Star-circling disks of dust and gas encourage astronomers to think of planets.
New evidence of some of these disks - which are common in the Sun's stellar neighborhood - back up the report last week of the detection of two actual alien planets. It reinforces the theoretical conclusion that planetary systems probably are common in our galaxy. In one case, it even hints at a ''solar system'' like our own.
The disks are left over from the parent stars' formation. They are the raw material of asteroids, comets, and planets. Studying them helps astronomers get at the basic questions of how planetary systems form and what types of stars are likely to have them.
That's the case with the disk around Beta Pictoris - a star only 50 light years away in the constellation Pictor (the Painter's Easel).
Even after a decade of studying planetary nebulae, it still can startle astronomers, said Chris Burrows of the Space Telescope Science Institute in Baltimore at the meeting last week of the American Astronomical Society here.
Studying Hubble telescope images, Dr. Burrows said ''we were surprised'' to find that ''something is ... twisting the disk'' around Beta Pictoris. He calls this ''strong, though indirect, evidence for the existence of planets.'' If Beta Pictoris did have a solar-system-like suite of planets, he says, ''it would produce a warp like the one we see.''
Meanwhile, Harold M. Butner of the Carnegie Institution of Washington, D.C., presented a different kind of indirect evidence. Studies made with the NASA Infrared Telescope Facility on Mauna Kea in Hawaii, suggest the presence of comets around several stars. Dr. Butner noted that spectra of the infrared radiation (heat) from Beta Pictoris's disk have already indicated the presence of particles typical of comets.
Now Butner and his colleagues have extended that kind of analysis to six other stars like Beta Pictoris. Three of them show evidence of particles with features similar to those of particles known to be associated with comets.
This makes these and other similar star systems good places for astronomers to hunt for evidence of at least embryo planet formation if not full fledged planetary systems, Butler explains.
While infrared radiation is the best means currently available for studying the planet-forming disks, ultraviolet (UV) light can also turn up tantalizing clues.
Carol Grady and Mario Perez of Applied Research Corporation in Landover, Md. and Karen Bjorkman at the University of Wisconsin at Madison did a disk study with a UV telescope on board the Astro-2 space shuttle mission last March. They studied a young star designated HD 163296, 550 light years away in the constellation Sagittarius.
Here too, they found evidence of comet-like activity. Material moving in toward the star looks like material associated with a small body or comet. The astronomers suggest that at least 35 comets a year may cross the star's disk as seen from earth.
Meanwhile, Lee Mundy at the University of Maryland in College Park and William J. Welch at the University of California, Berkeley have gotten a sharper infrared image of the young star HL Tauri in the constellation Taurus. A thick dust lane obscures the star. They now have the first image of that dust using the Berkeley-Illinois-Maryland Millimeter Wavelength Array at the Hat Creek Observatory near Cassel, Calif.
The two astronomers say they are seeing material participating in the early life of the star. ''Although our observations cannot directly detect planets,'' they explain, ''such disks are the most promising sites for future planet formation and give us insight into the conditions during the formation of our Solar System.''