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Research Points to Rapid Planetary Formation Around Distant Stars

ASHVILLE, May 26 — New telescope observations suggest that planets may form around distant stars more rapidly than previously thought, and some of these planetary systems could be far more extensive than the Sun's, astronomers say.

The research, reported here today at a meeting of the American Astronomical Society, showed that many huge disks of dust around young stars — leftovers of stellar formation out of which planets coalesce — appeared to dissipate within three million years of the star's formation.

The likely explanation, the researchers said, is that the dust has already collected into the dense building blocks of Earth-like solid planets, which would be undetectable with current telescopes.

If that is the case, the findings challenge the working hypothesis, based largely on studies of Jupiter in the solar system, that it takes at least 10 million years for planets to develop out of these protoplanetary disks of dust and gas. Such disks with enough mass to form planetary systems are relatively common with newborn stars.

"We will have to adjust our theories to understand how planets can form this quickly," Dr. Elizabeth Lada of the University of Florida said at a news conference.

Dr. Lada and Dr. Karl Haisch of the University of Michigan, both of whom conducted the observations with radio telescopes, also concluded that the gas in the disks probably dissipated on a time scale similar to the dissipation of the dust, a finding that indicated that large gaseous planets like Jupiter could be getting an equally rapid start.

Other astronomers said they found the research provocative and potentially significant in one of the most active fields of astronomy: the study of planet formation and comparative planetary systems. In the last eight years, astronomers have detected more than 100 stars and observed even more circumstellar disks where planets may be forming.

Commenting on the new findings, Dr. David Weintraub, an astronomer at Vanderbilt University, said it was "conceivable that the planet-forming process is well on the way" around three-million-year-old stars.

But it is also conceivable, Dr. Weintraub said, that all the dust has simply gone away. Some was probably sucked up by the star itself, and the rest could have been blown out into space by winds of stellar particles.

A Vanderbilt graduate student, Jeffrey S. Bary, working with Dr. Weintraub, reported X-ray detections of molecular hydrogen around young stars that could be further evidence of extremely early planet formation. Molecular hydrogen is the main constituent of protoplanetary disks. Its persistence around a star with no detectable protoplanetary disk could mean that there had been one and that the dust had now coalesced into planetary objects, Mr. Bary said.

Dr. Lada acknowledged that it was not possible with current technology to confirm that any planets or growing seeds of planets actually exist around the young stars that have lost their detectable disks.

Still, Dr. Stephen P. Maran, an astronomer who is a spokesman for the astronomical society, said the observations of how short a time protoplanetary disks persist provided "clues to enable you to develop a better theory" for how new planets develop.

The researchers led by Dr. Lada and Dr. Haisch examined dozens of young stars in the constellations Orion and Perseus, a region of prodigious star birth about 1,000 light-years from Earth.

They determined that the number of massive protoplanetary disks decreased as the age of the clusters of stars they studied increased. In one cluster of three-million-year-old stars, there were few such disks; by the age of five million years, almost none of the stars appeared to retain their disks.

In another discovery, a team of international astronomers using infrared telescopes was surprised to find similar circumstellar disks that were 10 to 100 times as large as the solar system or any planet-forming disks previously studied.

The team, led by Dr. Richard Elston of the University of Florida and Dr. Lada, who are married, calculated that each disk's diameter stretched thousands of times the distance from Earth to the Sun.

Dr. Elston said the observations suggested that it might be possible for planets to form at much greater distances from their stars than previously thought. This could throw theorists in a spin, but it would be good news for astronomers.

"Planets are notoriously difficult to detect near stars, which swamp their visible light," Dr. Elston said. "So if the more distant reaches of the disk are conducive to planet formation, they will be easier to find once astronomers start looking there."