Summary: In 1961 the Drake Equation put the search for alien civilizations on a scientific footing and launched the modern SETI movement. How do its numbers look today? What is the chance of finding aliens?
Searching for extraterrestrial life has become a hot topic among astronomers, biologists, and the general public. But not many remember how the subject was jump-started more than 40 years ago.
In September 1959, physicists Giuseppe Cocconi and Philip Morrison published a landmark article in the British weekly journal Nature with the provocative title, "Searching for Interstellar Communications." Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 centimeters (1,420.4 megahertz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe. Other intelligences might see this as a logical landmark in the radio spectrum where searchers like us would think to look.
Seven months later, radio astronomer Frank Drake became the first person to start a systematic search for intelligent signals from the cosmos. Using the 25-meter dish of the National Radio Astronomy Observatory in Green Bank, West Virginia, Drake listened in on two nearby Sunlike stars: Epsilon Eridani and Tau Ceti. His Project Ozma (named for L. Frank Baum's story Ozma of Oz) slowly scanned frequencies close to the 21-cm wavelength for six hours a day from April to July 1960. The project was well designed, cheap, simple by today's standards, and unsuccessful.
Following the Ozma experience, Drake organized a meeting with a select group of scientists to discuss the prospects and pitfalls of the search for extraterrestrial intelligence — nowadays abbreviated SETI. In November 1961, ten radio technicians, astronomers, and biologists convened for two days at Green Bank. Young Carl Sagan was there, as was Berkeley chemist Melvin Calvin, who received news during the meeting that he had won the Nobel Prize in chemistry.
It was in preparing for this meeting that Drake came up with his famous equation:
N = R x fp x ne x fl x fi x fc x L
Today this string of letters and symbols can be found on T-shirts, coffee mugs, and bumper stickers. It is simpler than it looks. It expresses the number (N) of "observable civilizations" that currently exist in our Milky Way galaxy as a simple multiplication of several, more approachable unknowns.
R is the rate at which stars have been born in the Milky Way per year, fp is the fraction of these stars that have solar systems of planets, ne is the average number of "Earthlike" planets (potentially suitable for life) in the typical solar system, fl is the fraction of those planets on which life actually forms, fi is the fraction of life-bearing planets where intelligence evolves, fc is the fraction of intelligent species that produce interstellar radio communications, and L is the average lifetime of a communicating civilization in years.
The Drake equation is as straightforward as it is fascinating. By breaking down a great unknown into a series of smaller, more addressable questions, the formula made SETI a tangible effort and gave the question of life elsewhere a basis for scientific analysis.
Astronomers and biologists alike have tried to "solve" the equation ever since. At first sight, coming up with a reasonable estimate for the answer might seem fairly straightforward. But the number of communicating intelligences can't be judged so easily. Several of the variables in the equation have been firmed up since 1961. But at least three remain very unknown.
The rate of star formation in our galaxy is approximately one per year, R = 1. The next factor, fp, is probably smaller than one: not every star can have planets. On the other hand, if a star has a planetary system, it seems plausible that two or three of its planets and moons will have liquid water and be potentially suitable for the origin of life, so maybe the product of fp and ne is close to 1.
Optimists would argue that life will form wherever it can (fl = 1), that the Darwinian process of natural selection eventually favors the evolution of intelligence (fi = 1), and that no intelligent civilization would exist for long without discovering electricity and radio and feeling the urge to communicate (fc = 1). In this most optimistic case, the Drake equation boils down to the simple observation that N = L (the average lifetime of technological civilizations, in years). If L is, say, 100,000 years, there would be about 100,000 chatty civilizations in our galaxy. And that's assuming that only one arises during a planet's entire multi-billion-year lifetime.
That figure of 100,000 would mean there is one radio-emitting civilization right now per 4 million stars — reason enough to tune in on the heavens and start hunting for them. If they were scattered at random throughout the Milky Way, the nearest one would probably be about 500 light-years from us. A two-way conversation would require a time equal to a good fraction of recorded human history, but a one-way broadcast might be audible.
However, 40 years of SETI have failed to find anything, even though radio telescopes, receiver techniques, and computational abilities have improved enormously since the early 1960s. Granted, the "parameter space" of possible radio signals (the possible frequencies, locations on the sky, signal strengths, frequency drift rates, on-off duty cycles, etc.) is vastly larger than the tiny bit that has yet been searched. But we have discovered, at least, that our galaxy is not teeming with very powerful alien transmitters continuously broadcasting near the 21-centimeter hydrogen frequency. No one could say this in 1961.
