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As far as I know, scientists seeking extraterrestial life are simply looking for plantets containing water in liquid state. To be honest, I find this weird.

It is true that life - in the form we know on Earth - cannot exists without water. But how can one proof that such life isn't but a special case out of a bunch of other different types of life?

Actually, I think we can come out with at least one example of a "organism-like thingy" that would not require liquid water. Well, we could build a simple robot, whose "natural ecosystem" would be a junkyard (for mechanical, electric and electronic garbage). That robot would would fuel itself with solar energy, and reproduce by seeking out appropiate parts in the junkyard and building a copy of itself from them. Of course, that's just a theoretical example, I know it has many faoults, but I still hope the idea is clear - it should be indeed possible for life to exists without water.

I have also heard (sorry, I can't remember the source) that theoretically, there could be an organism whose metabolism would rely on sulphur.

So, unless scientists can proove that no life can exists without water (any life; not just Earth-like life), why do they keep looking just for water?

Unless, of course, we define "life" as "any organism functioning like Earth organisms; therefore, metabolising organic structures with enzymes, built of protein, requiring water, etc"

BTW. This question is a little bit different than Why is liquid water considered a requirement for life? . The answers there were pointing out that water is a good solvent. And I'm (also) asking why is any similar solvent considered needed. See my example above.

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When you lose your keys on a dark night you look where the probability of finding them is highest (which is a combination of being likely to see them with their probability of being there), which is not necessarily where they are most likely to be. In the case of the search for extra-terrestrial life we have a fairly good idea what the signature of carbon based life using water as its primary solvent (conventional life) looks like. We also know that such life is possible. Other unconventional forms of life could exist but we have a very poor idea of what signature to look for, so we devote most of our effort to looking for the signature we will recognise.

We do not need to prove that unconventional life is impossible in order to spend most effort on searching for the conventional. It is sufficient to suspect that the probability of finding such life is higher than unconventional just because the probability of detection is so much higher. If we find nothing looking for conventional life, then a larger proportion of the effort will be devoted to searching for the unconventional.

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    $\begingroup$ That is exactly the thing I use in my lectures! My version has a policeman asking a drunk, who is on his hands and knees underneath a lamp post, what he's doing? "Trying to find my keys officer" says the drunk; "Where did you lose them?" says the policeman; "On the other side of the road", says the drunk, "but here's the only place I might find them"! $\endgroup$
    – ProfRob
    Commented Jan 15, 2015 at 21:38
  • $\begingroup$ That's a very old joke, @RobJeffries. I use it, too. The only difference is that I use "but this is where the light is shining" as the punch line. $\endgroup$
    – David Hammen
    Commented Jan 16, 2015 at 18:29
  • $\begingroup$ It is not a joke, it is an illustration of the underlying idea of search theory. The search concentrates on those regions where the product of probability of detection (if the target is there) with the probability that it is there is largest. The probability that the target is in a region is then modified using Bayes rule if it is not detected. $\endgroup$
    – Conrad Turner
    Commented Jan 16, 2015 at 18:36
  • $\begingroup$ True (of course), but the joke is there is no chance of finding the keys where you didn;t drop them. $\endgroup$
    – ProfRob
    Commented Jan 16, 2015 at 18:45
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    $\begingroup$ What is important is the product of the detection probability and the location probability. That one is low for a region does not stop the product being maximal there and so that being the best place to look. $\endgroup$
    – Conrad Turner
    Commented Jan 16, 2015 at 19:02
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All biological life requires an electrolyte.Water is the only none reactive solution that can carry the solute,(salts,acids,alkali,) that would cause reactive change. Water has limits in its ability to remain the solution(evaporation:thereby restricting chemical mobility,)or (crystalisation/freezing:thus causing precipitation of chemicals in the "solid state".) Biological life ceases to evolve.

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  • $\begingroup$ And have you read my example? $\endgroup$
    – gaazkam
    Commented Jan 16, 2015 at 20:43
  • $\begingroup$ Gaazkam,with respect as I do not know you.The film "Robots" was entertaining,however all components of the robots would require water in their manufacture.Automotive engineering,cars or robots is the same so spare parts would have to be processed.This takes us back to the basic requirement.(perhaps you joke)Cheers,Colin $\endgroup$
    – Colin Shorey
    Commented Jan 16, 2015 at 22:15

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