Astronomer Frank Drake, in the 1950s, came up with an equation that tried to estimate how many advanced civilizations there might be in the galaxy. You start with the 100 billion stars in the galaxy, and reduce that number with a series of reasonable assumptions. You reduce it by the fraction that have planets, the fraction that have planets with atmospheres, the fraction that have planets with atmospheres and oceans, the fraction that have planets with microbial life, etc. No matter how many or how few reasonable assumptions you make about these planets, the final number is usually in the
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