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Astronomer Stands By Discovery of Alien Planet Gliese 581g Amid Doubts

Despite the doubts raised recently over the existence of the potentially habitable alien world Gliese 581g, the planet's co-discoverer is standing behind his find.
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Despite the doubts raised recently over the existence of the potentially habitable alien world Gliese 581g, the planet's co-discoverer is standing behind his find.

Steven Vogt, leader of the team that detected Gliese 581g, said he respects the work of the researchers who questioned the planet's existence yesterday (Oct. 12). He said he cannot comment on the scientists' results, since he hasn't seen their data.

But he has confidence in his own team's conclusions.

"I stand by our data and analysis," Vogt, an astronomer at the University of California, Santa Cruz, said in an e-mail interview with "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. I feel confident that anyone independently analyzing this data set will come to the same conclusions."

Vogt added that he looks forward to reading the other team's results when they're published in a peer-reviewed journal. He's not necessarily expecting Gliese 581g to be yanked off the list of extrasolar planets, though.

"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 said. "We are doing our level best to keep it that way."

Questioning the claim

Vogt's team announced the discovery of Gliese 581g on Sept. 29. The planet, about 20 light-years from Earth, is the first rocky, roughly Earth-size alien planet found to orbit its star in the so-called "habitable zone" a just-right range that can allow liquid water to exist.

Since then, the discovery has received a lot of attention, from both the media and other researchers. One group of astronomers, led by Michel Mayor of the Geneva Observatory in Switzerland, performed a follow-up investigation in an attempt to confirm the existence of Gliese 581g. [ FAQ: 12 Questions (and Answers) on Planet Gliese 581g ]

At an astronomy conference this week in Torino, Italy, the Swiss team announced that it could not confirm Gliese 581g or 581f, another planet Vogt's team discovered in the same system. Though the researchers did not refute the existence of either planet, they did confirm the other four previously found around the star.

Vogt said he wasn't overly surprised to hear the news, since the two newfound planets' signals were quite weak.

Similar methods, different results

Both research teams used similar methods scrutinizing the parent star Gliese 581's movement, looking for the telltale gravitational tug of orbiting planets. And both teams analyzed some of the same data.

Vogt's team looked at 119 measurements made by the HARPS instrument on the La Silla telescope at the European Southern Observatory in Chile, as well as 142 measurements from the HIRES instrument on the Keck I telescope at Hawaii's Keck Observatory.

The Swiss team analyzed those same 119 HARPS measurements, and they looked at 60 additional HARPS observations as well. Vogt said those 60 new measurements could lead to a maximum potential detection sensitivity gain of only 23 percent or so, assuming all the new data points are useful and non-redundant.

The Swiss team didn't investigate any of the Keck data, however a fact Vogt finds puzzling, especially since his team concluded that both the HARPS and HIRES measurements had to be combined to reliably detect all six planets orbiting Gliese 581.

"As the Swiss group was given our data over a week ago now, I am also mystified why they have not already combined all the data together into a more complete analysis themselves," he said.

The Swiss team has not yet published its results in a peer-reviewed journal. Until that happens, it's hard to know what to make of the team's findings, Vogt said.

"As we have done, they must publish their data, analysis and conclusions in a peer-reviewed archival scientific journal for all the world and history to see," Vogt said. "Once they do, we will thoroughly analyze both the combined and individual data sets and extract what information they offer."