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M.J. Valtonen, J.Q. Zheng (Turku U.), C. Mileikowsky (KTH, Stockholm), L. Lindegren (Lund U.), H. Rickman (Uppsala U.), F.A. Cucinotta (JSRC, NASA), J.W. Wilson (LSRC, NASA), B. Gladman (U.Toronto), G. Horneck (DLR, Cologne), J. Melosh (U.Arizona)
We calculate the probability that large size ejecta from a major collision on a planet in an extrasolar planetary system find their way to our solar system and collide with the Earth. The production rate of ejecta is estimated from the conditions in Our Solar System, and the evolution of the environment of the Sun is derived on the assumption that the Sun was initially a member of a star cluster. The maximum survival times of colonies of bacteria in the interstellar space are estimated when the bacteria reside inside large meteoroids. Using the maximum survival times as the maximum travel time we derive the probability that a potentially life-carrying meteoroid from an extrasolar system has landed on the Earth during the Earth's history. The probability is less than unity, but not necessarily by many orders of magnitude.