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Sometimes in science fiction, you'll find aliens whose bodies replace carbon with silicon. In hard science fiction, the plausability of this trope rests on carbon and silicon being in the same family of elements: being able to form four bonds opens the door to a vast array of potential structures.
Silicon biochemistry would be wildly different though: you can't replace carbon with silicon in known biomolecules and get anything functional. Even the small "hydrosilicon" molecules are much more reactive than their carbon counterparts. Plus, carbon is actually not so common on the earth, with the most abundant element being...wait for it...silicon. If that kind of life were to form, it would have done so here.
It's somewhat more chemically plausible to base life on silicates (silicon oxides), which have a huge variety in nature. These species would either have to exist at molten-glass temperatures, or be very slow (literally geological). Silicate based lifeforms would definitely be more Starfish Aliens than Rubber Forehead Aliens.
In his 1962 essay "Not as We Know it: The Chemistry of Life", Isaac Asimov postulated that the hottest range of planets (Mercury-like planets) could theoretically be able to sustain life forms using fluorosilicone (hybrid molecules carbon- and silicon-based with fluorine attached to the carbon groups) in fluorosilicone instead of nucleic acid / oxygen-based proteins in water, like the life forms on Earth.
Most of the examples below are not hard science fiction. Authors have a lot of fun with the possibilities. Sometimes other elements are used, which don't have to be in Group 4 of the periodic table.
Anime and Manga
- The Tekkamen from Tekkaman Blade look like they are simply wearing Powered Armor after their Transformation Sequence, but it's explained in-series that they are actually transmuted into crystalline-based entities.
- Suzumiya Haruhi: Silicon-based Data lifeforms were responsible for the mysterious happenings in one of the two stories of the eighth volume. But they live on without their bodies. How? That's classified, apparently.
- Blame features a race called the "Silicon Creatures" or "Silicon Lives". Like the rest of the cast, they're actually just advanced cyborgs who "evolved" from humans, but they're so heavily altered that it's hard to say if the fleshy-looking parts of them are really flesh or not anymore.
- In Gundam 00 a Wakening of The Trailblazer, the Extraterrestrial Living metal Shape shifter is hinted to be this... for a series that never had aliens, they are supposed to be the Mecha-Mooks of the story.
- In Project: ARMS, the alien life form Azreal is silicon-based. The discovery of it is what gets the Egrigori to kick-start their various ARMS experiments. It also saves the day at the end, when it's revealed that Azreal is nearly immortal because of its silicon makeup, which leaves it very lonely and more willing to sympathize with the protagonists than Keith White (who wants to kill everyone on Earth and essentially condemn Azreal to an eternity alone).
- Rot Lop Fan, from Tales of the Green Lantern Corps Annual # 3, is stated to be a silicon-based life form in a galaxy with no visual light spectrum. He looks like a frog.
- In another DC Universe story by Alan Moore collected in the same book, there is a rare example of a more realistic portrayal of silicon lives as they would exist at Earth-like temperatures. When their planet is invaded by the power-hungry Spider Guild, the silicon beings, resembling giant stone statues of men, don't even notice because they move at a geologic pace.
- Blok from The Legion of Super Heroes, as well as Strata from its sister title, L.E.G.I.O.N.
- Stonians, from Astonishing Tales #22 and Strange Tales #74, are silicon-based lifeforms.
- Stargate Atlantis: In the Dark Frontier fanfiction, the Shade are a strange hybrid of carbon- and silicon-based life: they are mostly carbon-based but use a powerful acid to digest any mineral in order to build and repair a silicon-based exoskeleton. Their unusual composition also gives them the ability to use ammonia and sulfuric acid for what we use water. Oh, and they are cold-blooded, capable of draining anything from alkaline batteries to energy shields in order to stay alive for months without feeding in any environment. The biggest ones (few thousand miles across) strip mine entire planets for naquadah and trinium, also draining the planetary core for heat; when they move on, the planet is essentially a dead husk locked in an eternal ice age while the metals are used to build planetoids which serve as hatcheries. Despite being living creatures, they are apparently capable of entering hyperspace.
