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Planetary potential surrounds most stars.

A new survey of nearby stars has boosted the likelihood that life exists outside our solar system, according to astronomical oddsmakers. What has raised the probability, they say, is the finding that dusty disks swathe a majority of the young stars surveyed. Such a disk encircled the sun, theorists believe, during a crucial phase of planet formation.

The result implies that solar systems may be the norm and not the exception in the universe. "It signals the existence of a very large number of potential abodes of life," says astronomer Frank Drake, president of the SETI Institute in Mountain View, Calif. As a pioneer in the search for extraterrestrial intelligence, or SETI, Drake identified the variables that many scientists now use to calculate the odds of alien life.

Astronomers have no tools to directly observe planets outside our solar system, but they can look for the dust disks associated with planet creation.

Using an orbiting telescope called the Infrared Space Observatory, European astronomers surveyed 84 stars. A disk around a star glows in the infrared because the star warms the disk's dust grains.

The researchers identified the telltale infrared glow in spectra of light emanating from 14 of the stars. The instrument was not equipped to image disks, although other astronomers have used larger telescopes to view them (SN: 8/8/98, p. 91).

Scientists are surprised not by the disk tally but by the similarity in age among the stars that have disks.

The European astronomers used measurements of distance and brightness to estimate stars' ages, says Harm J. Habing, a member of the team who works at Leiden University in the Netherlands. To obtain these numbers, the team recruited data from other instruments, such as the satellite Hipparcos, which measured how stars' positions in the sky change subtly as Earth orbits the sun.

With their large sample size and age estimates, the European team was able to identify a trend that astronomers had not discerned before: Most young stars have disks, they found, while most older stars don't. The findings, which appear in the Sept. 30 NATURE, also suggest a typical life span for disks. Ninety percent of the stars with disks were less than 400 million years old.

Since stars usually live for billions of years, these results suggest that most of the vast number of middle-age stars may well have had disks in the past. Furthermore, the simplest explanation for the disappearance of the disks is that unseen, nascent planets swept up the dust, says theoretical astronomer Harold F. Levison of the Southwest Research Institute in Boulder, Colo. So, the new observations turn the majority of stars in the sky into candidate solar systems.

The source of the dust is collisions of comet- and asteroid-size objects, says astronomer Dana E. Backman of Franklin & Marshall College in Lancaster, Pa. Planets build up from softer impacts that permit the chunks to stick together, he says.

Levison hesitates to say that dust disks always signal the creation of planets. Yet he describes the disks as the best available evidence for other Earthlike planets. Of the new study he says, "It tells us that something akin to the planet-forming process is occurring around most stars."
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Author:Baker, O.
Publication:Science News
Article Type:Brief Article
Geographic Code:1USA
Date:Oct 9, 1999
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