I always here “life as we know it needs water to live” and like, that’s just life as we know it right?
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Life as we know it is made up of very long carbon chains. Basically all life that has ever existed on Earth uses these long carbon chains (proteins, fats, hormones, vitamins, etc.). We don’t know how life can exist without these long carbon chains.
However, most of the Earth’s atmosphere and surface (and that of many other planets) is made up of very small chains of several elements (2-10 atoms per molecule). To convert these small molecules into big molecules capable of supporting life, we need water.
Now onto why the water needs to be liquid: gas water can escape the atmosphere and leave the planet forever, solid water can’t be consumed as easily by organisms that need it to make long chains.
You are correct in that it is theoretically possible that there is life elsewhere in the universe that doesn’t require water. The issue is that we have no idea how to identify such theoretical signs of life. We do know that all life on Earth is based on water, and we know how to look for signs of water in space, so we focus on that for now, because that’s a direction we know to take.
Chemical properties of water make it a perfect solvent, and the best environment for chemical reactions. And what is life if not ‘tricky complex chemistry’
To unpack the statement, there is some logic behind it.
1. Life requires “liquid water” to live. If the water is all ice, it’s probably too cold to the chemistry required for life. If it’s all steam, everything is probably on fire, so no life.
2. Water is a fantastic chemical that can be used to do “tons of different things”. Look at us, we use water in everything from producing the energy we need to live, to transporting nutrients and oxygen in our blood, to removing waste, to focusing light in our eyeballs to see. Water is just a great chemical.
3. Water is both stable and contains huge energy. Water isn’t explosive, it doesn’t *want* to change form in chemical reactions easily. The opposite way to look at this is that the atoms that make water up, oxygen and hydrogen, have a ton of energy bound up in their bonds. This just makes water a great vehicle for storing and transporting energy, for example plants store sunlight by using it to force water molecules apart and making sugar. We use sugar and break it back down into water to get energy back out to live. Water is kind of like a really powerful rechargeable battery that living things can use for life.
4. So it’s not that it’s impossible that life existing without water, it’s just that water is sooo sooo good for life from our humble knowledge that we haven’t identified alternatives.
5. Finally, water is made of oxygen and hydrogen, which are super abundant chemicals in the universe. Given the conditions that we *believe* are required for life to existing (like temperature ranges, radiation ranges) you’re likely to find liquid water. So it’s circular. We look for water to find the conditions we believe are required for life, and when we find the conditions we believe are required for life, we find water.
We think that it probably doesn’t, but we only have a single example to go on and that one did.
We can envision all sorts of theoretical things that might work to qualify as “life”, bu the only life we have actually seen in practice is the one on our planet and that one got started in and requires water.
So of life as we know it we need water.
We have no idea what other types of life might be out there and thus no idea for what to look for those other types of life.
We do know what we need to look for for our type of life: water.
So we make that our base point. Could liquid water exist in place x, have we seen signs liquid water in place x? If yes than our kind of life could possibly work in that environment.
As far as we can tell, the absolute minimum for ‘life’ is some sort of pattern that causes copies of itself to come into existence and that these copies sometimes have errors.
That could theoretically include a whole lot of things that are very, very alien to us.
The problem is that we know how physics work to a degree, but we can’t predict what sort of phenomena might end up fitting that bill.
I mean if someone gave you complete knowledge of all the elements and the basics of how atoms interact with each other to form molecules and general chemistry, you wouldn’t necessarily be able to predict that RNA/DNA could be a thing let along that after a few billion years of copying with errors some of the results have the ability to think for themselves and wonder if they are alone in the universe.
Might there be life based on chemistry other than out own organic chemistry? Could life come into existence that isn’t chemistry based at all?
Who knows.
For all we know there are sentient sun-spots living on a star a few hundred light years away who have looked at our sun and dismissed the possibility of it harboring life because the conditions for life as they know it are now right.
In short we don’t know what to look for in regards to alien life so we look for life that is not that alien instead.
Water is a great solvent for life to use to carry out biochemistry reactions such as building proteins.
However, this is just for life as we know it. It may not be fair to assume all life works like the life on Earth does. It could be possible that some theoretical life forms may not need to depend on water to carry out their biological processes.
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