What does the uniquely（adv独特地） rational（合理的） 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（起源） of life that given（做） a planet only approximately like our own, life is almost certain to start. Of all the planets in our own solar（太阳的） 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（[天] 金星） far too hot, and so is Mercury（水银）. And the outer（外部的） planets have temperatures near absolute（绝对的） zero and hydrogen（[化学] 氢）-dominated atmospheres.

  But other suns, stars, as the astronomers call them, are bound（捆绑） to have planets like our own. And as the number of stars in the universe（ 宇宙） is so vast（巨大的） this possibility becomes virtual（[计] 虚拟的） certainty. There are 100,000 million stars in our own Milky（(Milky)人名） Way alone. And then there are 3,000 million other Milky Ways or galaxies（星系） in the universe（ 宇宙）. So the number of stars that we know exists is now estimated（估计） at about 300 million million million. Although perhaps only one percent of the life that has started somewhere will develop into highly complex and intelligent patterns, so vast is the number of planets that intelligent life is bound to be a natural part of the universe.