- Xenomorphs from the Alien franchise are apparently silicon based.
- Space Slugs and Mynocks are described as silicon based in various Star Wars sources.
- Gorignak from Galaxy Quest.
- Lost In Space features silicon-based spider-like lifeforms.
- The Silicates in Island of Terror.
- The aliens in the film Evolution were silicon-based
- Sentenced to Prism by Alan Dean Foster. Most life on Prism is silicon based. Foster has a lot of fun with the possibilities: natural Frickin' Laser Beams, rich colouration, lots of armour, a casual approach to being on land or underwater (the inhabitants consider water 'thick air') and so forth.
- The Isaac Asimov short story "The Talking Stone" was about Silicon-Based Life forms called "siliconies" that lived on asteroids. They survive by absorbing gamma rays from radioactive ores.
- Stanley G. Weinbaum's short story "A Martian Odyssey" had a silicon-based life form that consumed silicon and excreted silicon dioxide bricks.
- Discworld: Played for laughs, but Trolls qualify. It is specifcially noted that, as sillicon-based organisms, they function like supercomputers, with processing speed (and thus intelligence) being inversely proportional to temperature. One of them uses Hulk Speak at room temperatures, but when locked in a freezer for a while, almost came up with a unified field theory. They also count in either base four or binary because of that.
- The third Young Wizards book has a planet of silicon-based life forms, the fact of which ends up being extremely important. As it turns out, they can function like extremely complex computers.
- Later books mention in passing that to some silicon-based species, chocolate is an aphrodisiac.
- The Dancing Meteorite by Anne Mason includes a flashback to humans' first encounter with silicon-based life, which the human explorers think is just part of the landscape until it rises up and attacks them.
- Ullerans in H. Beam Piper's Terro-Human Future History. The concept is made more believable by using silicones (Si-O-Si-O-Si chains), some of which are pliable at what we consider room temperature.
- Also, the non-intelligent native life on Niffelheim, which has a fluorine atmosphere and a x-ray-emitting sun.
- Dark is the Sun by Philip Jose Farmer: Phremompit is a silicon based lifeform native to an asteroid, coming to Earth in a meteor shower. He eats radioactive rocks and moves on natural treads. Unfortunately, he drills through many people before learning his morse-code communication lazer is turned up a bit too strong for the mushy-bodied earthlings.
- In Ben Bova's Venus, it's discovered that such a lifeform inhabits the titular planet. It has a massive underground body, and tentacles a few kilometers long which it uses to search the planets surface, for food or whatnot.
- The vampiric beings from The Stress of Her Regard appear to be this trope, although it's slightly too early in history for the characters to describe them in terms of modern chemistry.
- Although no actual life form appears, Sheldon of The Big Bang Theory builds a model of a hypothetical silicon-based DNA analogue.
Live Action TV
- The Tholians of Star Trek, as seen in the page picture (taken from Star Trek: Enterprise). The original Trek just showed the head. The Horta are also this trope.
- Star Trek: The Next Generation:
- Tiny, sentient silicon crystals. To them, we're "ugly bags of mostly water".
- The extinct Martian insects that are sometimes displayed on computer screens were, according to certain reference books, also silicon-based lifeforms.
- The X-Files: The title creature from "Firewalker" was a silicon based fungus-like creature that exploded from people's throats to spread its spore.
- Doctor Who has several examples, including the Kastrians from "The Hand of Fear" and the Ogri from "The Stones of Blood".
- Disneyland: "Mars And Beyond" briefly mentions silicon based life in a segment about what life on Mars would look like. It suggests that it would take the form of crystalline structures that rise during the day, then shatter into oblivion in the cold Martian night.
- In Stargate Atlantis, the extinct Sekkari were apparently a silicon-based lifeform.
- Carl Sagan explored this question in this video: "I wonder if we'll ever find a specimen of life based not on organic molecules, but on something else — something more exotic." He goes on to describe himself as a "carbon chauvinist."
