Perhaps the least unusual "alternative" biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acids are almost universally of the L form and sugars are of the D form. Molecules of opposite chirality have identical chemical properties to their mirrored forms, so life that used D amino acids and/or L sugars may be possible.
The most commonly proposed basis for an alternative biochemical system is the silicon atom, since silicon has many chemical properties similar to carbon and is in the same periodic table group, the carbon group.
But silicon has a number of handicaps as a carbon alternative. Because silicon atoms are much bigger, having a larger mass and atomic radius, they have difficulty forming double or triple covalent bonds, which are important for a biochemical system. Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkane hydrocarbons, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymers of alternating silicon and oxygen atoms instead of direct bonds between silicon, known collectively as silicones, are much more stable. It has been suggested that silicone-based chemicals would be more stable than equivalent hydrocarbons in a sulfuric-acid-rich environment, as is found in some extraterrestrial locations. In general, however, complex long-chain silicone molecules are still more unstable than their carbon counterparts.
Another obstacle is that silicon dioxide (a common ingredient of many sands), the analog of carbon dioxide, is a non-soluble solid at the temperature range where water is liquid, making it difficult for silicon to be introduced into water-based biochemical systems even if the necessary range of biochemical molecules could be constructed out of it. The added problem with silicon dioxide is that it would be the product of aerobic respiration. If a silicon-based life form were to respire using oxygen, as life on Earth does, it would possibly produce silicon dioxide as a by product of this, assuming that the only difference between the two types of life is the presence of silicon in place of carbon.
Finally, of the varieties of molecules identified in the interstellar medium , 84 are based on carbon and 8 are based on silicon. Moreover, of those 8 compounds, four also include carbon within them. The cosmic abundance of carbon to silicon is roughly 10 to 1. This may suggest a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation upon which to build silicon-based biologies, at least under the conditions prevalent on the surface of planets.
Earth, as well as other terrestrial planets, is exceptionally silicon-rich and carbon-poor. However, terrestrial life is carbon-based. The fact that carbon, though rare, has proven to be much more successful as a life base than the much more abundant silicon may be evidence that silicon is poorly suited for biochemistry on Earth-like planets.
Even so, silica is used by some existing Earth life, such as the silicate skeletal structure of diatoms. See biogenic silica. This suggests that extraterrestrial lifeforms may have silicon based strucutre-molecules and carbon based proteins for metabolic purposes, therefore enabling the ability, to feed on a rather common ressource on a terrestrial planet like Earth for building up the silicone based part of their body.
It is also possible that silicon compounds may be biologically useful under temperatures or pressures very different from the surface of a terrestrial planet, either in conjunction with or in a role less directly analogous to carbon.
A. G. Cairns-Smith has proposed that the first living organisms to exist were clay minerals - which were probably based on silicon.
Nitrogen and phosphorus also offer possibilities as the basis for biochemical molecules. Like carbon, phosphorus can form long chain molecules on its own, which would potentially allow it to form complex macromolecules if it were not so reactive. However, in combination with nitrogen, it can form much more stable covalent bonds and create a wide range of molecules, including rings.
Earth's atmosphere is approximately 78% nitrogen, but this would probably not be of much use to a phosphorus-nitrogen (P-N) lifeform since molecular nitrogen (N2) is nearly inert and energetically expensive to "fix" due to its triple bond. (On the other hand, certain Earth plants such as legumes can fix nitrogen using symbiotic anaerobic bacteria contained in their root nodules.) A nitrogen dioxide (NO2) or ammonia (NH3) atmosphere would be more useful. Nitrogen also forms a number of oxides, such as nitrogen monoxide, dinitrogen oxide, and dinitrogen tetroxide, and all would be present in a nitrogen-dioxide-rich atmosphere.
In a nitrogen dioxide atmosphere, P-N plant analogues could absorb nitrogen dioxide from the air and phosphorus from the ground. The nitrogen dioxide would be reduced, with analogues to sugar being produced in the process, and waste oxygen would be released into the atmosphere. Animals based on phosphorus and nitrogen would consume the plants, use atmospheric oxygen to metabolize the sugar analogues, exhaling nitrogen dioxide and depositing phosphorus, or phosphorus-rich material, as solid waste.
In an ammonia atmosphere, P-N plants would absorb ammonia from the air and phosphorus from the ground, then oxidize the ammonia to produce P-N sugars and release hydrogen waste. P-N animals are now the reducers, breathing in hydrogen and converting the P-N sugars to ammonia and phosphorus. This is the opposite pattern of oxidation and reduction from a nitrogen dioxide world, and indeed from the known biochemistry of Earth. It would be analogous to Earth's atmospheric carbon supply being in the form of methane instead of carbon dioxide.
