Launched last March, NASA's Kepler spacecraft is designed to find Earth-size planets around other stars. The observing strategy is simple: use Kepler's big telescopic eye to stare at a patch of sky covering 105 square degrees near the Cygnus-Lyra border.
Any time a planet passes in front of one of the estimated 100,000 stars within the target area, the spacecraft records the temporary dip in the star's light. This gives the mission's scientists enough information to deduce the size of both the planet and its orbit.
In August, for example, chief scientist William Borucki and other team members showed Kepler's promise by recording the passage of a known exoplanet, dubbed HAT-P-7b, in front of its host star. The spacecraft was working just as planned.
But last week Borucki told a NASA advisory panel that there's a problem with some of the craft's light-sensing detectors. To cover this much sky, Kepler uses an array of 42 detectors, each divided in half for ease of data transfer. It turns out that three of those 84 detector channels are noisy, and the stars in these areas appear to flicker — not a good thing if you're trying to detect minuscule changes caused by transiting planets.
Apparently the Kepler team knew about the noisy channels prior to launch, but the cure (disassembling the flight-ready craft to replace the bad electronics) was deemed worse than the disease. Instead, for now output from the three channels will simply be ignored, and a computer program should be ready by 2011 to filter out the flickering.
In the meantime, since the other 81 detector channels are unaffected, the planet hunting goes on. Once one of the target stars winks and a candidate world is identified, the team will record at least two more transits before feeling secure about the discovery. For a planet in an Earth-size orbit around a Sunlike star, this confirmation might take three years — by which time the noise-canceling software should be in place.
By contrast, the "habitable zones" for lower-mass dwarf stars lie closer in, so the orbits of planets in those zones will be smaller and their orbital periods shorter. Should one of these candidate solar systems fall in one of the noisy fields, its discovery might be delayed for up to a year, according to Borucki.