- Dungeons and Dragons:
- Gunman Chronicles uses it as a plot point, where Silicon lifeforms can't digest Carbon lifeforms.
- The Chenjesu in Star Control are essentially sapient crystals. They are living hyperspace receivers more sensitive than most sentients who tried managed to build and make pretty much anything from crystalline materials.
- Master of Orion: The Silicoids. The wildly divergent biochemistry of silicon-based organisms is reflected in the gameplay. Silicoids do not need to farm to produce food, thus freeing up their population to conduct research or construction. On the down side, their slower metabolism means slower population growth.
- Also, the Space Crystal monster, which flies around and exterminates the population of colonized planets.
- Metroid Prime Hunters has this in Spire, who's race (the diamonts) are silicon based lifeforms. Super Metroid has Crocomire.
- The Gastro, from The Maw, are silicon-based organisms.
- Sword of the Stars has the Swarm, beelike things that spawn from asteroid hives and fire plasma, as well as sending out Silicoid Queens to build new hives. They are Demonic Spiders, especially in early game or with small groups, but reward you with a fair amount of resources if you clear a hive.
- Technically, most Rock-type Pokémon count, as several of them are literally made of rocks. Same goes for many Ground-type Pokemon.
- The Silacoids from X-COM: UFO Defense are simple silicon-based life forms that look like lumps of purple lava. They have rock-hard skin that is immune to fire damage and attack by biting. Because of their extremely high body temperature, they leave burn marks on the ground as they move, sometimes setting fire to nearby objects.
- The Meteor Heads from Terraria. They are flying Mooks that spawn at meteorites and attack en masse.
- Crystals in VGA Planets.
- Schlock Mercenary: A halfway house between this and regular life: Sergeant Schlock, who is carbosilicate (i.e. based around both carbon and silicon). In a bit of realism, his biology and biochemistry is pretty clearly shown to be fundamentally different from traditional carbon-based life.
- Invasion America has a genetically-engineered species called manglers. A baffled human scientist describes the discovered bones of one as a "silicon analog" rather than carbon-based.
- This trope is invoked in the episode of The Simpsons when Homer encounters an alien.
Homer: I'd be happy to answer your questions about the alien. Any questions at all.
- Diamondhead and his Expy, Chromastone, in Ben10.
- Silicon is more abundant than carbon on the Earth's surface, and yet Earth life is almost exclusively carbon-based. The reason for this is because using silicon instead of carbon is only an advantage in hellishly hot environments (around 300 degrees Fahrenheit), where silicon compounds would remain stable while carbon compounds would break down. It's speculated that such silicon-based life would actually be silicone-based life, as silicones can bond with organic compounds just like the carbon backbones in our biology (silicones are silicon-oxygen chains used in synthetic rubber, engine lubricant, and breast implants). It would use a substance such as sulfuric acid as a solvent in the same way we use water (as it remains liquid under high temperature) and breathe pure fluorine (which has an attraction to silicon, and unlike oxygen, doesn't combine with it to form sand). To compensate for the vastly different methods of energy production and consumption, it would probably have physiology most closely comparable to fungus if not something stranger (the more popular crystalline formations would only be found in pure silicon-based life, not silicone-based life). Hilariously, they'd be prone to exploding on contact with an earth-like atmosphere.
- Even weirder are the hypothetical sulfur and fluorosilicone based-life, who would live in even hotter environments, and at the hotter end, use molten rock as a solvent instead of water.
- While life on Earth is carbon-based in its molecular structure, one origin-of-life theory posits that organic molecules could've initially begun copying themselves on the surfaces of wet clays. As clays do contain silicon, this would make "silicon-based life" true in a literal sense.
- Some Real Life organisms use silicon in their skeletal structure (sponges) or as an abrasive to render themselves less palatable as a food source (grass).
