This year, Nature celebrates its 150th birthday. To mark this anniversary we’re rebroadcasting episodes from our PastCast series, highlighting key moments in the history of science.In the early 1990s, a team of astrophysicists led by Carl Sagan looked at data from the Galileo spacecraft and saw the signatures of life on a planet in our galaxy. Historian of science David Kaiser and astrobiologists Charles Cockell and Frank Drake discuss how we can tell if there is life beyond the Earth – and how optimism, as well as science, is necessary for such a venture.This episode was first broadcast in October 2013. For information regarding your data privacy, visit acast.com/privacy