Debate continues, as several aspects of a phosphorus-nitrogen cycle biology would be energy deficient. Also, nitrogen and phosphorus are unlikely to occur in the ratios and quantity required in the real universe. Carbon, being preferentially formed during nuclear fusion, is more abundant and is more likely to end up in a preferred location.
Arsenic, which is chemically similar to phosphorus, while poisonous for most Earth life, is incorporated into the biochemistry of some organisms. Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Both arsenate reduction and arsenite oxidation have been observed in microbes. Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy. It has been speculated that the earliest life on Earth may have used arsenic in place of phosphorus in the backbone of its DNA.
Chlorine is sometimes proposed as a biological alternative to oxygen, either in carbon-based biologies or hypothetical non-carbon-based ones. But chlorine is much less abundant than oxygen in the universe, and so planets with a sufficiently chlorine-rich atmosphere are likely to be rare, if they exist at all. Chlorine will instead likely be bound up in the form of salts and other inert compounds.
Sulfur is also able to form long-chain molecules, but suffers from the same high reactivity problems that phosphorus and silanes do. The biological use of sulfur as an alternative to carbon is purely theoretical. However, the biological use of sulfur as an alternative to oxygen is widespread -- strains of sulfur-reducing bacteria have been discovered in exotic locations on earth, and also not so exotic locations, such as aging water systems. These bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide. Examples of this type of metabolism are green sulfur bacteria and purple sulfur bacteria. Examples of micro-organisms that metabolize elemental sulfur can be traced back 3.5 billion years on Earth.
Physicists have noted that, while photosynthesis on Earth generally involves green plants, a variety of other colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of solar radiation than that received on Earth. These studies indicate that no photosynthetic plants would be blue-colored, because blue light provides some of the highest photosynthetic yields in the light spectrum (therefore it is important for blue light to be absorbed rather than reflected). The physicists base their conclusions not on chemistry, but on the physical quality of different frequencies of light produced by known types of stars.
One terrestrial example of energy conversion based on something other than ordinary light involves fungi that convert high energy (compared to visible light) gamma rays into useful energy using the pigment melanin. In most organisms, melanin, a black pigment, instead protects the organisms against ultraviolet and solar radiation. Ordinarily fungi derived their energy from decomposing other biomass, rather than by converting radiation into energy for itself.
The gasses present in the atmosphere on Earth have varied greatly over its history. Traditional plant photosynthesis has terraformed the atmosphere by sequestering carbon from carbon dioxide, increasing the proportion of molecular oxygen, and by participating in the nitrogen cycle. Modern oxygen breathing animals would have been biochemically impossible until earlier photosynthetic life transformed Earth's atmosphere, giving rise to the Cambrian explosion. Precambrian life had to have biochemistries that did not require oxygen.
Changes in the gas mixture in the atmosphere, even in an atmosphere made up predominantly of the same molecules of Earth's atmosphere, impacts the biochemistry and morphology of life. For example, periods of high oxygen concentrations determined from ice core samples have been associated with fauna of a larger scale in the fossil record, while periods associated with of low oxygen concentrations have been associated with fauna of a smaller scale in the fossil record.
Also, while it is customary to think of plants, which are on one side of the oxygen and nitrogen cycles as being sesile, and animals which are on the other as being motile, this is not a biological imperative. There are animals which are sessile for all or most of their lives (such as corals), and there are plants (such as tumbleweeds and venus fly traps) that exhibit more mobility than is customarily associated with plants. On a slowly rotating planet, for example, it might be adaptive for photosynthesis to be performed by "plants" that can move to remain in the light, while non-photosynthetic "animals," much like Earth's fungi, might have a lesser need to move from place to place on their own. This would be a sort of mirror image of Earth's biochemistry.
Many Earth plants and animals also undergo major biochemical changes during their life cycles as a response to changing environmental conditions, for example, by having a spore or hibernation state that can be sustained for years or even millennia between more active life stages. Thus, it would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it.
Similarly, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state, while desert frogs in Australia can become inactive and dehydrate in dry periods, losing up to 75% of their fluids, yet return to life by rapidly rehydrating in wet periods. Either type of frog would appear biochemically inactive (i.e. not living) in dormant periods to anything other than the most acute means of sensing this activity.
However, ammonia does have some problems as a basis for life. The hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension to be three times smaller, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. For these reasons, science questions how well ammonia could hold prebiotic molecules together in order to allow the emergence of a self-reproducing system. Ammonia is also combustible and oxidizable and could not exist sustainably in a biosphere that oxidizes it. It would, however, be stable in a reducing environment.
