Ok, this may be a little out on the loony fringe for this crowd (forgive me, I'm new), but this is something that has always bugged me.
Why do we assume that life on other planets will rely on water? Europa is often cited as a top candidate for life in the solar system (beyond Earth, obviously) because it has water. Why is this?
What is it about water that we think is so critical? Is it because of the theories about how life started (at a super-simplified level, that molecules in water were able to bump into each other to create replicating molecules)? Do we think the same could not happen in, say, a gas environment?
Is it something fundamental about biology? If so, why do we assume that other biologies have to follow the same structures as ours? In a different environment, might not another system have evolved?
Forgive me if this has been covered before. I am happy to read another thread on this if there is one.