【新编大学英语】（第一册）Unit 6 B 外星人（1/3）

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        Earth
        Milky Way
        3处破折号
As we all know, the Earth has everything that is needed for life to develop, which is why we are here. But four billion years ago, when the Earth was formed, what were the chances that life would emerge? Pretty small, according to biologists – about the same chance that you would have of winning the national lottery five weeks in a row. So we're quite lucky to be here.
What about the chances of life forming elsewhere – in our own galaxy, for instance? The Milky Way contains around 400 billion stars. Some, maybe most, of these have planets orbiting them, and the chances are that many millions or even billions of these planets could support life. So it does seem possible that there are some other lottery winners living in the same neighborhood.
And if you think that there are countless other galaxies out there – many of them far bigger than the Milky Way – then it seems obvious that there must be other forms of life somewhere in the universe.
But is it intelligent life, the kind of life that could build spaceships and pay us a visit?