Why haven't we discovered signs of life beyond Earth? As Carl Sagan said, the absence of evidence is not evidence of absence. This thought is well known in other fields of research. Astrophysicists, for example, spent decades studying and searching for black holes before accumulating today’s compelling evidence that they exist. The same can be said for the search for room-temperature superconductors, proton decay, violations of special relativity, or for that matter the Higgs boson. Much of the most important and exciting research in astronomy and physics is concerned exactly with the study of objects or phenomena whose existence has not been demonstrated.
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."
Recently, scientists are content to define life using the "chemical Darwinian definition" that involves "self-sustaining chemical systems that undergo evolution at the molecular level."
There are in fact a number of genetic-studies which purport to demonstrate that the common ancestors for Earthly life forms may have first began to form billions of year before the Earth was created. It has been speculated the first steps toward actual life may have begun with self-replicating riboorganisms whose descendants fell to Earth and other planets through mechanisms of panspermia, triggering the RNA world and then life as we know it. Life on some planets may be like life on Earth. Life on other worlds may have a completely different chemistry, and may not even possess a genetic code.
Life that may have been originated elsewhere, even within our own solar system, could be unrecognizable compared with life here and thus could not be detectable by telescopes and spacecraft landers designed to detect terrestrial biomolecules or their products. Life might be based on molecular structures substantially different from those on Earth.
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."
Organic molecules are now known to be common throughout the universe. Life, then, is assumed to be carbon-based.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”. 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 carbon-based.
As John Baross of the University of Washington has suggested, our present knowledge of physics and chemistry suggests that an organism could have an entire non-carbon-based metabolism such as silicon, which like carbon, can form four bonds. The greater reactivity of silicon compared with carbon may be an advantage in cold environments. Thus, its chemical and structural flexibility in non-aqueous environments can provide analogues to most of the functions of terrestrial biochemistry.
Silicon can form long chains as silanes, silicones, and silicates. Among them, silanes have been considered the most proper compounds to sustain life because they present the closest analog to hydrocarbons, which are so important to terrestrial life processes. However, such silicon-based life would have to be different from life as we know on Earth.
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.
A team of physicists have looked at the conditions necessary to the formation of carbon and oxygen two elements in the universe that are the foundation of life as we currently know it, and found that when it comes to supporting life, the universe leaves very little margin for error.
“The Hoyle state of carbon is key,” says North Carolina State physicist Dean Lee. “If the Hoyle state energy was at 479 keV or more above the three alpha particles, then the amount of carbon produced would be too low for carbon-based life. The same holds true for oxygen,” he adds. “If the Hoyle state energy were instead within 279 keV of the three alphas, then there would be plenty of carbon. But the stars would burn their helium into carbon much earlier in their life cycle. As a consequence, the stars would not be hot enough to produce sufficient oxygen for life. In our lattice simulations, we find that more than a 2 or 3 percent change in the light quark mass would lead to problems with the abundance of either carbon or oxygen in the universe.”
Both carbon and oxygen are produced when helium burns inside of giant red stars. Carbon-12, an essential element we’re all made of, can only form when three alpha particles, or helium-4 nuclei, combine in a very specific way. The key to formation is an excited state of carbon-12 known as the Hoyle state, and it has a very specific energy – measured at 379 keV (or 379,000 electron volts) above the energy of three alpha particles. Oxygen is produced by the combination of another alpha particle and carbon.
The international team -- Lee and German colleagues Evgeny Epelbaum, Hermann Krebs, Timo Laehde and Ulf-G. Meissner-- had previously confirmed the existence and structure of the Hoyle state with a numerical lattice that allowed the researchers to simulate how protons and neutrons interact. These protons and neutrons are made up of elementary particles called quarks. The light quark mass is one of the fundamental parameters of nature, and this mass affects particles’ energies.
In lattice calculations done at the Juelich Supercomputer Centre the physicists found that just a slight variation in the light quark mass will change the energy of the Hoyle state, and this in turn would affect the production of carbon and oxygen in such a way that life as we know it wouldn’t exist.carbon and oxygen production and the viability of carbon-based life.
In new lattice calculations done at the Juelich Supercomputer Center the physicists found that just a slight variation in the light quark mass will change the energy of the Hoyle state, and this in turn would affect the production of carbon and oxygen in such a way that life as we know it wouldn’t exist.
The researchers’ findings appear in Physical Review Letters.
The Daily Galaxy via NASA AStrobiology, North Carolina State University, New Scientist and Alternative Biochemistry
Image credit: Dean Lee. Earth and Mercury images from NASA and http://www.physics.sfsu.edu/~lwilliam/sota/anth/anthropic_principle_index.html