Have we overestimated the values of one or more of the Drake parameters? Is the average lifetime of technological civilizations short? Or have astronomers overlooked some other, more subtle aspect?
Let's reevaluate the Drake equation by analyzing each term separately. R, the rate of star formation in the Milky Way per year, is indeed currently about 1 — astronomers are quite sure of that. In fact, astronomers have recently determined that stars formed at a higher rate several billion years ago, when the stars that might now bear intelligent life were being born. So a value of R = 3 is more realistic.
However, astronomers and biologists are much less certain about the subsequent terms in the equation.
How Many Planets? fp
The second variable is fp, the fraction of stars that have planetary systems. Recent discoveries that many or most young stars are surrounded by planet-forming disks, and detections of scores of actual planets orbiting nearby Sunlike stars since 1995, confirm what astronomers had already suspected: planets are common.
So-called "protoplanetary disks" are routinely detected by infrared observations and are seen directly in, for instance, Hubble Space Telescope photographs of the Orion Nebula, one of the most prolific star-forming regions in our part of the Milky Way. Submillimeter-wave observations have shown much more tenuous dust disks around many older stars, including Drake's first target, Epsilon Eridani. Many of these disks are doughnut shaped. According to many theorists, the central holes can only be swept clear by planets accreting gas and dust from the disk's inner portion. In addition, some of the disks (including Epsilon Eridani's) show distortions that may directly indicate a planet circling in their outer regions.
As for actual planet detections, extrasolar-planet searches find (as of June 2003) that about 12 percent of Sunlike stars have a giant planet orbiting within 5 astronomical units of the star (Jupiter's distance from the Sun). At face value, this might imply that about 12 percent of stars have planets, so fp would be 0.12.
However, this is only part of the story; the current search techniques are sensitive only to massive planets, especially those in small, fast orbits. Solar systems like ours cannot yet be recognized (though they should be in reach within a few years). Very likely the fraction of single Sunlike stars with planets of some kind is much higher than 12 percent. Reasonable guesses right now might be 20 to nearly 100 percent. (A September 2003 paper by Charles Lineweaver and Daniel Grether delves into this.)
So what do these new observations tell us about fp? Although we don't yet have a final value, it's now clear that fp is substantial and is not a bottleneck in the Drake equation.
How Many Good Planets? ne
There's less definite news when we turn to the equation's next term, ne. This factor represents the average number of worlds in a typical solar system that have environments suitable for the origin of life (the "e" stands for "Earthlike"). In his 1992 book Is Anyone Out There?, Drake recalls that the participants in the Green Bank meeting concluded that the minimum value of ne lay between one and five. In other words, every planetary system was expected to contain at least one minimally Earthlike place (defined as where liquid water is possible), and that there might easily be three, four or five hospitable worlds per system.
This optimistic view was based on the assumption that our own solar system is typical. Today Mars and Jupiter's moon Europa are being considered as possible sites of early biology, making three possible "Earths" (by the Drake-equation definition) in our solar system. However, the extrasolar planets found in the last few years have taught us a humbling lesson. Our solar system, with lots of worlds and moons in nice, circular, stable orbits, may be the exception rather than the rule. For all we know, Earthlike planets with long-term stable orbits and climates may be quite uncommon.
How Many Origins of Life? fl
In scientific circles there's much less concern now than in the past about the value of fl, the fraction of habitable planets on which life evolves. The molecular building blocks of life — complex organic compounds and even amino acids — are abundant in the universe. They have been discovered in meteorites, comets, and interstellar gas and dust. There are vastly more amounts of amino acids, for instance, in interstellar space than in the Earth's biosphere. Although hydrocarbons and amino acids are not living organisms, there's little doubt that a lot of prebiotic evolution is going on in the dark clouds between the stars.
Most significant are the recent discoveries that microorganisms appeared on Earth only moments (geologically speaking) after the last devastating, ocean-vaporizing impacts of the planet's youth some 3.9 billion years ago. There is clear evidence that bacteria were already around by 3.5 billion years ago, and more disputed evidence from 3.7 and 3.85 billion years ago. Apparently, given the right conditions, the origin of life is a rather straightforward process that happens easily — at least when given a planet-sized laboratory and millions of years for the experiment to run. If the process were rare or difficult, one would not expect it to have happened at the first possible opportunity on our home planet, but somewhat later in Earth's history instead. Biologists now discuss whether life may have arisen several times separately. There's every reason to think that all living things today have a common ancestry, but other, independent lines could have formed and been wiped out early. If life does form wherever it can, then presumably fl = 1.
