"Life needs water in order to survive"

Says who?

Why do scientists rule out life on other planets/in other galaxies if they know there is no water to be found there?

Are they assuming that just because life on earth needs water, then “surely” life on other planets would need water too?

Isn’t that being a bit narrow-minded?

Wouldn’t that go against evolutionary theory? Why can’t life on other planets evolve in a way that they don’t need water in order to survive?

Less narrow minded than you might think. It’s certainly possible that life could exist without water, but water (and carbon) make life much more convenient. One reason is that water is less dense when solid, allowing water-born things to survive under the surface during a winter (either periodical or permanent.)

A related question I’ve always had, and was going to post as a separate thread until I saw friedo’s post, is why is it generally assumed that life has to be “carbon-based”? Again, it’s what we know, but are there no other possibilities?

If it’s “certainly possible that life can exist without water” then it is narrow-minded. No possibilities should be excluded. As for carbon, Asimov made some good points in substituting silicon for carbon.

We once thought that no life can exist in extremely thermal or frigid conditions, but we found life in extremely hot springs and in the polar areas. So we cannot be so smug in declaring that life needs water.

However, it is difficult to fathom how life can exist w/o water. What kind of life could it be? How could it be? We know all the elements in the universe. Which element or combo could support life? It is easy to replace carbon with silicon, but what could replace water? Since life on this planet needs water, it is reasonable to assume that all life would need water as all things in this universe are based on a common origin. If you are to posit life w/o water, you should also posit how such life can exist.

Well, it is possible to imagine a life form that uses liquid ammonia or liquid methane or liquid hydrogen as a solvent. The trouble is that these don’t seem to have the special properties water does.

The most common elements in the universe are hydrogen, helium, oxygen, nitrogen, and carbon. Helium is out, since it doesn’t form compounds. So, H+H makes hydrogen gas, C+H makes methane, N+H makes ammonia, and O+H makes water. These are the most common compounds in the universe.

We can imagine creatures using more exotic compounds, but it is hard to imagine how, say, chlorine might be segregated on a planet enough to form the basis of a biosphere.

And the trouble with silicon is that it really doesn’t form the same sorts of compounds that carbon does. Although silicon can form four bonds like carbon, it doesn’t bond as tightly. We have millions of highly complex carbon compounds on earth, but only a few complex silicon compounds. There might be environments that favor the formation of complex silicon compounds, but we have no idea what those might be. Carbon, on the other hand, sticks itself together all the time. The ability of carbon to form complex compounds is what sets it apart from the rest of the periodic table, silicon included. That’s why we call it “organic chemistry”.

First of course we have to define what we mean by life. I personally see no problems with life existing as electrical signals inside an inorganic/semi-organic matrix (ie basically a computer/robot) that wouldn’t require water.
Life could exist entirely in a gaseous or solid phase without the need for water as a solvent.
I think the idea that life has to somehow run around on legs to still qualify as life is very narrow-minded.
How about life existing as plasma within the atmosphere of a star?
Of course the problem with most of these lifeforms is we probably wouldn’t recognise them as life even if we saw them. We would recognise water/carbon based lifeforms.