Here’s an article by Fuz Rana at Reasons to Believe, talking about alternatives to carbon-based life. (H/T Tough Questions Answered)

Excerpt:

Life as we know it on Earth consists of carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur (CHONPS). But could other elements constitute life as we don’t know it?

Not merely a discussion topic for science-fiction buffs, this question bears on origin-of-life discussions and on the search for extraterrestrial life. Carbon-based life requires a strict set of conditions. But perhaps life based on an element like silicon can exist under more extreme conditions. Few places in our solar system, and presumably beyond, can conceivably support carbon-based life. But for life built upon silicon, habitable sites may well abound throughout the universe.

However, of the 112 known chemical elements, only carbon possesses sufficiently complex chemical behavior to sustain living systems.  Carbon readily assembles into stable molecules comprised of individual and fused rings and linear and branched chains. It forms single, double, and triple bonds. Carbon also strongly bonds with itself as well as with oxygen, nitrogen, sulfur, and hydrogen.

Carbon serves as the hub of complex molecules. You can join lots of different things to it so that they stay put. But the bonds are not so strong that you can’t break things apart if you really want to. That’s what makes it suitable for making complex life, and why people talk about “carbon-based life”.

The rest of the article explains why other kinds of elements like silicon and phosphorus are not suitable for creating life.

Is carbon synthesis fine-tuned?

Here’s an article by agnostic physicist Paul Davies explaining why people think that the production of carbon in the universe is an example of fine-tuning.

Excerpt:

One of the best-known examples of this life-friendly ‘fine-tuning’ of the laws of physics concerns carbon, the element on which all known life is based. The Big Bang that kicked off the universe coughed out plenty of hydrogen and helium, but no carbon. So where did the carbon in our bodies come from? The answer was worked out in the 1950s: most of the chemical elements heavier than helium were manufactured in the cores of stars, as the product of nuclear fusion reactions. It is the energy released by these reactions that makes the Sun and stars shine.

However, while the details of stellar nuclear reactions are fairly straightforward, there is a notable exception: carbon. Most nuclear reactions in stars occur when two atomic nuclei, rushing around at tremendous speed care of the searing temperatures, collide and fuse, forming a heavier element. But carbon cannot be made this way because all the intermediate steps from helium to carbon involve highly unstable nuclei. The solution, spotted by University of Cambridge astronomer Fred Hoyle, is for carbon to form from the simultaneous collision of three helium nuclei.

THERE IS, HOWEVER, a snag. The chances that three helium nuclei will come together at the same moment are tiny. So Hoyle reasoned that a special factor must be at work to boost the rare reaction and lead to our abundance of carbon. If not, then life in general, and Fred Hoyle in particular, would not exist!

Hoyle knew that nuclear reactions can sometimes be greatly amplified by the phenomenon of resonance, similar to the way that an opera singer can shatter a glass by hitting a certain pitch. Carbon nuclei can resonate too, if the masses and energies of the colliding particles that go to form it are just right. Hoyle worked backwards — he knew the particle masses and energies, and he used them to predict the existence of a carbon resonance.

He then pestered Willy Fowler, a nuclear physicist at the California Institute of Technology, to do an experiment to test the prediction. And sure enough, Hoyle was right. Carbon has a resonant state at exactly the right energy to enable stars to manufacture abundant carbon, and thereby seed the universe with this life-encouraging substance.

Hoyle immediately realised just what a close-run thing this mechanism is. Like Baby Bear’s porridge in the story of Goldilocks, the energy of the carbon resonance has to be “just right”. Too high or too low, and the consequences for life would be catastrophic.

So what determines the carbon resonance? Ultimately it depends on the strength of the force that binds protons and neutrons together in the nucleus. That force is one of the unexplained parameters of basic physics — one of the knobs on the Designer Machine if you like. If the strength of the force that determined the carbon resonance was only a fraction stronger or weaker, it is doubtful there would be observers in the universe to worry about the distinct absence of carbon.

Hoyle himself was deeply impressed by this discovery. “It looks like a put-up job,” he quipped. “A commonsense interpretation of the facts suggests that a superintellect has monkeyed with physics,” he later wrote. A similar conclusion was reached by the Princeton physicist Freeman Dyson: “In some sense, the universe knew we were coming.”

He doesn’t accept that God is the fine-tuner though, so the article just concludes with “it could be” speculations, which is all that naturalists can offer against the standard theistic arguments. Still, what he said about the finely-tuned synthesis of carbon is accurate.