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Planetary ingredients. The dust cloud around zeta Leporis, bottom, could give rise to a solar system similar to ours, top.

Asteroids Stir Up Planets-to-Be

PASADENA, CALIFORNIA--Asteroids are probably kicking up a surprisingly small cloud of swirling dust observed around a nearby star, astronomers reported at the American Astronomical Society meeting here on 4 June. The protoplanetary disk is the first ever found that is small enough to congeal into a near twin of our solar system--and it may already harbor a Jupiter-sized planet.

Astronomers, at least some of them, think they know how planets form: Chunks of rock orbiting a star in a protoplanetary disk collide and stick together, eventually clearing a path through the disk as most of the rocks in the orbit smack onto the growing planet. Clouds of dust billow out of these collisions and settle into a band of asteroids called a debris disk. In the past 2 decades, astronomers have detected infrared radiation from debris disks around several stars.

All these disks are much larger than the solar system, ranging in size from 70 to 100 astronomical units. (1 AU equals the distance from Earth to the sun.) Dust gets hotter the closer it is to the star, and a 10-year-old series of observations suggested that the dust surrounding zeta Leporis--a younger and slightly heavier version of our sun 70 light-years away--was hot enough to harbor an exceptionally small disk.

Now astronomers Christine Chen and Michael Jura of the University of California, Los Angeles, have snapped a precise photo of the disk. They find a thick disk of dust that extends to within 2.5 AU of zeta Leporis. "Dust usually spirals into a star within 20,000 years," says Chen. And because the first dust clouds would have formed with the star nearly 100 million years ago, Chen and Jura argue that "this disk must be continually replenished by asteroid collisions." They estimate that the zeta Leporis asteroid belt holds 200 times as much mass as the belt between Mars and Jupiter, enough to form a small planet.

At least one planet might already be there. Asteroids start life on circular orbits that rarely collide. The pull of a Jupiter-sized planet could be the instigator that keeps the grinding, planet-forming bumper-car game around zeta Leporis running, says astronomer Mark Sykes of the Steward Observatory in Tucson, Arizona.

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The twin 10-meter Keck Telescopes on Mauna Kea in Hawaii, used to observe zeta Leporis