A presumed planet orbiting another star has been determined not to exist, possibly calling into question a small number of other discoveries of planets around other stars.
The star, called HD 192263, is about 65 light-years from Earth. It's previously presumed planet, a gas giant something like Jupiter, was detected in 1999 independently by two teams, one based in the United States and the other in Switzerland.
Astronomers find extrasolar planets by indirect means, detecting the wobble of a star as it moves slightly to-and-fro as a result of the gravitational effects of its orbiting planet. But in the case of HD 192263, dark spots on the star almost surely fooled both research teams, according to astronomer Greg Henry at Tennessee State University.
Henry has previously worked with the U.S. team, led by Geoffrey Marcy of the University of California, Berkeley, to help confirm or refute the existence of planets the team detects.
New observations from Arizona observatories show variations in brightness of the star that correspond strongly to spots crossing the star's surface as it rotates. The spots are the equivalent of sunspots, dark and cooler regions that generate less light. The brightness varies with the same timing as the previously perceived changes in velocity of the star. Those velocity changes were used to make the wobble method discovery, which operates on the Doppler principle, which is identical to the changes in sound as an ambulance approaches you and then speeds away.
Henry reports his new observations in the Aug. 28 issue of the Astrophysical Journal Letters.
"HD 192263 should be removed from lists of stars with well-established planetary companions unless further observations and analysis can support the existence of the planet in spite of the star's intrinsic variations," the report concludes.
Planet not ruled out
"I'm very confident of these particular results, although there are some additional definitive studies that can and probably need to be done to absolutely clinch it," Henry said in a telephone interview.
That additional work would involve new wobble-method observations. The supposed planet orbited the star every 24 days. The presumed star spot rotated across the star every 24 days, too, and Henry assumes this is the rotation period for the star itself.
By comparing the initial wobble observations with new ones, researchers could fill in the intervening time and see if the orbit cycle is continuous. If not, then they would be sure that the two observations were created by different phenomena. This would nearly prove that one star spot caused the first set of observations and a different star spot caused the second set. As with our Sun, spots dissipate over time and new ones are continually generated.
Henry said both the U.S. team and the Swiss group considered the possible star spots when making their original announcements, and each contacted him at the time. But he had no data then to disprove their findings.
Robert Donahue and Sallie Baliunas of the Harvard-Smithsonian Center for Astrophysics participated in the new finding.
Other possible reversals
Some 100 planets have been detected around others stars, but not all of them have been confirmed to exist. Henry's reversal could mean a small percentage of other presumed planets will turn out to be star spots.
Henry and his colleagues say about 95 percent of planet discoveries are on sound footing, however. Many of the stars they orbit are older than HD 192263 and are not expected to have significant spots that would fool astronomers.
"It is possible, but not sure of course, that a few other planet candidates will disappear in a similar way," said Jean Schneider of the Paris Observatory, who tracks extrasolar planet discoveries. Schneider and other astronomers have been aware of Henry's doubts since May.
Schneider told SPACE.com that the reversal is "reasonably conclusive." Other astronomers involved in the original discovery did not dispute the new results.
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