To date, astronomers have announced their "discovery" of more than 100 planets orbiting nearby stars. But do all these planets really exist?

Not always, according to studies performed at Mount Wilson Observatory's 100-inch telescope and announced in the October 1 issue of the Astrophysical Journal.

A research team consisting of Gregory W. Henry of Tennessee State University and Sallie L. Baliunas and Robert A. Donahue of the Harvard-Smithsonian Center for Astrophysics has shown that a "planet" orbiting the star HD 192263, which had been announced by California and Swiss researchers in 1999, actually does not exist.

The reason why this planet could be illusory is that in all these "discoveries" you never actually see the planet. The evidence for it is indirect; you detect a rhythmic shift in the parent star's spectrum from blue to red and back again, as the star moves toward and away from us in response to the pull of the orbiting planet's gravitational force. But for stars like the Sun, with large "starspots," the same rhythmic spectral shifts can also be caused by the star's rotation.

When a large spot moves across the star's face as the star rotates, it also causes a rhythmic shift in the star's spectrum from red to blue and back again, mimicking the rhythmic shift in the spectrum that would be caused by a planet's gravitational pull, without any planet being present. Thus the planet searcher may believe he or she has found a planet when actually none exists.

Baliunas, who serves as Deputy Director of the Mount Wilson Institute, explained that data on sunspot-like activity cycles of HD 192263, gathered at Mount Wilson as part of a long-term program to observe sunspot-like activity on distant stars, were one of the key factors in proving that the rhythmic changes observed in this star's spectrum were indeed caused by the star's rotation and not by an orbiting planet.

This is not the first time Mount Wilson's starspot data has been used to disprove an extrasolar planet's existence. In 1998, Baliunas' team showed that the evidence for a supposed planet around the star HD 166435 was really produced by the star's rotation, and the planet did not exist.

The observing program at Mount Wilson Observatory that obtained the starspot data for HD 192263 (called the "HK Project" because it measures the intensity variations in the calcium H and K lines in the stellar spectrum) was inaugurated by Mount Wilson astronomer Olin C. Wilson in 1966. It has monitored the starspot activity cycles of more than 100 Sun-like stars continuously for more than 34 years. This makes the HK Project one of the longest-running continuous observational programs in the history of astronomy.