life on Earth, and the variety of possible sites for life on Earth and beyond. Most astro biologists would agree that the basic ingredients of life are not uncommon in the universe. In fact, they can be found almost anywhere! The elements for life are the products of stars. Several of the organic molecules necessary for life can form in many ways and places, even in interstellar clouds. In addition to the ingredients of life, scientists also believe that planets form as a natural "byproduct" of star formation. Over 3500 planets have now been identified beyond our Solar System and are being discovered at a rate of about several hundred every year. A number of these are Earth-like planets in the habitable zones of their Sun-like stars. Furthermore, there are several places within our Solar System where we may yet find microbial life, or at least traces of its past existence. As a result, astro biologists are confident that there is a multitude of probable sites for life beyond Earth. There is also evidence to suggest that life developed readily on the early Earth and quickly proliferated into a mass of evolutionary niches. We might, therefore, convince ourselves that life is not only an inevitable consequence but should also be quite prevalent in the universe.
Bearing in mind the powerful principle of mediocrity, we must carefully consider life in the universe. If we have learned anything from the history of astronomy, we will have learned that our place in the universe is not particularly special. We live in the outer regions of a typical spiral galaxy, one of billions scattered throughout the universe, on a giant rock orbiting an average, middle of the road type star. The same laws of physics apply everywhere in the universe; things will interact by the same rules on other planets as they do on Earth. The chemical elements that comprise our planet and living organisms are by no means rare. Can we, then, extend this principle of mediocrity to biological systems as well? Demonstrating that life in the universe is commonplace would be the ultimate step in the Copernican revolution.
If we were to apply simple statistics, the numbers alone would imply that life should be quite pervasive in the universe. There are over 1021 stars in the observable universe, and based on the statistics of searches for extra solar planets, a similar number of planets! Is it possible that life only arose on one planet, Earth? To some it may seem ridiculous to propose that we are alone in the universe. Yet we must remember that the principle of mediocrity is no more than an assumption. Life may indeed be a natural consequence of the evolution of stars and planets. However, it is equally possible that life, and in particular intelligence, has not taken place anywhere else in the universe. What if life is a "one time deal" resulting from a set of unlikely chemical and biological events? Until we have more data, we can only speculate.
Early life on Earth is thought to have arisen quickly once conditions were conducive. However, the long road towards intelligent life is another story; it was neither smooth nor direct. Natural selection is the process that directs evolution, but it does not necessarily dictate that life result in intelligence. In fact, it is the fate of most species to become extinct. Evolution depends on the local and global environment. It is has been punctuated at times by catastrophic events such as meteor or comet impacts. On our own planet, intelligence evolved in just a few species. Only one of those species has been able to create and utilize technology. It is that same species, humans, that has developed the ability to communicate through space. If we consider the development of intelligence on Earth, is it rational to be searching for intelligent extraterrestrial forms of life? What methods should we employ to communicate with them? Would we even recognize a message if it were sent to us?
This search for extraterrestrial intelligence, known by its acronym "SETI" dictates that we first determine the likelihood that intelligent life exists in the universe. American astronomer Frank Drake is one of the pioneers of SETI. He introduced a method for organizing information that would result in the logical arrival of an estimate for the number of intelligent, communicable civilizations in just the Milky Way Galaxy. The result was a fairly straightforward mathematical equation that has come to be known as the Drake equation.
The estimate achieved by the Drake equation is based on several factors such as the number of habitable planets expected to orbit a star. All of these factors are multiplied together to obtain an estimate of the number of civilizations with the capability for interstellar communication. If any single factor in the equation is completely unknown, no estimate can be made. Likewise, if any single factor is grossly over or underestimated, this will lead to an inaccurate estimate. Unfortunately, our ability to come up with estimated values for any of the factors is limited by our ignorance of the likelihood of life evolving intelligence and technology. Our predictions are based on one and only one planet that we know has life and only one species that uses technology to communicate across interstellar space. We must recognize that any numbers that result from our use of the Drake equation cannot reliably be extrapolated; we have but one example.
If we temporarily ignore the complexities involved in creating estimates for the factors of the Drake equation, we can instead consider the equation in a more qualitative light. Think of the Drake equation as a series of windows lined up. If we were to look through all the windows we would see the number N - representing the number of intelligent, communicating civilizations — written on a wall just beyond the final window. Each window can be thought of as a single factor in the Drake equation. If each window were to hold a perfectly clear pane of glass, we would be able to see the answer clearly. In this case, a perfectly clear window would represent a perfectly estimated factor in the equation. A poorly estimated or completely unknown factor would, therefore, be represented by a frosted or dirty pane of glass. If even one of the windows were obstructed, we probably wouldn't see the answer to N.
There are many people that do not view SETI as a scientific enterprise. With no empirical evidence to show that the probabilities we have estimated are reasonable, we are left holding nothing but a bag of speculation. Nevertheless, we can still consider the logical products resulting from "optimistic" and "pessimistic" estimates. For example, let us just assume for a moment that intelligent civilizations are distributed randomly among the stars and planets. If the number of communicable civilizations is small, then the average distance between them will be considerably large. If the number of communicable civilizations is large, then the average distance between them will be relatively small. If we make an optimistic calculation of N, then the closest civilization might only be 10 to 15 light years away, practically a next door neighbor from a cosmic perspective. Signals traveling at the speed of light could easily be exchanged within a single human generation. In contrast, if we make a pessimistic calculation of N, our civilization might be unique in the Milky Way Galaxy. Or civilizations may be so rare that we are operationally isolated in time and space. Two-way communication between two civilizations in separate galaxies could then take millions of years and might be vulnerable to the limited lifetimes of civilizations.
The question as to whether or not intelligent life exists beyond Earth has yet to be answered, and may never be. However, the implications of such a discovery would be profound, and the nature of human curiosity will continue to motivate our search.