After centuries of speculation and decades of tantalizing hints, two research teams believe they have found one of astronomy's ''holy grails'' - a planet circling another normal star.
A stream of discoveries over the past decade suggests that planetary systems are common in our galaxy. Disks of dust and gas that have been found around many stars are especially intriguing. But there has been no definitive observation. Now Paul Butler of the University of California at Berkeley says he finds the latest discovery so persuasive that ''it takes us out of fantasy and puts us into reality.''
It would be premature celebrate. The discovery is too new and there's still too much skepticism for that. Nevertheless, this does appear to be a historic observation.
On Oct. 6, Michel Mayor and Dicher Queloz of Switzerland's Geneva Observatory told a meeting in Florence, Italy, that they had found a planet around 51 Pegasus. That's a star similar in mass to our sun and 40 light years away. Last week, UC Berkeley announced that visiting scholar Geoffrey Marcy of San Francisco State University and Dr. Butler confirmed the Swiss findings. As the announcement put it: ''This is the first time a claimed discovery of a planet around a normal star has withstood scrutiny.''
That's the good news. The bad news is that it's a mighty peculiar planet. It appears to be about half as massive as Jupiter or perhaps a bit heavier. And, it's so close to the star it is within the star's hot corona (outer atmosphere) and whips around its orbit once every 4.2 earth days.
How a large planet could wind up in a stable orbit so close to its star is a mystery. Theories of planet formation never predicted this. It sends theorists back to their computers to find explanations that could revolutionize their understanding of planetary system evolution. That makes the theorists so uncomfortable they are firing off a barrage of electronic-mail messages spelling out their skepticism. But so far, Geoffrey Marcy says, none of the objections holds up to detailed scrutiny. He adds that he is convinced that ''this thing is just nailed down.''
It's not simply a matter of peeking through a telescope. The planet is too faint to be seen. Instead, both the Swiss and the Berkeley astronomers looked for the influence of the planet on the star. The gravitational interaction makes the star move backward and forward as seen from Earth. Astronomers can measure such motion precisely. What Marcy and Butler find is that the star's motion exactly fits the pattern theory says it should if a planet is present.
This is a bittersweet triumph for the Berkeley team. Dr. Marcy explains that they had been working on planet detection when the Swiss team scooped them. So he and Butler ''had a vested interest'' in proving the discovery wrong. ''To our chagrin,'' Marcy adds, they confirmed the discovery.
This discovery has major implications. Astronomers have found over 100 stars with the kind of encircling disks from which planets might evolve. Discovery of an actual planet strengthens the case for suspecting that the abundance of these disks implies an abundance of planetary systems. The new planet could reveal the existence of other planets in this star system. Their gravitational influence would show up in the new planet's motion. Butler says he expects to ''have something to say'' about this by the end of the year.
Astronomers have been expecting discovery of planets in other solar systems at any time. The technology is rapidly becoming available. Moreover, this search now has high priority in the National Aeronautics and Space Administration's science program.
If this discovery is as solid as Marcy and Butler say it is, it could mark the beginning of a flood of such discoveries over the next few years.