ELI5 Why is finding water on other planets one of the most important things for us to look for in the case that there is life there?

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So life needs chemistry to take place to exist. For chemistry to take place chemicals have to come into contact with each other. A chemical sitting on one rock and another on another rock won’t react with each other. So they need to mix in something. This best place for chemicals to mix is in a liquid. Of all liquids water is the best for this to happen in since water can dissolve more chemicals in it than any other liquid. Hence, they look for water.


The answer: All forms of life that we know of requires liquid water. The answer to *but that’s just because **we** need it*: Life has had roughly 4 billion years of evolution on this planet. If any species could have evolved not to use water they would have had a huge evolutionary advantage compared to all the other species on this planet. It would be the dominant species on this planet. Life evolved to fly, swim, hibernate, walk, run, hop, glide, swing between the trees, live without the Sun, without oxygen, *with* oxygen. But somehow nothing evolved to live without water. There must be a reason why. Hence we search for liquid water.


We have one reference for life. Here. Life here required water. The presence of water supports the current existing evidence for the possibility of life – it is a similarity to the existing example, here. There are some special things about water. Strong acids destroy proteins, so if you detect sulphides or the like, then the life there would need to be substantially different in nature to our reference example. Water is neutral, not too acidic not too alkaline. It means that any developing life don’t get destroyed by the conditions present. Water is also a solvent. All life must to be classified as life, metabolize. Its hard to eat rocks. Water can dissolve things that basic forms of life can metabolize.


Because as far as we know so far, water is essential to every form of life, under our current scientific definition. No water means no life as far as we know. So basically since the universe is so vast, we are only looking for the type of life that fits our current definition, since that is already hard enough. If we start looking for new definitions of life as well, we just made our job exponentially harder. We wouldn’t even know what to look for, so we might as well stick to what we know, at least until we can confirm that exists outside of earth.


100% of all planets with known life on them have a lot of liquid water on them. Now, we don’t know that water is *required* for life, but we do know that water *can and did* help create life on Earth. So given that there are 300,000,000,000 or so stars to look at in our galaxy, it’s a lot easier to look for water than it is to look for, say, oceans of molten boron, because we have no idea if life even could exist under those conditions. Liquid water is relatively easy to detect and is the one thing we know *could* support life… so why wouldn’t we prioritise looking for that?