Usage in Fiction
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 plausible.
In Arthur C. Clarke's short story "Technical Error" there is an example of differing chirality. This is not a case of alien life, rather it is an accident. The concept of reversed chirality also figured prominently in the plot of James Blish's Star Trek novel Spock Must Die!, where a transporter experiment gone awry ends up creating a duplicate Spock who turns out to be a perfect mirror-image of the original all the way down to the atomic level.
An example of silicon based life forms takes place in the Alan Dean Foster novel Sentenced to Prism in which the protagonist Evan Orgell is trapped on a planet whose entire ecosystem is mostly silicon-based.
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 life-forms 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 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.
One of the major sentient species in Terry Pratchett's Discworld universe are the "Earth"-based (ranging from Detritus to Diamond) Trolls. 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 life-form based on silicon and superfluid helium located in deep intergalactic space, processing its thoughts 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 life-forms, and sends out other probes to foment supernovae to do so.
Robert L. Forward's Camelot 30K describes an ecosystem 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 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. In Forward's Rocheworld series, an 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. 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 or Frederik Pohl's novel The World at the End of Time. The idea is that places where 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.
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 life-forms in his Xeelee series of novels and stories, including supersymmetric photino-based life that congregate in the gravity wells of stars, entities composed of quantum wave functions, and the Qax, who thrive in any form of convection cells, from swamp gas to the atmospheres of gas giants. In his book Manifold: Space, he also proposes natural robots, life forms made of iron, called the Gaijin, evolving from creatures in oceans of iron carbonyl.
In his novel Diaspora, Greg Egan posits 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 be based on some element other than carbon. In the same novel Egan describes lifeforms in the 6-D 'macrosphere' which use a collapsed atom chemistry with energetic processes of the same order as nuclear reactions, due to the peculiarities of higher dimensional physics.
In her novel Brain Plague, Joan Slonczewski describes a species of intelligent microrganisms with arsenic based chemistries that live symbiotically with human hosts.
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 appendages at will, they excel at 'closer-than-melee-range combat, primarily "meme-toxins" against other Amorphs.
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.
The eponymous organism in Michael Crichton's The Andromeda Strain is described as reproducing via the direct conversion of energy into matter.
Read more about this topic: Hypothetical Types Of Biochemistry
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“Americans will listen, but they do not care to read. War and Peace must wait for the leisure of retirement, which never really comes: meanwhile it helps to furnish the living room. Blockbusting fiction is bought as furniture. Unread, it maintains its value. Read, it looks like money wasted. Cunningly, Americans know that books contain a person, and they want the person, not the book.”
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