That leaves us with three remaining unknowns. How likely is the evolution of intelligence (fi)? How confident can we be that at least some intelligent extraterrestrials will broadcast radio or other signals we can detect (fc)? And what is the average lifetime of radio-capable civilizations (L)? These biological and sociological factors in the Drake equation are subject to greater scientific debate and uncertainty than the astronomical ones.
According to many life scientists, it is naive to suppose that evolution on another planet should necessarily result in intelligence as we know it. In his bestseller Wonderful Life, the late paleontologist Stephen Jay Gould (Harvard University) asserts, "We probably owe our own existence to . . . good fortune. Homo sapiens is an entity, not a tendency." Evolution is unpredictable, undirected, and chaotic. Gould has pointed out again and again that if we could rewind the tape of biological evolution on Earth and start over, it is impossible that humans would again appear on the scene. We are the result of too long a chain of chance flukes and happenstance.
Others counter, of course, that humans are not what we are looking for. No one expects to find men among the stars (little green ones or otherwise). Rather, the issue is whether any species evolve enough symbol-based intelligence to use tools, store and manipulate information, and develop societies that grow large and complex enough to discover the principles of science and electronics. To optimists this seems like a difference only in degree, not in kind, from the levels of intelligence, tool use, and purposeful behavior that have emerged independently in widely divergent species of animals on Earth, from apes to octopi.
But Gould notes that there is no overall pattern in evolution, no preferred direction. If some recently evolved animals are bigger and smarter than earlier ones, that could just be a fluke. Human levels of planning and technology may be even more so.
To some biologists and SETI proponents, the phrase "survival of the fittest" implies that greater intelligence inevitably boosts a species' chance to survive and spread by natural selection. But the renowned biologist Ernst Mayr (retired from Harvard University) has argued that many astronomers and physicists are too optimistic concerning the emergence of intelligence. "Physicists still tend to think more deterministically than biologists," wrote Mayr in the May 1996 issue of The Planetary Report. "They tend to say that if life has originated somewhere, it will also develop intelligence in due time. The biologist, on the other hand, is impressed by the improbability of such a development."
Strangely enough, optimists and pessimists base their claims on the same key observation — namely that technology has appeared on this planet in 4 billion years. Pessimists (or realists, as they would prefer to be called) like Mayr see this as evidence of the unlikeliness of intelligence as an evolutionary given. For optimists, it strengthens their belief in the existence of extraterrestrial civilizations.
This divergence stems in part from different specialists' intellectual backgrounds. To a biologist, something that happened once in 4 billion years is terribly rare. Astronomers take a wider view: something that happened once in less than a single planet's lifetime seems reasonable for planets generally.
Optimists have pointed out that by some estimates, Earth has at least 1.2 billion good years ahead before it will get broiled by the expanding Sun. This is several times longer than the time since the first simple creatures crawled out of the sea onto land. If the emergence of intelligence were difficult and rare, the optimists argue, it would not have happened relatively early in the time available for it to do so on Earth. Given humanity's early arrival in the long era expected for land life, it seems likely that entirely different intelligent creatures will emerge a few more times in the coming geological ages (and will find our fossils). This argument parallels the point drawn from the rapid emergence of microorganisms on the young Earth.
Pessimists reply that we don't really know how long the Earth will remain clement. The Earth's seemingly stable climate may actually be the result of a long run of lucky flukes that could give out at any time, geologically speaking. If so, humans have arisen late in the total span of time available. Given the fact that we are here at all to ponder the question, a late emergence in the time span available would indicate that the birth of intelligence is an improbable event.
Contrary to popular belief, the fact that intelligence has arisen once tells us nothing whatsoever about how often it happens — for the simple reason that we ourselves are the one case! We are a self-selected sample. Even if intelligent life is so improbable that it appears just a single time in one remote corner of the universe, we will necessarily find ourselves right there in that corner observing it, because we are it.
