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Most of the news about extraterrestrial search focuses on possibility for existence of water and temperatures like earth. Why do scientists feel this would lead them to extraterrestrial life?


After all, we are searching for something that doesn't exist on earth. Why do we assume it would have behavior like us? Maybe the matrix inside their cells is of acetone or something else. Water may not play the role in their lives as it does for us. Maybe they can sustain high temperatures, maybe they can live in extreme cold. Why should we look for earth-like planets, isn't there a baseless (of course it's not baseless, there must be some logic which I want to know) assumption that the aliens are human-like (or any species on earth)?

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    $\begingroup$ Water is made out of two of the three most common elements and is one of the very most common molecules in the universe. Water is a great solvent because it is a bipolar molecule. Methane is another very common molecule, but it is not bipolar, it is not a good solvent. Not very exciting for biology. On Earth, life is everywhere where there is liquid water. Given how very little is known about the origin of life, liquid water is a signal to look for. The alternative would be unknown processes in uncommon liquids, and I suppose it is hard to get research grants for looking for that. $\endgroup$ – LocalFluff Feb 23 '17 at 12:09
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    $\begingroup$ You have to distinguish between the science done and how reports are written about this science. Water and life is way easier to sell to people, than to say 'yeah, we found 7 planets around a star, all we know is their radii and maybe their masses'. But also there is the small field of Astrobiology dedicated to thinking about exactly the questions you addressed. $\endgroup$ – AtmosphericPrisonEscape Feb 23 '17 at 13:22
  • $\begingroup$ Recommend closing because this is a biology question, not astronomy. $\endgroup$ – Carl Witthoft Feb 23 '17 at 13:57
  • $\begingroup$ @CarlWitthoft Astrobiology is on topic here. $\endgroup$ – called2voyage Feb 23 '17 at 15:04
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    $\begingroup$ Same question, on space.se: space.stackexchange.com/q/13624 $\endgroup$ – Mark Feb 24 '17 at 0:33
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A lot of different "alternative biologies" have been considered, but if you analyze their required chemistry, this always falls apart at some point, some mechanisms necessary for life are simply impossible. Replace carbon with silicon, increase ambient temperature, and you're getting a lot of interesting reactions that could be conductive to life. But instead of CO2, which is a gas, you have SIO2 which is a crystal. You can't breathe crystal. It's awfully insoluble, so no simple hemoglobin counterpart. And so on. Won't work.

Water and carbon based biology is one most plausible. There are odd variants, like bacteria thriving on sulfite, manganese and copper oxide, but they still require water to live, and still use carbon as the fundamental building block.

While others might exist, chance for them to form is much lower than for one similar to ours. And since we haven't designed even one fully plausible alternate biology that doesn't involve water we don't even know what sort of detectable signatures it would produce. So we search for something we know might yield results; hint at possibility of life somewhat similar to ours - instead of a wild goose chase after the unknown.


Actually, there exist one "alternate biology" which is fully plausible even if its early origins would be Earth-like: Silicon based, artificial intelligence, robotic physiology, electrical power, proliferation through fully automated manufacturing processes. It's unlikely to produce any detectable chemical signature (at least distinguishable from half of the natural celestial bodies out there), but we are fairly sure its electromagnetic spectrum - radio waves signature - would be quite specific. Well, The SETI project is on it.

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    $\begingroup$ Of course, the latter case might be better described as "synthology". That is a rare term though; the more used one is "synthetic biology" which puts your terming on solid ground. $\endgroup$ – called2voyage Feb 23 '17 at 16:26
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To copy my answer from Space.SE:

Virtually all chemical reactions take place in a liquid or gaseous phase. Uncontained gasses aren't a likely candidate for life, so we're looking for solvents for life-related reactions to take place in.

A good solvent is (a) common, (b) dissolves a wide variety of substances, and (c) is liquid at a wide range of temperatures. Very few liquids meet even two of these criteria, much less all three. The likely solvents for life are water and ammonia, and maybe methane.

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