The Bricks of Life: Exploring the Idea of Alien Chemistry

Apr. 15, 2004
by Seth Shostak - Senior Astronomer

Body TextIts a question as common as brown dogs: will alien life be carbon-based?

Im asked this frequently, although Im not sure why the public is so hung up on the elemental basis of extraterrestrial life. In my experience, folks seldom inquire whether the Krebs cycle could be prevalent on other worlds, or if adenosine triphosphate might underpin the energy production of active aliens. Probably the fascination with vital soot is just a consequence of carbons high profile on Star Trek. The plot of this popular TV series gets viscous whenever the Enterprise detects "carbon-based life forms" on some God-forsaken planet deep in the Galaxys nether regions. If theyre carbon-based, well, they must be like us (and possibly edible, too).

Hype aside, as most astrobiologists or any one of a thousand books will tell you, carbon-based life is not simply a provincial conceit. Theres good reason why this element is the basis for life on Earth, and probably on most other worlds that shelter biology.

If you remember your high school chemistry, youll recall that carbon has half of its outer electron shell filled. In other words, each carbon atom is able (and eager) to bond with up to four other electron-sharing atoms (most atoms prefer to have a filled outer shell of eight electrons). As a common example, a single carbon atom will eagerly take on four hydrogen atoms to make methane (CH4). And because carbons outer shell is both half filled and half empty, it can handily hook up with other carbon atoms, creating the sort of elaborate molecular chains and rings that fuel companies love to pump.

Carbon, in other words, is adept at making complex structures. And complex structures are the bricks of life.

Are there other contenders? Is carbon really so special, or did it just get lucky here on Earth? If you have a periodic table handy, youll note that the element situated under carbon is silicon, which also has four electrons in its outer shell. Ergo, silicon might also seem to be an obvious basis for life, a point that was first made at the end of the nineteenth century by the German astrophysicist, Julius Scheiner. The optimistic Scheiner was certain that other planets in our solar system (including roasty toasty Mercury) sported life.

But his sunny attitude was misplaced when it comes to silicon-based beings. Silicon may be carbons chemical cousin, but its a poor relation. Because the silicon atom is larger, its bonds with other elements are weaker. While carbon hooks up with two oxygen atoms to make carbon dioxide, a nice waste product for both humans and SUVs, the silicon equivalent, silicon dioxide, quickly assembles itself into a crystalline lattice. Its better known as sand, and would make exhaling a gritty experience. The weaker bonds of silicon also preclude the easy formation of those long, same-atom molecular chains that underlie many biological compounds. A slew of complex carbon-based molecules are easily produced in comets, interstellar dust, and university glassware. But if you check out natures chemistry lab for silicon (consider volcanic lava), the products are far less interesting.

If thats not enough to dissuade you from silicon, consider this: theres just a lot more carbon around. Cooked up in the searing interiors of stars, the cosmic abundance of carbon is more than ten times that of silicon. And by the way, if silicon is a distant second in the biology sweepstakes, the elements under it in the periodic table germanium, tin, and lead are worse. Theyre less abundant, and less inclined to make biologically interesting compounds. The sole known example of tin-based life occurred in The Wizard of Oz, and it suffered from lack of lubricant.

Of course, one must always beware of hubris in speculating on the properties of extraterrestrial life. Earth is just one planet among many billions in our galaxy. Life, after all, is about organization, function, and accurate reproduction. At its heart is information processing, and there may be other ways to accomplish this beyond mundane chemistry -- based, as it is, on the social behavior of electrons.

But when the Enterprise boldly goes in search of life among the stars, theres good reason its scanners perk up at any sign of carbon-based chemistry. Its more than likely that overweight aliens will be watching their carbs and not their sils.