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That raises another interesting question: how quickly could life-bearing ejecta from Earth (or anywhere else) seed the entire galaxy? Hara and co calculate that it would take some 10^12 years for ejecta to spread through a volume of space the size of the Milky Way. But since our galaxy is only 10^10 years old, a single ejection event could not have done the trick. However, they say that if life evolved at 25 different sites in the galaxy 10^10 years ago, then the combined ejecta from these places would now fill the Milky Way.

Ref: arxiv.org/abs/1204.1719: Transfer of Life-Bearing Meteorites from Earth to Other Planets

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