The first world ever found beyond the solar system is but an illusion, a Canadian astronomer contends.

His colleagues are just fooling themselves if they think they've found a planet about half the size of Jupiter orbiting the distant star 51 Pegasi, David Gray charges in a paper to be published Thursday. When its discovery was announced 16 months ago, the so-called planet was thought to be the first ever found orbiting a star other than the sun.

What astronomers really see when they look at 51 Pegasi, Gray concludes, are regular pulsations of a lone star that mimic the effect of an orbiting planet. "Something else is going on, and it's no longer reasonable to talk about a planet pulling the star around," Gray, a professor of astronomy at the University of Western Ontario in London, said in a telephone interview.

Since the October 1995 announcement, astronomers have found evidence of planets orbiting about 10 sunlike stars. The planet discoveries have ushered in a new astronomical era, with researchers conjuring images of distant worlds similar to Earth's own solar system, perhaps even capable of supporting life. NASA Administrator Daniel Goldin has made it one of his agency's primary goals to snap a picture of a planet light-years away and collect basic information about its composition.

Gray's research doesn't threaten all of that - just some of it. Gray directly questions the existence of only one of the new planets - the one thought to orbit 51 Pegasi - and casts suspicion on two or three like it. Nevertheless, his suggestion that the planet that got things rolling really doesn't exist has pulled planet hunters down from their mountaintop observatories and onto their favorite battleground - cyberspace.

"The conclusion claimed by Dr. David Gray is extraordinarily premature," Swiss astronomers Michel Mayor and Didier Queloz, the discoverers of the 51 Pegasi planet, wrote on a Web page maintained by the University of Geneva astronomy department.

The issue is so contentious partly because astronomers can't really see planets orbiting distant stars. They use indirect methods, carefully gauging changes in the star's light, to suggest the presence of planets.

Lots of things can cause starlight to change slightly, but what planet hunters look for is an oscillation caused by a star's wobbling motion. A slight wobbling indicates that the gravitational pull of some invisible planet is tugging the star around.

So far astronomers have detected light variations that look like such wobbling in many distant stars. They've concluded there's a planet responsible in a few cases. Not so fast, says Gray, at least in the case of 51 Pegasi.

"Although at this stage, the cause of the ... variations in 51 Pegasi are not fully understood, the chance of their being caused by a planet is vanishingly small," Gray writes in Thursday's issue of the British journal Nature.

He observed the 51 Pegasi 39 times between 1989 and 1996, noting each time how a very precise color of light was shifted away from its normal value. The pattern of shifts he collected, more detailed than any other observations of the star, rules out a planet, Gray concludes. He argues that subtle features he detected could be caused only by light variations in the star itself.

If that's true, said Alan Boss of the Carnegie Institution in Washington then 51 Pegasi must behave very unusually for a star so similar to the sun.

Which makes Boss and many other astronomers suspicious. "The effect he's finding is just sort of barely there," Boss said. "I don't think it's quite time to declare 51 Pegasi dead as a planet."

The debate may be settled in the fall, when two astronomers from the University of Texas at Austin plan to check on the observations of 51 Pegasi. The star can't be observed until then, because it is in the sky during the day in spring and summer.

"It really needs to be confirmed," Texas scientist Artie Hatzes said of Gray's research. "It's potentially a very important discovery."