Well, I think the only reason we as atheists ask this question is to feel as if we are being scientifically rigorous, although we do know, with very good reason, that atheism is true.

But it's good to be sure, isn't it?

For every supernatural explanation it is possible to conceive an explanation whose claims are just as unfalsifiable and yet contradict and refute the claims of the first. Science, naturalism, is the only belief system that actually works on the grounds of evidence and incessant self-scepticism.

Life on other planets will have its own parochial religions, but we can see pretty easily that every species, on every conceivable world will have the atheists. Atheism and science are objective, religion is subjective.