Cinema sign saying Copyrighted image Credit: The Open University

Adam Hart-Davis presents this episode from the Allan Telescope Array (ATA), an impressive group of telescopes in the Mountains of Northern California. It’s at the cutting edge in the search for intelligent life. Adam teams up with Dr Seth Shostack (SETI Institute), an alien hunter with twenty years’ experience under his belt. We find out why the ATA is the most powerful telescope ever built - and learn about future plans for the hugely powerful Square Kilometre Array.

In San Francisco, Adam visits the HQ of the SETI Institute and catches up with the godfather of extraterrestrial hunting - SETI's Doctor Frank Drake. Frank’s been searching for aliens for over forty years. In 1961 he led a team that produced the “Drake Equation”. This mind-boggling mathematical formula estimates the number of intelligent civilisations in our galaxy. We reveal the answer - or at least the intelligent guess!

However, not all alien life would be intelligent. We travel to the pretty, seaside town of Beer in Devon. It’s probably the last place you’d expect to find life similar to that which could exist on other planets. But for astrobiologist Professor Charles Cockell of The Open University this beach on the South Coast is home to a form of life you could expect to find on alien worlds. We join Charles as he collects samples for the ultimate endurance test - a journey into outer space!

If Beer’s creatures survive the worst of the British weather and a journey into Earth orbit, what does this mean for the chance that similar life forms may exist in the extreme conditions on Mars? At the Space Research Centre, University of Leicester, Janet Sumner catches up with Dr Mark Sims and Dr Dave Cullen. They are developing brand new technology to detect life on Mars. Their “Life Markerchip” gets its inspiration from an unusual source – a pregnancy testing kit!

If we find life elsewhere in the Cosmos, what will it look like? Could Hollywood’s science fiction monsters stand up to a bit of science fact? We’ve teamed up space scientist Dr Phil Plait with evolutionary biologist Dr Lynn Rothschild to scrutinise some of the most famous Hollywood aliens.

Back at the ATA, Adam links up with Seth. He’s monitoring one hundred million channels at once. If he discovered an alien signal, what would it look like? Radio telescopes like the ATA are great for us to listen to aliens but can they hear us? Television signals have been leaking into the Cosmos for seventy years. If aliens had televisions, what could they watch on Channel Earth? Adam shows us with an ingenious demonstration - aliens could be watching the release of Nelson Mandela or even experiencing Beatlemania!

TV signals have been leaking into the cosmos but has anyone ever sent a direct, targeted message? Back in 1974, Frank Drake used the giant Arecibo telescope in Puerto Rico to beam a message into the unknown – and Voyager and Pioneer spacecraft had a gold record telling any potential alien who we are and where we come from. But what are the very latest plans for talking to ET?

Teaming up with Dr Doug Vakoch, we learn how we would communicate with aliens. But why are we still waiting for aliens to contact us?

Vakoch believes we should get in touch – though any message we receive may not be as friendly as the one we send. Frank Drake has been waiting forty years for a signal. He doesn’t only believe they’ll get in touch. Drake thinks there’s a chance they already have – we just don’t have the technology to detect the signals yet.

For Adam any discovery, however small, would bring a revolution in our understanding not only of ourselves but of our place in the Cosmos.

And if there’s no life out there, that’s pretty amazing too - because it would confirm that we really are alone.

First broadcast: Tuesday 7 Aug 2007 on BBC TWO