Kevin Sokolowski:  

calculation still gives a figure of 10 <14>.Shapley asks: how many stars in this truly 'astronomical' figure have an atmosphere suitable for life? One in a thousand? That would still leave the incredible figure of 10 <11> stars with the prerequisites for life.. Even if we assume that only every thousandth planet out of this figure has produced life, there are still 100 million planets on which we can speculate that life exists. This calculation is based on telescopes using the techniques available today, but we must not forget that these are constantly being improved. If we follow the hypothesis of the biochemist Dr S Miller, life and the conditions essential for life may have developed more quickly on some of these planets than on Earth. If we accept this daring assumption, civilisations more advanced than our own could have developed on 100,000 planets. Professor Dr Willy Ley, the well-known scientific writer and friend of Wernher von Braun, told me in New York: 'The estimated number of stars in our Milky Way alone amounts to 30 milliards. The assumption that our Milky Way contains at least 18 milliard planetary systems is considered admissible by present-day astronomers. If we now try to reduce the figures in question as much as possible and assume that the distances between planetary systems are so regulated that only in one case in a hundred does a planet orbit in the ecosphere of its own sun, that still leaves 180 million planets capable of supporting life. If we further assume that only one planet in a hundred that might support life actually does so, we should still have the figure of 1-8 million planets with life. Let us further suppose that out of every hundred planets with life there is one on which creatures with the same level of intelligence as homo sapiens live. Then even this last supposition gives our 'Milky Way the vast number of 18,000 inhabited planets.'Since the latest counts give 100 milliard fixed stars in our Milky Way, probability indicates an incomparably higher figure than Professor Ley puts forward in his cautious calculation. Without quoting Utopian figures or taking unknown galaxies into account, we may surmise that there are 18,000 planets comparatively close to the earth with conditions essential to life similar to those of our own planet. Yet we can go even further and speculate that if only 1 per cent of these 18,000 planets were actually inhabited, there would still be 180 left!".