The Rosetta mission’s historic landing of a robotic spacecraft on a comet has brought to a head one of humankind’s timeless questions: are we alone in the universe? For the project’s leading land scientist, that answer may soon be on the horizon. After a tense seven-hour wait, the lander module Philae touched down on the surface of Comet 67P/Churyumov–Gerasimenko at 16:03 GMT on Wednesday....