Under the Microscope: Enrico Fermi, the great Italian physicist once asked - "If the universe is teeming with intelligent life, why do we see no evidence of their existence?" Perhaps the reason is that we are alone in the universe.
For many years most scientists have thought that intelligent life is probably common in the universe because of the likelihood that many earth-like planets exist where life could begin and evolve. More recent investigations have concluded however that complex life is, at best, rare in our own galaxy and, probably, even rarer elsewhere in the universe. The reasons are outlined in the Scientific American publication Majestic Universe in an article by G Gonzales, D Brownlee and P Ward.
In any particular solar system it is possible to calculate the location of the zone that is optimal for biological life - the circumstellar habitable zone (CHZ). The limits of the CHZ are determined by the conditions that allow liquid water to exist on the planet potentially bearing life. Too close to the sun and all the water evaporates. Too far away and all the oceans freeze. In our solar system the narrow CHZ extends from somewhere between the orbits of Venus and Earth out to a small bit beyond the orbit of Mars. Of course the habitability of a planet is also affected by other factors. For example the location of the CHZ is irrelevant if the solar system resides in a part of the galaxy where general conditions are inimical to life, eg very high radiation levels.
On a larger scale the Galactic Habitable Zone (GHZ) describes the most hospitable places in the Milky Way galaxy for biological life. The dimensions of the GHZ are determined by the availability of materials necessary to build planets and a location remote from cosmic threats to life. Planets beyond our solar system have been detected in recent years. Those detected to date are all giant planets (about the size of Jupiter) because the resolution of the detection technique cannot see small planets.
The degree of "metallicity" of a star is an important parameter determining whether or not a planetary system will form around it. Two elements, hydrogen and helium, were formed in the big bang about 15 billion years ago. The remainder of the 92 natural elements were forged in the interior of stars and in supernova explosions. Elements heavier than helium are called "metals" and these metals are necessary to form planets.
No extra-solar planets have been found to date around a star with a "metallicity" less than 40 per cent of our sun's metallicity. But, too high a metallicity is also a problem as the resulting planets will be larger, with stronger gravity and consequent muted topography. Such planets would probably be covered entirely by water and this would hinder the development of life. The mixture of land and water on earth is important for the regulation of atmospheric temperature and other processes. It seems that stars with a metallicity near that of our sun are optimal for the production of earth-like planets in stable orbits.
Our Milky Way galaxy may be visualised as having the shape of a fried egg surrounded by a diffuse halo of stars. The central yolk corresponds to the galaxy bulge and the white of the egg to the galaxy disc - thicker towards the centre and thinner further out (bulge, thick disc, thin disc and halo). Stars in each region orbit the centre of the galaxy as stars in our solar system orbit the sun.
Stars in the halo and the thick disc were formed a very long time ago and are low in metallicity. It is not likely that earth-like planets have formed here. Stars in the bulge have a wide range of metallicities but cosmic radiation levels are high in this region.
Our sun is located in the thin disc of the galaxy. Stars are formed when gas condenses under gravity. There is less gas in the outer parts of the galaxy and consequently the rate of star formation is low and the build up of metallicity is slow.
When metallicity limits of between 60 and 200 per cent of our sun's metallicity to define GHZ are applied, the GHZ is seen to reside in a zone extending from 14,670 light years to 37,500 light years from the galactic centre.
This region contains only about 20 percent of the stars in the galaxy. This calculation, based on degree of metallicity alone, is an upper limit, because the closer you get to the centre of the galaxy the greater the chances of life-destroying bombardment by asteroids and comets and lethal high-level radiation. And, in the broader universe conditions amenable to complex life appear to be even rarer than in our galaxy.
The considerations used in this article apply only to complex life. Simpler forms of life such as microbes could exist in a much wider range of environments. But, vast though the universe is, it seems that the possibilities of life such as human life arising elsewhere are far more limited than we have heretofore imagined. And, indeed, it is not out of the question that we are the only intelligent life in the universe. If this is the case, then we are in a very lonely situation and we have a truly awesome responsibility to succeed.
Perhaps we should follow the advice in Monty Python's Galaxy song, "Pray there's intelligent life somewhere up in space . . . 'cos there's bugger-all down here on Earth."
• William Reville is associate professor of biochemistry and director of microscopy at UCC