Distant dust: asteroid belt or boiling comet?

Astronomers this week announced that they have discovered signs of an asteroid belt circling a sunlike star 41 light-years from Earth. If confirmed, the belt would be the closest known analog to the asteroid belt in the solar system and a possible indicator of rocky planets.

Charles Beichman of the California Institute of Technology in Pasadena and his colleagues base their findings on infrared observations of the star HD 69830 with the Spitzer Space Telescope. A spectrometer on Spitzer revealed that a thick disk of warm, micrometer-size dust particles surrounds the star, which has a mass and age similar to that of the sun.

Small dust particles orbiting a star rapidly dissipate, so for a disk to persist, it must be constantly replenished. Scientists say that the most likely source of particles is a belt of asteroids, too small to be seen, that bang together and create a spray of dust about once every 1,000 years.

Astronomers had found evidence of belts circling young stars, but this is the first time that they've found signs of a belt around a star similar in mass and age to the roughly 4.5-billion-year-old sun, says Beichman. He presented the findings during a NASA telephone briefing.

Spitzer didn't discern the disk, but its spectrometer measured the temperature, composition, and size of the dust grains. From the temperature, slightly above the boiling point of water, Beichman's team determined that the grains lie as close to the star as Venus does to the sun. Our solar system's asteroid belt is farther away, flanked by Mars and Jupiter, and contains only about 4 percent as much material as in the proposed belt around HD 69830.

The dust discovery by Spitzer is notable for another reason. It's the first time that astronomers have found fine-grain silicate crystals in a disk around a mature star. Similar particles were detected coming off comet Hale-Bopp when it journeyed into the inner solar system in 1997 (SN: 7/29/00, p. 72). By analogy, Beichman says, it's possible--though not likely--that the source of HD 69830's disk is a Pluto-size comet that is now slowly boiling away.

Astronomer Alycia Weinberger of the Carnegie Institution of Washington (D.C.) says that the dust may indeed have derived from rocks in an asteroid belt. But only an unusually violent collision would pulverize rocks into tiny silicate grains. Such collisions are rare and could explain why Spitzer hasn't found such grains in other dust disks, she says.

Weinberger suggests that Spitzer happened to observe HD 69830 just after a major asteroid smashup, before the fine dust dissipated. "It's as if this star suddenly sent us a flare, saying 'Here I am,'" says Weinberger. At an upcoming meeting, her team plans to report evidence of warm silicate dust around the star HIP 8920, a 300-milion-year-old version of the sun.