Cosmic Life: Is Only Carbon-Based Life Out There?

Abstract

This paper explores the concept of life as a cosmic phenomenon rather than a solely terrestrial one, providing definitions and explanations that are universally applicable to facilitate the investigation of life on other worlds and enhance the Search for Extraterrestrial Intelligence (SETI). The fundamental question of what elements life can be based on is examined, with a detailed analysis of why carbon-based life is expected to be the most prominent throughout the cosmos. Concurrently, this paper investigates the potential for non-carbon-based life, discussing the elements that could theoretically form its foundation. Due to silicon’s chemical resemblance to carbon, the possibility of silicon-based life and its unique chemical characteristics are thoroughly discussed. Furthermore, the potential for life forms based on other alternative elemental compositions is also examined. Ultimately, this paper concludes that a deeper understanding of these alternative biochemistries is crucial for framing our search for extraterrestrial life and identifying a broader range of potential biosignatures.

1. Life as a Cosmic Phenomenon

The notion of "life as a cosmic phenomenon" was scientifically articulated in 1977 by Fred Hoyle and Chandra Wickramasinghe through the theory of panspermia, yet the existence of other ‘worlds’ was speculated upon in ancient Greek philosophy. This idea was further developed by Roman philosophers, where ideas clashed between schools of thought—those who believed in terrestrial uniqueness and those who viewed life as a more universal principle. For his agreement with such views, the Dominican friar Giordano Bruno was burned at the stake in 1600. Over centuries, great scientific minds like Charles Darwin began to support ideas that positioned life as a natural outcome of universal laws. In the modern era, popular culture, through films featuring green, blue, or even purple beings on distant planets, has led to a gradual public acceptance of this concept. This cultural shift, though often rooted in fiction, has evolved from speculative sightings at "AREA 51" into a rigorous scientific discipline, expanding our scope to sending sophisticated satellites and rovers to search for life. The immense investment in such missions is driven by a fundamental quest to understand our own origins and our place in the universe.

To identify life elsewhere, we must first understand what life is. Defining life has remained a profound challenge for centuries, and while some consider it a philosophical dead end, the question becomes increasingly crucial as we embark on our search. We are not yet visiting planets with a microscope to examine microbes; instead, we are observing entire planets for large-scale signs of life, or biosignatures. Without a robust definition, differentiating a living process from a non-living one becomes exceptionally difficult. For example, the presence of atmospheric oxygen could be a product of photosynthesis (a biological process) or the photolysis of water by ultraviolet radiation (a non-biological process). An abundance of oxygen alone is not definitive proof of life.

In these situations, definitions that describe life based on its terrestrial characteristics—such as growth, metabolism, reproduction, and responsiveness—can create conceptual roadblocks. Many non-living systems can exhibit these features; crystals grow, fire has a metabolism, and certain chemical reactions can appear responsive. The core problem is our search for a single, objective definition, when the concept of life is inherently subjective, varying between individuals and scientific fields. A philosopher's definition of life will differ starkly from that of a chemist or a biologist.

Some notable attempts to define life include:

• A) The Organismic State: "An organismic state characterized by the capacity for metabolism, growth, reaction to stimuli, and reproduction." (Merriam-Webster's Collegiate Dictionary, 10th ed., 1993)

• B) The Thermodynamic Flow: Life may be described as “a flow of energy, matter, and information.” (Baltsheffsky, 1997)

• C) The Darwinian System: "Life is a self-sustained chemical system capable of undergoing Darwinian evolution." (NASA’s working definition; Joyce 1994)

• D) The Bounded Microenvironment: A chemical entity that consists of a bound microenvironment, capable of maintaining a low entropy state through energy and environment transformation, and capable of encoding and transferring information. (Schulze-Makuch and Irwin, 2004)
  

It is also important to note that the concept of ‘being alive’ is distinct from the concept of ‘life.’ As astrobiologist Steven Benner (2010) notes, if life is defined by reproduction, then a single sterile organism is not 'alive' in that context, but the species as a whole represents 'life.' This distinction further complicates the search for a simple definition. Perhaps there is no single, stipulative definition of life. As N. Friedman (2002) aptly put it:

2. The Concepts of Chemical Disequilibrium and Low Entropy

Chemical disequilibrium and low entropy are fundamental features of all known life. To grasp these concepts, we can expand upon the definition offered by Schulze-Makuch and Irwin, which views biology as a continuation of chemistry.

