For much of our scientific history, the answers to such questions were beyond reach and consigned to the realm of speculation and fiction. This state of affairs started to change with the publication of the first realistic strategy for a search for extraterrestrial intelligence in 1959. "Searching for Interstellar Communications," by Philip Morrison and Giuseppe Cocconi, published in Nature, advocated searching nearby stars for interstellar radio signals centered on the 1420 Mhz part of the radio spectrum, known as the 'Water Hole." This naturally quiet part of the electromagnetic spectrum, allowed directed microwave transmissions over vast distances using modest power levels.
Around this time, Frank Drake, a radio astronomer at the NRAO (National Radio Astronomy Observatory), also began independently planning a SETI search. Drake conducted the first systematic interstellar search for microwave signals in 1960. This search, called project Ozma, targeted two nearby star systems and listened in the 1420 Mhz frequency range for possible signal carriers. This seminal work was the beginning of the scientific search for other intelligences in our galaxy.
While planning the first SETI conference in 1961 at Green Bank, WV, Drake conceived of a formula that could help guide the conference agenda. This simple linear equation (which appears in detail on The SETI League website, and in all modern astronomy textbooks) is used to estimate the probable number of technological civilizations that might transmit radio beacons in our galaxy. For the past forty years all of the factors used in this equation were mere speculation, guesses. However, within the present decade, we should see this situation change. The Drake equation, instead of being a vehicle for speculation, promises to become a valuable tool; it will be used to determine many probabilities and to answer fundamental questions.
New technological abilities, scientific instruments and the research programs they generate, will deal with the Drake equation in a methodical way. Let's look at each of the factors that produce the result N, the number of communicating technological civilizations in the galaxy. Furthermore, let's project ahead ten years and try to determine what factors will be known and how accurate those estimates of the factors might be.
R* The rate of appropriate star formation is the average number of stars born each year in the galaxy. Many stars may form each year, but only a certain percentage will live long enough or can go on to become hospitable to life and evolution. Only certain stars will do, but since there are an estimated 400 billion stars in the galaxy, there will be tens of billions of candidate stars.
By 2010, using existing data and discoveries from planned space missions and instruments, this number will be known with the most certainty. Current best guess: 1.5 appropriate stars per year. Projected year 2010 error box for the value of R* might be +/- 1%.
Fp The fraction of those appropriate stars that have planets. Since it is believed that planetary formation is an integral part of star formation, this number should be high. Recent extrasolar planetary discoveries indicate planets are very common. Information for determining this factor is pouring in almost monthly. Already dozens of extrasolar planets have been discovered. New detection methodologies, instruments and programs are already starting to set the outside parameters for this factor. As the database grows, overall trends should become apparent. Professional expertise, theoretical models and computer simulations will add to these predictive and detection capabilities. Reasonable conjectures on the planetary systems of more distant stars and even the entire galaxy might then be possible.
Current best guess: 90% of the appropriate stars might have planets. Projected year 2010 error box for the value of Fp might be +/- 5%.
Ne The numbers of earthlike (terrestrial) planets per appropriate star. Planets must be in the zone around their star where liquid water can exist. This is the habitable area around a star, where at least it's possible for life to exist and where biospheres might form, thrive and persist.
Missions currently in the planning or development stages will place very sensitive instruments into space within this decade. These instruments will be able to detect terrestrial planets, and even biospheres, several tens of light years distant. By the end of the decade we hope to actually image an extrasolar planet the size of Earth from several light years away.
Current best guess: most of the appropriate stars that have planets have terrestrial planets, though we really don't know. Projected year 2010 error box for the value of Ne might be +/- 10%.
Fl The fraction of the above worlds that have life. This factor currently has no known value. To a large extent it depends on the nature of life. Is life an inevitable consequence of natural processes, like the progressive formation of more complex elements in stars? The creation and mixing of complex organic molecules, amino acids and even DNA precursors, have all been observed in interstellar nebula; this appears to be a common process in the universe. How common is it for this organic sludge to form life, at least carbon-based life? We do know it happened very quickly here; life arose on Earth almost from the moment it could exist.
The fecundity of life in the galaxy is a key factor in the Drake equation. Fl is the factor to watch in the coming decade. If life is abundant, then the probabilities of successful SETI goes way up. If life is very rare, then the odds of finding a transmitting technological civilization are extremely small. I estimate that there are a half billion biospheres similar to Earth in the galaxy (a guess, to be sure, but my own best guess). Dividing a high-end estimate for the number of biospheres into the volume of space our galaxy occupies, results in roughly one biosphere for every 16,000 cubic light years of galactic volume. This is approximately the volume of a sphere with a radius of 16 light years. If these biospheres are randomly spaced, some, at least, should be detectable by future space-based spectroscopic analysis of extraplanetary atmospheric gases for: ratios of water vapor, free oxygen, ozone and methane, that indicate the presence of life. It is assumed the first generation of these instruments could detect the spectroscopic signature of an extrasolar biosphere over two-dozen light years distant.