A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual for terrestrial life. Terrestrial life usually exists within the melting point and boiling point of water at normal pressure, between 0 °C (273 K) and 100 °C (373 K); at normal pressure ammonia's melting and boiling points are between −78 °C (195 K) and −33 °C (240 K). Such extremely cold temperatures create problems, as they slow biochemical reactions tremendously and may cause biochemical precipitation out of solution due to high melting points. Ammonia could be a liquid at normal temperatures, but at much higher pressures; for example, at 60 atm, ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K).
Ammonia and ammonia-water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based habitability zone. Such conditions could exist, for example, under the surface of Saturn's largest moon Titan.
Hydrogen fluoride, like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. Its melting point is -84 °C and its boiling point is 19.54 °C; the difference between the two is little more than 100 °C. HF also makes hydrogen bonds with its neighbor molecules as do water and ammonia. All of these things make HF a candidate to host life on other planets.
Not much research has been done on liquid HF in regards to its ability to dissolve and react with non-polar molecules. It is possible that the biota in an HF ocean could use the fluorine as an electron acceptor to photosynthesize energy.
A proposal has been made that life on Mars may actually exist and be using a mixture of water and hydrogen peroxide as its solvent. A 61.2 percent (by weight) mix of water and hydrogen peroxide has a freezing point of -56.5 degrees Celsius, and also tends to super-cool rather than crystallizing. It is also hygroscopic, an advantage in a water-scarce environment.
In 2007 V. N. Tsytovich and colleagues discovered the possibility of life-like behaviors being exhibited by dust particles suspended in a plasma, similar to conditions in interstellar space. Computer models showed that when the dust became charged the particles could self-organize into microscopic helical structures capable of replicating themselves, interacting with other neighboring structures, and evolving into more stable forms. Similar forms of life were speculated on in Fred Hoyle's classic novel The Black Cloud.
In the realm of science fiction there have occasionally been forms of life proposed that, while often highly speculative and unsupported by rigorous theoretical examination, are nevertheless interesting and in some cases even somewhat plausible.
Perhaps the most extreme example in science fiction is James White's Sector General: a series of novels and short stories about multienvironment hospital for the strangest lifeforms imaginable, some of them breathing methane, chlorine, water and sometimes also oxygen. Some of the species metabolise directly hard radiation and their environment doesn't differ much from the atmosphere of a star, while others live in near absolute zero temperatures. All of the life forms are classified according to their metabolism, internal and external features, and more extreme abilities (telepathy, empathy, hive mind, etc) with four letter codes. Humans from Earth share the DBDG specification with small furry beings called Nidians.
Pratchett has also written the science fiction novel The Dark Side of the Sun which features a range of extraordinary lifeforms, including a telepathic body of water, creatures called "Sundogs", which are capable of interstellar travel from birth, and a sentient planet: effectively a giant silicon-based computer.
Fred Hoyle's classic novel The Black Cloud features a life form consisting of a vast cloud of interstellar dust, the individual particles of which interact via electromagnetic signalling analogous to how the individual cells of multicellular terrestrial life interact. Outside of science-fiction, life in interstellar dust has been proposed as part of the panspermia hypothesis. The low temperatures and densities of interstellar clouds would seem to imply that life processes would operate much more slowly there than on Earth. Inorganic dust-based life has been speculated upon based on recent computer simulations.
Similarly, Arthur C. Clarke's "Crusade" revolves around a planetwide lifeform based on silicon and superfluid helium located in deep intergalactic space, processing its thoughts very slowly by human standards, that sends probes to look for similar life in nearby galaxies. It concludes that it needs to make planets more habitable for similar lifeforms, and sends out other probes to foment supernovae in order to do so. Clarke implies that this is what accounts for most supernovae having occurred in the same region of space and warns that the effort will eventually reach Earth.
Robert L. Forward's Camelot 30K describes an ecosystem existing on the surface of Kuiper belt objects that is based on a fluorocarbon chemistry with OF2 as the principal solvent instead of H2O. The organisms in this ecology keep themselves warm by secreting a pellet of uranium-235 inside themselves and then moderating its nuclear fission using a boron-rich carapace around it. Kuiper belt objects are known to be rich in organic compounds such as tholins, so some form of life existing on their surfaces is not entirely implausible–though perhaps not going so far as to develop natural internal nuclear reactors, as have Forward's. Fluorine is also of low cosmic abundance, so its use in this manner is unlikely.
