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Lately I have been thinking about possible forms of life on planets that would have radically different experiences than here on Earth. I was wondering if life based on transition metals was possible at all, with iron being probably the easiest to consider. I've always found the idea of metallic lifeforms very interesting even though I know there are obvious troubles that come with it.

Let's imagine this was a solar system with quite high concentrations of iron, nickel, cobalt, chromium, manganese, copper, and similar. Not so high to be implausible but high enough to be considered very metal rich by observers.

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  • $\begingroup$ This is hardly possible, because metals (unlike non-metals) are not able to form strong Me-Me bonds in molecules. Which means that the whole biochemistry has to be non-molecular. $\endgroup$
    – Alexander
    Jan 14, 2019 at 5:47
  • $\begingroup$ Humans have an iron-based biology. It's one of the reasons why our blood is red. Do you mean iron rather than carbon as the basic chemistry? Please note that, insofar as we understand chemistry, there are precious few atoms that can take the place of carbon - iron isn't one of them. Iron-hydrogen bonds are not as numerous nor as stable as carbon-hydrogen bonds. $\endgroup$
    – JBH
    Jan 14, 2019 at 6:15
  • $\begingroup$ Incidentally, I've read before that as you get closer to the galactic core, you will tend to find more metal richness. More heavier elements further in, lower density further out. That is, there's a whole lot more supernova waste floating around in the inner galactic region, so your high metal planet might be pretty common closer to the core. $\endgroup$
    – JamieB
    Feb 24, 2023 at 20:47

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It's certainly possible but it's also unlikely because of a chemical property called Valency. Put simply, carbon makes a great basis for organic compounds because it's naturally tetravalent, or in other words can combine with 4 other elements or atoms to form new molecules. This means that you can quickly build very complex molecules with substantially different properties from each other, just sticking with a few common elements.

CHNO (Carbon, Hydrogen, Nitrogen, Oxygen) compounds are the basis of complex life on Earth and from these we get carbohydrates, LSD, alcohol, caffeine, morphine, and many other types of compounds that when used within a complex lifeform, have massively different effects.

Iron is at best trivalent, as I understand it usually bivalent, meaning it doesn't quite have the same ability to build complex organic molecules as carbon. If it could, we'd probably be iron based life forms because iron is more plentiful in our environment than carbon is.

Also, because iron oxidises VERY well, it may well restrict the capacity of the lifeforms to metabolise and oxidise because any oxygen that comes near the iron in the lifeform is likely to be trapped exceedingly well. This is in fact how our hemoglobin works to get oxygen through our bodies right now. If the entire lifeform was iron based, it may not be as efficient at getting oxygen to all parts of it.

This is one of the key reasons why silicon is generally seen as the most likely candidate for alternative biochemistry, although it's not as prevalent as carbon. Another possibility might be Chromium (because in certain conditions it can be hexavalent), but that would be even more exotic a lifeform to form and it is difficult to imagine how it might work.

So possible? Yes. Practically possible? Probably not, at least not if you want complex diverse life on your planet.

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    $\begingroup$ Witness me, Shiny and Chromium!!! $\endgroup$
    – Joe Bloggs
    Jan 14, 2019 at 10:41
  • $\begingroup$ "If it could, we'd probably be iron based life forms because iron is more plentiful in our environment than carbon is." <- while generally true, there is also much more silicon than carbon on Earth and it is also tetravalent. So, while being tetravalent is helpful, it does not seem to be the only factor. $\endgroup$
    – Nosajimiki
    Oct 3, 2022 at 18:45
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One of the things that most of these answers assume are life forms in an earth like environment. Yes, iron forms 2-3 bonds at room temperature and oxidizes well like as the other answer stated. But in certain conditions iron can form up to 6 bonds, I don't think carbon has that capability.

What if you had the right conditions where iron could form more bonds than 3 easily? Could a high temperature, high pressure environment with lots of oxidizing agents like oxygen, nitrogen or sulphur allow iron to easily form more bonds? Would there be more complex structures formed with iron in the environment which could then be the basis of iron-based life?

I'm very sure that we could see some really unorthodox chemistry with other environmental settings. My money is on iron being the base for life because it can be just as robust as carbon in forming structures given the right conditions.

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