By DENNIS OVERBYE
Published: June 10, 2013
ESO
A wide-field view of the sky around Alpha Centauri, 4.4 light-years from Earth, the star system closest to our own.
As Geoffrey W. Marcy, an astronomer at the University of California, Berkeley, said at the time, “This is close enough you can almost spit there.” Close enough, some astronomers said, to send a scientific probe that would get there in our lifetime. The new planet, only four million miles from its home star, would be too hellishly hot for life, but, astronomers said, where there is one planet there are likely to be others, more comfortably situated for life.
Now, however, in a shot across the bow of cosmological optimism, a new analysis of the European data has cast doubt on whether there is actually a “there” there at Alpha Centauri B.
Writing in The Astrophysical Journal last month, Artie P. Hatzes, the director of the Thuringian State Observatory in Tautenburg, Germany, who was not part of the original discovery team, reported that he could not confirm the planet when he went looking for it in the European data on his own. “Sometimes it is there, other times not,” depending on the method he used to reduce the statistical noise, he said in an e-mail.
That doesn’t mean the planet does not exist, Dr. Hatzes wrote, but “in my years of experience in extracting planet signals, this simply does not ‘smell’ like a real planet.”
Dr. Hatzes’s skepticism proved catching. Suzanne Aigrain of Oxford University quoted Carl Sagan’s dictum that extraordinary claims require extraordinary evidence, saying that Dr. Hatzes’s paper “certainly casts doubt on the original evidence.”
Xavier Dumusque of the University of Geneva, who led the original discovery effort, said that Dr. Hatzes’s challenge was healthy for science. “Calling to question a detection is always something fruitful,” Dr. Dumusque wrote in an e-mail. But he added that it was clear in his team’s paper that “the signal we are searching for is at the limit of the data precision.”
More data, everyone agrees, is essential, and luckily there will be more data, according to Debra Fischer, a Yale astronomer who has studied the Alpha Centauri system. Both her group and the Geneva team of which Dr. Dumusque is a member obtained more observations in May.
May was a bad time for exoplanet astronomers. NASA’s planet-hunting Kepler satellite lost its pointing capability.
Quoting Jeff Lebowski, the hero of the Coen brothers movie “The Big Lebowski,” Dr. Hatzes said, “Life is indeed ‘gutters and strikes,’ and this week were some gutters.”
The Alpha Centauri B planet is hardly the first promising world to slip into the shadows of uncertainty. Astronomers are still arguing about the existence of Gliese 581g, a “Goldilocks” planet said to be almost a sure bet for life when it was discovered in 2010.
It all goes to show just how devilish the details have become as astronomers close in on the goal of finding Earth-like planets.
Dr. Dumusque and his colleagues first found the planet by the so-called wobble method — perfected over the years by Michel Mayor and his colleagues at the Geneva Observatory — which measures planets’ masses by how much they tug their host star to and fro as they orbit it.
The group uses a specially built spectrograph called Harps on a 140-inch-diameter telescope at the European Southern Observatory in La Silla, Chile, to measure those wobbles, which show up as slight rhythmic shifts in the wavelength of starlight.
The Earth imparts a kick of about four inches a second to the Sun as it orbits around, but that is much smaller than the jitter caused by sunspots and magnetic activity.
The result is that success in detecting low-mass planets depends more and more on digging a small signal out of a much larger background of noise in a reliable manner.
Kepler, which uses the blink method to find planets when they cross in front of their stars, is running up against limits of time.
On the grounds that three blinks were needed to verify an orbit, Kepler’s astronomers once thought they would need three years to verify the existence of planets in comfortable orbits like our own.
But the stars turned out to be noisier than predicted; a year ago, Kepler’s mission was extended so that more blinks could be collected, but the failure of a reaction wheel that allows the telescope to point precisely has probably brought an end to that.
There is still an enormous amount of data in Kepler’s pipeline, including 132 confirmed planets and, as of last week, 3,216 planet candidates.
Waiting in the wings for a 2017 launching is the Transiting Exoplanet Survey Satellite, TESS, which will monitor about two million nearby stars for exoplanets. “In fact,” the project’s leader, George R. Ricker of the Massachusetts Institute of Technology, said during a talk at Google a few years ago, “when starships transporting colonists first depart the solar system, they may well be headed toward a TESS-discovered planet as their new home.”
It might even be in Alpha Centauri.
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