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

Abstract

This paper explores life as a cosmic phenomenon rather than a solely terrestrial one, setting out definitions and principles that are meant to be universally applicable so that they can guide the investigation of life on other worlds and support the Search for Extraterrestrial Intelligence (SETI). The fundamental question of which elements life can be based upon is examined, with a detailed analysis of why carbon-based life is expected to be the most prominent chemistry throughout the cosmos. At the same time, this paper investigates the potential for non-carbon-based life and the elements that could theoretically form its foundation. Because of silicon's chemical resemblance to carbon, the possibility of silicon-based life and its distinctive chemical characteristics are examined closely, alongside a survey of other candidate elements such as boron and nitrogen. The argument is deliberately connected to the environments and missions that now make these questions testable, including Saturn's moon Titan and NASA's Dragonfly rotorcraft, and to the emerging field of agnostic biosignatures. This paper concludes that a deeper understanding of alternative biochemistries is crucial for framing our search for extraterrestrial life and for identifying a broader range of potential biosignatures, so that genuine evidence of life is not mistaken for a geological curiosity.

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 thinkers, where views clashed between those who believed in terrestrial uniqueness and those who saw 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 the centuries, scientific minds such as Charles Darwin came 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 encouraged a gradual public acceptance of the concept. This cultural shift, though often rooted in fiction, has evolved from speculative sightings at Area 51 into a rigorous scientific discipline, one that now sends sophisticated spacecraft 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 important as we embark on the search. We are not yet visiting planets with a microscope to examine microbes. Instead, we observe 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, which is a biological process, or of the photolysis of water by ultraviolet radiation, which is not. An abundance of oxygen alone is not definitive proof of life.

This ambiguity is not merely a thought experiment. It is exactly the problem that has played out in the most closely watched exoplanet result of recent years. In 2025 a team led by Nikku Madhusudhan reported a tentative detection of dimethyl sulfide, a gas produced almost exclusively by marine microbes on Earth, in the atmosphere of the sub-Neptune K2-18b using the James Webb Space Telescope. Within months, independent reanalyses found the signal to be far weaker than the standard required for a claim, and demonstrated that photochemistry in a hydrogen-rich atmosphere could produce comparable amounts of the same molecule without any biology at all (see Astronomy magazine and the Astronomy and Astrophysics reanalysis). A parallel debate has surrounded the reported detection of phosphine in the sulfuric acid clouds of Venus (Greaves and colleagues, 2020). Both cases show, in real time, why a single candidate molecule read against an Earth-centred assumption is fragile, and why the definitional groundwork below matters.

In these situations, definitions that describe life by its terrestrial characteristics, such as growth, metabolism, reproduction and responsiveness, can create conceptual roadblocks. Many non-living systems display 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 context-dependent and varies 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:

• 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)
• The Thermodynamic Flow: Life may be described as "a flow of energy, matter, and information." (Baltscheffsky, 1997)
• The Darwinian System: "Life is a self-sustained chemical system capable of undergoing Darwinian evolution." (NASA's working definition; Joyce, 1994)
• 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 the astrobiologist Steven Benner (2010) notes, if life is defined by reproduction, then a single sterile organism is not alive in that sense, 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 Carol Cleland and Christopher Chyba, and later N. Friedman (2002), have put it:

"Life is what the scientific establishment (probably after some healthy disagreement) will accept as life."

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 build upon the definition offered by Schulze-Makuch and Irwin, which views biology as a continuation of chemistry.

A chemical entity that consists of a bound microenvironment, capable of maintaining a low entropy state by energy and environment transformation, and capable of encoding and transferring information.

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, for example 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 towards a state of maximum entropy, or 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, for example by eating food or absorbing sunlight. In doing so, they increase the entropy of their surroundings, for example by releasing heat and waste products, and thereby obey 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 such as DNA and RNA, but other information-bearing polymers could serve this function elsewhere.

It is worth stressing that none of these three requirements specifies water, carbon or DNA. They specify a boundary, a thermodynamic gradient and a memory. This is the crucial move for the rest of the paper: the search for life becomes a search for those functions, in whatever chemistry can deliver them. That reframing is precisely the logic now formalised in the study of agnostic biosignatures, discussed in the conclusion.

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), in their combinatorial survey of biochemical space, found that a large fraction of core metabolic reactions are redox, or reduction-oxidation, reactions (see the PubMed record). This requires an element that can exist in multiple oxidation states, which immediately weakens the case for the highly reactive alkali metals of Group 1 and the alkaline earth metals of Group 2.

Second, the compounds of life must be stable yet dynamic. This requirement strongly favours covalent bonds. Ionic bonds are generally too unstable in a solvent, and metallic bonds, with their delocalised electrons in a lattice structure, cannot form the vast diversity of stable, complex molecules that life needs. This rules out most metals. We can also set aside the noble gases, because of their chemical inertness, and the halogens, which typically form only one bond and do not build complex structures. Furthermore, elements from the fourth period and below are generally unsuitable for forming the primary backbone of life, because 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 thought to play a role in some prebiotic chemistry but is cosmically rare. The exclusion of silicon is more puzzling, since it shares many properties with carbon and makes up approximately 28 per cent of Earth's crust. We will return to 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 building the stable structures needed for compartmentalisation (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 because of its unparalleled versatility in forming a vast number and variety of stable compounds. This chemical diversity is essential for the complexity that life demands.

