PLANET-HUNTING astronomers have crossed a watershed. Encouraged by detection of Jupiter-class planets orbiting sun-like stars, they are turning from mere hopeful exploration to the more demanding task of gaining a deeper understanding of how planetary systems form.
''We're at a turning point ... we're [actually] finding new worlds!'' exclaims William Borucki of the NASA/Ames Research Center in Mountain View, Calif. ''We're going from the discovery phase to trying to understand the science,'' explains William Cochran of the University of Texas at Austin.
Yet the challenge now is to find examples of more extensive planetary systems, says David Black, director of the Lunar and Planetary Institute in Houston. Scientists need analogs of our own solar system's suite of multiple planets to test their theories, he said. He expects the recent discoveries to spur on efforts to do this.
Such is the impact of Wednesday's report to a meeting of the American Astronomical Society that Geoffrey Marcy of San Francisco State University and Paul Butler of the University of California at Berkeley have detected two Jupiter-size planets that could support liquid water. They orbit the stars 70 Virginis in the constellation Virgo and 47 Ursae Majoris in the Big Dipper.
This gives astronomers three examples of such planetary systems to whet their scientific appetites. The third example - a Jupiter-class planet orbiting 51 Pegasi in the constellation Pegasus - was reported last October by Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland. Drs. Black, Borucki, and Cochran joined several other astronomers in briefing the press on new possibilities to add to this list.
So far, the best clue to a planet's presence has been its gravitational interaction with its parent star. This causes the star to wobble in a way that reveals the planet's size and orbit. That's the method Marcy, Butler, and the Swiss astronomers used. While this technique is being refined, other methods are becoming practical.
For example, Borucki explained that even Earth-size planets can be detected by the way they dim a star's light when they move in front of the star's disk. He and his colleagues are proposing a mission in the National Aeronautics and Space Administration (NASA) Discovery series of low-cost projects. They want to orbit a one-meter-diameter telescope that could monitor several thousand candidate stars for four to eight years. He said there is a good probability this would detect several dozen Earth-size planets.
As another example, Michael Werner of the Jet Propulsion Laboratory in Pasadena, Calif., explained that the European Space Agency's infrared telescope now on orbit and a comparable infrared telescope NASA plans to orbit in 2001 could make detailed studies of dust surrounding many stars. Such studies could indicate the presence of planet-forming material or even of planets themselves.
BRYAN BUTLER of the National Science Foundation's National Radio Astronomy Observatory (NRAO) described yet another possibility using radio telescopes. He explained that NRAO plans to build an array of 40 radio telescopes, each 8 meters in diameter. Operated in concert, their signals would be combined to enable them to detect finer detail than can any existing optical or infrared telescope. The array would be able to image planetary systems in the earliest stages of their formation. An enhanced version of the design could directly detect very young giant planets in the nearest star-forming regions.
The National Science Foundation provided initial design funding for such an instrument last year. NRAO now is looking at two possible sites. One is 16,500 feet high in northern Chile. The other is on Hawaii's Mauna Kea mountain where other observatories already are located.
Meanwhile, planned upgrades to the agency's existing 27-antenna Very Large Array radio telescope near Socorro, N.M., promise also to advance planet hunting. New receivers would allow the array to study the inner parts of disks of dust and gas surrounding young stars. This could provide insights into early stages of planet formation.