If we find life surviving on one of the planets that we've been exploring, and it's so similar as to be indistinguishable from Earth life, then it almost certai

If we find life surviving on one of the planets that we've been exploring, and it's so similar as to be indistinguishable from Earth life, then it almost certainly came from Earth on one of our missions. Even if an asteroid transplanted something that managed to take root millions of years ago, it would have endured some pretty severe evolutionary pressure by now (and probably didn't have any competition from native life).