In Forward's Rocheworld series, a relatively Earth-like biochemistry is proposed that uses a mixture of water and ammonia as its solvent. In Dragon's Egg and Starquake, Forward proposes life on the surface of a neutron star utilizing "nuclear chemistry" in the degenerate matter crust. Since such life utilised strong nuclear forces instead of electromagnetic interactions, it was posited that life might function millions of times faster than typical on Earth.
Gregory Benford and David Brin's Heart of the Comet features a comet with a conventional carbon-and-water-based ecosystem that becomes active near the perihelion when the Sun warms it. Brin's own novel Sundiver is an example of science fiction proposing a form of life existing within the plasma atmosphere of a star using complex self-sustaining magnetic fields. Similar sorts of plasmoid life have sometimes been proposed to exist in other places, such as planetary ionospheres or interstellar space, but usually only by fringe theorists (see ball lightning for some additional discussion). Gregory Benford had a form of plasma-based life exist in the accretion disk of a primordial black hole in his novel Eater.
The suggestion that life could even occur within the plasma of a star has been picked up by other science fiction writers, as in David Brin's Uplift Saga. Any place in which reactions occur–even an incredible environment as a star–presents a possible medium for some chain of events that could produce a system able to replicate itself.
The Outsiders in Larry Niven's Known Space universe are cryogenic creatures based on liquid helium. They derive thermoelectric energy from a temperature gradient by basking half their body in sunlight, keeping the other half in shadow and exposed to interstellar vacuum.
Stephen Baxter has imagined perhaps some of the most unusual exotic lifeforms in his Xeelee series of novels and stories, including supersymmetric photino-based life that congregate in the gravity wells of stars, and the Qax, who thrive in any form of convection cells, from swamp gas to the atmospheres of gas giants.
In his novel Diaspora, Greg Egan posits the existence of entire virtual universes implemented on Turing Machines encoded by Wang Tiles in gargantuan polysaccharide 'carpets.' The sentient ocean that covers much of the surface of Solaris in Stanislaw Lem's eponymous novel also seems, from much of the fictional research quoted and discussed in the book, to based on some element other than carbon.
Sergeant Schlock is one of the lead characters in the webcomic Schlock Mercenary. His species, Carbosilicate Amorphs, evolved from self-repairing distributed data storage devices, and as such, redundantly distribute their 'brain' throughout their body. They are highly resistant to Hard Vacuum, explosive decompression, projectile weapons, chemical-based explosives, and dismemberment. Their only specialty organ is their eyes, which they harvest as fruit from the Ghanj-Rho eye-tree on their home planet. While the Amorphs have the ability to move fast, quietly, and sprout apendages at will, they excel at 'closer-than-melee-range combat, primarily "meme-toxins" against other Amorphs.
A more farcical example comes from the Hitchhiker's Guide to the Galaxy, where the Hooloovoo are a hyperintelligent shade of the colour blue.
Alien warriors recruited by the god Klael in David Eddings' "Tamuli" trilogy are noted by their human opponents to breathe marsh-gas (methane). Within Eddings' universe, this limits their capacity for exertion in an oxygen atmosphere, and also determines the tactics used to fight them and eventually to destroy them in their encampments.
A well-known example of a non–carbon-based lifeform in science fiction is the Horta in the original Star Trek episode "Devil in the Dark". A highly intelligent silicon-based creature made almost entirely of pure rock, it tunnels through rock as easily as humans move through air. The entire species dies out every 50,000 years save for one who tends all the eggs, which take the form of silicon nodules scattered throughout the caverns and tunnels of its home planet, Janus VI. The inadvertent destruction of many of these eggs by a human mining colony led the mother Horta to respond by murdering the colonists and sabotaging their equipment; it was only through a Vulcan mind meld that the race's benevolence and intelligence were discovered and peaceful relations established.
Star Trek would later offer other corporeal lifeforms with an alternative biochemistry. The Tholians of "The Tholian Web" are depicted and described, in that episode and later in the Star Trek: Enterprise episode "In a Mirror, Darkly" as being primarily of mineral-based composition and thriving only in superheated conditions. Another episode from TOS's third season, "The Savage Curtain", depicted another rock creature called an Excalbian, which is believed in fanon to also have been silicon-based.
Later on, in Star Trek: The Next Generation, the Crystalline Entity appeared in one episode, Silicon Avatar and was referenced in another, Datalore. This was an enormous spacefaring crystal lattice that had taken thousands of lives in its quest for energy. It may have been unaware of this, however, but it was destroyed before communications could be established at a level sufficient to ascertain it.
