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Caitlin: People talk about the search for alien life all the time: why we haven’t found any yet, and what we should be looking for. But somewhere out there in the universe, there might be creatures so different from us that we wouldn’t even recognize them as alive.

All living things that we know about have some basic things in common: they grow, reproduce, respond to stimuli in their environment, and evolve over time. They also all share the same basic biochemistry: they’re made out of long chains of carbon molecules that hang out in a water-based medium. But it’s also possible that there’s life that’s not carbon based. And it would be a lot different from what we’re used to.

Carbon’s ability to form long chains makes it the perfect base for building molecules complex enough to keep a living thing... alive. But even though carbon is great for building these big, complicated molecules, it’s not the only element that can do this. Silicon, which sits just under carbon on the periodic table, shares many of the same chemical properties. Just like carbon, the outer layer of a silicon atom has four unpaired electrons ready to form molecule-building bonds.

Silicon can also form long chains and bond to oxygen--again, just like carbon. But a lot of the bonds formed by silicon are weaker than those formed by carbon--especially the silicon-silicon bonds that it would need to make those long chains. And even when silicon-silicon bonds do form, they’re generally unstable if there’s oxygen around. Still, because carbon and silicon have so much in common, some scientists think that this silicon-based life could theoretically be possible, even though we’ve never found any.

Like carbon-based life, silicon-based life would grow, reproduce, respond to stimuli in its environment, and evolve over time. But we might not even recognize it as life at first. A silicon life-form might look more like a pile of rocks than a plant or animal, and it would probably do some pretty weird things.

When silicon reacts with oxygen, for example, it turns into quartz, so silicon-based organisms that breathed oxygen would exhale quartz! Silicon-based bonds are most stable at high temperatures, so if silicon-based life does exist, the best place to look for it would be in very hot places, like deep beneath a planet’s surface. But silicon-based life probably wouldn’t be very complex, because of its unstable bonds.

Still, science fiction writers have had a lot of fun with this idea, like, in one episode of the original Star Trek series, Kirk and Spock run into a tunnel-dwelling silicon-based creature that’s terrorizing miners on an alien planet. But organisms with such different biochemistry might be hard to even detect -- they could look more like rocks or crystals than anything we’d recognize as alive. So, some scientists have proposed the idea that there could be a so-called “shadow biosphere” of non-carbon based life living alongside us right here on Earth.

After all, silicon is the second most common element in the Earth’s crust after oxygen. But that silicon is bound up in rocks, where it would be hard for organisms to incorporate it into their biochemistry. If silicon-based life does exist on Earth, it might be in the form of silicon-based microbes living within magma deep inside the Earth’s mantle.

But no one’s ever found any actual evidence of this. Our best bet for finding silicon-based life is probably to just look somewhere else. The James Webb Telescope, launching in 2018, will look for signs of life in the atmospheres of planets outside our solar system.

Photosynthesis and respiration affect the amounts of oxygen and carbon dioxide in Earth’s air. Silicon-based organisms could leave traces of their existence in their own planets’ atmospheres, too. Meanwhile, some scientists are working on making their own version of silicon-based life -- or at least, the first steps toward it.

In March, Caltech scientists announced that they’d found a species of thermophilic bacteria-- bacteria that thrives in extreme heat--with an enzyme that in very rare cases incorporated silicon molecules into its carbon-based molecules, a sort of chemical accident. Using artificial selection, they were able to modify the bacteria in a lab to produce the molecules with silicon in them 2000 times as often. Someday, artificially-created microbes like this may be used to produce complex silicon molecules that chemical companies can turn into glues and sealants.

And even cooler, Research like this also helps scientists understand more about how life can use silicon. So the next time you are outside and and stop to look at a cool rock, you might want to look for signs of life. You never know.

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