“Its skin was of an oily smooth grayness. Its motions were slow, as became a creature who burrowed in stone and was more than half stone itself. There was no writhing of muscles beneath that skin; instead it moved in slabs as thin layers of stone slid greasily over one another. It had a general ovoid shape, rounded above, flattened below, with two sets of appendages. Below were the ‘legs,’ set radially. They totaled six and ended in sharp flinty edges, reinforced by metal deposits.”
The “silicony”—a creature made from silicon—features prominently in “The Talking Stone,” a science fiction short story written by Isaac Asimov. This tale was first published in 1955 in The Magazine of Science and Horror. It later appeared in the 1968 collection of short stories of science fiction mysteries called Asimov’s Mysteries.
By some accounts, this story served as the inspiration for the Star Trek episode entitled “The Devil in the Dark,” written by Gene L. Coon. This classic episode, which first aired in March of 1967, features an encounter between the crew of the Enterprise and a silicon-based life-form called a Horta.
Thanks to these well-known pieces of science fiction, many laypeople hold the view that there could be life in our universe that is unlike that which we know here on Earth—for example, life based on silicon, instead of carbon.
The first scientific proposal for silicon-based life extends back to 1891 and the ideas of German astrophysicist Julius Scheiner. Since that time scientists have debated the prospects of silicon-based life—with some embracing its plausibility and others dismissing it. But as a team of astrobiologists from MIT have recently pointed out, to date no one has systematically and comprehensively assessed the capacity of silicon to support life in both a terrestrial environment and plausible nonterrestrial settings. They tackle this problem in a 2020 review article published in the journal Life in which they present a detailed evaluation of silicon’s life-support capacity.1
This work has obvious implications for astrobiology and origin-of-life models. But the implications extend beyond science. The research also highlights the design of the universe, our solar system, and the biochemical systems that constitute life.
Life’s Requirements Before it’s possible to assess the usefulness of silicon as a chemical framework for life, it’s necessary to identify the general chemical requirements for life. The team from MIT notes that any life-supporting chemical element must display sufficient chemical diversity. This chemical diversity is required to produce the chemical complexity necessary to generate the diverse collection of molecular structures and chemical operations required to originate and sustain living systems.
There are two facets to this diversity: the capacity for an atom to produce molecules with a variety of shapes and the capacity of the atom to form compounds with a range of functional diversity. Both features are illustrated by carbon-based (organic) compounds.
Carbon atoms form four covalent bonds. This bonding capacity can be expressed as (1) four single bonds, (2) a double bond and two single bonds, or (3) a triple bond and a single bond. Each of these bonding configurations produces distinct geometries: tetrahedral, trigonal planar, and linear, respectively. Under specific conditions, carbon atoms can also form a special type of bonding arrangement called aromatic bonding.
Bonds between carbon atoms and carbon-hydrogen atoms are highly stable. As a consequence, carbon can form compounds with lengthy carbon-carbon chains. These chains can be linear or branched. Carbon can also form compounds with carbon-carbon bonds that form rings. These compounds can consist of single rings and fused rings.
Collectively, these properties make carbon (and hydrogen) an ideal atom to serve as the molecular scaffolding that can result in a wide range of molecular shapes and sizes.
Carbon can also form stable bonds with atoms such as oxygen, nitrogen, sulfur, and phosphorus. These so-called heteroatoms can bind to carbon atoms in a variety of ways to yield a wide range of groups that impart a diversity of function to carbon-containing compounds.
Carbon forms compounds that are both soluble in water and water-insoluble. These two solubility profiles are necessary to sustain living systems. Water-soluble compounds dissolve in water and are readily available to take part in the chemical reactions needed to sustain living systems. Compounds that are water-insoluble can aggregate in an aqueous setting, resulting in molecular complexes that form the basis for cellular structures. The water insolubility of some carbon compounds also results in a phenomenon known as the hydrophobic effect, which leads to the formation of cell membranes, drives the formation of the DNA double helix, and causes proteins to fold and interact with other proteins to form protein complexes and supramolecular structures.