Strangely enough, both camps accept the so-called Copernican principle, which claims that humankind enjoys no preferred position in time or space. Skeptics like Mayr say it is anthropocentric to believe that humanlike intelligence has appeared over and over again in the universe. Believers like Drake are unwilling to accept our uniqueness, because this would put us on a very un-Copernican pedestal.
Christopher Chyba, chair of the SETI Institute's Center for the Study of Life in the Universe, sums it up: "It's an argument that turns on the comparative importance of contingency versus convergence in evolution." In other words, how many evolutionary tendencies are random flukes, and how many repeatedly drive in a particular direction? "Are there data sets that we can analyze to actually help focus and quantify this argument?" Chyba continues. "The answer, it appears, is a resounding 'yes.' We don't have to guess about these questions, but can begin to quantitatively assess some of them using well-understood, quantifiable tools." The SETI Institute is assembling researchers to tackle this problem.
For now, however, fi is one of the most controversial factors in the Drake equation. Some scientists believe it is almost certainly next to zero; others are convinced it's close to one. There seems to be no middle ground.
Even if intelligence is a likely consequence of evolution, fi will probably be much lower than 1, based on recent insights into the stability of solar systems and planetary climates. Just because a planet starts out good for life doesn't mean it will stay that way forever.
Computer simulations by Fred Rasio and Eric Ford (Massachusetts Institute of Technology) among others show that Earthlike planets are probably unable to survive the gravitational tug-of-war in a system with two (or more) massive, Jupiterlike giants. They would be slung out of the system or sent careening into the central star.
Conversely, systems with no giant planets at all might also have dire consequences for life-bearing planets. Computer simulations by George Wetherill (Carnegie Institution of Washington) indicate that Jupiter acts as the solar system's gravitational vacuum cleaner, efficiently thinning out the population of hazardous comets that venture into Earth-crossing orbits. Without a Jupiter the current impact rate of comets would be about 1,000 times higher, says Wetherill, with truly catastrophic collisions (like the one that killed the dinosaurs 65 million years ago) happening about once every 100,000 years. This would surely frustrate any slow evolutionary progress from simple life forms to higher intelligences.
Also, dynamical studies by Jacques Laskar and Philip Robutel (Bureau des Longitudes, Paris) have shown that rocky, Earthlike planets show chaotic variations in orbital tilt that could lead to drastic climate changes. Fortunately, Earth's chaotic tendencies are damped by tidal interaction with the Moon. Without a relatively large satellite, Earth might have experienced variations in axial tilt similar to those of Mars, possibly as large as 20° to 60°. This would cause extreme variations in the patterns of the seasons. According to one analysis of planet formation, a world like Earth has only about a 1 in 12 chance of ending up with a nice, mild axial tilt that is safely stabilized by a large moon. (On the other hand a moonless Earth might have retained its original rapid spin, which would tend to stabilize its axis.)
It's anyone's guess how large axial swings would influence the evolution of life and the chance for the emergence of intelligence. Change and stress actually promote the emergence of new, versatile, adaptable species, biologists say. For instance, Paul F. Hoffman (Harvard University) and three colleagues proposed in 1998 that the series of intense global ice ages between 760 and 550 million years ago were the crisis that drove the remarkable "Precambrian explosion" of new life forms around or shortly after that time. The disastrous great extinctions later in Earth's geologic record were always followed by vigorous recoveries, eventually spawning more species than existed before. (Complete recovery from any great extinction, regardless of size, always seems to take about 10 million years.) Humanity's own emergence as a species during an unusual run of ice ages is sometimes cited as an example of stress-driven evolution leading to adaptability and intelligence. So a planet with a tippy axis might actually speed evolution along.
But planetary crises that are too extreme or frequent would kill off everything, or keep life beaten down to a low level. In any case, our existence here and now seems to be the accidental result of a number of astronomical coincidences that were unimagined in 1961.
Such coincidences are discussed in the book Rare Earth by Peter Ward and Donald Brownlee (Copernicus Books/ Springer, 2000). Ward and Brownlee argue that only very rarely will a good planet form and remain life-friendly for the billions of years that advanced creatures took to appear on Earth. Seth Shostak of the SETI Institute argues in a rebuttal essay that some of their points are overstated, that once life is established it is probably adaptable enough to thrive in un-Earthly conditions, and that it therefore need not require a planet with a narrowly Earthlike history.
