All of which means that you shouldn't be too confident in your probability distribution in such a foreign situation, but you still have to come up with a probability if it's relevant at all for action. Bad priors can hurt, but refusal to treat your uncertainty in a Bayes-like fashion hurts more (with high probability).
Yes, but in this situation you have so little information that .5 doesn't seem remotely cautious enough. You might as well ask the members of Strigli as they land on Earth what their probability is that the Red Sox will win at a spelling bee next year - does it look obvious that they shouldn't say 50% in that case? .5 isn't the right prior - some eensy prior that any given possibly-made-up alien thing will happen, adjusted up slightly to account for the fact that they did choose this question to ask over others, seems better to me.
We've had these for a year, I'm sure we all know what to do by now.
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