On the trail of dead planets: dust ring around a white dwarf.

Infrared observations have just depicted the dusty vestiges of a planetary system dancing around a dead star. Researchers say that the dust is generated by collisions among comets that outlived both their parent star and the star's innermost planets.

In detecting the relic dust, researchers may have glimpsed the fate of our solar system some 5 billion years from now. That's when the sun will run out of hydrogen fuel and briefly swell to enormous proportions, burning Earth to a crisp or swallowing it altogether. The swollen star will then blow off its outer layers, leaving its core to shrink to a cinder called a white dwarf.

The Spitzer Space Telescope recently recorded glowing dust that rings the hot white dwarf at the center of the Helix nebula, 700 light-years from Earth. With its cocoon of shimmering clouds--the material cast off by the dying star--the widely photographed Helix resembles a giant eye. The Milky Way is littered with such colorful celestial corpses, dubbed planetary nebula.

Kate Su of the University of Arizona in Tucson and her colleagues were initially puzzled by their Spitzer study of the Helix nebula. The team expected that when the dying star expelled its outer layers, it would have swept away any lingering dust. However, further observations with Spitzer confirmed that dust resides between 35 to 150 astronomical units (AU) from the white dwarf, the team reports in the March 1 Astrophysical Journal Letters. One AU is the distance between the sun and Earth.

Su's group suggests that the dust comes from comets that were originally in orderly orbits around the aging, sunlike star, just as comets in the Kuiper belt orbit the sun beyond Neptune. When Helix'S dying star east off its outer layers, it disturbed the comets' orbits and spawned dust-generating collisions, the team proposes.

Astronomers had previously found dusty rings around a handful of other, more mature white dwarfs, which were old enough that their planetary nebulae had dissipated long ago. Spitzer observations of the white dwarf G29-38, only about 45 light-years from Earth, revealed dust with a composition "just like Halley's comet" circling the dead star, notes Mare J. Kuchner of NASNs Goddard Space Flight Center in Greenbelt, Md. He and his colleagues reported that study in 2005. The Helix nebula may represent an earlier phase of these dusty white dwarfs, he notes.

Other researchers have found about 40 white dwarfs with an excess of sodium and other metals in their atmospheres. Because metals take only about a million years to sink below the atmosphere of a white dwarf, any metals seen in the spectrum of a billion-year-old white dwarf "must have been recently supplied"--perhaps by the capture of planetary debris, says Kuchner.

"We're starting to think [that the metals] are probably the smile on the crocodile, the last bits remaining of a dead planetary system, fed to the white dwarf," Kuchner says.