Dusty Disks May Reveal Hidden Worlds; on the Trail of Extrasolar Planets

Article excerpt

Like a crab scuttling through sand, an orbiting planet leaves a telltale trail in the dust surrounding its parent star. Astronomers began scouring nearby stars for such trails nearly 2 decades ago, but telescopes provided only fuzzy images. With the keen-sighted instruments available today, however, the dust trails are coming into sharper focus, opening the way to finding and characterizing the properties of hundreds of planets beyond the solar system.

Doughnut-shape patterns of dust, or debris disks, are much easier to detect than planets themselves because the disks have a much larger surface area. Recent pictures of debris disks around 10 or so nearby stars show gaps, arcs, rings, warps, clumps, and bright patches.

Some of these structures are serving as guideposts, helping astronomers home in on planets they hope to directly image. Others suggest the presence of planets that can't currently be detected by other means.

Although the most successful planet-detection protocol so far has been the measurement of the slight wobble that extrasolar planets induce in the motion of their parent stars, the technique has limitations. It fails for stars that rotate rapidly, have insufficient mass, or lie at great distances from their parent star.

"Debris disks can definitely point you to the existence of a planet that you might not be able to detect in any other way," notes Alycia J. Weinberger of the Carnegie Institution of Washington (D.C.). Although she cautions that astronomers haven't proved that the features they see have been sculpted by planets, theorists are using the newest data to refine estimates of the mass of proposed planets, their distance from their parent stars, and the shapes and tilts of their orbits.

Trying to discern the properties of planets through the patterns they generate within debris disks is an arduous task, says Charles A. Beichman of NASA Jet Propulsion Laboratory in Pasadena, Calif. But the increasing resolution of images taken at near-infrared and longer wavelengths is making the job easier.

DUSTY DOINGS Astronomers have known since 1984 that searching for planets can be a dusty business. That's when they began analyzing data from the first spacecraft to survey the entire sky at mid- and far-infrared wavelengths. Launched in 1983, the Infrared Astronomical Satellite (IRAS) detected much more infrared light around the brilliant star Vega than could be accounted for by the star's own radiation. About 15 percent of the stars surveyed by IRAS showed a similar excess of infrared radiation.

Researchers deduced that dust must encircle such stars, soaking up the stars' ultraviolet and visible light and reradiating the energy at longer, infrared wavelengths. A swirling distribution of dust around a star flattens into a debris disk. These disks are considered the remains of much denser, gas-rich rings of dust, known as protoplanetary disks, that swaddle infant stars. During the first few million years of the life of some stars, the gas, dust, and ice within a protoplanetary disk clump together to form planets and smaller bodies, such as asteroids and comets. These processes drastically thin out the material in the disk.

Calculations indicate that most of the grains in a debris disk quickly spiral toward the star or get ejected from the system. That means that for the ring to survive, the dust must continuously be replenished. Beichman cites three possible sources of this dust: asteroids smashing into each other, comets evaporating, and material drifting in from the outer edge of the original protoplanetary disk.

Although IRAS lacked the resolution to image debris disks, the excess infrared radiation it detected sparked a hunt for what came to be called Vegalike stars. Astronomers then used larger telescopes to examine these stars for evidence of planet-bearing debris disks. …