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Objective atheism only refers to not believing in the existence of a God or god. I've never heard anyone say that ghosts=gods, so by definition an atheist can believe in a ghost. Of course, this is unlikely, because the existence of ghosts is often associated with ideas of afterlife and supernatural gods etc, but it depends on what you believe a ghost is; they aren't necessarily exclusive beliefs.
I agree with you (and @
Witty) that aliens are far more likely to exist, but as of today there is just as much actual scientific evidence for alien existence as there is for the existence of a deity. IE none. so in terms of evidence or data-driven analysis, aliens are equivalent to a god insofar as belief is concerned.