Two of the world's most accomplished teams of exoplanet hunters are at odds over one group's claimed detection of a world billed as the most habitable planet yet discovered outside the solar system.

In question is the existence of Gliese 581g, which a group of American researchers announced in a September 29 teleconference that they had detected orbiting a small, dim star 20 light-years away. The planetary system around the star Gliese 581 was already known to harbor four planets; Steven Vogt of the University of California, Santa Cruz, and Paul Butler of the Carnegie Institution of Washington announced that they had inferred the presence of two more by the wobble of the host star as the gravitational pull of orbiting planets tugged it about.

The U.S. group used 122 of their own measurements of the star Gliese 581, taken at the 10-meter Keck I telescope in Hawaii, as well as 119 publicly released measurements obtained by a competing group using the European Southern Observatory's 3.6-meter telescope at La Silla Observatory in Chile. Together the data implied the presence of six planets in total, with the new worlds assigned by convention the names Gliese 581f and Gliese 581g. The latter planet made quite a splash; at just a few times the mass of Earth and with an orbit inside the star's temperate, so-called Goldilocks zone, liquid water and—just possibly life—could potentially exist there.

But at an International Astronomical Union symposium on planetary systems being held this week in Turin, Italy, a member of the European team said that the Goldilocks planet did not appear in a larger, updated data set from La Silla. According to Science, the European data now include 180 observations of Gliese 581. "We do not see any evidence for a fifth planet," Francesco Pepe of the Geneva Observatory said in an e-mail to Science, adding that the precision of the measurements was insufficient to rule out its presence. In e-mail correspondence with Astrobiology Magazine, Pepe noted that his group could not confirm the presence of the other newfound planet, Gliese 581f, either.

Both Butler and Vogt have told reporters that they cannot comment on their competitors' results, because the new data have not been published. But the impending stand-off pits two of the field's most renowned research groups against one another. Pepe's collaborators Michel Mayor and Didier Queloz in 1995 became the first astronomers to find an exoplanet orbiting a sunlike star, a discovery that triggered an avalanche of research that has in the intervening years produced nearly 500 new planetary discoveries.

Butler, Vogt and their colleagues are no slouches, either, having contributed heavily to that total. "In 15 years of exoplanet hunting, with over hundreds of planets detected by our team, we have yet to publish a single false claim, retraction or erratum," Vogt wrote in an e-mail to He told the Web site that he stands by the data and analysis that led to the proposed existence of Gliese 581g: "I feel confident that we have accurately and honestly reported our uncertainties and done a thorough and responsible job extracting what information this data set has to offer."

Schematic showing the proportions of the Gliese 581 system in relation to the solar system: Zina Deretsky/NSF