In his famous lecture, "Life in the Universe," Stephen Hawking observed that what we normally think of as 'life' is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorous. 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."
One of the founders on modern genetics, Cairs-Smith, argued that layers of crystalline silicates functioned as a primitive form of life on early Earth, before they evolved into carbon-based life forms.
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, as we mentioned above 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.
While some of these scenarios may seem the stuff of science fiction, it's important to keep in mind that the foundations of life on Earth, the association of a protein with a nucleic acid when view abstractly, does little to convey the endgame wonders such as blue whales and Mozart's operas.
A billion years from now our descendants may have discovered other systems of physical life such as plasma within stars which would be based on the reciprocal influence of patterns of magnetic force and the ordered motion of charged particles. In fact, such life may well exist within our Sun.
Another form would be based on radiation emitted by isolated atoms and molecules in a dense interstellar cloud similar to the one physicist Fred Hoyle described in his scifi thriller, The Black Cloud. Such clouds can have a long lifetime lasting millions of years before they collapse.
Our personal favorite at The Daily Galaxy is the possibility of life in Neutron stars which wouldfbe based on the properties of polymeric atoms which which could form chains that could store and transmit information in a way that bears an uncanny similarity to the functions of nucleic acids -the molecules that carry genetic information or form structures within cells.
The Daily Galaxy
Image Credit: antwrp.gsfc.nasa.gov