'Astrobiology' is a brand new field of science, encompassing research into the origins and limits of life on our own planet, and where life might exist beyond the Earth. But what actually is 'life' and how did it emerge on our own world? What are the most extreme conditions terrestrial life can tolerate? And what would an alien actually look like - how realistic are the life-forms envisaged by science fiction novels and films over the years? Join Lewis on a tour of the other planets and moons in our solar system which may harbour life, and even further afield to alien worlds orbiting distant stars, to explore one of the greatest questions asked: are we alone?