1. Bound Microenvironment: Life requires a boundary to separate its internal chemistry from the external environment. This boundary, such as a cell membrane, is crucial because if an organism were in perfect equilibrium with its surroundings, all the chemical reactions necessary for life would cease. The microenvironment creates a state of disequilibrium (e.g., different concentrations of ions inside and outside a cell), which fuels the chemical processes essential for existence. The environment must be "micro" because a small size maintains a high surface-area-to-volume ratio, allowing for the efficient transfer of energy and matter.

2. Low Entropy: Entropy is a measure of disorder or randomness. The universe naturally tends toward a state of maximum entropy (chaos), as described by the Second Law of Thermodynamics. Living beings, however, are "islands of low entropy" in this chaotic universe; they are highly ordered and complex systems. To maintain this low-entropy state, organisms must constantly input energy from their surroundings (e.g., by eating food or absorbing sunlight). In doing so, they increase the entropy of their surroundings (e.g., by releasing heat and waste products), thus obeying the universal law.

3. Information Encoding and Transfer: All organisms have a finite existence. To ensure the continuation of life, information dictating structure and function must be stored and passed on to the next generation. On Earth, this is accomplished through nucleic acids like DNA and RNA, but other information-bearing polymers could serve this function elsewhere.

3. Elements that Could Potentially Give Rise to Life

Carbon is the fundamental building block of every life form known to us. To understand why, it is instructive to examine why other elements are less suitable.

First, a central element for life must support complex metabolic reactions. Bains and Seager (2012) found that of 787 core metabolic reactions, 291 are redox (reduction-oxidation) reactions. This necessitates an element that can exist in multiple oxidation states, which immediately eliminates the highly reactive alkali metals (Group 1) and alkaline earth metals (Group 2) from the periodic table.

Second, the compounds of life must be stable yet dynamic. This requirement strongly favors covalent bonds. Ionic bonds are generally too unstable in a solvent, and metallic bonds, with their delocalized electrons in a lattice structure, cannot form the vast diversity of stable, complex molecules needed for life. This eliminates most metals. We can also rule out the noble gases (due to their chemical inertness) and halogens (which typically form only one bond and do not create complex structures). Furthermore, elements from the 4th period and below are generally unsuitable for forming the primary backbone of life, as their larger atomic size results in weaker, less stable covalent bonds.

After this process of elimination, we are left with a handful of candidates from the second and third periods: H, B, C, N, O, Si, P, and S.

Among these, C, N, O, H, S, and P are the familiar "building blocks of life" on Earth. This leaves Boron (B) and Silicon (Si). Boron is believed to play a role in some prebiotic chemistry but is cosmically rare. The exclusion of silicon is more puzzling; it shares many properties with carbon and makes up approximately 28% of Earth's crust. We will address this in a later section.

Regardless of the central element, life anywhere will almost certainly be macromolecular and polymeric. This is a fundamental requirement for creating the stable structures needed for compartmentalization (membranes), catalysis (enzymes), and the transmission of information (genetics). It is also plausible that biomolecules could have a backbone composed of more than one element. Chains of Boron-Nitrogen (B-N), Silicon-Carbon (Si-C), or Silicon-Oxygen (Si-O) are chemically possible and could form the basis for exotic life.

4. Why Carbon-Based Life is Advantageous

Carbon holds a special status in the periodic table, primarily due to its unparalleled versatility in forming a vast number and variety of stable compounds. This chemical diversity is essential for the complexity required by life.

Key advantages of carbon include:

• Catenation: Carbon has an exceptional ability to form long, stable chains and rings by bonding with itself. This property, known as catenation, is due to the high bond energy of the C-C single bond (348 kJ/mol). While silicon also exhibits catenation, the Si-Si bond (226 kJ/mol) is significantly weaker and more reactive.

• Bonding Versatility: Carbon can form stable single, double, and triple bonds with itself and other elements, allowing for the construction of an immense array of molecular structures, from linear alkanes to complex aromatic rings like benzene. This enables the formation of countless functional groups (-COOH, -OH, -NH2, etc.) that dictate the function of biomolecules.

