新概念第四册 Lesson 43 Are there strangers in space?

　　Lesson 43 Are there strangers in space?

　　宇宙中有外星人吗？

　　First listen and then answer the following question.

　　听录音，然后回答以下问题。

　　What does the 'uniquely rational way' for us to communicate with other intelligent beings in space depend on?

　　We must conclude from the work of those who have studied the origin of life, that given a planet only approximately like our own, life is almost certain to start. Of all the planets in our solar system, we are now pretty certain the Earth is the only one on which life can survive. Mars is too dry and poor in oxygen, Venus far too hot, and so is Mercury, and the outer planets have temperatures near absolute zero and hydrogen-dominated atmospheres. But other suns, start as the astronomers call them, are bound to have planets like our own, and as is the number of stars in the universe is so vast, this possibility becomes virtual certainty. There are one hundred thousand million starts in our own Milky Way alone, and then there are three thousand million other Milky Ways, or galaxies, in the universe. So the number of stars that we know exist is now estimated at about 300 million million million.

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