Carbon-Based Life: The Cosmic Chemistry of Extraterrestrial Possibilities
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It’s a question as common as brown dogs: will alien life be carbon-based? I’m asked this frequently, although I’m 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…Probably the fascination with vital soot is just a consequence of carbon’s high profile on Star Trek. The plot of this popular series gets viscous whenever the Enterprise detects "carbon-based life forms" on some planet. If they’re carbon-based, well, they must be like us (and possibly edible, too).
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Most astrobiologists will tell you, carbon-based life is not simply a provincial conceit. There’s good reason why this element is the basis for life on Earth, and probably on most other worlds that shelter biology. Carbon has half of its outer electron shell filled. Each carbon atom is able to bond with up to four other electron-sharing atoms. And because carbon’s 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.
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Is carbon really so special, or did it just get lucky here on Earth? If you have a periodic table handy, you’ll note that the element situated under carbon is silicon, which also has four electrons in its outer shell. 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 sported life. But his sunny attitude was misplaced when it comes to silicon-based beings.
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Silicon may be carbon’s chemical cousin, but it’s 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, the silicon equivalent, silicon dioxide, quickly assembles itself into a crystalline lattice (better known as sand). 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 nature’s chemistry lab for silicon, the products are far less interesting.
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If that’s not enough, consider this: there’s 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 i.e. germanium, tin, and lead are worse. They’re less abundant, and less inclined to make biologically interesting compounds.
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Of course, one must beware of hubris in speculating on the properties of extraterrestrial life. Earth is just one planet among many billions. 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. But when the Enterprise goes in search of life among the stars, there’s good reason its scanners perk up at any sign of carbon-based chemistry. It’s more than likely that overweight aliens will be watching their carbs and not their sils.
