【新编大学英语】（第一册）Unit 6 B 外星人（2/3）

英语自学网

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        Earth
        Milky Way
        X
        一处连字符,三处冒号
Here on Earth there have been billions of species, but only one of them has turned out to be intelligent enough to develop technology and fly into space. And even we're not particularly intelligent: the human race will probably blow itself up long before it learns how to visit other parts of the galaxy.
But let's suppose that there are advanced civilizations here in our own galaxy, who have developed fast-moving flying saucers. Would they visit the Earth?
First we need to ask: why should they want to? The most likely answer is that their own sun is at the end of its life, and they need to find somewhere else to live.
And the next question is: why here? The diagram shows the part of the Milky Way which probably has the largest number of habitable planets. The oldest ones and therefore ones which are most likely to have advanced forms of life are in the shaded area. We're towards the edge marked X.