In another episode, Home Soil, intelligent crystals that formed a "microbrain" were discovered during a terraforming mission, and they described the humans they encountered as "ugly bags of mostly water."
"The Disease", an episode of Voyager featured some artificially-engineered silicon-based parasites, and an Enterprise episode, "Observer Effect", also presented a lethal silicon-based virus. In another Voyager episode, "Hope and Fear", a xenon-based lifeform was mentioned. In the Enterprise episode The Communicator, an alien species is encountered whose blood chemistry, while not explicitly stated, is sufficiently different from terrestrial organisms that it is not red and iron is toxic to it.
In the Star Wars movie The Empire Strikes Back, two lifeforms were encountered by the characters that were non-carbon based entities. Although details of their physiology were not mentioned on screen, the Space slug, (a giant worm-like creature that lived on asteroids in the freezing vacuum of space), and the Mynock, (pesky bat-like vermin that would attach themselves to spaceship hulls and chew through power conduits to feed off the raw energy), are said to be silicon-based organisms in expanded universe sources.. Also from The Empire Strikes Back, the bounty hunter Zuckuss is a member of the Gand race, an ammonia-based lifeform. However, it is worth nothing that the Gand are divided into two subspecies, only one of which breathes at all, the other drawing all their required sustenance from food intake and producing speech by means of essentially modulated flatulence.
Of a purely expanded universe creature is the Spice Spider of Kessel is a silicon-based creature that spun crystalline webs that miners harvested as Glitterstim Spice, an illegal psychoactive narcotic. The spider used the webs to catch Bogies, tiny energy creatures that it fed upon.
In the movie Alien the science officer Ash notes that the facehugger creature replaces its cells with polarised silicon in order to give it "prolonged resistance to adverse environmental conditions". Both stages of the alien cycle also have a highly corrosive blood, normally understood to be some kind of acid, which would be inconsistent with any known terrestrial biochemistry.
In the movie The Monolith Monsters, a silicon meteor reproduces itself, draining silicates from everything it touches. It needs water to start its cycle and contains molecular structures typical of many kinds of rocks, mixed together. A geologist says that its structure is nearly impossible. The meteor is killed by salt water, which can stop the cycle.
In the Stargate SG-1 fourth season episode "Scorched Earth", a Human society known as the Enkarans are threatened on their new homeworld by an alien ship that is terraforming the planet to be suitable for the sulfur-based Gadmeer species.
Indiana Jones and the Kingdom of the Crystal Skull (2008) introduces thirteen "extra dimensional beings" with crystal skeletons, who founded a city that became the basis of the El Dorado myth. Though their flesh has died and rotted away, their minds still live on within their skeletons, which communicate telepathically.
In the Command & Conquer real-time strategy games, both the gameplay and storyline revolve heavily around the introduction to Earth of an extraterrestrial mutagen called Tiberium via meteor, which displays strikingly lifelike behaviours such as self-replication, evolution, and homeostasis, without necessarily undergoing anything like common carbon-based metabolic cycles, and which appears to be colonising the Earth, converting it into an environment unsuited to carbon-based biology. Earth creatures (such as animals, plants and even humans) exposed to Tiberium can either be killed because of the radiation or be transformed into Tiberium-based lifeforms, to whom Tiberium radiation is curative rather than toxic. It is later revealed that Tiberium was introduced to earth by the Scrin, an extremely advanced race of Tiberium-based aliens bent on mining the planet after the Tiberium deposits have reached maturity.
In the Halo video game series, a race of Covenant aliens named "Grunts" by humans require a breathing apparatus while fighting the humans in an Earth-like atmosphere. According to the novelizations of the video game, the Grunts' apparatus allows them to breathe the methane they need to survive. In the Master of Orion series of space strategy games, there exists an extraterrestrial race called Silicoids, whose appearance (and presumably composition) is similar to crystalline mineral structures. The game posits that this grants them immunity to the effects of hostile environments and pollution, at the expense of impeding their reproductive rate and their ability to interact with other intelligent species.
In the Star Control series, the Chenjesu, are intelligent, peaceful silicon-based lifeforms that were the backbone of the Alliance of Free Stars. Also, there are the Slylandro who are gas beings resideng in the upper atmosphere of a gas giant.
In the game of Xenosaga, artificial life forms known as Realians have been created using silicon-based chemistry. They resemble humans in every aspect, except they are considered to be lower than humans on the social ladder.
In "Mass Effect" the alien Turians and Quarians , are both dextro-amino acid-based organisms, as opposed to humans, a deoxyribonucleic acid lifeform. There are also the Volus, an amonia based species that must wear pressure suits to survive in environments suited to the other races.