• Ideal Stability: Carbon compounds strike a perfect balance between stability and reactivity. They are stable enough to form reliable structures like DNA and proteins but not so stable that they become inert. This allows for controlled, dynamic metabolic reactions to occur.

• Chirality: Carbon atoms bonded to four different groups are chiral, meaning they exist in left-handed and right-handed forms (enantiomers). Life is stereochemically specific (e.g., using only L-amino acids and D-sugars), a property that is a strong indicator of biological origin.

• Favorable Waste Product: The oxidation of carbon compounds for energy produces carbon dioxide (CO2​). As a gas at most terrestrial temperatures, CO2​ is easily expelled from an organism and can be recycled within a planet's atmosphere and oceans. In contrast, the oxidation of silicon produces silicon dioxide (SiO2​), a solid (sand/quartz), which is difficult to dispose of as a waste product.

• Cosmic Abundance: Carbon, along with hydrogen and oxygen (the components of water), are among the most abundant elements in the universe, created in the hearts of stars. This makes the combination of carbon chemistry and a water solvent a statistically probable basis for life.
  

Given these powerful advantages, it is reasonable to expect that most life in the universe will be carbon-based. However, our search should not be limited by this expectation.

5. Silicon as a Building Block of Life

Silicon is the most frequently proposed alternative to carbon. It is in the same group as carbon, with four valence electrons and a similar electronegativity. Why, then, despite its abundance on Earth, is silicon not the basis for life here?

The primary obstacle is silicon's interaction with oxygen and water. The Si-O bond is extremely strong and stable, leading to the formation of silicates and silicon dioxide (SiO2​)—essentially rock. On a wet planet like Earth, any reactive silicon compounds would quickly and irreversibly be converted into inert minerals, making them biologically inaccessible.

However, this does not rule out silicon-based life in environments radically different from our own. While the oxidation of silicon with oxygen produces a solid waste product (SiO2​), this process releases more energy than the oxidation of carbon to CO2​. In an environment without oxygen but with other potent oxidizers, this could be an advantage. Furthermore, silicon biochemistry would likely require a solvent other than water, such as liquid methane or ethane, which are found on Saturn's moon, Titan.

The advantages and chemical possibilities of silicon include:

1. Variable Valencies: Silicon can exist in multiple coordination states (4, 5, and 6), allowing for diverse chemical interactions.

2. Stable Covalent Bonds: It forms stable covalent bonds with itself and other key elements like C, N, P, O, and S.

3. Complex Structures: Silicon can form branched and unbranched chains (silanes) and ring systems (cyclohexasilanes). It also forms complex cage-like structures known as silsesquioxanes, which could protect a reactive core.

4. Polymers and Self-Aggregation: Oligosilanes (chains of up to 26 Si-Si bonds) can be synthesized. When side chains like carboxyl groups (-COOH) are attached, these molecules become amphiphilic in water, meaning they can self-assemble into vesicles and micelles—structures analogous to cell membranes.

5. Electronic Properties: While silanes cannot form pi-conjugated systems like benzene, they can form sigma-conjugated polysilanes. These compounds exhibit unique electronic properties, including electroluminescence and light-activated effects, which could theoretically be harnessed for processes similar to photosynthesis.
   

Organosilicon compounds, which feature Si-C bonds in their backbone, are particularly intriguing. These polymers (silicones) are highly stable at extreme temperatures (up to 400 °C), repel water, and are resistant to UV radiation. While unsuitable for a water-based world, they could be the basis for life on a much hotter planet with a non-polar solvent.

6. Potential Environments for Silicon-Based Life

Based on its chemistry, we can hypothesize about the kinds of environments that might support silicon-based life.

6.1 Life Based on Silanes

Silanes (Sin​H2n+2​) are the silicon analogues of alkanes. For them to be stable and serve as a basis for life, the environment would require:

• A reducing atmosphere with little to no oxygen to prevent the formation of silica.

• An absence of liquid water for the same reason.

• Low temperatures and/or high pressures to maintain their stability.

• A suitable non-polar solvent, such as liquid methane or ethane.
  

