The planet-hunting space telescope Kepler has been hobbled by a broken wheel, say scientists at Nasa. Two of four reaction wheels are now faulty. At least three are needed to orient the telescope correctly. "I wouldn't call Kepler down and out just yet," said Nasa administrator John Grunsfeld, saying scientists were working on the problem.

Kepler was launched in 2009 and last month identified two distant planets that Nasa said could be habitable. So far, the $600m mission has identified 132 "exoplanets" outside our solar system, and another 2,700 possible candidates.

Image: A United Launch Alliance Delta 2 rocket is prepared for the launch of the Kepler Telescope  Photograph: Reuters