Post Author: Bill Pratt
The scientists at Reasons to Believe have always argued that finding advanced life on other planets is extremely improbable, considering the hundreds of conditions that must be simultaneously met for a planet to support advanced life. Earth is quite unique in this regard.
When I’ve mentioned this to friends and family, they sometimes express skepticism and claim that maybe there are life forms that could exist with different chemistry than the carbon-based life on earth.
In an article published last year, Dr. Fazale Rana argued that non-carbon based life forms are just not feasible. Rana explains:
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.
What about silicon? This is the most common idea I hear. Rana replies:
Silicon belongs to the same chemical group as carbon and should display similar chemical properties, prompting some astrobiologists to propose that life could be based on this element. But while silicon does form rings and chains, these structures lack the stability and the range of complexity found in carbon-based compounds. Silicon-silicon bonds are much weaker than the corresponding carbon-carbon bonds, and unlike carbon-carbon bonds, they are susceptible to oxidation.
Silicon won’t work. Then what about arsenic or phosphorus? Rana explains:
Though arsenic and phosphorus share some chemical properties, the two elements display significantly different chemistries as well. Phosphorus is a nonmetal. Arsenic is a metalloid. Phosphorus reacts with oxygen to form chemical compounds called phosphates. These groups take part in the linkages that constitute the backbone of the DNA molecule by reacting with the sugar, deoxyribose. The sugar-phosphate linkages are described as phosphate esters. Arsenic will also form a compound called arsenate by reacting with oxygen. Arsenates and phosphates display some chemical similarities, but esters formed with arsenates are unstable. They could never be used to construct the backbone of DNA––considered indispensable for life––or another similar compound.
The bottom line is that based on all the known chemical elements, scientists are fairly confident that advanced alien life would have to be carbon-based, which places severe limitations on the kinds of environments that our E.T. friends could live in. Don’t hold your breath for an alien visitation any time soon.