On the 50th anniversary of NASA, Stephen Hawking, Newton’s heir as the
Lucasian Professor of Mathematics at the University of Cambridge,
was asked the question, "Are we alone?"
His answer was short and simple; “probably not.”
Hawking outlined three possibilities. One, being that there is no life
out there, and two – somewhat pessimistically, that when intelligent life gets smart
enough to send signals in to space, it is also busying itself with stockpiling nuclear bombs.
Hawking, known not only for his sharp mind,
but his also for his biting 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.”
We should be careful if we ever happen upon extraterrestrial life, Hawking warns. Alien life may not have DNA like ours: “Watch out
if you would meet an alien. You could be infected with a disease with
which you have no resistance.”
What we normally think of as ‘life’ is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorous, Hawking observed in his lecture, Life in the Universe. We can imagine that one might have life with some other chemical basis, such as silicon, “but carbon seems the most favorable case, because it has the richest chemistry.”
The Earth was formed largely out of the heavier elements, including carbon and oxygen. Somehow, Hawking observes, “some of these atoms came to be arranged in the form of molecules of DNA. One possibility is that the formation of something like DNA, which could reproduce itself, is extremely unlikely. However, in a universe with a very large, or infinite, number of stars, one would expect it to occur in a few stellar systems, but they would be very widely separated.”
Other prominent scientists 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 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 fete.
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 Casey Kazan with Rebecca Sato.
This is the fourth and final post in a four-part series on Stephen Hawking’s views on life in the universe.
Related Galaxy posts:
Stephen Hawking: “Humans Have Entered a New Stage of Evolution”
Stephen Hawking: “Asteroid Impacts Biggest Threat to Intelligent Life in the Galaxy”
MIT Asks: How Would Extraterrestrial Astronomers Study Earth?
“The Great Silence” -A Galaxy Insight
Harvard-Smithsonian Scientists Zero In On Key Sign of Habitable Worlds
Sources:
http://www.rationalvedanta.net/node/131
http://www.physorg.com/news128057557.html
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