The insistence on water is not so much a peculiar egocentricity or some sort of narrow-mindedness that keeps scientists from imagining fundamental alternatives to life as we know it. It's simply a limitation imposed by the laws of chemistry, McDonald said. If you're going to do biochemistry, you have to move things around. And it's a lot easier to move molecules around in liquid than it is in solid. It is not likely that the complex chemistry required to form living organisms can occur when molecules are locked in a solid matrix.
Vapor poses another barrier to developing life.
If you're a cell, you've got to keep your cell machinery together. So just on a physical basis, it makes the most sense to have life in a liquid medium. Scientists have suggested a few other solvents that might work in biochemistry for forming living organisms. Ammonia is the most promising of these, but liquid ammonia exists well below the freezing point of water, at temperatures where molecules and chemical interactions move only very slowly. If life did exist in a liquid-ammonia environment, it would face major challenges doing the simple chemistry it needs to fuel its own metabolism, McDonald said.