It’s one of the greatest puzzles of the universe, and one that has vexed humanity ever since we first gazed at the stars and thought of other worlds. Is our Earth the sole place that harbours life, or might it be found elsewhere, among the trillions of planets, star systems and galaxies? As Arthur C. Clarke put it: ‘Two possibilities exist: either we are alone in the universe or we are not. Both are equally terrifying.’
