and a to a large number, its not a bad way of approximating things. If you do it based on the knowledge we have... the answer should be 1 of course, us! Though with some knowledge of the Universe you can put in some stuff. When I do it I tend to get around 2-3.... so more reasonable than you lot. Though the number of planets we have discovered that are habitable is surprisingly zero apart from the Earth.. give us 10 years though.