BENEATH ALIEN SKIES, PART ONE
MAURA FLYNN, DATA ANALYST
6.5 minute read
A Truly Alien Formation
Lately, on many nights, I have stood out under the stars and found constellations. I often wonder what planets might surround the great stars of Aldebaran, Pollux and Castor, and the rest of the celestial multitude that embroider our nighttime sky. My thoughts inevitably turn toward the unimaginable alien life which may march on, mirroring our own, until we find each other– but I have decided 'unimaginable' is a quitter's word, and I would like to invite you along for this resultant ride. We cannot predict alien life, but we can imagine it. How might it be different– or similar– to life on Earth? In this column, we discuss the barest necessities of life from a curious, candid perspective, preparing ourselves to explore truly alien physiology and ecology. When discussing the possibilities of life in the universe, it is easy to narrow one’s scope based on what we know– the biodiversity of Earth. We do have some requirements for life, but we ought to take as few concessions as possible.
To be alive, an organism needs to be… well, its own thing. This initially requires defining what makes that organism unique, and this definition (DNA, RNA, etc) brings replication to the checklist of life with it. Tangibly, this distinctness keeps organisms separate from the rest of the world. Humans, ant colonies, slime mold, aspen groves, and even viruses (which may be life, but that’s a separate debate) have some sort of membrane. The exceptions, like lichen's fungi/plant hybrid and humans' relationship with mitochondria, result from previously successful lifeforms symbiotically joining – a solid step after the rudimentary list we are making. The factoid "humans are just big bags of water" alludes to the benefits of a membrane – we generally do not spill ourselves all over the place (at least until allergy season), and neither do the dangers of the outside world have free access to our juicy innards. So life needs a boundary, a container: a membrane. This can be flexible or rigid, and it is often specialized and complex, like animal skin. Concertly, a membrane allows the lifeform to keep an optimal environment, or homeostasis, for its chemical processes. This includes a sort of solvent in which these reactions can occur, something plentiful in its environment (spoiler alert: for Earth life, this will always be water).
Next, life is an effortful thing, and it needs energy. Diffusion across membranes allows for the transmission of energy-containing nutrients and the ridding of resultant waste. This criterion can be achieved in a wide variety of ways; even on Earth, approaches range from passive methods like filter feeding and photosynthesis to active ones like breathing, circulation, and the primitive ‘ejection method’ used by amoeba. The bold, open vascular systems of clubmosses and ferns provide another sort of solution somewhere between active and passive, freeing our imaginations from the dichotomy toward which we might be tempted.
Lifeforms then need to process that energy, which we call metabolism. This implies the need for a storage system, too. Fat cells, bone marrow, gas chambers, chloroplasts, and even the membrane itself can store energy in the case of single-celled organisms. This storage need not be long-term, but only the ability to somehow store energy until used or released. Higher life forms with more delicate systems also need to clean their energy, as seen with organs like livers and kidneys.
Life is hunger—deer gnawing at bark in winter’s void. (Canva)
Finally, life needs to be able to react to the world around it. A truly passive life form does not exist. Life is hunger; the fight for survival is universal. Senses allow life to perceive the world around it and react to opportunities and dangers, whether that means moving to or away from something, engaging a defense mechanism, or answering the stimulus with predatory behavior. Reproduction is something worth keeping in mind, as mentioned earlier, but it is so widely approached that it does little to constrain our dreams beyond the need to somehow separate one's material– something a dynamic membrane, genetic instructions, and a storage system can certainly figure out. So we conclude life needs, at the very least, these three pieces plus transport systems, metabolism, and senses.
With this list firmly in hand, we consider the building blocks which might give us that precious result. Carbon-based life is all we truly know; each living thing on Earth features carbon as the stable center of its molecules. Carbon is the favored elemental choice for several reasons– its balance of stability and reactivity, its diversity in molecular structures, and its abundance are the biggest– but the universe is a wild place. Curiosity spurs us onward and outward, and shows us other possibilities. While several other compositions have been theorized for life– boron-based life, for example, or life swimming in ammonia– one option keeps topping the list in interest and potential: silicon.
Silicon nanowires observed with an electron microscope. (Canva)
Ah, silicon, the golden child of futuristic dreamers everywhere. It makes four bonds, like carbon, is fairly small (for an element), and the variety of structures we already know– rubbers, plastics, rocks of all sorts, and more– are broad and intriguing. Could it stand in for carbon in the dizzyingly complicated dance of life? Current theory suggests it is unlikely. Silicon structures struggle to naturally develop in any but very curated environments. However, the argument for silicon life is still quite compelling. Science is what we know so far, not all there is to know, so let us honor our meticulous sides and consider both silicon's obstacles and its possibilities.
