Until we have bigger space based telescopes we cannot determine what the odds are of finding earthlike planets (right size, right mass, right temperature, with water in liquid form) for all we know it could be 1 in 10 or 1 in 50 or 1 in 100 or 1 in 1000 star systems. I think it is reasonable to assume that earth is not only planet like this in the whole galaxy but how many planets like this are there? At the moment that is anyone's guess.

Where a planet like this exists then we are still unable to determine how probable is it that biological activity gets started. Once again we only have our sample of one. It might be reasonable to assume that given enough time some form of life may well get started. What becomes more difficult is to determine if it ever develops the complexity to become a technological species. There have been large multicellular life forms on this planet for nearly 600 million years. For most of that time the "Wild Life" got along just fine without much intelligence. And if one particular group of "tree swingers" had not needed to adapt to climate change at a critical point in the history of life they would never have gone on to develop stone tools, fire, steam engines and the internet. It is perfectly possible that were it not for some little chance event in geology and climate then this planet today would have no roads or cities but just "wildlife". I think we must be prepared to accept that there are a fair proportion of planets that may develop large multicellular species but which never go on to evolve spacefarers before their sun finally scorches them at the end of its life.

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Note 1. All requests for planetary demolition must now be submitted in quadruplicate on form UX-565/B4 and be counter-signed by the assistant administrative officer for interstellar traffic calming - department QG-7. Subject to approval by the chief planning officer and the infrastructure development coordination sub-committee.