We live on a small blue planet that circles a medium-sized star. Our suburban solar system is just one among the many that make up the vast stellar metropolis we call the Milky Way. Beyond our galactic shores, billions of other galaxies compete for time and space in an ever-expanding universe. Exact sums are difficult to deduce, but conservative estimates put the total number of planets in the trillions.

With such an abundance of universal real estate, is it conceivable that Earth is the only place where life has put down its anchor? Are we truly alone in the Universe, unique freaks hurtling through the boundless vacuum of space on an archaic piece of rock? Or are there other planets out there with their own equivalent of our bacteria, birds, and merchant bankers?

Nobody doubts that, in our minds, if not yet in reality, the existence of extra-terrestrial life remains a potent possibility. The recent success of Steven Spielberg's War of the Worlds, a cinematic take on a century-old sci-fi novel, shows that alien life remains a perennial crowd pleaser. But let's not get ahead of ourselves here. Certain fundamentals need to be negotiated before we can get to grips with the sticky issue of whether those alien colonists of the future will be with us, or against us.

For science, if not its fictional cousin, the issue of alien worlds is an invaluable conduit through which questions on life, the Universe and everything, can be channelled. How easy, for example, is it to make life, and can it be done in the laboratory? Planets may be abundant, but how many of them would make suitable homes? Would life on these planets follow the same biological rules as life on Earth, and if so, can we make any predictions about what we might find there?

Answers to these questions may be as evasive as alien life itself. But the search is most definitely on, with ambitious plans for the future. Over the next 20 years, new probes and telescopes will blast off from Earth, to boldly go where no probe or telescope has gone before. If life on other planets really does exist, then perhaps this may be the century to find it.

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