I have seen many articles that talk about a star's "habitable zone" as the ring around it which is just the right temperature for planets to have liquid water, which is necessary for life. My question is, why does a planet need to have liquid water in order to sustain life? Who says that extraterrestrial life can't live off of some other kind of sustenance?

  • $\begingroup$ Nice question, I guess this could get really philosophical and head towards asking "what is life?" - it's just what we define it as yet you are right, extraterrestrial objects might exist that under other definitions could be called life! $\endgroup$ – rg255 Jun 26 '13 at 20:55
  • $\begingroup$ @GriffinEvo Really? The current definition of "life" involves surviving using water? $\endgroup$ – Daniel Jun 26 '13 at 21:17
  • $\begingroup$ @GriffinEvo What are you asking? I don't understand the word "life" to require the use of water to survive. $\endgroup$ – Daniel Jun 26 '13 at 21:40
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    $\begingroup$ This is really just a side point because I started thinking about it a bit... I think finding life that survives well outside of the conditions we define as being necessary for life would force us to reconsider what defines life. Part of our definition of life (google "define life") is the ability to respire (a process within "metabolism"), this requires oxygen, but what if we find something that doesn't respire in a way we recognize? i.e. not using oxygen. Can we call that a living thing? (I think we would have to call it life) $\endgroup$ – rg255 Jun 26 '13 at 22:00

Your question is not complete. I would suggest to slightly change the title to something like:

"Why must a planet be in a star's “habitable zone” to have life as we know it from our experience?"

You are right, when you are assuming that there could be another than carbon based life as we know it from our planet Earth.

There are many hypotheses that life could be based on Si or other elements.

But carbon is still the easiest element to create bounds with other elements and carbon too. Why? Here is the answer from wikipedia Carbon-based life:

There are not many other elements which even appear to be promising candidates for supporting life - for example, processes such as metabolism - but the most frequently suggested alternative is silicon.3 This is in the same group in the Periodic Table of elements and therefore also has four valence bonds. It also bonds to itself, but generally in the form of crystal lattices rather than long chains. Silicon compounds are generally stable but do not support the ability readily to re-combine in different permutations in a manner that would plausibly support lifelike processes.

There is a great documentary from a physicist Brian Cox published by BBC this year called Wonders Of Life which deals with the basic questions how carbon based creatures were "born" on this planet ;) + other interesting things about life.

Brian talks there why the first organisms were created probably in water and how it could be.

Why water? http://www.pbs.org/wgbh/nova/evolution/liquid-of-life.html

And why is water the best liquid to do the job? For one thing, it dissolves just about anything. "Water is probably the best solvent in the universe," says Jeffrey Bada, a planetary scientist at the Scripps Institution of Oceanography in La Jolla, Calif. "Everything is soluble in water to some degree." Even gold is somewhat soluble in seawater. (Before you get any ideas about extracting gold from the oceans, I should add that, according to Bada, the value of dissolved gold in a metric ton of seawater comes to about $0.0000004).

If you take a route that there could be a different form of life outside of habitable zone, then many things are possible. E.g. life on asteroids, ouside of solar systems, life in other dimensions, life based on silicon or some other elements, energy based life? Or even dark matter and dark energy based life or gods/God-like creatures and all kinds of sci-fi ideas ;).

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    $\begingroup$ It should also be noted that other heat and energy sources are plausible replacements for stellar radiation. E.g., tidal warming of a Jovian moon and vulcanism. $\endgroup$ – Paul A. Clayton Jun 26 '13 at 22:59
  • $\begingroup$ Why does the fact that water is a good solvent make it necessary for life? $\endgroup$ – Daniel Jun 27 '13 at 5:27
  • $\begingroup$ @Daniel It is necessary for the life as we know it on our planet. If you want to speculate then many other things are possible. But the scientists take in count the fact how the elements are spread through the Cosmos. And they came to the conclusion that hydrogen and oxygen are pretty common. E.g. you have ice on Mercury, Earth, Mars and on Jupiter's and Saturn's moons, asteroids, comets and probably many other objects ad planets in space. More about the importance of water is e.g. in the 1st or 2nd episode od Wonder of Life, when Brain talks about the "cell = batteries" and "waterfalls";) $\endgroup$ – Derfder Jun 27 '13 at 6:30
  • $\begingroup$ @Daniel here is a summary of what the water is capable of and why it is so important for life (as we know it) and why astrophysicists are looking mostly for planets with water as a possible life candidate youtube.com/watch?v=7sA_JJ9gj2E Btw. the thing with God is exagerating, but everything else is a nice summary of some articles. So, if you want to watch a 10 min talk about how the water is good for life, watch that video. $\endgroup$ – Derfder Jun 27 '13 at 6:41

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