This discussion of carbon’s diverse features leads to the question: Are there other atoms that display chemical diversity on par with carbon that could serve as the molecular basis for life?
Silicon Chemistry Based on its position in the periodic table, at first blush silicon is expected to have the best chance of any other chemical element to rival carbon as a life-support system. Silicon has similar chemistry to carbon. It has a valence of four and forms Si-Si and Si-H bonds. It also forms bonds with heteroatoms such as oxygen.
But make no mistake, silicon chemistry only superficially resembles carbon’s chemistry. In many respects, silicon displays fundamentally distinct chemistry from carbon. This difference, I’ll explain, undermines silicon’s capacity to support life, at least in an aqueous environment.
As the authors of the Life paper state, “Silicon and carbon are ‘false twins.’ Their similarities are superficial and insufficient to mitigate their crucial differences. Chemistry that is stable and normal for carbon is unstable and exotic for silicon, and, similarly, chemistry that is unstable and impossible for carbon is stable and routine for silicon. Silicon’s distinct chemical characteristics and reactivity make it a challenging choice for life.”2
Some of the reasons for the difference in life-support capacity between carbon and silicon are:
Silicon has a much larger atomic radius than carbon. This difference impacts the bond angles, bond lengths, and bond strengths when silicon forms covalent compounds. For example, Si-Si bonds are much longer and weaker than C-C bonds, again because of the difference in atomic radii of these two atoms. The same is true for Si-H bonds. The weaker bonds make Si-Si and Si-H bonds much more chemically reactive than the corresponding C-C and C-H bonds.
Unlike carbon, silicon generally doesn’t form double and triple bonds, limiting its chemical diversity.
Unlike carbon, the electronic configuration of the silicon atom yields low energy, unfilled 3d orbitals. These unfilled orbitals allow silicon’s valence to extend beyond four, leading to compounds in which silicon forms five or six bonds. While this feature expands the chemical diversity of silicon-containing compounds, it also makes the Si atom highly chemically reactive, resulting in a relatively unstable Si-Si bond compared to the C-C bond.
Thermodynamic properties also distinguish carbon-based and silicon-based compounds. Silicon-based compounds have a much higher heat of formation than organic compounds. This property makes it much more difficult for silicon-based compounds to form than carbon-based compounds. It also renders silicon-based compounds less stable and much more chemically reactive.
Silicon is more electropositive than carbon. This property impacts the stability of the chemical bonds that silicon forms with heteroatoms. Because atoms of silicon don’t hold electrons as tightly as carbon atoms, the bonds silicon forms with heteroatoms are more polarized, less stable, and more reactive than the corresponding bonds carbon forms with heteroatoms.
Silicon reacts aggressively with oxygen. In fact, silicon’s natural propensity is to form Si-O bonds in the presence of oxygen or oxygen-containing compounds. In contrast, carbon’s propensity is to form C-C bonds. This difference means that in the presence of oxygen, silicon-based compounds will have a tendency to react with oxygen, forming silicon dioxide.
Solvent Effects Up to this point, the analysis of silicon’s suitability as a life-support element assumes water as life’s matrix. That’s for good reason. One would be hard-pressed to make the case that any other solvent could replace water in this vital role. With that caveat in mind, we can still ask: What about other possible solvents? The MIT researchers probed this possibility for silicon as well.
They conclude that the same chemical challenges facing “silicon-based” life in an aqueous setting would also exist for solvents such as ammonia. They also conclude that aprotic (incapable of acting as a proton donor) solvents, such as methane and ethane, would be unsuitable for carbon-based life and hypothetical silicon-based life because of the low solubility of both carbon-based and silicon-based materials.
