Non-Carbon Species: Will We Overlook Extra-terrestrial Life? A Galaxy Classic

Carbon is great molecular glue—there’s not doubt about it. Just add water and you’ve got life. Well, maybe it’s not quite that simple, but carbon and water do seem to be a winning combo, at least on planet Earth. That may be why we’ve been limiting ourselves in our search for . The carbon/water combo has worked so well for our own conditions, that we simply can’t imagine anything else supporting life.

Currently, our search for extra-terrestrial life forms has been focused on planets similar to ours. The perplexing idea exists, however, that what would be death to us on Earth, may be life to other beings. What we’re looking for may not lie in our version of the “sweet spot”.

It is definitely worth considering that other options do exist besides water and carbon. Alternative biochemists speculate that there are several atoms and solvents that could potentially spawn life. It is also worth considering that because humans are carbon-based beings, who do their lab work under conditions on planet Earth, we may be a bit biased towards carbon thinking.

Not everyone is a “carbon chauvinist”, however. So far, scientists have already hypothesized several interesting alternatives to carbon. Various elements become more stable and capable of forming complex molecules when under strange (from a human perspective) thermal and atmospheric conditions. For example, silicone-based chemicals would be more stable than equivalent hydrocarbons in a sulphuric-acid-rich setting, which has been noted in some extraterrestrial environments.

Even counter-intuitive elements such as arsenic may be capable of supporting life under the right conditions. Even on Earth some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Several other small life forms use arsenic to generate energy and facilitate growth. Chlorine and sulfur are also possible elemental replacements for carbon. Sulfur is capably of forming long-chain molecules like carbon. Some terrestrial bacteria have already been discovered to survive on sulfur rather than oxygen, by reducing sulfur to hydrogen sulfide.

Nitrogen and phosphorus could also potentially form biochemical molecules. Phosphorus is similar to carbon in that it can form long chain molecules on its own, which would conceivably allow for formation of complex macromolecules. When combined with nitrogen, it can create quite a wide range of molecules, including rings.

So what about water? Isn’t at least water essential to life? Not necessarily. Ammonia, for example, has many of the same properties as water. An ammonia or ammonia-water mixture, stays liquid at much colder temperatures than plain water. Such biochemistries may exist outside the conventional water-based "habitability zone". One exciting example of such a location would be Saturn's largest moon Titan.

Hydrogen fluoride methanol, hydrogen sulfide, hydrogen chloride, and formamide have all been suggested as suitable solvents that could theoretically support alternative biochemistry. All of these “water replacements” have pros and cons when considered in our terrestrial environment. What needs to be considered is that with a radically different environment, comes radically different reactions. Water and carbon might be the very last things capable of supporting life in some extreme planetary conditions. In any case, it is not beyond the realm of feasibility that our first encounter with extra-terrestrial life will not be a solely carbon-based occasion.

Posted by Rebecca Sato.

Links:

New Scientist
Alternative Biochemistry

Comments

[Jack Butler]

The micro-organisms which use sulfur for metabolism on Earth use it in place of oxygen, not in place of carbon, so they do not demonstrate the feasibility of long-chain molecules based on carbon.

That said, I agree that we are somewhat provincial in our definition of life. No reason to suppose there is not carbon-based life on other worlds, and plenty of it, but that may well not be the only kind.

May I suggest that our provincialism does not merely consist of a bias toward carbon-based life-forms, but toward molecular life-forms? It is ironic that in the very age in which we are attempting to reproduce consciousness in nonorganic materials, we are limiting our search for life to the organic.

Perhaps the definition of life needs tweaking. Is the sun alive? It feeds, grows, uses energy, remains stable but complex over extremely long intervals. It is extremely complex in its workings, arguably as complex as we are. We even refer to the life-cycles of stars. Perhaps it is not merely a metaphor.

[Jack Butler]

Oops--I meant sulfur, not carbon at the end of paragraph one above.

[Navneeth]

I've always wondered about this: Why do scientists restrict themseleves to finding life that is similar to life on this Planet? Indeed, there are many places right here on Earth that are not supposed to be congenial to life but are thriving with micro-organisms and small crustacean.

[Martin Wisse]

Navneeth: perhaps because deep in their hearts they want to find other humans, rather than other lifeforms?

[Michael]

Its easy to understand why we only look for life in a small niche so rarely found in the universe: ITS OUR NICHE! If we look outside it, and find 10 foot tall rockmen of astonia 4 we could end up like Tim Allen. "Its a rock! It doesnt have any weak spots!" Just kidding but I think the important thing here is to remember just how delicate a balance life is. I think we should continue to look for life in the carbon field until we accidentally find life outside of it. Lets stick with what works using our meger resources, and at a later time when these projects are better funded widen our horizons.

[Christo van der Bergh]

Time is another factor. We are used to 24 hour diurnal cycles and lifespans of around 50 to 80 years (for humans). I have wondered about life forms elsewhere in the universe with much shorter or longer lifespans, like seconds (or milliseconds or microseconds) or millennia.

[Jason]

I reckon any extraterrestrial life would be fairly obvious by its alteration of its surroundings.

[knoxvilledaniel]

Even scientists who should know better are somewhat afraid - or perhaps just reluctant - to think outside the box & imagine other lifeforms that don't function like carbon - based life forms. Think of it as " carbon chauvinism ". As long as we only imagine life similar to what we know of on Earth, it would be hard to recognize a truly " ALIEN " species if we encountered one, or at least the evidence of one, someday. Imagine a planet of crystal life forms, what would they consume ? More importantly, how would they think ? We need to envision scenarios like that, not just green - skinned, scaly humanoids & others that were the staple of old fashioned science - fiction magazines.

[celesteon]

IT ALL DEPENDS ON WHAT YOU CLASSIFY AS LIFE! Animals, Plants, Bacterias, Fungui, Protazoans, ...Stars. Virus's arn't even classed as life, when they are clearly a set of cellular-parasitic micro-organisms! If Virus's arn't life, then what are they? Rocks? Liquid? ..Air? And what objects are "allowed" to be life? The fact is, "life" is just a word used by us humans to describe what we perceive as "life", and not anything more. The real thing and the word are two entirely different things. If "Life" is something that moves, then planets, moons and stars are also alive. If "life" is something that grows, then crystals, volcanoes and balloons are also alive. If "life" is something that is part of an "ecosystem", then the sun, water and the air are also alive. So, again; IT ALL DEPENDS ON WHAT YOU CLASSIFY AS LIFE!