An artist's concept of FUSE in space. Photo: JHU

Another team of scientists announced Wednesday that they had found evidence for a cloud of millions of comets that they believe has formed around the young star Beta Pictoris.

In a paper published in Thursday's issue of the journal Nature, American and French scientists reported that a lack of molecular hydrogen in disk surrounding the star is evidence that hydrogen and other compounds have condensed to form a swarm of comets around the star, which is still in the process of forming a planetary system.

That conclusion is based on data from NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft. Astronomers used FUSE to look for the telltale absorption of ultraviolet light caused by molecular hydrogen, but failed to detect such absorption around Beta Pictoris.

The non-detection puzzled astronomers, who had previously detected carbon monoxide around the star using the Hubble Space Telescope. Since hydrogen is estimated to be 100,000 times more abundant than carbon monoxide in interstellar gas clouds, and that carbon monoxide has a short lifetime ­ about 1,000 years ­ before sunlight breaks it apart, astronomers believe that both are locked away in some kind of reservoir.

The most likely reservoir, said scientists, is a swarm of millions of comets analogous to the Kuiper Belt in the outer reaches of our own solar system. "If a similar comet swarm surrounds Beta Pictoris, the comets would still be warm enough to slowly release carbon monoxide, but far too cold to release molecular hydrogen, which would remain locked up as water ice," explained Paul Feldman of Johns Hopkins University, one of the coauthors of the Nature paper.

"We are witnessing the birth and evaporation of millions and millions of comets," said FUSE project scientist George Sonneborn.

The FUSE data, however, are at odds with observations by another spacecraft. ESA's Infrared Space Observatory (ISO) did find evidence for molecular hydrogen around Beta Pictoris by detecting the telltale emission of infrared light by such molecules.

The two observations can be reconciled, Feldman said, if hydrogen is not evenly distributed in the disk but rather scattered in clumps bright enough to be seen by ISO but not large enough to block enough ultraviolet light to be noticed by FUSE. "The molecular hydrogen clumps could be left over gas from the formation of the star, or perhaps from failed protoplanets," he said. "The initial carbon monoxide would no longer be in gaseous form but rather condensed into cometesimals or comets. All we can say for certain is that carbon monoxide must be continuously generated in the disk."

While there is no direct detection of planets around Beta Pictoris, astronomers believe the FUSE data are providing good insights into the formation of planetary systems. "This discovery gives new information on the last stages of the planetary formation," said Alain Lecavelier of the Institut d'Astrophysique de Paris. "The cleaning of the residuals of planetary formation takes a long time, in the form of collisions between asteroids, bombardment of planets by planetesimals, and evaporation of millions of comets as observed today around Beta Pictoris."