The Search for Life on Other Planets

The National Aeronautics and Space Administration is preparing for the first large-scale search for radio signals from alien civilizations


IN AUGUST OF 1924 THE PLANET MARS CAME UNUSUALLY close to Earth. Convinced that this proximity represented the best opportunity for many years to prove the existence of Martians, David Todd, a professor emeritus in the astronomy department of Amherst College, in Amherst, Massachusetts, embarked on a highly publicized campaign. He sought to persuade all the radio stations on Earth to shut down for certain five-minute periods so that the stations and their listeners could tune in to messages from the Red Planet. Although few commercial stations went along with Todd’s request, the United States military complied; the executive officer of the Army Signal Corps solemnly announced that the service’s chief decoder would stand by to decipher any communiques received. In the excitement it was inevitable that signals would be picked up—and indeed they were. A radio station in Vancouver, British Columbia, caused a flurry of speculation when it reported having received not just one but a series of inexplicable broadcasts. After a few weeks, however, the code was shown to have come from the other side of the border.

Today astronomers smile at the notion of catching the Martian equivalent of Amos ‘n Andy on ordinary AM radios. But they do not dismiss the idea of using more sophisticated equipment to listen for signals from other planetary systems. Today, sixty years after the Martian alert of 1924, the National Aeronautics and Space Administration (NASA) is gearing up to begin the first broad, systematic search for extraterrestrial life. The search, which will be conducted piecemeal at observatories all over the world, will dwarf Todd’s effort—and all others since—in cost, sensitivity, and scale. The project will not reach the listening stage until sometime after 1988; it will run for at least five years after that, and possibly until the end of the century. Patiently and slowly, astronomers will be searching every corner of the sky, in the hope of answering a question that has intrigued mankind for thousands of years: Are we alone? For most of the past two millennia, opinion on the possibility of life on other worlds has been, by and large, positive; those people who have thought about the matter at all have tended to assume that the cosmos is teeming with aliens. For example, in the first century B.C. the Roman thinker Lucretius remarked (in the midst of an epic poem explicating atomic theory as conceived by the ancients): it cannot by any stretch of the imagination / be thought that ours is the only earth and sky created / ... . you must admit that other worlds in other places exist, / and other races of men and animals.

This is still the primary argument for the existence of living creatures on other worlds: The Sun has planets and life; there are many, many stars; it is unlikely that not one of these stars has a planet on which there is life; thus it is probable that other civilizations are out there.

The counterargument (as articulated by such eminent biologists as Ernst Mayr and the late Theodosius Dobzhansky) is equally straightforward: Intelligence on Earth was made possible only by a four-billion-year chain of evolutionary accidents; the chance that this sequence of events could ever be repeated is incredibly small; thus earthly life must be unique.

For all the time that astronomers, philosophers, and theologians have spent arguing over points like this, it is only in the past century or so that anyone is known to have tried to resolve the dispute by going out and looking. The first serious use of the telescope as a means of searching for alien life probably did not occur until 1877. In that year the Italian astronomer Giovanni Schiaparelli observed markings on Mars, which he called canali. When rendered in English as “canals,” the term, by which Schiaparelli meant to designate mere channels or grooves, implied that these features had been built by someone or something. The finding a few decades later that what astronomers had taken for canals was mostly the result of their own eyestrain caused considerable public disillusionment. Because of the flap over the Martian canals, and the failure to make contact with Mars by radio, extraterrestrial life came to be classified in popular as well as scientific opinion with UFOs, parapsychology, and the lost, lamented civilization of Atlantis.

In 1933 Karl Jansky, an engineer for Bell Telephone Laboratories, discovered that a certain amount of broadcast interference here on Earth was caused by radio emissions from outer space. It soon became clear that the static was caused by the natural activity of stars, nebulae, and galaxies. Within twenty years astronomers realized that such interference could be a valuable clue to the behavior and evolution of stellar objects, and Jansky’s discovery blossomed into the discipline of radio astronomy. The first radio astronomers were frustrated by the extreme weakness of unearthly radio emissions. Moreover, radio telescopes were not accurate enough to enable astronomers to pinpoint the sources. But by the late 1950s electronics had advanced so far that it became worthwhile for the first large dish antennas to be constructed.

In the summer of 1959 Giuseppe Cocconi and Philip Morrison, two prominent cosmic-ray physicists from Cornell University, sent the British scientific journal Nature an article in which they argued that the available technology was just sophisticated enough for contact with alien civilizations to be made, and that therefore a search for extraterrestrial signals should be undertaken. Somewhat to the surprise of Cocconi and Morrison, Nature accepted the article and published it that September.

