Astronomers search the skies for other planetary systems

PLANETARY astronomers, who now have seen most of the solar planets at close range, are pushing against their next major research frontier - the study of other planetary systems orbiting other stars. Discovery of disks or shells of debris around several dozen stars over the past few years suggests strongly that such systems exist. Sharper-eyed instruments planned for operation in Earth orbit promise to reveal at least some planetary systems by the century's end.

The announcement, earlier this month, of the first visible light pictures of the debris around the young star Beta Pictoris encourages astronomers to think this space-based search will be fruitful.

As reported at the American Astronomical Society meeting in Pasadena, Calif., Francesco Paresce of the Space Telescope Science Institute and Christopher Burrows of the European Space Agency found debris particles ranging in size from fine dust to pebbles. They don't know if there are larger bodies. But, by studying the way the debris particles reflect Beta Pictoris's light, Paresce says, ``the observations show unequivocally that an agglomeration process is in an advanced state, where fine interstellar dust grains stuck together to form larger clumps.''

So far, the only other place where such stardust clumping is known to have occurred is our own solar system. Here, comets, asteroids, and planets are considered the end product of clumping.

The planet quest is more than a mere search for other worlds. It's aimed at answering one of the great cosmic questions: Is the solar system a rare quirk of nature or are such systems a common aspect of star formation?

Many astronomers once thought the planets may have condensed from a plume of debris pulled from the Sun by a passing star. That would make Earth and its companions a product of chance. In the currently popular theory of star birth, an interstellar cloud of dust and gas collapses to form both a central star and a flattened disk of dust and gas where planets form. The theory is well worked out. Now astronomers want to test it by studying as many other star systems as they can.

They first began to sight such systems with the American-British-Dutch InfraRed Astronomy Satellite (IRAS) that operated in 1983. IRAS data identified some 40 stars with associated debris. They include such familar stars as Vega and the red giant Betelgeuse of the constellation Orion - now prominent in the Northern Hemisphere night sky - as well as Beta Pictoris.

But the IRAS data are too coarse to show whether the debris is spread out in a shell or is more tightly organized in a disk, as expected in a planet-forming collapse. Subsequent studies, mainly with ground-based instruments, have found Beta Pictoris the most promising subject.

Shortly after IRAS found it, Bradford A. Smith of the University of Arizona and Richard J. Terrile of the NASA Jet Propulsion Laboratory produced an infrared picture showing the Beta Pictoris debris as a well defined disk. There is some suggestion that the inner part of the disk - a region about the size of our solar system - may be relatively clear. This may imply that planets have formed there, sweeping the region clean in the process. But this is uncertain. Now the visible light images made by Burrows and Paresce show evidence for an agglomeration process that might, indeed, involve planet formation.

Astronomers couldn't do much better than this with the equipment and observing techniques available up to now. Space-based instruments will sharpen their view.

The Hubble Space Telescope, which the shuttle Atlantis currently is scheduled to orbit in November 1988, should carry specialized instruments that can track a star precisely enough to detect wobbles in its motion across the sky due to accompanying planets. Looking farther ahead, the University of Arizona and NASA Ames Research Center are developing a very accurate position-measuring telescope to be attached to NASA's space station. Called the Astrometric Telescope Facility, it should be able to track stars for 10 to 20 years. This would produce a data base from which planet-induced wobbles could easily be picked out.

If finally approved for deployment, this instrument should become one of the most important scientific tools on the space station. It would ultimately be supplemented by systems that, free of Earth's distorting atmosphere, could directly image large planets around the nearer stars or even pick out an Earthlike planet by detecting its oxygen.

By this century's end, we may know definitely whether or not our solar system is unique. It's an exciting prospect.