If extra-terrestrials do exist they must have a home. Horizon tells the story of the race to find out where in the Universe this might be. The answer, for scientists across the world, lies in the hunt for planets around distant stars.Stars which are trillions of miles away from our own solar system.But the history of the planet hunters is littered with failure. Centuries of searching had thrown up nothing. It was time for the new style planet hunters to step in. However, it is only in the last ten years that these scientists have had the technology to succeed. Even now looking for these distant planets is far from straightforward.The planets themselves are so faint that they cannot be seen, even by the most powerful telescopes ever built. Instead the astronomers must devise ingenious ways to search for clues to their presence. They examine stars just like our own Sun, across the galaxy, for any give-away characteristics that might indicate that they too have planets circling around them.A Swiss team finally struck gold in 1995 - convinced they'd detected a star that must have its own planet. Their discovery was the first of its kind but not the last. Other teams started to get lucky and suddenly it seemed like there were stars with planets everywhere. But the scientific community soon became restless. All they had done so far was detect the presence of alien planets - without seeing one, it was impossible to work out what the planet was like.If these planets really did exist it was time the scientists caught a glimpse of one of them. Only then would they be able to learn about the planet - its surface and its atmosphere. And only then would they know whether it could sustain life as we know it.Horizon follows the trials and tribulations of the planet hunters and shares in the triumph of the Scottish team who, just a few months ago, became the first to achieve the ultimate goal - to capture the image of an alien planet. It is orbiting another star, 55 light years away from Earth.The question is - how similar is this planet to our own and could it be home to alien life? Horizon uncovers the answers.