New dust sheds new light on planetary birth.

Astronomers once thought that planets could be born of only a single star. Now, researchers have discovered a disk-shaped layer of dust orbiting a mature binary star system-evidence that planets could have two or more stellar parents.

If other astronomers confirm the disk's existence, the number of possible planets in the galaxy could double, since more than half of the stars in the galaxy are part of binary or multiple systems.

Working at optical wavelengths, astronomers had previously spotted just one dust disk orbiting a mature star, Beta Pictoris, says Paul Kalas of the Max Planck Institute for Astronomy in Heidelberg, Germany. For some years, Kalas and David Jewitt of the University of Hawaii in Honolulu have been searching for other optically visible disks. In 1995, they found a second such disk. Their report appears in the March 6 Nature.

Visible dust disks survive for only a short period during a star's lifetime, says Kalas. When an inward-collapsing cloud of matter first shapes a star, it also gives rise to a disk. In turn, the disk begets small bodies called planetesimals, which can later cluster together through gravitational attraction and eventually grow into comets, asteroids, or planets.

The disks orbiting young stars can only be seen through an infrared telescope, which reveals the heat absorbed by the dust particles. Such telescopes tell astronomers little about the shape of the disk, however. That information is crucial because only thin, dense disks support planet formation. Astronomers need an optical telescope to determine a disk's shape, but because the disks shrink as stars reach maturity, astronomers have scant time to study them.

A dust disk doesn't always result in planets. As in human conception, many factors affect planet formation.

"To form a planet, one probably needs a relatively quiet environment so that dust can settle into a thin, dense disk layer," Kalas says. For that reason, scientists until now have doubted that a binary system could support planet formation; the competing pulls of gravity and pushes of radiation from two stars should, in theory, provide too turbulent a womb.

Kalas suspects that the newly discovered disk has already formed its planetesimals. He theorizes that the rest of the binary system's original dust has been sucked back into the stars or pushed away by radiation. Disintegrating comets and crashing asteroids then replenished the dust in the disk, he says.

Jack J. Lissauer, an astronomer at NASA's Ames Research Center in Mountain View, Calif., disagrees. Lissauer theorizes that planetesimals have not yet formed from the disk. He adds that this disk may be a poor candidate to sire planets. Its color indicates that the dust particles are about one-tenth the size of the particles in the Beta Pictoris disk. Such small dust particles provide inefficient building blocks for planets, he says.