Determining if other life exists at all, let alone knowing its possible frequency of occurrence, would be a monumental scientific discovery. Whenever we find new biospheres, we will use the Drake equation to reestimate the number of potential homes there might be for higher life forms in the galaxy. We should then be able to determine the factor Fl with even greater accuracy over time.
NASA has its gaze firmly transfixed on the quest for life. It is looking within the solar system for signs of life and is also starting to look toward the nearby star systems. Biology is a big part of NASA's future, and it's getting almost as important as astronomy. Biology and biochemistry have made major strides during the past couple of decades in understanding the basic nature and processes of life. The detection of just one extrasolar biosphere, or other life in the solar system, will found the field of exobiology and a host of others we can't even imagine yet.
Current best guess: We don't know enough about the origin of life to even guess. Projected year 2010 error box for the value of Fl might be +/- 20% if we detect other extrasolar biospheres, and an error box would be inapplicable if we detect none.
Fi The fraction of biospheres that develop intelligence. This equation factor will be one of the most difficult to determine. On Earth, with each successive dominant species, there has been a trend towards complexity and increased intelligence, over billions of years of evolution. Increased awareness leads to more flexible behaviors and diets; this has survival value in an often rapidly and frequently changing environment. Higher intelligence could be a lucky fluke on Earth, or it could be an inevitable consequence of rapid and frequent changes in the biosphere and therefore is quite common. We know that awareness developed as a competitive adaptation to a rapidly changing environment. Intelligence might be a faster way of adapting to changes than simple awareness or genes and might be of survival value. The jury is still very much in session and awaiting evidence on the question of other intelligences.
Current best guess: We don't know enough about life or the survival value of intelligence to even guess. Projected year 2010 error box is not applicable.
Fc The fraction of intelligence hosting worlds that are communicating. SETI optimists will tell you they expect some kind of evidence within ten years. Possible results would be more forthcoming with more effort and that comes with more money. In particular, larger radio telescopes and more dedicated observing time would allow us to see further and listen longer.
Current best guess: We will know when we hear from them. Projected year 2010 error box is not applicable.
L The average length of time a transmitting technological civilization, or its progeny (machine, genetically designed, or a mix, or something else), or its transmitting artifacts, will persist, measured in years. What's the average life time of civilizations that transmit interstellar signals? This is a question that only successful contact with other civilizations can answer and even then, they might not know.
Projected year 2010 error box is not applicable. Current best guess: We don't have one and probably never will. So, in 2010 what will the big picture look like? Will SETI pioneers be judged scientific saints or sinners? Will the detection of a nearby extrasolar biosphere, or several, have a significant impact on the pace of SETI research? Will it cause the sudden injection of billions of dollars of government money into SETI and bioastronomy research? Should the SETI community make contingency plans for a possible deluge of government funding? Should SETI researchers plan for other career goals or ways of beating off the swarms of descending media? A wave of popular interest and speculation arising out of possible discoveries could see the pendulum swing to either extreme.
Researchers want to see any funding spent wisely and efficiently. Governments, institutions and businesses put resources behind those approaches that are likely to yield results. Though they, like all of us, are swayed by natural curiosity and the desire to experience the mysterious. Governments in particular are especially moved by self-interest and the public's attitude towards such things. The discovery of a verdant, new world has been a powerful idea in our recent past and is firmly entrenched in our mythology. Will scores of pristine biospheres serve to impel humanity forward, towards the nearby stars? Will new discoveries motivate us to spend more money on SETI in order to listen longer, harder and more often? Or will a paucity of life in the galaxy revive skepticism about SETI research?
The advent of space travel, and the ability to put sensitive instruments like infrared interferometers into space, will impact SETI research, one way or the other. If extrasolar biospheres are found anywhere within the detection range of these new astronomical instruments, it will greatly stimulate further study along many fronts, including SETI. If no biospheres are found, or if we determine there is a paucity of life in general, it will lower the ceiling on N, the number of possible transmitting civilizations in the galaxy. If we don't receive a signal from the stars in the first decade of this millennium, then by the end of it we will have a much better idea of what the odds will be of ever receiving such a signal. The prospect of having the ability to determine such things as the origin and abundance of life in the galaxy, a mere 50 years after beginning the serious scientific investigation, seems almost as miraculous as receiving radio signals from a civilization living around another star.
Perhaps when we look up at the night sky ten years hence, we will have fewer things to wonder about and more things to wonder at. The magnificent night sky will not be less mysterious than before, but more. Soon we may be able to look out across the sea of stars and point to nearby harbors of life. Whatever the result of this decade's SETI and life searches might be, we will gaze upon the stars more knowingly, but certainly not less reverently.
entire website copyright © The SETI League, Inc.
this page last updated 28 December 2002
Top of Page