But how often does that have to happen? They only looked at about 150,000 stars. There are hundreds of billions in our galaxy alone, and if alien civilization developed even 1% earlier than ours, they'd have had time to colonize the entire Virgo supercluster, so long as they start near the center.
I'd say that at this point we are largely ignorant of the odds of intelligent life existing in a solar system. While at least some basic forms of life ought to be plentiful in the galaxy, the conditions for evolution from simple life to intelligent life (that is, civilization-building life) just aren't understood to the level that would be required for ANY probability estimate to be given. Note that I'm not saying intelligent life is rare; I'm just saying that both scarcity and abundance of intelligent life are consistent with our current state of knowledge.
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