The possibility of silicon-based life
Jemima Hodkinson contemplates life as we don’t know it
Many searches for extra-terrestrial life concentrate on uncovering signs of life similar to our own – the availability of liquid water, or the existence of complex compounds made from carbon (the base element for life on Earth). But what if life could be built from different stuff?
There is an element sitting very close to carbon in the periodic table which some think could be an alternative chemical basis for life. We are more used to finding it in computer chips, sand and lubricants. What might a silicon-based life-form look life?
Life on Earth
All known life on Earth is based on the element carbon. It is the fourth-most abundant element in the universe; in our bodies, it makes up around 18.5 per cent of our mass. It is an essential component of biological molecules such as lipids, proteins, carbohydrates and nucleic acids.
Carbon’s importance results from its rich chemistry. Each carbon atom can form four bonds with other atoms, unlocking countless possibilities for building complex molecules. For example, it can form long, stable chains – hydrocarbons – which are found in the lipid membranes of cells. It can also easily form and break bonds with oxygen, which occurs during respiration, releasing energy from food.
Complex chemistry and sci-fi creatures
Silicon (Si) sits just below carbon in the periodic table. Like carbon, it can also form bonds with four other atoms at once, form long chains (polymers), and bind to oxygen. There are huge amounts of it on Earth – in fact, it is the second-most abundant element in the Earth’s crust after oxygen.
It is these chemical similarities between silicon and carbon that have planted the idea of silicon-based life. It has certainly proved a fertile source for life in science fiction: silicon-based life-forms have been a sci-fi favourite for decades, from sentient crystals to rocky, lava-dwelling creatures and aliens that excrete bricks of silica. However, some important differences between the chemistry of silicon and carbon mean that we are unlikely to be invaded by these creatures any time soon.
Although silicon can form chains, these are not stable like hydrocarbon chains; neither does silicon share carbon’s ability to easily make and unmake bonds with oxygen. When energy is released from a carbon compound during respiration it is ‘oxidised’, and the waste product is carbon dioxide – an easily excretable gas. When silicon compounds go through the same process, solid silica is produced as a by-product – less easy to remove, although the brick-excreting sci-fi aliens seem to have the answer.
A brave new world?
The complex chemistry of carbon is vastly favourable to life under conditions on Earth. However, that does not mean that silicon-based life-forms would be impossible under different planetary environments.
In theory, planets far, far away may have environmental conditions that favour silicon chemistry over carbon chemistry. For example, silicon bonds are much more stable than carbon at high temperatures. Perhaps silicon-based life could arise on a planet that is too hot for carbon-based life.
What would it look like?
If silicon-based life-forms did exist, how would be look for them – would we even be able to recognise when we’d found them? The search for silicon-based life would certainly not be easy; astronauts of the future will not be stumbling across the tracks of a rock-monster or the sandy excreta of a silicon creature.
A silicon-based life-form would likely be very primitive, and the signs of its existence much more ambiguous – such as groups of silicon molecules appearing in an unexpected place. It might not even be found on a planet’s surface – the very hot, hydrogen-rich, oxygen-poor conditions deep inside a planet may be more conducive to complex silicon chemistry.
In the search for life on other planets, perhaps the truth will turn out to be stranger than science fiction.Lead image:
huangjiahui/Flickr CC BY NC ND
- Scientific American: Could silicon be the basis for alien life?
- TV Tropes: Silicon-based life in sci-fi
- Harvard-Smithsonian Center for Astrophysics: Alternative biochemistries
- Universe Today: Non-carbon astrobiology
Questions for discussion
- Are there theories about elements other than silicon and carbon which could be the basis of life-forms on other planets?
- What methods are used currently to search for life on other planets?
- Carbon is the basis of all life on Earth. What other elements are common in living organisms on Earth? What are they essential for?
- How did life on Earth begin?
- PDF of The possibility of silicon-based life [PDF 76KB]