After launch, however, scientists discovered that most stars are more variable than the Sun (see Nature 477, 142–143; 2011). The mission would need more time to distinguish a random fluctuation in a star’s brightness from a dip in starlight due to a passing planet, so last year, NASA extended the mission until 2016. But around the time of that decision, the first reaction wheel went bad. With the loss of the second wheel, says Troeltzsch, “the science we were doing with Kepler, as we were doing it, is over”.
Although engineers will try to reactivate the failed wheels in the next couple months, Troeltzsch is not hopeful. “It’s very unlikely that it can be restored to any kind of usefulness,” he says. Thrusters on board the craft may be able to serve as a crude stand-in, but they were not designed to provide the precision orientation needed. Efforts to bring the mission back to life will probably continue until the end of autumn, says Sobeck.
In the meantime, Borucki’s team is analyzing the two most recent years of Kepler data, which seem likely to add hundreds of candidate planets to the mission’s current tally of more than 2,700 (see ‘Kepler’s bounty’). Because the most recent data contain information on planets with orbits of similar length to Earth’s, Borucki is optimistic that they will reveal a reasonable estimate of the frequency of Earth analogues in the Galaxy.
Sara Seager, an exoplanet theorist at the Massachusetts Institute of Technology in Cambridge, is not so sanguine. She says that the data may yield decent statistics about Earth-sized planets in orbits up to about 200 days, but not all the way up to orbits in the ‘habitable zone’ of a Sun-like star, which are around 365 days. She is hopeful, however, that a few such planets are buried in the remaining data. “It’s possible, and even likely.”