We've talked a lot about life in the universe in this season — from where we might look, to what it might conceivably be, to the question of what even we mean by the word "life" anyway. And now, finally, it's time to grapple with the huge problem of what astronomers are actually looking for: the signals, the signs, the smoking gun of life on another planet. We're talking biosignatures, and Emily has some bad news: turns out, it's really hard to take a precise measurement of a conclusive biosignature of exolife ... mainly because, it's not clear any such conclusive biosignature even exists. But she has some good news, which is that we're getting a lot better at detecting atmospheres on exoplanets — so there's still hope!

Finally — finally! — Chris and Emily get to the meat of the matter in this season on life in the universe. Very special guest Dr Kelly Redeker from the Department of Biology at the University of York breaks down the principles of life, both on Earth and on other worlds (or "slimy rocks" as Emily insists on calling them). Do you need a Frankenstein-esque spark? Is life inevitable? What about complex life? Intelligent life? And Emily asks Kelly the big question: will we find life outside our planet?