Planet-hunting scientists were dealt a major blow Wednesday when NASA officials announced that a crucial wheel on the Kepler space telescope had ceased to function and that the craft had been placed in safe mode. Even as NASA officials raised the possibility that they could get the telescope back up and running, scientists began mourning the potential loss of a spacecraft that they said had fundamentally altered our understanding of alien planets in the Milky Way—and Earth’s place in an increasingly crowded galaxy. “Tears are coming to my eyes on and off,” said UC Berkeley astrophysicist Geoff Marcy, a co-investigator...