Things don’t look good for Gliese 581g, the first planet found orbiting in the habitable zone of another star. The first official challenge to the small, hospitable world looks in the exact same data — and finds no significant sign of the planet.
“For the time being, the world does not have data that’s good enough to claim the planet,” said astro-statistics expert Philip Gregoryof the University of British Columbia, author of the new study.
The “first habitable exoplanet” already has a checkered history. When it was announced last September, Gliese 581g was heralded as the first known planet that could harbor alien life. The planet orbits its dim parent star once every 36.6 days, placing it smack in the middle of the star’s habitable zone, the not-too-hot, not-too-cold region where liquid water could be stable.
Planet G was the sixth planet found circling Gliese 581, a red dwarf star 20 light-years from Earth. A team of astronomers from the Geneva Observatory in Switzerland found the first four planets using the HARPS spectrograph on a telescope in Chile. The team carefully measured the star’s subtle wobbles as the planets tugged it back and forth.
Two more planets, including the supposedly habitable 581g, appeared when astronomers Steve Vogt of the University of California, Santa Cruz and Paul Butler of the Carnegie Institution of Washington added data from the HIRES spectrograph on the Keck Telescope in Hawaii. They announced their discovery Sept. 29.
Just two weeks later, the HARPS team announced they found no trace of the planet in their data, even when they added two more years’ worth of observations. But it was still possible that the planet was only visible using both sets of data.
Now, the first re-analysis of the combined data from both telescopes is out, and the planet is still missing.
“I don’t find anything,” Gregory said. “My analysis does not want to lock on to anything around 36 days. I find there’s just no feature there.”
Unlike earlier studies, Gregory used a branch of statistics called Bayesian analysis. Classical methods are narrow, testing only a single hypothesis, but Bayesian methods can evaluate a whole set of scenarios and figure out which is the most likely.
Gregory wrote a program that analyzed the likelihood that a given planetary configuration would produce the observed astronomical data, then ran it for various possible configurations.
For the HARPS data set, he found that the best solution was a star with five planets, which orbit the star once every three, five, 13, 67 and 400 days. The 36-day habitable world wasn’t there.
When he looked at the HIRES and the combined data sets, the best solution was a star with two planets. Only when he included an extra term in the HIRES data did Gregory find more, which he suspects means the HIRES instrument isn’t as accurate as thought.
“There may be something in the telescope…that’s contributing to the error,” he said.
Gregory’s model finds the probability that the six-planet model is a false alarm is 99.9978 percent. None of the planets Gregory’s analysis turned up are in the habitable zone. The results are in a paper submitted to the Monthly Notices of the Royal Astronomical Society and published on the physics preprint website arxiv.org.
Other astronomers seem impressed with Gregory’s analysis.
“That’s the right way to do it,” said exoplanet expert Daniel Fabrycky of the University of California, Santa Cruz. “I think everyone would agree that that is the most sophisticated analysis that you can do, and as much as you could hope to do.”
“The Gregory paper is by far the most complete statistical analysis to date that has been made public,” said exoplanet and astro-statistics expert Eric Ford of the University of Florida. “It’s by far the most rigorous analysis.”
But most astronomers are not yet ready to close the book on Gliese 581g.
“I’m not going to admit that it’s a dead planet yet,” said exoplanet expert Sara Seager of MIT. “No one will be able to sort this out today … it will take some time.”
Vogt still firmly believes the planet is there. “I’m standing by our data,” he told Wired.com.
He said there are two ways to interpret the signals from Gliese 581. Sometimes a single planet with an elongated, or elliptical orbit can look the same as two planets that trace perfect circles around their stars. One of Gliese 581’s planets, planet D, could be one of these “eccentric impostors,” hiding an extra planet within its signal.
Part of the reason it’s so difficult to tell these two scenarios apart is that spotty observations make fake signals in the data. These signals, which show up because the telescope can’t watch the star continuously, look like they could actually be planets, but they would disappear if we could observe round the clock.
In a paper that’s still in preparation, astronomer Guillem Anglada-Escudé and Harvard graduate student Rebekah Dawson tackle these issues, and conclude that the habitable planet still has a chance. “With the data we have, the most likely explanation is that this planet is still there,” Anglada-Escudé said.
Everyone agrees that the problem can only be resolved with more data. In particular, astronomers are anxious to see the extra data that the HARPS group used to conclude Gliese 581g is a mirage.
“I don’t think anything will change significantly until the Swiss publish their data,” Anglada-Escudé said. “Nobody else has seen their data. We’re waiting to see that, just to settle down the problem.”
By Lisa Grossman