says Charles Beichman, a researcher at the Jet Propulsion Laboratory in Pasadena, Calif., who leads both teams. "We're now pushing into the asteroid belts."

Both efforts rely on the Spitzer Space Telescope, the last of NASA's four orbiting "Great Observatories." Spitzer detects infrared emissions from objects in space - in this case, from dust generated as asteroids and more-distant chunks of planet wannabes collide and crumble.

Dust whose signature could mark an asteroid belt was detected around a star 41 light-years away known as HD69830. From the relatively hot emissions, the team calculates that the asteroid belt lies within 93 million miles (1 A.U.) of the star. Our sun's asteroid belt, by contrast, is centered some 2.7 A.U. away. Moreover, HD69830's purported asteroid belt appears to hold between 18 to 64 times more mass than the sun's - turning the region into a cosmic billiards hall.

If the belt were to exist in our solar system at our asteroid belt's distance, the dust would appear as a brightly glowing arc across the night sky.

The team acknowledges other possibilities, such as a "supercomet" that spews gas and dust as it periodically swings by the star. The dust contains minerals similar to those detected in the 1997 Hale-Bopp comet.

But the dust grains are so small that pressure from the star's radiation would sweep the region clean within 1,000 years. So either astronomers displayed impeccable timing and spotted the dust during a supercomet "flyby," or the dust is being replenished through asteroid collisions.

Astronomers should be able to make a final call after they try to detect gases in the so-called "volatiles" that comets also emit.

If the dust has asteroidal heritage, it could hint at the presence of a planet. Previous surveys suggest the star has no planet - at least in Jupiter's class or larger. But the team notes that the dust's infrared signature shows a sharp drop in temperature with distance, implying a clear outer boundary. Thus, an object farther out - perhaps with the mass of Saturn or less - may be shepherding the belt.

The second team's Kuiper-Belt evidence comes from similar, but far cooler, infrared signatures beyond 10 A.U. from their parent stars.

For planet hunters, the results represent a milestone. "This result is extremely exciting," notes Debra Fischer, an astronomer at the California State University at San Francisco and a member of a prolific planet-hunting team.

She suggests that the star's lack of a gravitational pull from Jupiter-class planet may explain why this asteroid belt survived. It also serves as a stark reminder that some type of planetesimal growth takes place even where large planets haven't been detected. "It seems important to remember this when we give statistics for the fraction of stars that have planets," she holds.