Scientists may be making a colossal mistake by deciding to contact extraterrestrial life rather than just listening.

One of my favourite films is Mars Attacks!, the 1996 science-fiction comedy from Tim Burton, in which Martians with Blairite grins swoop to Earth and pick off the suited earthlings that have assembled to welcome them. "Nice planet. We'll take it!" they snigger, as the body count rises.

Science doesn't work like the movies, though, and the idea of hostile aliens, armed with metal and malice, seems laughable. Or does it? Fifty fruitless years of scanning the heavens for signals has prompted some astronomers to abandon their roles as passive listeners and to start shouting across the cosmos.

There are plenty of observers who believe such hopeful hollering to be deeply unwise. Sir Bernard Lovell, the founding director of Jodrell Bank, together with the physicist Freeman Dyson and the intellectual Jared Diamond, believe that the terrestrial hand of courtesy should be extended with extreme caution. Their hesitancy is based on uncertainty about whether any extraterrestrials belong to the Spielberg school of cute'n'cuddly, or the Tim Burton institute for the murderously deranged.

This week's Royal Society conference to mark the 50th anniversary of the Search for Extra-Terrestrial Intelligence (SETI) mostly circumvented the issue, although Simon Conway Morris, the evolutionary biologist, did suggest that any life out there is likely to have followed the same evolutionary path and therefore be just as potentially nasty as humankind. The conference chose instead to discuss whether the hunt for alien lifeforms should focus on inhospitable niches on Earth, or on other planets. But this pressing question - the wisdom of proclaiming our existence to other inhabitants of the cosmos, of whom we know nothing - refuses to go away. Two years ago two scientists resigned from a SETI study group, pointing out that the potential risks of deliberate transmissions to other planets was being ignored.

Knowing there are other civilisations out there would be a defining moment for humanity; but half a century of deep-sky radio searches has resulted in a deafening silence. Despite the lack of so much as a celestial hiccup, many eminent scientists, including Lord Rees of Ludlow, president of the Royal Society, believe it is perfectly possible that we have company in the loneliness of space.

So there has grown a contingent of impatient alien-hunters who, instead of just listening, want to talk. If ET won't come to them, runs their logic, then they must go to it. This approach, called active SETI or METI (Messages to Extra-Terrestrial Intelligence), involves pumping out powerful signals, essentially turning our planet into a highly visible beacon - or a target.

One such hunter is Aleksandr Zaitsev, from the Russian Academy of Science, who enjoys access to one of the world's most powerful telescopes. He dispatched one such earthly telegram from Ukraine in 2008; in 2029, whoever or whatever is inhabiting the planets circling the red dwarf Gliese 581 will be the bemused recipients of a digital time capsule containing messages from the social networking site Bebo. Dr Zaitsev's rationale is that if every civilisation just listens, then SETI is doomed. Somebody, he reasons, has to make the first move. Two years ago British astronomers took a gamble on an extraterrestrial predilection for corn-based snacks, and flashed a Doritos ad in the direction of a star 42 light years away in Ursa Major.

Dr Zaitsev's defence is that even if we want to remain celestially anonymous, our cover is blown: we leak evidence of our existence all the time, in the form of radio, television and military transmissions. Others, such as David Brin, the physicist, novelist and futurist, argues that it is a very different matter to direct the human megaphone to potentially inhabited regions of the Universe, and to do so without consulting bodies such as the United Nations.

Dr Brin also asks: if there really are other sentient beings out there, why have they not contacted us? Why are they keeping quiet? Why do we assume they are benign?

To paraphrase one contributor to the BBC Focus magazine forum this week: if the Universe is 14 billion years old and the Earth just 4.5 billion, we newcomers to the Universe might have grasped the concept of civilisation rather late in the day. Upon meeting us, our technologically superior, interstellar pen pals might deem us quaint cosmic toddlers - or expendable cosmic savages.

If we are to extend that terrestrial hand of courtesy, its fingers should be crossed, in the hope that any passing aliens simply fancy adding a few unreconstructed earthlings to their Bebo accounts.