As the authors of the Life paper conclude, “Silicon-based life that uses Si exclusively as a scaffold element is often portrayed in science fiction. . . . However, silicon-based life that uses Si exclusively as a scaffold element is almost certainly impossible.”3
Scientific and Theological Implications The insight of the MIT investigators about silicon’s life support capacity has wide-ranging scientific and theological implications that intertwine.
The recognition that: (1) silicon cannot form life’s scaffolding, and (2) aprotic solvents, such as methane and ethane, lack the solubility to serve as a suitable matrix for living systems strikes a blow to the view that life might have originated and currently exists in “extreme” and “exotic” locations, whether on Earth or other sites beyond Earth. In other words, if the MIT scientists’ insights are taken seriously, then attempts to answer the origin-of-life question and research in astrobiology should be restricted to carbon-based life existing in aqueous settings.
This discovery also helps address a common objection raised when design proponents present the theistic implications of the cosmological anthropic principle and the rare Earth hypothesis.
The cosmological anthropic principle refers to the discovery that the numerical values associated with the fundamental constants of physics (quantities that define the universe) must assume precise, exacting values for life to exist in the universe. If any one of these numerical quantities deviates ever so slightly (in some cases imperceptibly) from their current values, the universe would be unsuitable for harboring life.
The rare Earth hypothesis refers to the large number of just-right features of Earth, the Sun, the solar system, the Milky Way Galaxy, etc. that appear to be necessary for life to exist on Earth. If any of these characteristics differs, Earth would become inhospitable for life. The likelihood of a planet such as Earth existing in the universe is exceedingly remote. In fact, by all rights, a planet such as Earth should be rare, if not nonexistent, in our universe.
One interpretation of the cosmological anthropic principle and the rare Earth hypothesis is that the universe and Earth have been intentionally designed to harbor life. To put it another way: the universe displays a fitness for purpose and that purpose appears to center around the advent of life.
One response skeptics level at this claim goes something like this: “This conclusion assumes life as we know it. What about life as we don’t know it?”
Indeed, the fine-tuning of the constants of physics and the just-right conditions of Earth’s habitable zone assume carbon-based life residing in an aqueous matrix. It is reasonable to think that if life exists “as we don’t know it,” perhaps the set of physical constants that define the universe wouldn’t need to be so exacting. Or, alternative sets of physical constants could exist that would support “life as we don’t know it.” It is also possible that a different set of just-right conditions on Earth or another astronomical body could support alternative forms of life.
Yet, as we demonstrated, the best chance for “life as we don’t know it” is silicon-based life. And, based on the analysis by the MIT researchers, life can’t be silicon-based. It must be built around carbon and must reside in an aqueous setting. This recognition bolsters the theistic implications of the anthropic principle and the rare Earth hypothesis.
This understanding also highlights a set of anthropic coincidences associated with carbon. The properties of this element are prescribed by the universe’s exacting physical constants. And carbon’s properties are precisely those needed for life to be possible in the universe. Carbon’s properties are also unique. This set of circumstances is a bit eerie and suggests an underlying teleology to the universe that points to its purpose—namely, life’s advent.
It is doubly eerie to recognize that apart from hydrogen and helium, carbon is one of the most abundant elements in the universe (along with oxygen, nitrogen, sulfur, and phosphorus). And the triple-alpha process that makes carbon in the nuclear furnace of stars requires fine-tuning of the resonance among the nuclear energy levels of beryllium, carbon, and oxygen along with the fine-tuning of other physical constants such as the decay rate of the beryllium nuclei once it forms from the collision of two helium nuclei.
In short, the fiction that permeates our culture about the prospects of silicon-based life—when carefully considered—leads us to a remarkable reality: A Mind must be responsible for the design of the universe. This Mind has a purpose for the universe—the advent of life.
Janusz Jurand Petkowski, William Bains, and Sarah Seager, “On the Potential of Silicon as a Building Block for Life,” Life 10, no. 6 (June 10, 2020), 84, doi:10.3390/life10060084.
Petkowski, Bains, and Seager, “On the Potential of Silicon.”
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