What dictates that extra terrestrial life requires water in the same way as we do? Even on earth we have fish who can’t live in the open air, and people who can’t live underwater.
What is to say that ET can’t eat space rocks to obtain all of its nutrients, or even more, what is to say that they require nutrients at all?
Edit: Thanks for all the wonderful answers. Makes perfect sense. They aren’t so much saying we HAVE to have water to create life, more that we only know how that works and looking for the unknown in the vast expanse that is the universe would be the worlds largest needle in a haystack game.
In: Planetary Science
On earth, literally all life depends on water.
Now, there’s no guarantee that all life *in the universe* depends on water. But if you’re looking for something, and you don’t know where it is, it makes sense to look in the most likely places first.
We dont know for certain that no live can exist without water but first of all, all life that we do not exists needs water to exist so its a pretty good starting point. And second of all life is when you get to the real basics chemistry. And for chemistry to happen you need a fluid medium otherwise the ingredients needed for life can be there but wont build life because they wont interact. And water has some pretty awsome propeties that makes a lot of chemistry possible. This isnt the case for most other fluids. So we search for water not because we know for certain that its the only possiblity but because its the most likely.
As far as we know, water is essential for life, so we look for planets with water so we can narrow it down in our search.
Water is good at chemistry. If you want to mix a bunch of random molecules together, then you need some sort of liquid to dissolve them in. Otherwise they aren’t really going to mix. Think baking a cake with just dry ingredients. If we accept life as a result of random chemistry then you look for the thing that’s best at facilitating that, and water is uniquely good at that while being simple enough to be relatively abundant.
So, there is a lot going on here. We’re looking for signs of life. Obviously you can conceive of life in many ways but we know of only one way it does work — carbon based life. That kind of life needs water for celular processes to work.
Is it possible there are other life forms that are not carbon based? Yes, indeed there are other possible bases for life — silicon, and I’ve also seen metal oxides being hypothesized to be able to be the basis of a form of life.
But again, those are hypothesized life forms. They would exist in very different forms and conditions than ours. As far as I know we are looking into investigating possible silicon based life form on Titan which has a lot of amonia to act as a solvent.
Aso to your other question — how do we know that life needs nutrients. Well that’s more simple to explain. You can’t have something that makes any kind of action function — living cell basically being alive, cellular processes and such, without expending energy. Energy is a must, and you get energy with chemical procceses. For these processes you need to expend some nutrients. No nutrients, no processes possible without the organism dying in short order.
What else are we going to look for then? Look, we know that water is necessary and fundamental to life as we understand it. We also know that water is fairly abundant (surprisingly so), and we know what it looks like when viewed from light years away. So it’s our “low-hanging fruit” and since we cannot look at everything all the time, we’ve reasonably chosen to pick the low-hanging fruit first. At some point, maybe we can expand our search if we learn some new physics things that lead us to believe non-water environments can support life, but for now, we’re using our limited resources to look at what is our current best guess when it comes to a potential for life.
Carbon has a cool property that it likes to make big fun and complex molecules (atoms that stick together because they have shared interests), and water is a good place to do that – we’re pretty sure that’s where life on our planet came from, we already have 1 planet we can make this assumption from (earth), so we assume other places might also come to a similar path if they have the same bits present for long enough
so we look for places with water and carbon, because water is a good solvent (stuff mixes up in it) and carbon being mixed and mashed with stuff could lead to a similar little proteins/molecules that form the stuff that eventually made the first little grumblers in our oceans
Any kind of biological live is based on carbon, there are a lot of studies, great thoughts about that. Even the next possibly chain builder the silicon is not able to build stable long molecule chains. So the possibility that (higher) live elsewhere runs on or at least started with carbon is very valid.
Why now water? Because other natural “solvers” would harm the long carbon chains and so kill live.
I’m pretty sure live could be very different especially highly developed ones that may even leave the barriers of biology. But the start of life will always be biological and that will be carbon based.
[https://en.wikipedia.org/wiki/Carbon-based_life](https://en.wikipedia.org/wiki/Carbon-based_life)
We know what water-based life forms look like. We know pretty much everything we consider a life form needs water, and is made up of it more or less. We do not even know what to even look for, to find non-water-based life forms, if anything like that does exist (big IF). This is really the main thing.
Let’s say I tell you I want you to spot me something magnetic, but also please do it from a mile away (scaled down to make it feasible to the layman, but you know). You know metal is magnetic, so you might look for something metal. Might there be SOMETHING that is not metal, that is magnetic? Sure! But you likely wouldn’t know what that is, so you wouldn’t even know what to look for, and would look for what you do know. And of course, finding metal may not mean it is for sure magnetic, but it’s a pretty good start and your best lead there.
Life, in every form we’ve ever understood it to be, is a series of chemical reactions. Water is a solvent the allows those chemical reactions to happen. Without water, the range of things that can happen is far, far more limited, and the opportunity for life to arise is much more difficult. It’s like trying to build a lego death star with rocks instead of actual lego. Could you do? Maybe? Kind of? But it can’t be very much. Same reason we look for carbon based life. Carbon can bond with so many different things that it gives life what it needs – flexibility and opportunity
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