The biggest problem with silicon is its difficulty with water. Silicon bonds with common elements like oxygen and nitrogen are more polar than the analogous bonds carbon makes. Water, with its comparatively freewheeling polarity, tears apart most types of molecules silicon naturally forms. Unlike carbon, silicon loathes to form double or triple bonds, which are strong, but still reactive enough to allow for change, which leads to growth. The strongest, most favorable bond silicon forms is that of silicon dioxide, a uniform crystalline lattice which leaves no room to explore possibilities of life. This reduces the number of structures available, and so reduces the diversity of functions they can perform, too. On Earth and other watery planets, silicon is doomed to naturally progress toward these coffins of rigid lattices. Just go outside: you will see silicon’s graves everywhere in the rocks and gems of the world. Water is a wonderful solvent for carbon, hence why our bodies are filled with it, but it is a poor choice for silicon. Silicon's problems become far less worrisome in solvents we balk at, like sulphuric acid. In very cold solvents like liquid nitrogen, for example, silicon can access more of its structural diversity than in any other. This is due to liquid nitrogen's aprotic nature: how it lacks charged protons, which contribute to hydrogen bonding (water's protic nature is why it's so damaging to silicon molecules). To truly consider silicon life, we will have to go to some very organically hostile, but remarkable places.
Like silicon atoms in the vast lattice of possibility–drawn close, reaching, almost connecting–but never quite bonding. A dance of proximity and potential, suspended in the space between. (Adobe)
One common complaint about silicon biology is its lack of homologous, or directly comparable, structures with carbon. Silicon tends to form structures with very similar, repetitive configurations when restricted to the types of structures carbon tends to make, and these silicon structures can't hold a candle to their organic counterparts' functionality. There are a few; silane-silane chains, for example, can transfer electric charge like the potassium-sodium pumps of our own neurons, but there are lamentably few functions we observe from a long chain of silicon– note the key word 'observe.' Again, we lock silicon's value to how it functions in a watery atmosphere. If we can look with fresh eyes upon silicon and release it from our carbonic expectations, some surprisingly familiar possibilities emerge. Siloxene rings, for example, have no analog carbon-centric structure and thus are often ignored when considering life, but they could feasibly perform functions similar to those of heme in our blood and chlorophyll. Silicon has huge potential for functional diversity, once we stop constantly comparing it to carbon.
Close up human blood vessels. (Adobe)
A final consideration is the possibility of carbon-silicon hybrids. Consider a world with some oxygen, but not nearly enough to support life as we know it. An atmosphere with little carbon and little oxygen could still see them populating the molecules of life, but if silicon is more plentiful, it could become the main scaffolding atom instead of carbon. Conversely, molecules with carbon at their center and supporting silicon atoms laced throughout could also be present and functional. Planets with higher gravity and thin atmospheres could develop life that utilizes the benefits of both silicon and carbon to survive. The conditions around silicon strongly dictate its ability to flourish from a biodiverse perspective; therefore, the types of planets we visit will decide the true likelihood of life featuring silicon, either as a main player or supporting carbon in a hybrid fashion.
The universe awaits. Having done my due diligence, I can now fly wildly into space, hindered only by my imagination. The irresistibly romantic adage "truth is stranger than fiction" drives me, and I shall err on the side of the impractical to allow possibility to reign. Silicon-, carbon- and hybrid-based lives may all surface in my heady explorations. I strap myself into my ship and prepare for launch, pointing out into the sea of stars to search for another island. Who knows what bizarre, diverse ecosystems are waiting beyond our hazy atmosphere? The thrusters fire, and soon the pale blue dot is behind me. I, the Xenologist, begin my mission: soon, I will review my scanners' findings. In the next issue I shall chart my course to four exciting planets and select the first which might hold a truly alien formation.
Through the porthole window. (Canva)
REFERENCES
Petkowski, J. J., Bains, W., & Seager, S. (2020). On the Potential of Silicon as a Building Block for Life. Life, 10(6), 84. https://doi.org/10.3390/life10060084
Recommended columns to read next: Sports in Space and Why We Choose to Go to Space: Human Imagination