In a recent article, mathematician Aczel argues convincingly that the number of planets suit able for life is extremely large. There are about 300 billion stars in our galaxy (银河系) and possibly 100 billion galaxies in the universe. Some may quarrel with Aczel’’s claim that as many as half of these stars can be presumed to have planets, and that on average one in nine of those planets is suitable for life; but few, I think, will disagree with his conclusion that there are a very large number of planets suitable for life.
The trouble starts when we come to estimate the probability that life will emerge on any of these planets. The problem is that we don’’t have a good theory of the origin of life on earth. Without one, it is anybody’’s guess how likely this event actually was. Out of thin air Aczel conjures (想像) the figure of 1 in a trillion for this likelihood and concludes that the probability of life existing on at least one other planet is virtually 1.
Statistics are extremely powerful and important, and Aczel is a very clear and capable exponent (倡导者) of them. But statistics cannot substitute for empirical (经验主义的) knowledge about the way the universe behaves. We now have no reasonable way of arriving at robust estimates for the probability of life arising spontaneously when the conditions are right. So, until we either discover extraterrestrial (外星球的) life or understand far more about how at least one form of life--terrestrial life--first appeared, we can do little more than guess at the likely hood that life exists elsewhere in the universe. And as long as we’’re merely guessing, we should not dress up our interesting speculations as mathematical certainties.
The term "Out of thin air" in Line 3 of Paragraph 2 can be best replaced by ______.