Planet Signs? Sifting a dusty disk

Twenty years ago, astronomers peering at the young star Beta Pictoris got their first glimpse of a disk of dusty debris—the sign that planets, asteroids, and comets are forming and then banging together and releasing an abundance of dust.

ROCKY VIEW. Depiction of a possible planet (upper left) flanked by bands of dust within the debris disk surrounding the star Beta Pictoris (upper right). ©K. Kanba, ISAS/JAXA

Such debris disks have now been found around many young stars, but the one surrounding Beta Pictoris remains the most revealing about how planets form and evolve.

Recording spectra of the disk at midinfrared wavelengths, Yoshiko K. Okamoto of Ibaraki University in Japan and his colleagues now have gleaned new details about the size, composition, and crystal structure of dust particles. The data, which indicate three distinct bands of dust within the Beta Pictoris disk, suggest the location of a possible planet as well as of a trio of asteroid or comet belts. The astronomers describe their study in the Oct. 7 Nature.

The team used an infrared camera on the 8.2-meter Subaru Telescope on Hawaii’s Mauna Kea. Okamoto and his collaborators found that small dust particles without a crystal structure—such as amorphous silicate grains—collect into belts within the Beta Pictoris disk. These dust bands reside at 6.4, 16, and 30 astronomical units (AU) from the star. An AU is the distance between the sun and Earth, or roughly 150 million kilometers. Images taken by other researchers had discerned the outermost two bands, but the new study is the first to reveal their composition, Okamoto notes.

It takes less than 100 years for the pressure exerted by photons streaming from Beta Pictoris to blow tiny dust particles from the disk into more rarefied regions of space. The persistence of the particles in the belts suggests that they are continually replenished, probably by the evaporation of comets or the collision of asteroids, says Okamoto. The team’s observations, which reveal that very few fine-grain silicates lie close to the star, favor the asteroid scenario, he adds.

After analyzing the forces exerted on the three dust bands, the team concluded that the tug of an unseen planet may have kept the belts intact over millions of years. The proposed planet would lie at 12 AU from the star, slightly beyond Saturn’s distance from the sun.

The team also reports that the inner part of the Beta Pictoris disk, within a few AU of the star, has a different composition from material that lies farther out. The crystal form of the mineral olivine is more concentrated near the star, as are silicate grains with diameters greater than a few micrometers.

Alycia J. Weinberger of the Carnegie Institution of Washington (D.C.) says she’s impressed by the mapping of the minerals in the star’s disk. For grains to acquire a crystal structure, they must be heated to several thousand kelvins, she notes. That typically occurs extremely close to a star such as Beta Pictoris—no farther than about 0.1 AU, or one-fourth Mercury’s separation from the sun. Finding crystals at greater distances than that, as Okamoto’s team did, suggests that some force is mixing the material in the disk.

Mapping the composition of debris disks may eventually explain “what determines the composition of the planets,” says Weinberger. The Beta Pictoris observations provide a “new piece of the puzzle of how solar systems and Earthlike planets form,” says Steve Desch of Arizona State University in Tempe.

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