On the 50th anniversary of NASA, Stephen Hawking, Newton's heir as the Lucasian Professor of Mathematics at the University of Cambridge, answered the question, “Are we alone?”
His answer was short and simple; probably not!
Hawking presented three options. One, being that there is no life out there, and two – somewhat pessimistically, but subsequently, a little too realistic – being that when intelligent life gets smart enough to send signals in to space, it is also busying itself with making nuclear bombs.
Hawking, known not only for his sharp mind, but his sharp sense of humor, prefers option number three. "Primitive life is very common and intelligent life is fairly rare," he quickly added: "Some would say it has yet to occur on earth."
Alien abductions, in Hawking’s view, are nothing more than claims made by “weirdos,” but we should be careful if we ever happen upon an alien. Because alien life may not have DNA like ours, Hawking warns "Watch out if you would meet an alien. You could be infected with a disease with which you have no resistance."
Other prominent astrobiologists have warned that we humans may be blinded by our familiarity with carbon and Earth-like conditions. In other words, what we’re looking for may not even lie in our version of a “sweet spot”. After all, even here on Earth, one species “sweet spot” is another’s species worst nightmare. In any case, it is not beyond the realm of feasibility that our first encounter with extraterrestrial life will not be a solely carbon-based occasion.
Alternative biochemists speculate that there are several atoms and solvents that could potentially spawn life. Because carbon has worked for the conditions on Earth, we speculate that the same must be true throughout the universe. In reality, there are many elements that could potentially do the trick. 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 example of such a location would be right here in our own solar system on 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.
Posted by Josh Hill with Rebecca Sato.