What are the actual chances of finding alien life? The idea of meeting an extra-terrestrial has ignited imaginations for hundreds of years, and it has also inspired real science: the Search for Extra-Terrestrial Intelligence - or Seti - is an organisation that brings together researchers across the world in pursuit of distant life forms. This same dream is on the mind of listener Andrew in Yorkshire in the UK, who has been looking into the sheer size of the universe, and wants to know: how many stars are there in existence, how many planets, and how many planets that could harbour life? Presenter Marnie Chesterton sets off on a space odyssey to answer these questions. She starts at Jodrell Bank Observatory in Cheshire, where University of Manchester astrophysicist Eamonn Kerins tells her the number of stars in the universe, and explains the Drake Equation - the mathematical formula that underpins SETI’s work. It is a series of seven numbers that combine to give you the probability of making contact with an alien civilisation. The next step after stars is the number of planets; Michelle Kunimoto of MIT, who works on Nasa’s TESS mission, explains the transit technique for finding distant worlds. Supposedly anyone can learn to use this technique, so Michelle puts Marnie to a test of her planet-hunting prowess. Distant planets are a huge leap forward - but not all of them will be hospitable to life. Eamonn breaks down how scientists define a habitable planet, as well as how to determine habitability using telescope observations. Marnie speaks to Mary Angelie Alagao from the National Astronomical Research Institute of Thailand about a cutting-edge piece of optical kit designed to block out the light from stars so you can take direct images of the planets next to them. Finally, it is time to put everything together and get some actual numbers for listener Andrew - as well ask how long it could take to find proof of alien life. Presenter: Marnie Chesterton Producer: Phil Sansom Production Coordinator: Jonathan Harris (Photo credit: Mark Garlick/Science Photo Library/Getty Images)