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There’s also a debate over what percentage of those sun-like stars might be orbited by an Earth-like planet (one with similar temperature conditions that could have liquid water and potentially support life similar to that on Earth). Some say it’s as high as 50%, but let’s go with the more conservative 22% that came out of a recent PNAS study. That suggests that there’s a potentially-habitable Earth-like planet orbiting at least 1% of the total stars in the universe—a total of 100 billion billion Earth-like planets. a<V=C
还有一个争论就是有多少类日恒星附近有类地行星（温度、有液态水、可能存在地球式生命）。有人说数量高达50%，让我们保守点，22%（最近PNAS的研究）。也就是说有至少1%(10^20颗）恒星被可居住类地行星绕轨而行。 |7zm!^t $
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So there are 100 Earth-like planets for every grain of sand in the world. Think about that next time you’re on the beach. MjI}fs<
想象一下你在沙滩上，地球上每有一粒沙,宇宙中就有100颗类地行星。 )N-+, Ms
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Moving forward, we have no choice but to get completely speculative. Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life (if that’s true, every grain of sand would represent one planet with life on it). And imagine that on 1% of those planets, the life advances to an intelligent level like it did here on Earth. That would mean there were 10 quadrillion, or 10 million billion intelligent civilizations in the observable universe. zYOPE 6E
下一步只能推测了。十亿年后1%的类地行星上生命继续发展（每一粒沙代表一个有生命行星），这些类地行星的再1%，有了像地球一样智慧生命。在现在可观察到的宇宙范围内，那将会有10^15个智慧公民。
