Kepler glitch may lower odds of finding Earth's twin

Does Earth have a twin in the galaxy? Our chances of finding an Earthlike planet any time soon may have just shrunk due to a glitch that could limit the lifetime of NASA's exoplanet-hunting spacecraft Kepler.

Since its launch in 2009, Kepler has glimpsed nearly 3000 potential exoplanets in a plethora of configurations around their host stars. This has revolutionised our understanding of the nature of solar systems and planets that lie beyond our own ? and spurred the search for alien life.

But on 16 July, Kepler sent down data showing that one of its four reaction wheels, whose spin controls the probe's orientation, had stopped turning. Ground controllers shut down the malfunctioning wheel and briefly stopped gathering scientific data.

Accurate orientation is critical to Kepler's success. The spacecraft monitors more than 100,000 stars for changes in brightness caused by transits ? dips in starlight caused by the passage of an orbiting planet in front of its star, as seen from Earth. The probe tries to keep a given star on the same few pixels of its light detectors for months on end. This avoids variations in the data since not all pixels respond in exactly the same way to light.

Precision pointing

"You have to correct the data to be able to find planets," says Sara Seager, an exoplanet researcher and Kepler mission participant at the Massachusetts Institute of Technology. "There could be a point where the data is too noisy to even correct. That's what would happen if you can't control the pointing very precisely."

Kepler only needs three wheels to function ? one to control the probe's motion along each axis ? and the probe resumed its observations on 20 July. "Kepler is functioning very well on three reaction wheels," says mission manager Roger Hunter of NASA's Ames Research Center in Moffett Field, California.

If the glitch can't be fixed, though, Kepler will be left without a backup wheel. "This is reducing the odds of making the mission go as long as we can," says Kepler chief scientist William Borucki of NASA Ames, who doubts that Kepler could point accurately enough to look for transiting planets if reduced to two reaction wheels. "It was a disappointing surprise to find this wheel stopped so early."

Extending the mission's lifetime is crucial for finding Earth-size exoplanets in the habitable zones around sun-like stars, he says. Like Earth, such planets should take about a year to circle their stars, and several orbits are necessary to confirm a planet's existence.

Longer observations

Even though Kepler is just a few months short of its planned 3.5-year lifetime, mission scientists have discovered that sun-like stars vary in brightness more than expected, which means they have to be observed for longer to filter out noise and pinpoint a planet's signal.

"We need to operate the mission for another four years to find Earth-size planets in the habitable zone of stars exactly like the sun," says Borucki.

Still, he points out, the mission has already turned up nearly 3000 candidate planets: 2300 have been reported so far, and another 500 will be announced this week. In addition, nearly a year's worth of observations have yet to be analysed.

"The mission has been a tremendous success, but there is still critically important work to be done by extending the mission," he says.

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