Aliens are found in space. Now all that is needed is some proof...

"Scientists discover extraterrestials floating in the air above our heads" could have been the mother of all announcements. Like many extraordinary claims, however, all that was lacking was the evidence to prove it.

A British scientist has told a scientific conference in California that he had found the first positive identification of extraterrestial microbial life in air samples taken miles above the tropopause, an invisible barrier in the Earth's atmosphere.

Professor Chandra Wickramasinghe, an astronomer at Cardiff University said that a balloon flying at an altitude of 41km (25m) picked up the unmistakable signs of life. There was only one realistic possibility: they must be aliens from outer space, he told a meeting of the International Society for Optical Engineering in San Diego.

"There is now unambiguous evidence for the presence of clumps of living cells in air samples from as high as 41 kilometres, well above the local tropopause (16km), above which no air from lower down would normally be transported," the professor said.

The findings clearly supported the controversial proposition that microbial aliens are drifting through space, thumbing a lift on passing comets and "seeding" life on distant planets, including our own.

"We have argued for more than two decades that terrestrial life was brought down to Earth by comets and that cometary material containing micro-organisms must still be reaching us in large quantities," said Professor Wickramasinghe.

So, the balloon experiments have finally proved it is true. Or have they?

The research was carried out last year from a research base at Hyderabad in India. It involved sending up equipment that could sample air at up to 41km high. Scientists then ran tests on any floating particles for signs of life by looking for changes in electrical voltage across cell membranes.

The scientific paper presented to the conference and co-authored by a team of 10 scientists, including Professor Wickramasinghe, was, however, a little more prosaic than the press release that was issued by Cardiff University, which trumpeted "the first positive identification of extraterrestial microbial life".

The paper stated: "Unambiguous evidence of living cells came from examining micropore filters on which the samples were recovered with the use of voltage sensitive lipohilic dyes that could detect the presence of active cells.

"Clumps of viable cells were found at all altitudes using this technique, and this conclusion was found to be consistent with images obtained from electron microscopy," the scientists wrote. "Since the 41km sample was collected well above the local tropopause, a prima facie case for a space incidence of these micro-organisms is established."

But what appears at first sight to be evidence of aliens might well not stand up to the rigours of further scientific investigation, a point accepted by David Lloyd, professor of microbiology at Cardiff and one of the senior authors of the study.

"What we can say is that bacteria have been discovered at a height of 41km but where they come from is an unknown. I'm certainly not saying we know where they come from," Professor Lloyd said yesterday.

"No one has sampled air at this height before. We wouldn't have detected these microbes until now because no samples at this altitude have been taken. What we do know is that wherever people have looked for bacteria on Earth, they have found them, even in the most hostile environments," he said.

If the samples are truly living microbes there are three possible explanations. Either the detection equipment was contaminated on the ground, which the scientists dispute, or earthly microbes have somehow broken through the tropopause barrier, perhaps riding on a rocket. Or, of course, the bugs really are from outer space.

It is the last possibility that has generated intense interest in what would otherwise be an obscure experiment in the rarefied field of high-altitude physics. It would, after all, support the romantic notion of "panspermia" – the notion that the seeds of life are being carried through the cosmos.

Panspermia has a history going back to classical times but the idea was first aired seriously by a Swedish chemist, Svante Arrhenius, nearly 100 years ago. He put it forward as a possible explanation for the origin of life on Earth.

However, the hypothesis was rejected by most scientists following experiments showing that organic molecules – notably amino acids, the building block of proteins – can be generated in a test tube using nothing much more complicated than a simple "primordial soup" of simple chemicals and a spark of electricity acting as a jolt of lightning.

Why argue for panspermia, when the constituents and conditions for life existed here on the Earth 4.5bn years ago? After all, Occam's Razor – a general rule in science – states that the simple explanation is more likely than the complex.

Two scientists, however, have consistently fought this established view of the origin of life. One is Professor Wickramasinghe himself, the other is his one-time mentor, Sir Fred Hoyle, the eminent cosmologist who greatest achievement was to work out how elements are created in the nuclear furnaces of the stars.

Sir Fred said: "Although the ideas of life existing outside the Earth and of panspermia have had a history stretching back over many centuries, the concept of comets carrying microbes began only with our own researches."

"This work has been published in a series of books and papers since the mid-Seventies. Prior to 1974, there were, as far as we could find, no references to organic materials in comets, let alone to a connection between comets and life," he said.

Recent observations, such as the discovery of signs of organic matter in comets, has lent support to the panspermia principle, but few scientists, outside of the Hoyle-Wickramasinghe circle, are fully paid-up members of the panspermia club.

Nevertheless, the US National Aeronautics and Space Administration (Nasa) has embarked on its Stardust mission to bring back a bit of cometary material from Comet Wild2 in 2006. Early results have already found evidence to suggest that comets contain chemical material similar to the cell walls of living organisms.

In Britain, meanwhile, several teams of scientists are engaged in experiments to test the possibility that alien life might have survived the high temperatures involved in entering the Earth's atmosphere

At the University of Kent at Canterbury, a light-gas gun is being used to propel objects carrying living bacteria into various targets to emulate the impact of cometary material on the atmosphere. Other scientists are analysing the changes that occur when extraterrestial rocks that might harbour life suffer the intense heat of atmospheric re-entry.

But all this would pale into insignificance if the Indian balloon experiment is shown to have caught living microbes from deep space. "We are trying to culture them, but it's proving quite difficult," Professor Lloyd said.

"They look like clumps of normal bacteria found on Earth. It may be that they are just common ordinary terrestrial bacteria, but we don't know how they could have got up to these heights," he said.

"On the other hand many developments have happened recently which make the idea of extraterrestial organisms believable."

Just how believable, however, remains to be seen.

The search for life on other planets

* The British astronomer Jocelyn Bell Burnell discovered a rapid series of radio pulses in 1967 that were occurring every 1.337 seconds. Her team kept it secret and nicknamed the discovery LGM for Little Green Men, thinking there was an outside chance it could be an alien signal. It was later proved to be a natural pulse from a neutron star.

* The search for life on Mars was galvanised in 1971 when the satellite Mariner 9, which was in orbit around the planet, beamed back pictures that showed dozens of winding channels. Some scientists said they had probably been created by water flowing across the planet. Spacecraft are due to return Martian rocks to earth in 2008.

* The Galileo space probe that travelled around Jupiter in 1995 showed that Europa, one of the planet's four major moons, was apparently covered in ice. Europa, which is about the same size as our moon, is a brilliant shade of white and littered with cracks that look like floating pack ice. Nasa plans further missions there.

* A potato-sized meteorite that landed in Antarctica 13,000 years ago and was discovered in 1984 was at the centre of a fierce scientific debate over the possibility that Mars teemed with life 3.5 billion years ago. Nasa scientists claimed in 1996 that the rock, ALH84001, showed microscopic traces of life forms that could only have come from Mars.

* Scientists found evidence of running water on Mars last year. Gullies and landslides picked up by the Mars Global Surveyor could have been caused by a reservoir seeping to the surface of the planet. The images were taken as evidence that there had been liquid water on Mars within the past million years, which increases the chances that primitive life forms had survived there.

* The discovery of the first planet outside our solar system in 1995 by Michel Mayor and Didier Queloz of Geneva Observatory raised the intriguing possibility that there might be earth-like planets orbiting distant suns. Extra-solar planets may possess liquid water, the basic ingredient of life.