From Eric Kvaalen, Les Essarts-le-Roi, France
In your leader (3 October), you say that because there are so many planets, “even if the odds of life arising on a particular world are tiny, there is a good chance it has happened many times”. Let’s assume that there are 1012 planets in our galaxy and 1012 galaxies in the visible universe, making a total of 1024 planets. If the chance of life arising on any planet is tiny, say 10−27, the chance of another planet with life in the whole visible universe is just one in 1000. I just picked 10−27 out of the blue. Itcould be 10−100.