- Evolution has nitrogen-based life. Since this makes them giant piles of azides, they are literally Made of Explodium. Cleverly enough, evolution has turned this into an advantage: extreme heat catalyzes their rapid cellular reproduction and growth. Combined with rapid mutations, this is why they seem to evolve before our eyes (such as a field of them suffocating in our atmosphere, with one able to gasp for life long enough to start producing more offspring already better able to breathe our air, then growing wings and flying away)..
- Animorphs: While they don't discuss what element they're based on, the Venber in one story are "not carbon-based" as they melt above freezing temperatures. Probably ammonia. It's been speculated that ammonia-based life could exist on extremely cold planets.
- Arthur C. Clarke
- A nineties story references "germanium-based consciousness," albeit as a long-complete Assimilation Plot by previously carbon-based aliens. (Germanium is directly below silicon on the periodic table.)
- The lifeforms on Jupiter in 2010 Space Odyssey are hydrogen based.
- Another story explores the possibility of hydrogen based creatures on the surface of the sun.
- In one Isaac Asimov story, it's mentioned that some of the native bacteria on the Spacer planet Aurora have fluorocarbon rather than hydrocarbon chemistry.
Live Action TV
- Another X Files episode featured ammonia-based alien brain worms that rode to Earth on an asteroid in prehistoric times and were hiding out at the North Pole. How they were able to take over the bodies of the human scientists that were trying to study them, despite the fact that not only is their biochemistry obviously incompatible, but being inside a human body for any length of time should have melted the little bastards is, of course, never adequately explained.
- Doctor Who
- The Krotons are tellurium-based.
- The Raxacoricofallapatorians (the Slitheens' species) are calcium-based.
- Stargate has sulfur-based aliens. The aliens themselves didn't show up as anything more than a picture, their automated probe was xenoforming an earth like planet into something their ecosystem could inhabit.
- GURPS: Space has suggestions for hydrogen and sulfur based lifeforms as well as silicon based ones.
- Star*Drive uses a page in its Alien Compendium sourcebook to explain six different classes of biochemistry, based on "liquid medium," "reagent for cellular respiration," and "compounds or elements that can create very complex organic molecules," which correlate well with the GURPS supplement described above. Furthermore, each type is only found on certain classes of planets. These classes are Class I: Terran (habitable), Class II: Minimal (minor life support required due to climatic extremes, atmospheric conditions, etc.), Class III: Extreme (major life support required due to intolerable climate or atmosphere), Class IV: Space (including asteroids, rings, etc.), and Class V: Jovian (extreme life support required).
- The series of life are Series I: Water medium, Oxygen reagent, Carbon structure, Class 1/2 environment; Series II: Ammonia medium, Hydrogen reagent, Hydrocarbon structure, Class 3 environment; Series III: Water medium, Chlorine reagent, Carbon structure, Class 2/3 environment; Series IV: Sulfur dioxide medium, Sulfur trioxide reagent, Carbon structure, Class 3 environment; Series V: Sulfuric acid medium, Oxygen reagent, Silicone structure, Class 3 environment; Series VI: Sulfur medium, Sulfur dioxide reagent, Fluorosilicone structure, Class 3/5 environment; and Series VII: anything that doesn't fit into the biochemistries described above.
- The Volus of the Mass Effect universe have a biochemistry based on Ammonia. As a result, they're forced to wear environment suits anywhere that's not a planet catered specifically to their biology. They just can't breathe in an atmosphere that isn't ammonia-rich. Their flesh will also split apart and rupture because they evolved on a planet with an extremely high pressure atmosphere, so their skin can't hold together under lower air pressures.
Truth In Television
- A NASA astrobiology team recently found microorganisms in a toxic (to everyone but them) lake in California which use Arsenic in place of Phosphorus in their biomolecules, such as DNA or ATP. However, the announcement was contested because of issues with the announcement. Regardless, these lifeforms are still primarily C-H-O-N based, like everything else on Earth, and can use Arsenic in place of Phosphorus if they're forced to, with a performance decrease. It's not as if they like it. They just don't die from it, unlike most other Earth lifeforms.
- In 2011 it was proven that metal oxides can function in a manner similar to carbon compounds in the formation of life.