Saturn-Like Rings Spotted Around Stars

The Hubble Space Telescope has spied a bright ring of dust around a star called HR 4796A, about 220 light-years away. Astronomers believe that planet-sized bodies shepherd the ring, the first such structure seen around another star.

AUSTIN, TEXAS--Like a planetary jewelry case, our solar system glitters with a dazzling variety of rings. Now, astronomers have unveiled the first evidence that dusty rings adorn other mature stars on an equally diverse but far grander scale. New images from the Hubble Space Telescope, released here today at a meeting of the American Astronomical Society, reveal a bright ring of dust around one star and a dark gap within a broad disk around another--features that may offer the strongest clues yet to the properties of large young planets there.

Astronomers already have spotted many dust disks around baby stars. Once stars become young adults, fierce stellar winds are thought to blow away most of the dusty flotsam to leave behind nascent planets. Over millions of years, these planets become shrouded by orbiting debris generated from the collision of comets, asteroids, and other objects. Last year, a ground-based infrared study of one such cloud, around a star called HR 4796A, revealed a "hole" in the debris cloud, perhaps indicating the birthplace of planets. But the image was too grainy to discern more details.

The new image of HR 4796A from Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) came as a surprise, says astronomer Bradford Smith of the University of Hawaii, Honolulu. The hollow disk actually resembles a ring, about as wide as the distance between the orbits of Mars and Uranus. "When we first looked at this, we thought, 'Wow, it looks just like Saturn,' " Smith says. Such a ring cannot persist without being shepherded in place by planet-sized bodies, Smith believes. If those objects are bigger than Jupiter, longer exposures with NICMOS could reveal them as faint points of light, notes team member Glenn Schneider of the University of Arizona in Tucson.

Hubble also spied a dark gap dividing an even larger disk of debris around a star called HD 141569, the first such clearing seen within a disk. Large planets could be sweeping particles out of the gap or kicking them out via gravitational interactions, says astronomer Alycia Weinberger of the University of California, Los Angeles. The feature is very wide, she notes: "The whole radius of our solar system would fit within the disk's gap."

The images may fill "one of the missing links" in the chain from embryonic stars to mature planetary systems, says astronomer Steven Beckwith, new director of the Space Telescope Science Institute in Baltimore, Maryland. Both stars are only about 10 million years old, so further study of the two disks should help researchers refine models of how giant planets arise by the time a star reaches early adulthood. "But these pictures by themselves are not proof" that planets spawned the distinctive ring and gap, he notes. Schneider agrees: "Nearby stars or low-mass companions that we haven't yet seen could be influencing the dynamics of these disks."