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One science fiction short story I read (I don't remember the title or author, sorry!) featured some small creatures (unnamed) that were formed from metal, with the argument that "just like life developed on Earth with a carbon base, why shouldn't life be able to form elsewhere, with a metal base?"

Is this realistic?
Is there anything similar to this that already exists on Earth
(ie, life based on something other than carbon)?

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  • $\begingroup$ Hate to say it Shokhet, but I believe that is currently realistic. Not that we can prove they could exist, but our lack of knowledge on other lifeforms that developed outside of life as we know it is nearly 0...not even sure if our knowledge of chemistry remains valid as we reach certain points in our galaxy where interactions with forces we are unaware of could have huge implications. Horrible answer I know (tis why it's a comment), but I consider our complete lack of knowledge of life outside of carbon life extreme enough to make it realistic until we know more. $\endgroup$ – Twelfth Oct 22 '14 at 21:30
  • $\begingroup$ @Twelfth That thinking was a large part of what prompted this question. However, in Jewish learning there's a rule called "לא מצאתי אינו ראיה," loosely translated as "absence of proof is not a proof" .....if I were to be writing hard science fiction (which, at the moment, I'm not), I wouldn't feel comfortable using creatures like these in my world based on your line of reasoning alone. $\endgroup$ – Shokhet Oct 22 '14 at 22:25
  • $\begingroup$ I agree with that, hence why I left the answer a comment...however in your question you are not asking for proof, you are asking for 'realistic'. I strongly believe that lifeforms existing outside the realms of our knowledge is more than realistic given the relatively narrow scope of what we do know. (should add that it isn't my intent to discredit irigi as that's a well done post to the best of our current knowledge). $\endgroup$ – Twelfth Oct 22 '14 at 23:12
  • $\begingroup$ @Twelfth Fair enough. $\endgroup$ – Shokhet Oct 22 '14 at 23:39
  • $\begingroup$ An interesting article is here; newscientist.com/article/… $\endgroup$ – user9057 Apr 17 '15 at 13:58
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There is very detailed discussion about this issue in the field of Astrobiology. There is many constrains for "reasonable" life. I will try to go through most of them in short points.

Let me use following approximate definition of life in my answer:

Life is very complicated machinery of proteins (or other sufficiently complicated molecules or structures) that sustain themselves out of thermodynamical equilibrium in proper conditions, are capable of replication and of undergoing darwinian evolution.

To me, this seems quite general, and (I think) it does not trigger obvious counter-examples. For example fire is out of thermodynamical equilibrium, it is capable of replication, but does not undergo darwinian evolution nor consists of sufficiently complicated structures. Mule or other sterile animals are not alive per se according to the definition, but the environment in which they live and which produced them definitely contains life. This is a minor bug of this definition, but I do not know any better.

Life needs fermions

This criterion seems quite obvious. Fermions are elementary particles that follow the Pauli exclusion principle. Without it, everything collapses into one point or passes everything else and it is very difficult to imagine that there would be any reasonable complicated structured process involved. For example life based purely on electromagnetic waves is very implausible, since photons pass each other without noticing and they therefore cannot form structures.

Obvious candidates are electrons, which create atoms and molecules together with nuclei. But for example in the Forward's book Dragon's Egg, there are aliens inhabiting a neutron star and they are composed of neutrons and some protons in very dense degenerate neutron matter. This seems plausible in principle, since neutrons are fermions and could theoretically form structures in this environment.

Life needs liquids and membranes

If we investigate life based on chemistry, it seems life needs liquids. This is not an absolute criterion, however it seems that life as we know it strongly depends on liquid environments. There are incredibly complicated chemical processes in a cell, that require very precise conditions (pressure, concentrations of chemical compounds, density, etc.) In a gas, like in a nebula in space, density fluctuates heavily and it would be very difficult to do anything so complicated. Moreover, it seems that life needs to separate the relevant complex molecules it synthetized from the outer environment.

Plasma would be problematic too, since:

  • It is as volatile as gas
  • It is too hot for any complex molecules to be created

Metals and other solids have different problems. Complex molecules created in one part of the solid cannot be easily transported to another place, where they are required. In a cell full of liquid, every molecule floats around in a membrane or inside the cell and can randomly encounter other molecules and trigger chemical reaction. In solids, such mixing does not happen or would happen after very, very long time. (Of course now we are not talking about life looking like intelligent robots, that are assembled in factories. These would be probably plausible - but we are talking about life as a chemical process that could evolve on its own from more basic chemistry.)

