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Best Evidence Yet Points to Other Planetary Systems

June 19, 1987|LEE DYE | Times Science Writer

VANCOUVER, Canada — Scientists Thursday revealed what they described as the best evidence yet that other stars have planets--a major breakthrough in the search for life elsewhere in the universe.

No existing telescope can actually "see" such planets because they would be too dim compared to the far brighter star they orbit. But Canadian astronomers say they have done the next best thing by showing that some stars appear to be wobbling just as they would if they were being pulled back and forth by the gravity of orbiting planets.

The results of the six-year study, announced here during a joint meeting of the Canadian and American astronomical societies, are particularly significant because two--and possibly as many as seven--of the 16 stars studied appear to have planetary systems.

"This is the best evidence to date for planetary systems," Bruce Campbell of the Dominion Astrophysical Observatory here said shortly after revealing his data to several hundred other astronomers. He added, however, that it will take many more years of research to be sure of the finding.

Frank Drake of the University of California at Santa Cruz, a pioneer in the search for extraterrestrial intelligence, termed Campbell's work "tantalizing," but added that it is "inconclusive." Drake has been disappointed in the past by claims from other scientists who thought they had discovered other planetary systems.

But many scientists attending the session said they were impressed by Campbell's conclusions, partly because the astronomer is highly regarded for the quality of his work, and partly because the findings result from six years of study in Hawaii using a system believed to be far more precise in measuring the movements of stars than previous technologies.

The research is based on the fundamental physics of orbital mechanics, as demonstrated in our own solar system.

The planets in this system do not just orbit around the sun. Instead, the planets and the sun orbit around the center of their collective mass. If it were possible to travel far beyond Pluto and look back at the solar system through precise instruments, the sun would appear to move back and forth slightly as it is tugged one way and then the other by the planets, particularly Jupiter, the most massive.

The sun also would appear to move toward the viewer and then away as it revolved in its tiny orbit. That movement should show up through the Doppler effect--the shrinking and stretching of light waves--as the sun approached and retreated.

Scientists have long sought a way to use their telescopes to measure that Doppler effect in other stars, as viewed from the Earth, and thus tell how much the star is moving. But their instruments have not been accurate enough. The margin of error has been greater than the expected movement.

But by using a new system at the 3.6-meter Canada-France-Hawaii Telescope on Mauna Kea, Campbell and his colleagues--Gordon Walker and Stephenson Yang of the University of British Columbia--have greatly increased the accuracy of that measurement, the astronomers said.

The system uses hydrogen fluoride gas to create a grid to calibrate the instrument for the Doppler effect, a little like using cross-hairs to aim a gun.

"The older techniques are just not very accurate," Campbell said, yielding an error factor so large that a star could be moving toward the Earth at more than 3,000 m.p.h. and yet that movement could escape detection.

The new system is so accurate that if the star were approaching the Earth at 25 m.p.h., that movement could be measured, Campbell said.

When they studied the 16 stars, selected because they appear to be much like the sun, the astronomers found that two of them showed orbital movement when plotted over the six-year period. The stars are Epsilon Eridani, which is about 11 light-years away, and Cephei, which is 50 light-years away. (A light-year is the distance light travels in one year, or about 6 trillion miles.)

Five other stars also showed some sign of movement, but need further study, Campbell said.

The results are still not conclusive because it will take observations over many years to determine if the stars continue to move in an orbital pattern.

However, the findings constitute a critical chapter in the expanding search for other planetary systems.

Several years ago, astronomers thought they had detected similar movements of a star, leading some to conclude that the first planetary system had been found. But other scientists could not duplicate the results, and subsequent examination of the original data has shown that it was in error, according to Steve Maran, spokesman for the American Astronomical Society.

Last January, during the society's annual meeting in Pasadena, scientists presented evidence that dust clouds around some stars have formed into rotating disks, suggesting the early stages of planetary formation.

Now Campbell and his colleagues have taken it a major step forward with evidence of two planetary systems, and possibly more.

And until someone comes up with a means of photographing dim planets near the blinding light of distant stars, the evidence inferred from changes in velocities of stars may be the best evidence available, several astronomers said.

Drake said that, to be conclusive, the movement of the stars will have to be measured long enough for the planets--if that is indeed what they are--to complete six or seven orbits.

"That could take a hundred years," he said.

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