Abstract
UFO reports cannot be regarded as establishing that extraterrestrials have visited the earth. On the other hand, if other biospheres exist in the galaxy, it is conceivable that life forms might migrate from one star to another even though the distances between stars are more immense than is realized by most people.
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Images courtesy of Lance Benner, JPL-NASA
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Halley, J.W. (2012). Colonization and Panspermia. In: How Likely is Extraterrestrial Life?. SpringerBriefs in Astronomy. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-22754-7_6
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