Ward and Brownlee's associate Guillermo Gonzalez advocates the idea that there is only a narrow "habitable ring" in the Milky Way where conditions allow life-bearing planets. Closer to the galaxy's center, conditions are supposedly too violent; farther out there aren't enough heavy elements to make planets. This idea has been roundly criticized as a gross exaggeration. Heavy elements are in fact distributed widely throughout a galaxy's disk (the evidence is in plain view: dark dust clouds of carbon and silicates riddle most parts of most disks), and stars with a fairly wide range of heavy-element concentrations have been discovered to have planets. Dangerous radiation from an active galactic center would be blocked by a planet's thick atmosphere; that's why our own X- and gamma-ray telescopes have to be put in orbit. David Darling notes in his book Life Everywhere: The New Science of Astrobiology (Basic Books, 2001) that Gonzalez argues from his religious conviction, expressed in other writings, that God designed one world for one intelligent race, and that Gonzalez's astronomical views should be understood in this light.
How Would Aliens Communicate? fc
Suppose that extraterrestrial intelligences are rare but do exist. Could we expect them to communicate with us through radio signals? What fraction of civilizations are able — and motivated — to broadcast in a way we can detect? In other words: what is the value of fc? SETI advocates tend to believe that it is large: that sooner or later, any civilization curious and inventive enough to become technological at all will discover that radio is an efficient way to communicate over astronomical distances, and will choose to do so.
Might there be a naive form of anthropocentrism at play here? Is it reasonable to expect that wildly different beings on another planet, even if they are older, smarter, and more capable than us, will choose to build radio telescopes and send signals to the larger universe? Maybe we just don't appreciate the true diversity of biological evolution, or the uniqueness of humans' monkeylike curiosity. Or maybe radio is hopelessly primitive compared to something we have yet to discover.
With fi and fc completely undetermined, we're left with the last term of the Drake equation: L, the average lifetime of communicating civilizations. Here also, optimists and pessimists are far apart.
The optimists claim that a stable, intelligent society could last for tens of millions of years, if not forever. This would certainly mitigate the effect of any bottleneck earlier in the Drake equation. In addition, a long-lived species might have time to spread to many stars, multiplying its presence. The pessimists point out that humans invented radio technology only a century ago, and that the human race has been on the verge of destroying itself (through nuclear war or ecological overload) for much of that time. The same technological power that enables interstellar communication also enables rapid self-destruction.
But others have pointed out that the human animal (as opposed to human civilization) would be almost impossible to kill off completely at this point. People have become too widespread and too capable; a few pockets of individuals would find ways to survive almost any conceivable war or global catastrophe. These survivors would be enough to fully repopulate the Earth, to numbers in the billions, in just a few thousand years. And a second technological civilization would arise more readily than our first one has done, because there would be a precedent. Maybe this will happen many times.
Which brings up a little-noticed point. The value of L properly does not refer to the lifetime of one radio-transmitting civilization, but instead to the sum of all those that ever appear on a planet once it develops its first.
The long-term future of humanity and Earth's biosphere is explored in Peter Ward's book, Future Evolution.
The Fermi Paradox
Perhaps the pessimists' most telling argument in the 40-year SETI debate stems not from theory or conjecture but from an actual observation: Earth has not already been overrun with aliens (contrary to some popular opinion). This is a more profound observation than it might at first seem.
A civilization lasting for tens of millions of years would have plenty of time to travel anywhere in the galaxy, even at the slow speeds foreseeable with our own technology. The drive to fill up all available territory seems to be a universal trait of living things. And yet the Earth shows no sign in its fossil record of ever having been colonized by an alien high technology in its long history, much less today. This is known as the Fermi paradox, after the nuclear physicist Enrico Fermi, who as early as 1950 asked (in a lunchroom discussion about aliens at, ironically, a nuclear weapons lab), "Where is everybody?"
(UFO believers might reply that we are being overrun right now. But scientists and other careful investigators who have examined the UFO movement's claims conclude almost universally that nothing is going on here but human misperception, tale-telling, and willful folly. More than 50 years after it was born, UFOlogy remains barren of a single tangible result despite thousands of loud claims, which suggests that we can sit out the next 50 years of it and not miss anything.)
