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To Seek Out New Worlds--From Scratch?

26 February 1997 7:30 pm
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Over the past 15 months, astronomers have spotted hints of perhaps a dozen planets orbiting other sunlike stars, making them seem almost commonplace. But just in case anyone was getting complacent, a respected Canadian astronomer writing in today's issue of Nature labels the first and most dramatic of these discoveries a case of mistaken identity. David Gray of the University of Western Ontario says a subtle wobble in the spectrum of the star 51 Pegasi, originally thought to be due to the gravitational tug of a roughly Jupiter-sized planet, actually reflects a complex sloshing on the star's surface. "The planetary hypothesis simply can't explain the observations," says Gray. "It's gone. Period."

In a riposte on the World Wide Web, the planet's original discoverers, Michel Mayor and Didier Queloz of the Geneva Observatory, along with Geoff Marcy and Paul Butler of San Francisco State University, call that claim "extraordinarily premature," pointing out that Gray has analyzed just 39 measurements of the star spread over about 7 years. But other astronomers think it deserves a hearing. "It raises an alarm bell of sorts," says Aleksander Wolszczan of Pennsylvania State University.

The dispute centers on dark absorption lines in the spectrum of 51 Pegasi--valleys in its spectrum that result as elements like nitrogen, iron, and calcium in the star's atmosphere soak up light coming from below. The planet searchers had monitored hundreds of these lines and found a minute frequency shift due to the Doppler effect--the same effect that shifts the pitch of a passing siren--implying that the star was wobbling toward and away from Earth every 4.23 days as a planet tugged it to and fro. When Gray monitored just one iron line at much higher resolution, however, he found that it seemed to change its shape and its depth over the same 4.23-day period. The wobble due to a planet should shift the wavelength of the line without altering its appearance; only the star itself could be responsible for changes in the shape of the line, says Gray.

Few other astronomers are ready to accept Gray's explanation for these changes: slow, persistent upheavals of the star's surface roughly analogous to ocean swells. Such oscillations have never been seen on the sun or other stars, says Timothy Brown, who studies stellar pulsations at the High-Altitude Observatory of the National Center for Atmospheric Research in Boulder, Colorado. Instead, astronomers are searching for other explanations--including some that involve a planet after all. Says Queloz, "I don't know who is right and who is wrong. This is just science. You try to do your best."

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