See full article: Nature 477, 142-143 (2011) | doi:10.1038/477142a

Excerpt: The Kepler spacecraft has hit an unexpected obstacle as it patiently watches the heavens for exoplanets: too many rowdy young stars. ... an analysis of some 2,500 of the tens of thousands of Sun-like stars detected in Kepler's field of view has found that the stars themselves flicker more than predicted, with the largest number varying twice as much as the Sun. That makes it harder to detect Earth-sized bodies.

As a result, the analysis suggests that Kepler will need more than double its planned mission life of three-and-a-half years to achieve its main goal of determining how common Earth-like planets are in the Milky Way. ... "I'm concerned that programmes like Kepler might be squeezed out ..." says Kepler's chief data analyst, Jon Jenkins at the SETI Institute in Mountain View, California. "We need an eight-year mission to reach our goal of understanding whether there are other worlds out there like Earth." ... Earlier this year, Kepler scientists announced that the craft had discovered 1,235 candidate exoplanets, 68 of which are Earth-sized. Fifty-four of the total number of planets detected, all of them larger than Earth, orbit within the habitable zone of their parent stars, a region temperate enough for water to remain liquid on the surface of a rocky body….

In an article … currently in press in The Astrophysical Journal, Kepler scientist Ron Gilliland of the Space Telescope Science Institute in Baltimore, Maryland, and his collaborators describe Kepler's noise dilemma (R. L. Gilliland et al. preprint at http://arxiv.org/abs/1107.5207; 2011)….

Before Kepler's launch in early 2009, the team had assumed that the Sun-like stars viewed by the craft would be about as quiet as the Sun, with luminosity fluctuating in a range near 10 parts per million (a 0.001% variation) over the key timescale of about 6.5 hours. But Gilliland and his colleagues found that the noise in the Kepler data is much larger. … whatever the source of the stellar noise, Gilliland says that Kepler will have to detect twice as many transits of Earth-like analogues than planned — an average of six passages per planet rather than three, at intervals of roughly a year — to be sure that a dip in starlight is a true signal of an Earth-sized body. This means that the probe will need a full eight years to attain its original goal of finding about 75% of Earth analogues among its target stars.
"We need an extended mission because the detection of Earth-sized planets hangs in the balance," says Geoff Marcy an astronomer at the University of California, Berkeley, and a member of the Kepler team….

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