Alien hunting isn't just tabloid fodder anymore. Over the last few years, astronomers have discovered thousands of planets outside the Solar System, suggesting that the galaxy is teeming with worlds - at least as many as one planet per star, on average.
The existence of so many planets raises the odds that at least one of them has life - and it's possible there may even be an Earth "twin" - making alien-hunting a bona fide scientific endeavour. "We're now ready to make the transition from 'are there planets?' to 'is there life on these planets?'" says Nick Siegler, the chief technologist of Nasa's Exoplanet Exploration Program. "That's a huge shift in how Nasa's thinking about the search for life and what's next in the world of planetary science."
The trouble is, it's harder than it seems to spot life from millions of miles away, especially if it's not intelligent. Last week in our series "The Genius Behind", we told the story of Sara Seager, a scientist looking for signs of life on second Earths, who believes that the key is to scrutinise the atmospheres of these alien worlds.
Find out why in the film here.