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I guess it all boils down to how you define life, and you define intelligence. Given that planetary systems are almost becoming passé these days, I'd say there's a very good chance that in most planetary systems orbiting G type stars (or similar) that some terrestial style planets have formed in the life zone. Resonance orbits seem to be quite the norm from what observational scientists are currently seeing, so I don't believe that there's any reason that similar planetary systems to Sol's don't exist.
Of course, evolution is the big question. Will intelligent species always develop on planets like Earth? We'll probably never know.
As to aliens, why on Earth would they want to show themselves to a bunch of unintelligent, war mongering, greedy, self centered, violent humans? If I was an alien, and was monitoring us, I'd sure as hell make sure that I remained hidden.
Dave
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