Melvyn Bragg examines Extra Terrestrials. New planets have been observed far beyond our solar system and telescopes are being built that will enable us to look for water and oxygen on these distant planets. If water and oxygen are present, there is every reason to suppose that some form of life might also exist there. It has even been suggested that we might find life within our own solar system. One of Jupiter’s moons, Europa, appears to be covered in an ice-crusted ocean and there is evidence that water once flowed on Mars. On our own planet, there are forms of life that don’t need the sun, living instead on energy from volcanic vents on the ocean floor. This discovery has changed our concept of what life needs in order to survive. Could life only exist on another planet like ours and what are our chances of ever discovering such a planet? If we find life, will it be intelligent, or little more than green slime? And if intelligent aliens exist, why aren’t they here? With Simon Goodwin, Researcher in Astronomy, Cardiff University; Heather Couper Space expert; Ian Stewart, Professor of Mathematics, Warwick University.