How many intelligent alien civilizations are out there among the hundreds of billions of stars in the spiral arms of the Milky Way? According to a new calculation, the answer is 36.

That number assumes that life on Earth is more or less representative of the way that life evolves anywhere in the universe — on a rocky planet an appropriate distance away from a suitable star, after about 5 billion years. If that assumption is true, humanity may not exactly be alone in the galaxy, but any neighbors are probably too far away to ever meet.



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