Astronomers set to home in on far-off planets

Astronomers hunting other planetary systems are beginning to close in on their quarry. Up to now this has been a frustrating task, using the inadequate technology that has been available. But planet-hunters should soon be getting tools that can do the job.

David Black of the Ames Research Center of the National Aeronautics and Space Administration outlined the new prospects at the recent annual meeting of the American Association for the Advancement of Science. Some examples:

* NASA is studying an infrared (IR) telescope, to be orbited by a space shuttle, which should be able to pick out planets around nearby stars by the IR (heat) radiation they emit.

* Ground-based equipment should be available within the next few years which can begin to look for planets by the effect their gravity has on the motion of their parent star as the latter moves through space.

* Using this and other equipment under study, the prospects for finding planets have progressed from being virtually impossible to being ''very, very good.''

* A new era in solar system study is about to begin as well. For the first time, astronomers will be able to test their theories of how the solar system formed against other examples of planetary systems.

Dr. Black explained that ''it is not a trivial task . . .to try to find evidence for something as insignificant as a planet around a distant star.'' Up to now, no telescope has been able to see one directly. Starlight reflected by the planet is swamped by the light coming directly from the star itself. Jupiter , for instance, is roughly a billion times dimmer than the sun.

The situation is better when searching for planets by the IR they emit. At IR wavelengths, the sun is only about 10,000 times brighter than Jupiter. Even this has been too much of a contrast between star and planet to be detected by equipment so far available. But the new telescope design studied at NASA-Ames should be sensitive enough to do the job, according to Black.

Called the large deployable reflector, it would have a reflector - an IR mirror - 15 to 30 meters (roughly 50 to 100 feet) in diameter. Placed in orbit by shuttle, this system, especially in the 30-meter class, should be able to detect planets around nearby stars, if they exist.

Planets should also be detectable by the way they interact gravitationally with their stars. While a planet is spoken of as orbiting its star, actually both star and planet orbit their common center of mass. This cyclical motion of the star shows up as a slight reddening of its light when it moves toward Earth (red shift) and by a slight bluish tinge to its light when the star moves away from Earth (blue shift).

Such red and blue shifts would be too faint to have been detected until now. But Black said that major technical improvements should enable astronomers to begin to make meaningful surveys in another year or two using this technique.

Having a company of planets can also make a star appear to wobble as it moves through space. There have been some claims that such wobble was detected in the past. But Black said that so far ''we have no unambiguous evidence for the existence of a single other planetary system.''

Here again, it now seems possible to build equipment that should be quite capable of detecting any wobble for a number of relatively nearby stars. A Lockheed Aircraft Corporation study shows it should be feasible to put a specialized telescope in orbit which would have a resolution for this kind of measurement a thousand times as sensitive as has been possible before.

The telescope should be able to measure angular displacements as small as a millionth of a second of arc. That is equivalent to measuring the thickness of a sheet of paper seen edge on from several thousand miles away.

Once other planetary systems begin to be found (or if a thorough search finds none of them), astronomers will have a touchstone for their theories of solar system and star formation.

Black explained that currently stars are thought to form when a swirling cloud of dust and gas collapses. This is thought to leave a disk of debris about the young star from which planets form. In fact, he said, ''We really don't know how a star is formed.'' The theory has never been tested.

If it is correct, this theory predicts that planetary systems should be common. However, if a thorough search shows them to be rare, Black said, ''we will have to very strongly rethink our theories of star formation.''