Alternative biochemistry
Alternative biochemistry is the speculative biochemistry of alien life forms that differ radically from those on Earth.  It includes biochemistries that use elements other than carbon to construct primary cellular structures and/or use solvents besides water. Theories about extraterrestrial life based on alternative biochemistries are common in science fiction.

# Chirality
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 nearly identical chemical properties to their mirrored forms, so life that used D amino acids and/or L sugars may be possible.

# Atoms other than carbon
Scientists have speculated about the pros and cons of using atoms other than carbon to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the molecular machinery necessary for life.  Since humans are carbon-based beings and have never encountered any life that has evolved outside the earths environment, excluding the possibility of all other elements may be considered carbon chauvinism.

## Silicon biochemistry
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 as of 1998, 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.
The 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.
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 biochemistry
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.

## Other exotic biochemical elements
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.
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.

# Non-Green Photosynthesizers
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.

# Non-water solvents
In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. It is sometimes assumed that water is the only suitable chemical to fill this role. Some of the properties of water that are important for life processes include a large temperature range over which it is liquid, a high heat capacity useful for temperature regulation, a large heat of vaporization, and the ability to dissolve a wide variety of compounds. There are other chemicals with similar properties that have sometimes been proposed as alternatives.

## Ammonia
Ammonia is perhaps the most commonly proposed alternative. Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has some chemical similarities with water. Ammonia can dissolve most organic molecules at least as well as water does, and in addition it is capable of dissolving many elemental metals. Given this set of chemical properties it has been theorized that ammonia-based life forms might be possible.
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
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. However, HF suffers from the fact that it is an acid, with a pH similar to Sulfuric acid.  See Hydrofluoric Acid.
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.

## Other solvents
Other solvents sometimes proposed include methanol, hydrogen sulfide and hydrogen chloride. The latter two suffer from a relatively low cosmic abundance of sulfur and chlorine, which tend to be bound up in solid minerals. A mixture of hydrocarbons, such as the methane/ethane lakes detected on Titan by the Cassini spacecraft, could act as a solvent over a wide range of temperatures but would lack polarity. Isaac Asimov, the biochemist and science fiction writer, suggested that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane or liquid hydrogen.  Other solvents such as formamide might also be suitable as a solvent that would support alternative biochemistry.
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.

# Interstellar dust-based life
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.