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My question may not be suitable here, because it's more of astrobiology. Life as we know it is carbon based.

  1. Is life based on silicon possible?
  2. What would the conditions for habitability for silicon based life be?
  3. Can both carbon- and silicon-based life co-exist?
  4. Is any silicon-based life found on Earth? Can any be grown in the laboratory?
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closed as off-topic by dmckee Feb 28 at 20:05

  • This question does not appear to be about physics within the scope defined in the help center.
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I think astrobiology questions are completely relevant. It's a hot topic in astronomy. –  dagorym Jun 19 '11 at 15:52
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This question appears to be off-topic because it is about hypothetical biological analogs, not physics. –  Brandon Enright Feb 27 at 21:16
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I'm voting to reopen. This is best asked in biology but I don't see the need to close it, and I really don't see the point of closing old questions. The worst that can happen is that there is no one here with the expertise to answer it. –  Emilio Pisanty Feb 28 at 9:59
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@EmilioPisanty exactly, I generally never saw the point in going after old well received and nicely answered questions to close them today just for the heck and fun of it, either ... –  Dilaton Feb 28 at 10:31
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The question is actually a list of question and while some of them are physics questions the core issue is not and the existing answers are about chemistry and biology. I'm re-closing it. And the reason for closing old questions is to leave a sign-post for future visitors about what does and does not go. –  dmckee Feb 28 at 20:05

2 Answers 2

You might be interested in following this link to a discussion of the subject in a NAI featured question:

The short answer is probably not. Silicon simply doesn't have the moves. While carbon is perfectly comfortable in a variety of different structures (rings, long chains, multi-ring chains, and double-bonded carbon catenations), silicon’s analogous structures are comparatively unstable and sometimes highly reactive. Additionally, such analogous silicon compounds may never occur in nature; the largest silicon molecule ever observed had only six silicon atoms. In contrast, some carbon-based molecules can have tens of thousands!

(Old link here unfortunately no longer maintained; available here through the Wayback Machine.)

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"the largest silicon molecule ever observed had only six silicon atoms. In contrast, some carbon-based molecules can have tens of thousands"- this is very wrong. Polysiloxanes are long chains $(-Si-O-)_n$. –  DavePhD Feb 28 at 17:49

ONE. This question is very speculative, so I don't think a specific and technical answer is available. Scientists are extremely hesitant to call anything impossible (including both silicon-based life and a conclusive answer to this question). Certainly no one has yet proven that life can't be based on silicon, and because it has the same number of valence electrons in its p orbital (four), it is tempting to think that you could just swap out carbon for silicon.

Unfortunately, carbon-based chemical compounds tend to be just the right amount of "floppy." That is, large structures built on carbon like muscle tissues have enough cohesiveness and structure to maintain their architectural integrity when stressed, but enough flexibility to act as, well, muscles. Silicon-based structures, on the other hand, tend to fall on either side of the "Goldilocks" structure- either much too rigid (like silica sand), or much too gooey (silicone, as in breast implants). That is obviously a very broad answer, and I would welcome comments or edits on the subject.

TWO. This is an even longer extrapolation from known science, but we can probably guess. Someone with a stronger chemistry background than I might be able to assist. However, in discussing carbon-based life, we always talk about the critical importance of liquid water. I would imagine that a liquid solvent would be just as critical in the development of silicon-based life. Chemists, would water still be the gold standard for silicon solvents? Some other?

...So, for whatever solvent silicon life used, you'd need temperatures in its liquid range. Besides that, certainly an energy source. Light from a star, or chemical energy, or a heat gradient from the planet's warm core? Hydrogen, oxygen, and nitrogen will likely play the same roles for silicon life as carbon. They are the most abundant chemicals with their respective numbers of valence electrons.

THREE. Humans might be able to live peacefully side-by-side (or spacecraft-by-spacecraft) with silicon-based aliens, but there's a good chance that free carbon-based and silicon-based ecosystems on a single planet would be poisonous to each other. Even if not, they would have to compete for energy and available chemicals as mentioned in the previous question, so I think one would necessarily go extinct as it was out-competed.

->This is a really cool concept! Evolutionary biologists have examined competition and survival of the fittest at the genomic, organism, population, species levels, but survival of the fittest biochemistry? You might be able to make some predictions on the basis of efficiency of chlorophyll photosynthesis vs. the silicon energy source.

The only way I could see natural coexistence is if the two ecosystems were almost entirely non-interacting and non-competing. For instance, perhaps a land-, carbon-, and photosynthesis-based ecosystem coexisting with a deep sea or subterranean silicon- and chemical energy-based ecosystem.

FOUR. None whatsoever. Silicon-based life is purely speculative at this point.

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You said carbon based compunds are the right amount of floppy. Won't a more rigid based structure be more suitable for planet with higher gravity? –  Jonathan. Jun 19 '11 at 20:22
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@Jonathan, not the right kind of floppy. :) This is floppy in a chemical sense (for chemical reactions) not in a structural sense. –  Larian LeQuella Jun 19 '11 at 22:34

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