Or: His face is shaped like a tear-drop, his lips are pinched, his nostrils are tiny slits, he has long skinny arms and dark, almond-shaped, chilling eyes.

Maybe he is something repugnant, slimy or tentacular, a menace to us humans.

But then, what if he's a Star Child, invisible to our eyes and our technology, an ethereal being ordained with godlike lives?

Well, whatever form the extraterrestrial takes, why -- if he exists -- doesn't he get in touch with us?

That question has sparked a furious debate ever since it was raised by the physicist Enrico Fermi in 1950 in a challenge to the post-World War II sci-fi frenzy.

Hopes that intelligent life is trying to contact us briefly rose a notch last week when New Scientist reported that an intriguing radio signal was being closely examined by the Search for Extraterrestrial Intelligenceproject.

The signal, named SHGbo2+14a, apparently emanating from a point between the constellations Pisces and Aries, had been picked up twice by an ingenious SETI scheme which harnesses screensaver programmes on millions of personal computers to sift through cosmic noise picked up by a giant radio telescope in Puerto Rico.

Alas: There's nothing there to show SHGbo2+14a is E.T.'s calling card.

Indeed, it is only one of a batch of low-grade "candidate" signals that have been sifted from trillions -- and, given the risk that it could be a statistical freak or the result of equipment interference, it has so far not even been upgraded to the category of "promising," says SETI's Seth Shostak.

New Scientist "has inadvertently wandered into a sticky vat of hyperbole," Shostak says wearily.

So nearly six years of scouring the skies by SETI@home have drawn a blank. Ditto the result notched up by Project Phoenix, which has had its ears open for radio signals from some 800 nearby stars for the past 10 years.

These and several other professional searches can be traced to a proposal sketched in 1959 by two Cornell University scientists that the radio telescopes then under construction also be used to listen out for any signals from aliens.

They argued that radio waves are the best way to communicate over interstellar distances, even if such a signal takes tens of thousands or even millions of years to get here.

What cosmic listeners would love to hear is a repeat signal or a signal that says, in effect, "I'm an intelligent lifeform -- are you, too?"

Particular interest is being focussed on the so-called water hole, a usually tranquil region in the radio spectrum where the white noise comes from emissions by hydrogen atoms in the cosmos.

The theory goes that extraterrestrials may send their message via the water hole, since (at least in human experience) all life is connected with water, whose components are hydrogen and oxygen.

But would intelligent life necessarily manifest itself this way?

No, according to rival theories aired last week in the British weekly science journal Nature, which suggest we have been looking for evidence in the wrong place.

To send a long message on a radio beam, repeatedly over a very long time and across a wide galactic scale, in the hope that someone eventually receives it, is a hugely inefficient use of energy.

It would be more energy-efficient to send out space probes or inscribed artefacts, "effectively messages in a bottle," suggests Woodruff Sullivan, an astronomer at the University of Washington in Washington state.

Whichever method is used, the quest for E.T. has many critics.

They say it is rooted in an ancient human reflex -- the dread that we could be all alone in the Universe, and to compensate for this fear we create gods, demons... and aliens.

"Nothing except statistics supports the idea that life, or at least intelligent life, exists anywhere else but the Earth," the British science fiction writer Brian Aldiss wrote in a bleak commentary in 2001.

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