The math is dead simple: it seems that the frequency of planets able to support life is roughly one percent. In other words, a billion or more such worlds exist in our galaxy alone. That's a lot of acreage, and it takes industrial-strength credulity to believe it's all bleakly barren.
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About Seth Shostak
Seth Shostak was a contemporary American astronomer and author. Seth Shostak is an American astronomer and author, and is currently the senior astronomer for the SETI Institute. Read more on Wikipedia →