Rocky finding: evidence of extrasolar asteroid belt.

Astronomers report that they've obtained the best evidence yet for an asteroid belt beyond the solar system. Such a belt would suggest that the star Zeta Leporis, which lies just 70 light-years away, possesses not only asteroids but rocky planets like Earth.

The new measurements pinpoint the location of a disk of warm dust that surrounds Zeta Leporis. The dust lies about the same distance from the star as the solar system's asteroid belt lies from the sun, Margaret M. Moerchen and Charles M. Telesco of the University of Florida in Gainesville and their colleagues report in an upcoming Astrophysical Journal Letters.

Most previously observed disks have been cool and lie much farther from their parent stars, in the region that corresponds in the solar system to the locale of Pluto and the reservoir of comets known as the Kuiper belt.

The close-in dust around Zeta Leporis probably arose when several asteroids bumped into each other, grinding rock into a fine spray of particles, or when a large asteroid, perhaps 100 kilometers in diameter, suffered a cataclysmic wallop, Moerchen and Telesco say.

"The [precise] measurement of the Zeta Leporis disk is a very exciting result," says Charles Beichman of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We now have direct evidence for structures around other stars that are directly analogous to the asteroid belt in our solar system."

Zeta Leporis entered the limelight in the 1980s, when a satellite revealed that the star and its surroundings emitted much more infrared light than was expected from the star alone. That's a sign that dust swaddles Zeta Leporis.

In 2001, Christine Chen and Michael Jura of the University of California, Los Angeles observed the star with one of the telescopes at the Keck Observatory on Hawaii's Mauna Kea. They found that the dust is probably confined to a disk with a radius no larger than 6.1 astronomical units (AU)--slightly greater than Jupiter's distance from the sun (SN: 6/16/01, p. 375).

In February 2005, the team led by Moerchen and Telesco viewed Zeta Leporis with the Gemini South telescope atop Cerro Pachon in Chile. Those observations for the first time enabled researchers to precisely gauge the size of the dust disk.

The team finds that most of the dust is concentrated at a distance of 3 AU from Zeta Leporis. That's similar to the location of the solar system's asteroid belt, which stretches between 2.1 and 3.3 AU from the sun.

Because asteroids are leftovers from the planet-making process in the solar system, the new study "supports the thought that Earthlike planets may exist" outside the solar system, says Jura. Compared with our sun, Zeta Leporis is a youngster, but it's still old enough to have formed planets.

Moerchen's team is planning further observations to reveal the Zeta Leporis disk's shape. If it's circular and uniform in density, the disk probably formed by the slow grinding of asteroids over thousands of years. A more distorted shape would suggest that the dust was generated by a collision between two large chunks of rock only about 100 years ago, Telesco says.

"For years we've been studying Kuiper belt-like disks; now, we're investigating the architecture of the inner asteroidal regions" around stars. "This is kind of new territory," Telesco says.