Optimists have replied to the Fermi paradox in many ways. Maybe any culture that is civilized enough not to destroy itself turns away from imperialism, or maybe the imperial drive runs out of steam after settling just a few thousand planets. Maybe we live in an uninteresting area of the galaxy — the equivalent of a backwoods area in the United States, a country that has been "completely settled" since the frontier was officially closed in 1890 but where you can find plenty of places where no one else is in sight. Or maybe aliens are thickly settled around us but obey, as in Star Trek, a prime directive "not to interfere" with living planets, which are kept off limits as nature preserves. This is the so-called zoo hypothesis. Or perhaps interstellar travel is really as expensive in effort and energy as it appears to us right now to be, and anyone capable of it has better things to do with the resources — such as investigating the universe by astronomy or radio.
A more sophisticated rejoinder to the Fermi paradox was published by William I. Newman and Carl Sagan in Icarus for September 1981. They analyzed how fast a spreading interstellar civilization would actually expand through the galaxy, based on mathematical models covering everything from the diffusion of molecules in a gas to the spread of animal species introduced into virgin territories on Earth. They found that how fast the galaxy fills up depends surprisingly little on the speed of interstellar travel; there are too many planets to be settled and populated along the way. "The expansion velocity of the colonization front is several orders of magnitude smaller than had been previously anticipated," they wrote; filling the galaxy might even take a time comparable to the age of the universe. Summing up, they quipped "Rome was not built in a day, although one can cross it on foot in a few hours."
But others have called their argument a stretch, because it assumes that population growth rates are always fairly low. In the end, the fact that aliens are not camped in your bedroom may truly mean that we are alone in the Milky Way. Perhaps almost every galaxy is either completely barren or settled in every inch. If so, we can expect the first radio signals we hear to come from beyond the Milky Way.
For more on this topic, see David Brin's influential 1983 essay "The Great Silence" (PDF format, 2.1 megabytes), Geoffrey Landis's analysis of partial, patchy galactic colonization based on percolation theory, and Stephen Webb's new book If the Universe Is Teeming with Aliens. . . Where Is Everybody? Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life (Copernicus Books, 2002).
"Success Can't Be Predicted"
Where does all this leave us? Can we still believe that N = L? Probably not. What about N = 0? To many people that extreme is inherently unacceptable, but of course the universe isn't obliged to live up to our hopes and expectations. Maybe there is some truth in the saying that nothing happens only once. Maybe alien civilizations are out there, and some are trying to announce themselves via radio transmissions. But their number could be very, very small.
In the preface to Is Anyone Out There? Frank Drake wrote that he wanted to "prepare thinking adults for the outcome of the present search activity — the imminent detection of signals from an extraterrestrial civilization. This discovery, which I fully expect to witness before the year 2000, will profoundly change the world." That was written in the heady days when NASA's now-aborted radio searches were about to get under way. In July 1996, at the fifth international bioastronomy conference in Capri, Italy, Drake confessed: "Maybe I was a little bit too optimistic. Success can't be predicted." Cocconi and Morrison already told him so in their 1959 Nature article: "The probability of success is difficult to estimate, but if we never search, the chance of success is zero."
Meanwhile, the Drake equation still stands as the best-known icon of one of the most forward-looking endeavors of the intelligent species here on Earth: the scientific search for coinhabitants of the dark emptiness of the cosmos, and for a wider, truer perspective on our place in space and time and on the meaning of our life. The "alien equation" has served this effort well by providing a rational basis for the search, by focusing our attention on the fundamental issues, and by defining a clearly visible goal.
We're a long way from that goal. The first term, R, has been known for decades, and we're now coming to grips with the second, fp. That leaves us with two medium-size question marks and three big ones — and a lot of speculation.
Moreover, the equation is showing its age, looking a little frayed around the edges by not explicitly treating newer issues that we now consider important, such as the rates of planetary catastrophes or the effects of slow, one-way changes in the universe itself that could either boost or diminish the abundance of aliens in our present era (see for instance the recent paper about this by Milan M. Cirkovic).
But maybe the Drake equation isn't to be solved after all. Its real value may lie in those thought-provoking question marks. Uncertainty and curiosity will keep the search going for years to come. Maybe the real payoff for SETI will not be to yield a yes-or-no result, at least not in our lifetimes, but to help us discover more about ourselves.
Alan M. MacRobert is a senior editor of Sky & Telescope. Govert Schilling is an astronomy writer in Utrecht, The Netherlands. His book Tweeling aarde: De speurtocht naar leven in andere planetenstelsels (Twin Earth: The search for life in other planetary systems) was published in 1997.