When NASA’s Kepler space telescope launched in March 2009, astronomers had no proof that any star other than the sun harbored an Earth-sized planet (with a diameter within 25 percent of Earth’s). By May 2013, when the telescope suffered a mechanical failure that ended its planet hunt, Kepler scientists had discovered 10 such worlds and identified hundreds of yet-to-be-confirmed Earth-sized candidates.
Kepler’s death blow came when the second of four wheels used to orient the telescope failed. The spacecraft requires three wheels to home in on a patch of sky and search for planets that block out some of the light of their stars. Mission engineers tried to repair the wheels but gave up in August (SN: 9/21/13, p. 18).
While the telescope is probably done hunting planets, its data will keep astronomers busy for at least several years. William Borucki, Kepler’s principal investigator, speculates that data not yet analyzed contain signals of the ultimate catch: Earth-sized worlds in the habitable zones of sunlike stars that could possibly support life. Already scientists have found intriguing worlds such as Kepler-62e and Kepler-62f, two planets slightly larger than Earth that could harbor liquid water on their surfaces (SN: 5/18/13, p. 5).
When Kepler was at full health, Borucki claimed that it was the greatest unmanned mission NASA has ever flown. That’s up for debate — fans of the Hubble Space Telescope can make a strong case — but there’s no question that Kepler has changed the way scientists view the galaxy. Statistical calculations based on Kepler data suggest that the Milky Way contains hundreds of billions of planets. And in April NASA approved the Transiting Exoplanet Survey Satellite mission, due for launch in 2017, to find the nearest ones to Earth.