[weikel]

If most stars have planets and there are around 200 billion stars( I think thats what Ive heard) in this galaxy alone the odds are there are some worlds with oxygen/nitrogen atmospheres and plantlife( at least I don't think an earthtype world is that improbable).

[RightWhale]

There is a book titled "Rare Earth" written in the popular style by a couple of actual scientists that points out just how unlikely the combination really is that made earth the great place it is. It's a quick and easy read but has a lot of material, a good overview of the situation as it was known a couple of years ago.