A team of scientists from India and the United Kingdom claims to have found extraterrestrial microbial life in the earth's stratosphere, but others in the astrobiology community have their doubts. The team presented its findings on Sunday at the 46th annual meeting of the International Society for Optical Engineering (SPIE) in San Diego, Calif.

"There is now unambiguous evidence for the presence of clumps of living cells in air samples from as high as 41 kilometers, well above the local tropopause, above which no air from lower down would normally be transported," said investigator Chandra Wickramasinghe of Cardiff University. The tropopause separates the tropospherethe layer of the atmosphere closest to the earth's surfacefrom the stratosphere. Wickramasinghe maintains that the cells have rained in from space.

But, says Max Bernstein, a space scientist associated with the SETI Institute and NASA Ames Research Center, "it would strain one's credulity less to believe that terrestrial organisms had somehow been transported upwards than to assume that extraterrestrial organisms are falling inward." Although Bernstein doesn't rule out the possibility that the findings are valid, he says that "one is reminded of Carl Sagan's caution that remarkable claims require remarkable evidence."

Wickramasinghe and colleagues based their conclusions on air samples gathered in the stratosphere using a balloon from the Indian Space Research Organization (ISRO). To test if the air contained living cells, they used fluorescent cyanine dye, which only living cells absorb through their membranes. The results suggested they had discovered living cells. "We have argued for more than two decades that terrestrial life was brought down to earth by comets," Wickramasinghe says, "and that cometary material containing microorganisms must still be reaching us in large quantities."

But not all scientists subscribe as completely to the panspermia theory that life descended from really on highin space. "He [Wickramasinghe] has been a strong advocate of this most extreme version of panspermia for a while," Bernstein adds, "and I think that since the claim comes from him, it is regarded with perhaps a little more skepticism than if it came from someone who was a little more critical of the theory."