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A large problem with any of this is that we have a very, very, very limited dataset with regards to life. We know of one planet, in one solar system, in one ga

• dotancohen 6y ago
  I agree with you, but the counterpoint is that anything other that this might not fit any defensible definition of life.
• dheera 6y ago
  Agreed. I would think that silicon-based life is possible and would require an entirely different set of elements to sustain. I would argue that a better definition of life should be based on (a) entropy and (b) propagation. Living systems (a) actively counteract entropy-increasing forces within their system as long as they are alive, and (b) are able to propagate indefinitely given the necessary energy inputs. I've heard other definitions of life including dependence on water and I think those aspects should not be part of such a definition.
  • adrian_b 6y ago
    There are 2 constraints for the set of chemical elements required for life, one is the ability to build very complex molecules from them and the other is the existence of some exothermic chemical reactions between the simple substances that are produced abiotically, i.e. the existence of some chemical reactions that can provide energy. It is quite likely that there could be some alien life forms that would use much more varied silicon compounds than those used on Earth, but it is quite unlikely that there are environmental circumstances where silicon could replace carbon as the main structural element. Silicon without carbon can provide only a much less variety of molecules and all the naturally occurring compounds are too stable, i.e. you cannot extract energy from them by rearranging the atoms, like you can in carbon compounds. The most important condition for the appearance of life is an energy source. There are various abiotic processes that produce dihydrogen, carbon monoxide and carbon dioxide, and these gases can enter exothermic chemical reactions (e.g. dihydrogen + carbon monoxide => acetic acid), which can sustain life. There are no such energy sources from silicon compounds. When oxygen is in excess, silicon forms silicates, otherwise it forms carbides and nitrides. All these are inert. The only environment where silicon could play a role in the appearance of life would be a place with a very large abundance of hydrogen, i.e. a large planet like Jupiter, where it might happen that all the elements are present as compounds of hydrogen, so the silicon would be present as silanes, and there are differences in stability between various silanes that might be exploitable as an energy source if there exists any abiotic process that generates less stable silanes instead of the more stable silanes. This possibility remains open just because I have not seen yet any model of such an environment to determine what compounds may appear and what chemical reactions are possible, so it cannot be completely excluded that silicon could play a role there, unlike on medium-size planets or smaller bodies, where the hydrogen is less abundant and silicon does not combine with it, but with the more electronegative elements. I am not aware of any attempt to analyze what chemical reactions are possible in an environment with too much hydrogen, but I suspect that both the too oxidized environments with too much oxygen and the too reduced environments with too much hydrogen make impossible the occurrence of chemical reactions that could be used as a source of energy for life forms, so I think that life on Jupiter-like planets is unlikely.
    • dheera 6y ago
      If a set of robots could, on their own, heal themselves of certain amounts of damage, maintain internal order, as well as reproduce by mining and purifying silicon and assembling it into new robots, without aid from anything else but raw materials and, say, solar energy, as well as evolve their own design as they assemble new robots, I would claim that that also qualifies as silicon-based life. To be able to mine for materials and manufacture new copies of itself is a more macroscopic way of creating those complex molecules than the smaller-scale processes used in Earth life, but I don't think that should be the differentiating qualifier for what constitutes life. It's rather just a different reproduction process than mitosis and meiosis but is a valid reproduction process. To some extent I think we will invent silicon-based life or some other life to carry on our legacy after the solar system dies. Carbon-based Earth life is far too sensitive to environmental conditions, and what I envision of silicon-based life is something that can handle a much wider variety of temperatures, pressures, radiation, and gases, which would be much more suitable for a spacefaring continuation of humanity.
      • adrian_b 6y ago
        You are right that self-reproducing automata can be designed and built using silicon and/or many other chemical elements besides those used in living beings. However we do not know any mechanism that would allow the spontaneous apparition of such living beings by natural processes, like those that exist for H, C, N, O and S as the main bioelements. After some life forms appear spontaneously using a set of chemical elements very similar to that used on Earth, it might be possible at some point to transition to the use of other chemical elements, because the original constraints to start from substances that can be generated in abiotic conditions no longer apply. So I agree that silicon-based life can exist, but there is no known process that could make silicon-based life forms appear spontaneously from non-living matter, unlike the case for carbon-based life. Actually carbon-based life is a misnomer, because there are several other chemical elements that are strictly required for various reasons, so there is a minimum set of about 8 to 12 chemical elements that are needed for life, not only carbon. While H, C, N, O and S are needed to build the minimal set of organic molecules and several transition metals are needed as catalysts, there are also a few other irreplaceable elements, e.g. potassium is needed to neutralize the excess negative charge of the organic acids over the positive charge of ammonium derivatives. (Other elements cannot replace potassium because they would form insoluble compounds).