Life needs complex polymers

If we investigate life based on chemistry, it seems it also needs stable polymers with rich chemistry. And here is a point at which carbon seemingly beats the alternatives:

  • Carbon: Very rich biochemistry with many polymers with hydrogen, nitrogen, sulphur, phosphorus, and others. Bonds C-H, C-O, C-C, C-S, C-N have similar energy, which allows for big complexity.
  • Silicon: Usually considered "the second best choice". Silicon, however, binds to oxygen much more strongly than to other atoms. This leads to a situation where either (on smaller temperatures) are silicon-oxygen bonds locked and cannot change, which makes things very difficult for complex chemistry, or (on higher temperatures) all bonds but the oxygen-silicon are broken and the complex chemistry must happen only between silicon and oxygen. (Silicones.) In absence of oxygen (which might be a problem, since oxygen is very abundant in the universe), and at very low temperatures, silane chemistry could be viable alternative for life. Zeolites at high pressures and temperatures seem like potentially viable option.
  • Phosphorus, sulphur, boron, nitrogen: These elements form polymers, but they are very unstable. Life utilising them would have to work at very low temperautes, where it could be problem to find a viable solvent. (Life needs liquids). Moreover, these elements would much more likely form simple stable molecules than polymers, which would strip such life of a building material.
  • Metal oxides: According to Wikipedia, oxides of metals have very rich chemistry, which could form a basis for life at very high temperatures and pressures (Like in Earth's mantle, for example).

As a conclusion: The universe seems to be quite restrictive about where life is plausible and what forms can it take. This list is not complete nor is it by any means certain, but it can give some idea what are the most probable candidates for life from the point of view of science.

Very good source can be found here - it is a short table listing polymer-forming elements and this is paired with typical solvent, that could take role of the liquid solvent in which the life based on given element could form. There is also another, even more extensive source on alternative biochemistries here.

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  • $\begingroup$ Thanks for your answer! ....you clearly put a lot of work into it, both in research and writing it down in a clear manner. You rock! $\endgroup$ – Shokhet Oct 20 '14 at 14:07
  • $\begingroup$ By technicality, the term "reproduce" when used in the context of defining life is not strictly referring to creation of additional members of the specie. Rather it can reproduce itself by making new cells etc. so an animal like a mule or steer is still alive because it can heal wounds which requires the "reproduction" of cells. $\endgroup$ – HadesHerald Apr 12 '15 at 1:28
  • $\begingroup$ Regarding absence of oxygen... isn't the abundance of free oxygen (as on our earth) only because we have (plant) life? (Ref. Great Oxygenation Event) And wouldn't bound oxygen (e.g. in the form of CO2, from volcanic activity) be "safe" for Silicon based life? Then again... what would they breathe if there is no free oxygen... $\endgroup$ – DevSolar Nov 23 '17 at 8:58
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There are several reasons why carbon based lifeforms thrive:

  1. Carbon molecules can form polymers, such as amino acids. This allows for proteins to form.
  2. Carbon molecules are stable, meaning that lifeforms won't have to regenerate their chemicals as rapidly as with a different chemical base.
  3. Carbon molecules react easily. This means that a lifeform will not need to put too much effort into keeping itself alive.

Silicon based life is often referred to as a possible base for life. This is inaccurate though as silicon does not have adequate properties for life to be formed from it.

You may have heard about arsenic based bacteria found. This has not only been disproved, but also miscommunicated - the arsenic was not replacing the carbon in its DNA, but the phosphorus.

In short, carbon is your best bet if you want to create a lifeform. It's unlikely that another chemical would form complex life.

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  • $\begingroup$ What about nitrogen? $\endgroup$ – Ashalynd Oct 19 '14 at 13:29
  • $\begingroup$ Nitrgen gas is very stable. Nitrogen-Nitrogen bonds in molecules are comparitively weak. This means a nitrogen based lifeform is comparitively unlikely because those same polymers would not form. Ones based on Nitrogen and sulphur or nitrogen and silicon are more likley but still fantastic. $\endgroup$ – kaine Oct 20 '14 at 17:10

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