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, roughly 348 kJ/mol. While silicon also exhibits catenation, the Si-Si bond, at roughly 226 kJ/mol, is significantly weaker and more reactive.
• Bonding Versatility: Carbon can form stable single, double and triple bonds with itself and with other elements, allowing the construction of an immense array of molecular structures, from linear alkanes to complex aromatic rings such as benzene. This enables the formation of countless functional groups, for example -COOH, -OH and -NH2, that dictate the function of biomolecules.
• Ideal Stability: Carbon compounds strike a good balance between stability and reactivity. They are stable enough to form reliable structures such as DNA and proteins, but not so stable that they become inert. This allows for controlled, dynamic metabolic reactions.
• Chirality: Carbon atoms bonded to four different groups are chiral, meaning they exist in left-handed and right-handed forms, or enantiomers. Life is stereochemically specific, for example using only L-amino acids and D-sugars, a property that is a strong indicator of biological origin.
• Favourable 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. By contrast, the oxidation of silicon produces silicon dioxide (SiO2), a solid such as sand or 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, forged 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. Our search, however, 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 sits 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 such as Earth, any reactive silicon compounds would quickly and irreversibly be converted into inert minerals, making them biologically inaccessible.

This does not, however, 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 free oxygen but with other potent oxidisers, this could be an advantage. Silicon biochemistry would also probably 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 adopt multiple coordination states (4, 5 and 6), allowing for diverse chemical interactions.
2. Stable Covalent Bonds: It forms stable covalent bonds with itself and with other key elements such as 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 around 26 Si-Si bonds, can be synthesised. When side chains such as 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 show unusual electronic properties, including electroluminescence and light-activated effects, which could in principle be harnessed for processes similar to photosynthesis.

Organosilicon compounds, which feature Si-C bonds in their backbone, are particularly intriguing. These polymers, the silicones, are highly stable at extreme temperatures, up to around 400 degrees Celsius, repel water and resist ultraviolet 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.

A caution is worth stating plainly, because it strengthens rather than weakens the case for looking. Silicon's weaker bonds and its appetite for oxygen mean that a silicon biochemistry is chemically demanding and, on present evidence, remains hypothetical: no silicon-based organism or self-replicating silicon system has been observed anywhere. The value of the analysis is that it tells us where such chemistry could survive, and therefore where to point our instruments.

6. Potential Environments for Silicon-Based Life

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

6.1 Life Based on Silanes

Silanes, with the general formula SinH2n+2, are the silicon analogues of alkanes. For them to be stable and to serve as a basis for life, the environment would require:

• A reducing atmosphere with little or 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 chemistry.

Titan also offers something the rest of this discussion mostly lacks: direct observational support that the required building blocks are present. Using the Atacama Large Millimeter/submillimeter Array (ALMA), Palmer and colleagues (2017) reported the first definitive detection of vinyl cyanide, also called acrylonitrile, in Titan's atmosphere, with related nitrile mapping by Thelen and colleagues. This matters because modelling by Stevenson, Lunine and Clancy (2015) had shown that vinyl cyanide could self-assemble into a stable membrane, an azotosome, in liquid methane at Titan surface temperatures near minus 179 degrees Celsius, playing the role that phospholipids play in water on Earth. The significance is not that Titan hosts life, which remains unknown, but that the bounded microenvironment required in Section 2 is chemically achievable in a non-aqueous solvent, using molecules that Titan is observed to contain. This is the strongest current, real-world case that the paper's central premise, life beyond carbon-and-water, is testable rather than merely conceivable.

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 in principle support silicone-based organisms.

6.3 Life Based on Silicates

The idea of life based on silicates themselves seems counterintuitive, given 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 a chemistry rich with possibilities. It can form chains and rings, and its bond with nitrogen (B-N) is isoelectronic with the C-C bond, giving rise to analogous compounds such as borazine (B3N3H6), often called inorganic benzene. The primary obstacle for boron-based life, however, 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 revert explosively to N2 gas. Nitrogen could, however, 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 agnostic biosignatures, that is, signs of life that are not tied to a specific biochemistry. This is now a defined research programme rather than a slogan: NASA's Laboratory for Agnostic Biosignatures pursues life detection through chemical complexity, unexpected elemental accumulation in compartments, and distinctive energy-transfer signatures (see NASA Astrobiology and a recent review), and machine-learning methods can now distinguish biological from abiotic samples largely independently of their specific chemistry (Cleaves and colleagues, 2023). The advantage of considering non-carbon life from the outset is exactly this: it lets us recognise potential evidence that we might otherwise dismiss as a geological anomaly, and it guards against the opposite error, seen in the K2-18b and Venus debates, of over-reading a single familiar molecule.

As we prepare for missions such as NASA's Dragonfly, a rotorcraft confirmed for a July 2028 launch and arrival at Titan in 2034, which will study prebiotic chemistry common to Titan and the early Earth (NASA), we must keep our minds open. Life could be present in our cosmic backyard, built from a chemistry we have only just begun to imagine. The continued exploration of our solar system, the refined observation of exoplanetary systems such as TRAPPIST-1, and inventive laboratory research here on Earth will bring us closer to answering one of humanity's oldest questions: are we alone?

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