(Adapted from the December 13, 1997, issue of New Scientist. Posted by permission of New Scientist. This post also appears at http://www.ccnet.com/~galaxy")


by Ken Croswell, author of Planet Quest

Planet hunters can breathe a sigh of relief--it looks as if there really is a planet orbiting the star 51 Pegasi. Their delight comes at the expense of a rival theory which claimed that the planet was simply a mirage.

In 1995, Michel Mayor and Didier Queloz of Geneva Observatory reported that 51 Pegasi is wobbling from the gravitational pull of an orbiting planet. It was the first planet ever found around another Sunlike star.

Early this year, however, David Gray of the University of Western Ontario in Canada asserted that 51 Pegasi's planet did not exist. Instead, he said, the star is pulsating, which causes the periodic change in its spectrum that planet hunters had mistaken for a planet.

Other astronomers have now searched for the alleged pulsation but failed to find it. One team, led by Timothy Brown of the High Altitude Observatory in Boulder, Colorado, used the 60-inch telescope at Mount Hopkins in Arizona to obtain high-quality spectra of 51 Pegasi. If the star were pulsating, the shapes of its spectral lines should vary. But they remained constant. The results, which several other teams have confirmed, will appear in Astrophysical Journal Letters.

The planet's discoverers have welcomed the news. "We are very glad," says Mayor.

Gray was not rejoicing, however. "It's disappointing that it looks like we're not going to be able to analyse oscillations on this star," says Gray, who notes that such oscillations would have probed the star's interior. "On the other hand, the people who study planets will be delighted to have one of their planets back."

FEBRUARY 28, 1997


According to David Gray, an astronomer at the University of Western Ontario, the planet-sized companion to the star 51 Pegasi does not exist! This is the star that started the current rush of extrasolar- planet discoveries in 1995. Astronomers Michel Mayor and Didier Queloz then claimed that subtle shifts in the star's spectrum showed that the star was nodding back and forth relative to Earth, the effect of an object with at least half Jupiter's mass circling nearby every 4.2 days. But Gray, who is an expert on stellar spectroscopy, believes that 51 Pegasi is causing the effect itself. He found that the shape of an absorption line was tilting one way, then the other, in lock step with the putative planet's orbit. Orbital motions can't change the shape of a spectral line, only its position. "Therefore," Gray wrote in Nature for February 27th, "the planet hypothesis is no longer an adequate interpretation of the data."

Instead, says Gray, 51 Pegasi seems to undergo complex pulsations that somehow tilt the lines back and forth. However, planet-hunters are rising to 51 Peg's defense, arguing that the pulsations Gray envisions can't become strong enough to reproduce the observed behavior. The outcome of this debate will affect the viability of planets presumed to circle three other stars too.

Reply to Dr. Gray's paper.

University of Western Ontario
London, Ontario

February 26, 1997

Planet Theory 'Exploded' By New Research

A new planet, discovered and announced to much excitement two years ago, probably doesn't exist at all, indicates research by a University of Western Ontario astronomer.

In an article being published in the Feb. 27 issue of the international science journal Nature, astronomy professor David Gray reports that the likelihood of a planet being next to the 51st star in the Pegasus constellation is "vanishingly small."

The detection of the planet was first reported by researchers in Nature in late 1995. Although the researchers had not observed the planet directly, they based their conclusion on the periodic movements in the position of lines in the spectrum of light surrounding the star, 51 Pegasi (pronounced Peg-a-see). The movements, it was proposed, could have been caused by the presence of a nearby planet.

Gray's examination of the spectral lines around 51 Pegasi indicates that they are not only moving, but changing shape.

"While the presence of nearby planets can make spectral lines move, it is well established in astronomy that they cannot make them change shape," says Gray.

"This new research explodes the theory that there must be a planet near 51 Pegasi. A planet could not produce the phenomena that I observed and recorded around the star," he says.

"Spectral lines are known to change shape when a star pulsates or has spots on its surface. That may be all that's happening in this case," he says.

Since the original discovery in 1995, approximately 10 other planets outside our solar system have been reported using the same research and data collection techniques. Four of them are similar to 51 Pegasi.

"They may be hot telescope subjects in the next year," says Gray.

Gray's report in Nature was based on work conducted between 1989 and 1996 in the University's Elginfield Observatory near Lucan, Ont.

Although faint, the Pegasus constellation is visible to the human eye and is about the size of the Big Dipper.

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