Could alien life be based on the element silicon rather than on carbon? This idea has been seriously proposed in the scientific literature for more than a century. Not surprisingly, then, silicon-based life has also been a staple of science fiction.
For example, the Star Trek franchise features Tholians, creatures that appear to be composed of crystalline minerals, such as quartz (silicon dioxide). The Tholians require very high temperatures (207 °C/404 °F) and suffer, even die, in substantially colder conditions. Even the hardiest known microorganisms would die quickly at such scorching temperatures.1
Tholians reflect popular speculation that there could be extraterrestrial life that is radically different from anything currently known (Earth life is exclusively carbon-based). We refer to all such unconventional organisms as “weird life.” Earlier this week, we discussed the properties that make carbon an ideal base for life chemistry. Now we turn to silicon to determine whether or not it too could provide an adequate base for life chemistry. (For the full details, see our paper.)
Scientists have long viewed silicon as the leading candidate for hosting non-carbon-based life. Because silicon is located just below carbon on the periodic table, it shares a number of carbon’s useful traits. Like carbon, silicon has the ability to form up to four single bonds, allowing it to form a wide range of molecules. In addition, many silicon compounds are able to withstand higher temperatures than typical carbon compounds; so hypothetically, silicon-based life, like the fictional Tholians, might be able to thrive on extremely hot planets.
Silicon is also good at self-linking (bonding directly to other silicon atoms), albeit to a considerably lesser degree than carbon. This enables it to form chains, branched chains, and ring structures similar to carbon. A further advantage of silicon is its abundance—it composes 28 percent of Earth’s crust (second only to oxygen)—making it approximately 1,000 times more abundant than carbon. Unlike carbon, large amounts of silicon are likely present on most small rocky planets.
Life Based on “Plain” Silicon
One feature that dominates silicon chemistry is silicon’s ability to bond strongly to oxygen, but only weakly to fellow silicon atoms. Because of that, we will treat silicon oxides as a special case (see below). Here we will examine “plain” silicon (i.e., molecules with silicon bonded to elements other than oxygen).
Plain silicon is an attractive prospect for building organisms because it supports many structures analogous to what scientists find in carbon-based biochemistry. However, several critical problems are associated with silicon’s chemical properties.
- Silicon-silicon bond is weak. Because of this, silicon compounds are typically less stable than corresponding carbon compounds.
- Silicon lacks strong multiple bonds. This feature greatly limits the number and types of molecules that silicon can form in comparison to carbon.
- Plain silicon is highly reactive. Silicon compounds are generally more reactive than their carbon-based counterparts and, thus, make building and maintaining biomolecules more challenging.
- Plain silicon is susceptible to oxidation. Because of silicon’s preference for bonding with oxygen, silicon biochemistry would likely require an environment free of both oxygen and water, which is a very restrictive limitation.
- Silicon-hydrogen bonds are unstable. Hydrogen atoms bonded to silicon atoms are highly reactive and tend to destabilize the molecule.
- Silicon has constraints on forming long chain structures. Silicon is able to link together to form chains and ring structures like carbon, but to a far lesser extent. It can form some simple monotonous polymers of great length, but these are not likely to be biologically useful.
Despite some promising capabilities, plain silicon is just too limited.
Life Based on Silicon Oxides
Silicon oxides avoid several of the pitfalls of plain silicon. First, silicon oxides are ideally suited for very high temperatures (above 1000 °C/1800 °F), such as might be expected on a planet very close to its star. Second, they form structures that are almost always highly stable. Third, they are generally not vulnerable to oxygen or water. These characteristics sound great—but silicon oxide chemistry has its share of deficiencies, too.
- Silicon oxides form crystalline structures rather than polymers. The most common forms of silicon oxides on Earth are silicate rocks. Such crystalline structures are poorly suited for constructing complex biomolecules.
- They are too This stability makes it difficult for silicon oxides to engage in metabolic reactions.
- They are inaccessible in the environment. Given the strength of silicon-oxygen bonds, once silicon oxides are formed they tend to stay locked in that configuration.
- Byproducts would be difficult to eliminate. Silicon dioxide (a solid) would likely be a byproduct of silicon-oxide-based metabolism, just as carbon dioxide is a byproduct of carbon-based metabolism. Advanced silicon-oxide-based life would have difficulty efficiently excreting this solid byproduct. It would certainly be more challenging than exhaling a gas like carbon dioxide.
- Reactions are too rapid. Reactions occur faster at higher temperatures. Given the high temperatures required for silicon-oxide chemistry, critical life reactions would occur so fast that it would be extremely difficult for organisms to control them.
- Lack of an adequate solvent. Life requires a liquid medium in which the key chemical reactions can take place. It is difficult to imagine a substance capable of remaining liquid at extreme temperatures and still able to support the necessary biochemical reactions for silicon-based life.
Based on these deficiencies, we find that life based on silicon oxides is even more problematic than it is for plain silicon.
What conclusions can we draw from this information? First, silicon is far less versatile than carbon. For perspective, chemists currently recognize only about 20,000 silicon compounds. In contrast, carbon is known to form some 10 million compounds with the potential to form a virtually unlimited number more. Therefore, silicon is at least 500 times less versatile than carbon, which for life chemistry is a huge deficiency. Second, plain silicon is too reactive to form a stable basis for life, whereas silicon oxides are too stable and form extremely long-lasting crystalline structures, such as rocks, instead of useful polymeric structures. Lastly and most significantly, silicon is far more limited than carbon in its ability to form the long chains needed for complex information-storing molecules (like DNA) required by any conceivable organism. Taken together, silicon is unlikely to fulfill all the roles needed for life, even single-celled life.
Sorry, sci-fi fans, but, as chemists, we can say with confidence that the Tholians and other silicon-based life only exist in our imaginations.
- Currently, a hyperthermophilic bacterium holds the record for the highest temperature (122 °C/252 °F) at which a microorganism can live and still be able to reproduce.
By John Millam and Ken Klos
John Millam received his doctorate in theoretical chemistry from Rice University in 1997, and currently serves as a programmer for Semichem in Kansas City.
Ken Klos received his MS in environmental studies from the University of Florida in 1971, and worked as an environmental/civil engineer for the state of Florida.