The closest known environment matching these conditions is Saturn’s moon Titan. Its thick, nitrogen-rich atmosphere, extreme cold, and lakes of liquid methane make it a prime candidate for hosting such exotic life.

6.2 Life Based on Silicones

Silicone polymers are highly stable at high temperatures. Life based on these molecules could exist on a hot, rocky world, but finding a suitable liquid solvent would be a major challenge. A world with a methane-based solvent cycle but at a much higher temperature than Earth could potentially support silicone-based organisms.

6.3 Life Based on Silicates

The idea of life based on silicates themselves seems counterintuitive due to their stability. However, at extremely high temperatures, such as in the molten mantle of a planet, silicates are liquid and chemically dynamic. It is conceivable that complex, self-replicating crystalline structures could emerge in such an environment, representing a form of life completely alien to our water-and-carbon-based understanding.

7. Other Alternatives for the Building Blocks of Life

While silicon is the leading contender, other elements could potentially form the basis for life under specific conditions.

• Boron: Boron forms stable covalent bonds and has an chemistry rich with possibilities. It can form chains and rings, and its bond with nitrogen (B-N) is isoelectronic with the C-C bond, leading to analogous compounds like borazine (B3​N3​H6​), often called "inorganic benzene." However, the primary obstacle for boron-based life is its extreme cosmic rarity. It is one of the least abundant elements in the universe.

• Nitrogen: Nitrogen can form chains, but the extreme stability of the dinitrogen molecule (N2​) with its triple bond means that long nitrogen chains are highly unstable and tend to explosively revert to N2​ gas. However, nitrogen could form stable backbones when paired with other elements, such as boron.

Conclusion

While the chemical advantages of carbon and water are immense, making carbon-based life a probable standard across the cosmos, our search for extraterrestrial life must not be constrained by this assumption. The universe is vast and offers a dizzying array of environmental conditions—from the cryogenic methane lakes of Titan to the sulfuric acid clouds of Venus and the searing surfaces of exoplanets orbiting close to their stars. In such alien settings, alien chemistries may arise.

By studying the potential of silicon, boron, and other elements to serve as the foundation for life, we broaden our perspective. This theoretical work is critical for identifying potential agnostic biosignatures—signs of life that are not tied to a specific biochemistry. The advantage of considering non-carbon life from the outset is that it allows us to recognize potential evidence that we might otherwise dismiss as geological anomalies.

As we embark on exciting missions like NASA’s Dragonfly, which will explore the surface of Titan, we must keep our minds open. Life could be thriving in our cosmic backyard, built from a chemistry we have only just begun to imagine. The continued exploration of our solar system and the refined observation of exoplanetary systems like TRAPPIST-1, combined with innovative research in our laboratories here on Earth, will bring us closer to answering one of humanity's oldest questions: Are we alone?

References

1. Schulze-Makuch, D., & Irwin, L. N. (2018). Life in the Universe: Expectations and Constraints (3rd ed.). Springer Praxis Books.

2. Kolb, V. (Ed.). (2018). Handbook of Astrobiology. CRC Press.

3. Westall, F., & Brack, A. (2018). The Importance of Water for Life. Space Science Reviews, 214(2), 50.

4. Scorei, R. (2012). Is Boron a Prebiotic Element? A Mini-Review of the Essentiality of Boron for the Appearance of Life on Earth. Origins of Life and Evolution of Biospheres, 42(1), 3-17.

5. Des Marais, D. J., Nuth III, J. A., Allamandola, L. J., Boss, A. P., Farmer, J. D., Hoehler, T. M., ... & Spormann, A. M. (2008). The NASA Astrobiology Roadmap. Astrobiology, 8(4), 715-730.

6. Wickramasinghe, N. C. (2015, September). The Beginnings of Life as a Cosmic Phenomenon. In Instruments, Methods, and Missions for Astrobiology XVII (Vol. 9606, p. 960602). SPIE.

7. Benner, S. A. (2010). Defining Life. Astrobiology, 10(10), 1021-1030.

8. Sabater, B. (2022). Entropy Perspectives of Molecular and Evolutionary Biology. International Journal of Molecular Sciences, 23(8), 4098.

9. Greenwood, N. N., & Earnshaw, A. (1998). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann.