纵观人类历史，每次我们以为自己知道宇宙是什么时，结果发现宇宙不止是其中一个，而是很多个。起初，我们认为宇宙是地球、太阳和月亮，天空中有许多神秘的光点。后来发现有很多行星都有自己的卫星。还有很多太阳，它们有自己的行星:很多太阳系。
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