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Tiny organisms were discovered by University of Sheffield experts on a research balloon they had sent 27km (16.7 miles) into the atmosphere during last month's Perseids meteor shower.
The microscopic bugs were detected when the balloon landed back on the ground in Wakefield, West Yorkshire.
But the scientists insist the samples could not have been carried from the Earth's surface into the stratosphere - the second layer of our atmosphere, which stretches up to 50km (31 miles) from the ground.
Strict tests were taken to avoid any contamination, they said.
Professor Milton Wainwright, who led the team, said: "Our conclusion then is that life is continually arriving to Earth from space, life is not restricted to this planet and it almost certainly did not originate here."
The findings are to be published in the Journal of Cosmology.