A reason for believing something is not the same thing as knowing a thing. I have good reason to think that there are habitable extrasolar planets somewhere in the milky way galaxy (with the amount of planets out there in the milky way there is bound to be some that are habitable). But I have no knowledge of this. Currently it is beyond the scope of our knowledge and ability to find out one way or another. Thus my position is one of agnostic belief.
MTF
You had me stumped there for a minute. But then I realised that the only reason you have for believing in the possibility of other habitable planets is the
knowledge you have that one already exists. You
know for certain that habitable planets do exist, since you're living on one, and that becomes the basis/reason/foundation of your speculation concerning the rest of the universe.
If we're talking about deities, it's a different ballpark. If we use similar grounds to your extra-solar planets example, an agnostic theist would still need their theist reasoning grounded in some form of firsthand experience, something that would render their agnosticism void.