Researchers led by the Observatory of Geneva in Switzerland had already discovered four planets circling the star Gliese 581 by observing subtle motions of the star that are induced by the gravitational tugs of any orbiting planets when an American team announced a fifth, believed large enough to hold an atmosphere and the right distance from its star to allow for liquid water, ScienceMag.org reported.
But the original Swiss team says it can find no reliable evidence of a fifth planet in Gliese 582's habitable zone.
They used an expanded data set of 180 measurements made over six years, more than they utilized for the determination of the four planets they say they found.
"We do not see any evidence for a fifth planet," astronomer Francesco Pepe of the Geneva Observatory said.
On the other hand, "we can't prove there is no fifth planet," he added.
No one yet has the required precision in their observations to prove the presence or absence of such a small exoplanet, he said.
Astronomer Paul Butler of the Carnegie Institution for Science in Washington, a member of the U.S. team announcing the fifth planet, says more observations will likely be needed to solidify the existence of Gliese 581g.
"I would expect that on the time scale of a year or two this should be settled."