[Bit#126] Alien Biology Unveiled: What Forms Could Extraterrestrial Life Take Beyond Earth?

Introduction

Why should alien life necessarily look like humans or Earth animals? Beyond Earth, the forms and biochemical structures of living organisms enter a realm of infinite possibilities. From the core components of organic matter to the gases they breathe and the media they inhabit, elements can combine in ways completely different from those on Earth. From a astrobiological perspective, we delve into the unimagined biological structures of extraterrestrial life. This will serve as an opportunity to encounter an entirely new paradigm of life itself.

1. Possibilities and Chemical Limits of Silicon-Based Life

What if life somewhere in the universe was made of an element other than carbon? The most prominent candidate element mentioned is silicon. On the periodic table, silicon sits directly below carbon. Like carbon, it can form up to four chemical bonds. This means it possesses the fundamental condition required to construct complex organic molecules.

Then why did Earth organisms choose carbon over silicon? The secret lies in bonding strength and flexibility. Carbon bonds hold a level of strength that is suitable for efficiently storing and breaking down energy. In contrast, bonds between silicon atoms are somewhat unstable compared to carbon. Particularly in environments where water is present, silicon bonds are easily destroyed.

Furthermore, the byproducts generated during the respiration process show a massive difference. Carbon-based life forms inhale oxygen and exhale carbon dioxide in gaseous form. However, when silicon combines with oxygen, it turns into solid silicon dioxide. The very material we commonly see as sand or quartz is this exact substance. If a silicon life form were to undergo oxygen respiration, it would face the tragedy of stones accumulating inside its body every time it breathes.

Why is that? For silicon-based life to exist, an environment fundamentally different from Earth is required. If it were a polar environment containing oceans of liquid methane or ammonia instead of water, the story would be different. In exoplanetary environments maintaining ultra-high temperatures of several hundred degrees, silicon bonds could actually remain more stable than carbon.

Can you imagine an organism living with hard rocky skin while breathing high-temperature gases? Scientists believe that this structure, which seems impossible on room-temperature Earth, could be entirely realistic in the extreme environments of deep space. The moment we abandon our carbon-centered terrestrial biases, our perspective on extraterrestrial life finally expands.

2. Biological Systems Operating on Methane and Ammonia Instead of Water

All life on Earth uses water as an essential medium. This is because water is the ultimate solvent for triggering biochemical reactions. But is water absolutely necessary on other planets in the universe as well? Scientists are paying attention to completely different life systems that operate based on liquid methane or ammonia instead of water.

Let us think of exoplanets or moons with temperatures much lower than Earth. A representative example is Saturn’s moon, Titan. The surface temperature of Titan reaches minus 179 degrees Celsius. In an environment this cold, water freezes solid like rock. Instead, methane and ethane exist in liquid states, forming oceans and rivers. In extreme environments where water turns into gas or solid, liquid methane can serve as an excellent biochemical solvent that withstands sub-zero cold.

How then would life in methane oceans breathe and maintain its cells? Earth organisms form water-friendly, water-soluble cell membranes. On the other hand, organisms in methane environments are highly likely to compose cell membranes using organic nitrogen compounds like azotosomes instead of lipids. These cell membranes remain flexible without being destroyed even in ultra-low temperatures of minus 180 degrees Celsius. They trigger chemical reactions that extract energy by inhaling hydrogen instead of oxygen and exhaling methane.

Planets harboring oceans of ammonia instead of water are also strong candidates. Ammonia possesses chemical properties remarkably similar to water. It is an outstanding solvent capable of dissolving numerous organic substances. The temperature range in which ammonia exists as a liquid is between minus 77 degrees Celsius and minus 33 degrees Celsius. Even on planets much colder than Earth, complex organic systems can emerge as long as there is an ammonia ocean.

Why have we persistently insisted on water as an essential condition for life? The universe contains far more planets with sub-zero ultra-low temperatures than those with Earth’s mild climate. At the bottom of frozen methane oceans on exoplanets, organisms swimming while running biochemical engines completely different from Earth might exist at this very moment. The moment we step outside of water, the habitable zones of the universe expand dozens of times over.

3. Bizarre Extraterrestrial Body Structures Shaped by Extreme Gravity and Pressure

The most powerful physical factors determining the appearance of extraterrestrial life are planetary gravity and atmospheric pressure. Let us imagine a Jovian exoplanet with gravity several times stronger than Earth’s. In such a place, even reaching upward or standing straight consumes enormous amounts of energy for an organism. High gravity pulls everything strongly toward the ground.

What would life existing in this environment look like? Long legs or tall stature are absolutely disadvantageous for survival. Instead, it is highly likely to evolve into a form flattened against the ground. Legs would transform into dense joint structures like numerous wheels or centipedes, and skeletons would acquire densities close to metal to withstand gravity. A strange life form with a short height and a broad, plate-like body is born.

Conversely, what if it were inside a gas planet with extremely high atmospheric pressure or a dense atmosphere? High pressure exerts force across the entire body equal to being deep underwater. Having an hydrodynamically perfect streamlined body like Earth’s deep-sea fish or floating and swimming using atmospheric density would be advantageous.

Why is that? Because the dense atmosphere itself provides strong buoyancy. Instead of legs or wings, giant organisms equipped with large fins or buoyancy bladders holding gas could exist, swimming through the atmospheric ocean.

Gravity and atmospheric pressure are absolute laws that dictate the size and appearance of life forms. The bizarre physical structures of extraterrestrial organisms we imagine are actually biological outcomes perfectly adapted to the physical laws of those planets.

4. Astrobiological Clues Found in Earth’s Extremophiles

There is a way to gain clues about astrobiology without traveling directly to distant exoplanets. It is to observe special microorganisms living in extreme environments on Earth. Scientists call them extremophiles or extreme environment microorganisms. They sturdily continue life even under harsh conditions that defy imagination.

Shall we look around deep-sea hydrothermal vents where water temperatures boil above 100 degrees Celsius? Microorganisms reside here, enduring immense high temperatures and strong acidity while using sulfur as an energy source. They have built an ecosystem without sunlight in complete darkness where light does not reach at all. Microorganisms also exist that survive inside glaciers of Antarctica frozen below minus 20 degrees Celsius or inside nuclear reactors emitting lethal radiation.

Why is that? It is because their cellular structures and DNA repair mechanisms are completely different from ordinary Earth organisms. Extremophiles produce special proteins to prevent cellular damage even in situations where organic matter would normally be destroyed. Earth itself is proving the biological possibility of surviving in space where cosmic rays pour down or on the Martian surface with severe temperature fluctuations.

Ultimately, Earth’s extremophiles serve as a great compass for the search for extraterrestrial life. The environments of exoplanets that we concluded were uninhabitable might be the most perfect habitats for someone else. The wondrous vitality shown by microorganisms is the strongest evidence establishing astrobiology as a realistic science.

Conclusion

Life existing in the universe could look completely different from what we know. When we lay down Earth’s standards of carbon and water, infinite possibilities of astrobiology finally open up.
If we were to face an extraterrestrial life form with a completely different biochemical structure, could we truly recognize them as life just like us? How far can the definition of life be expanded?