Noise confounds NASA mission to find an Earth twin.
Kepler, NASA's mission to search for planets around other stars, will not be able to spot potentially habitable Earth-sized planets until 2011, according to the mission's team. The delays are caused by noisy amplifiers in the telescope's electronics. The team is racing to fix the issue by changing the way data from the telescope is processed, but the delay could mean that ground-based observers now have the upper hand in the race to be the first to spot an Earth twin.
"We're not going to be able to find Earth-size planets in the habitable zone — or it's going to be very difficult — until that work gets done," says Kepler principal investigator William Borucki, who revealed the problem on Thursday to the NASA advisory council at a meeting at Ames Research Center in Moffett Field, California.
Kepler, which launched on 6 March, is staring at 100,000 stars in a specific patch of sky. The telescope is designed to look for the slight dimming of light that occurs when a planet transits, or crosses in front of a star.
The problem is caused by amplifiers that boost the signals from the charge-coupled devices that form the heart of the 0.95-metre telescope's 95-million-pixel photometer, which detects the light emitted from the distant stars. Three of the amplifiers are creating noise that compromises Kepler's view. The noise affects only a small portion of the data, Borucki says, but the team has to fix the software — it would be "too cumbersome" to remove the bad data manually — so that it accounts for the noise automatically. He says that the fix should be in place by 2011.
The noisy amplifiers were noticed during ground testing before the device was launched. "Everybody knew and worried about this," says instrument scientist Doug Caldwell. But in the end, he says, the team thought it was riskier to pry apart the telescope's electronic guts than to deal with the problem after launch.
Borucki points out that the team was probably going to have to wait at least three years to find an extrasolar Earth orbiting in the habitable zone anyway. Astronomers typically wait for at least three transits before they confirm a planet's existence; for an Earth-sized planet orbiting at a distance similar to that between the Earth and the Sun, three transits would take three years. But Borucki says that the noise will hinder searches for a rarer scenario: Earth-sized planets that orbit more quickly around dimmer, cooler stars — where the habitable zone is closer in. These planets could transit every few months.
Kepler, and the Convection, Rotation and Planetary Transits (COROT) mission, a French satellite that also looks for transits, are in a fierce race with ground-based telescopes to spot Earth-like planets. Whereas Kepler and COROT rely on transits to determine a planet's size, the ground-based telescopes identify planets by their mass. They look for tiny wobbles in the motion of the parent stars caused by the planets' gravity, a technique known as 'radial velocity' measurement. Greg Laughlin, an astronomer based at the University of California at Santa Cruz, says that the delay for Kepler makes it "more likely that the first Earth-mass planet is going to go to the radial-velocity observers".
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Hand, E. Planet hunt delayed. Nature (2009). https://doi.org/10.1038/news.2009.1051