The authors proposed seven nearby stars as likely targets for a listening project. They also considered the baffling question, Which of the millions of frequencies should astronomers listen to first? Like ordinary television and radio receivers, the receivers that astronomers use pick up electromagnetic waves. These waves rise and fall in strength in much the same way that ocean waves do. The distance between two neighboring wave crests or troughs is called a wavelength, and the number of wavelengths crossing a given point in a second is called a frequency. Electromagnetic waves are classified into “bands” of frequencies. For example, radio waves, which are long and whose frequencies are therefore low, occupy one band; xravs, which are short and whose frequencies are therefore high, occupy another. Cocconi and Morrison pointed out that most of the low-frequency bands are cluttered with interstellar static, and that the high-frequency bands are absorbed by the earth’s atmosphere, but that one of the bands in between—the microwave band—is relatively unobstructed. And in the middle of that band, they wrote, “lies a unique, objective standard of frequency, which must be known to every observer in the universe”—the frequency naturally emitted by single atoms of hydrogen. Hydrogen is by far the most abundant substance in the universe, and any civilization capable of attracting our attention would know that hydrogen atoms produce microwaves that are twenty-one centimeters long. This wavelength, Cocconi and Morrison said, might serve as an interstellar landmark.

After the paper appeared, several scientists remarked that the frequency of the microwaves emitted by hydroxyl (OH) is near to that of the microwaves emitted by hydrogen (H). H and OH combine to make water, and so the zone between their frequencies began to be called the waterhole. (Search aficionados today like to imagine galactic civilizations talking around the waterhole as if they were tribespeople meeting peaceably at an oasis.) Some astronomers have argued that because water is of some interest to all known living things, we should also listen to the microwaves emitted at the water-molecule frequency.

The Nature article surprised many scientists, but it flabbergasted the staff of the National Radio Astronomy Observatory, in Green Bank, West Virginia, where a young astronomer named Frank Drake was planning exactly the type of search that Cocconi and Morrison had described. Working independently of Cocconi and Morrison, and using reasoning entirely different from theirs, Drake had picked out twenty-one centimeters (the hydrogen wavelength) as the frequency of choice and had decided to listen to Tau Ceti and Epsilon Eridani—two of the seven stars that Cocconi and Morrison had listed as targets. (That Cocconi and Morrison and Drake came to the same conclusion about the suitability of the hydrogen frequency could be an indication that aliens, if they exist, would reach this conclusion too. Or it could show merely that human scientists tend to think alike.)

Before dawn on April 8, 1960, Drake switched on a set of electronic receivers and began what he called Project Ozma, after the princess in the Oz books. Over the course of the next three months Drake and other astronomers at Green Bank pointed their eighty-five-foot antenna at the two stars. They found nothing. Still, Drake was pleased. “For all we knew, every star in the sky had a booming civilization,” he says now. “It would have been foolish not to take a look.”

A YEAR AND A HALF AFTER PROJECT OZMA, DRAKE CONvened a small conference—ten scholars in all—to take stock. Ozma had elicited violent reactions, both positive and negative. Some praised it as daring and visionary; others attacked it as a senseless outlay of federal money (a charge that lost some of its sting when it was disclosed that the total expenditure had been less than $2,000). Drake held his conference without fanfare; he wanted to discuss how to go about a search that he recognized would be lengthy and expensive.

Drake proposed, as a means of organizing the discussion, the following equation:

N = R* x fp x ne x fl x fi x fc x L

This equation states that the number of civilizations (N) that might talk to us is calculated by multiplying the average rate at which stars form in the lifetime of the galaxy (R#) by the fraction of stars with planets (fp) by the mean number of planets suitable for life (ne) by the percentage of planets on which life occurs (fl) by the fraction of those planets on which intelligent life occurs (fj) by the fraction of intelligent civilizations that are in a phase during which they are willing and able to make contact (fc) by the mean lifetime of those civilizations (L). Drake knew full well that only one of these variables (R*) had been assigned even a rough value; today, scientists think that R* is about ten stars per year, and they have gone on to make a stab at fp. A march from left to right across the equation is a journey from tentative knowledge to sheer ignorance.

Since Project Ozma the scientific field defined by Drake’s equation has acquired its own acronym: SETI, for the “search for extraterrestrial intelligence.” From 1979 to 1982 it even had its own magazine: Cosmic Search. At least thirty-five searches, of varying size, seriousness, and intensity, have been undertaken. A surprising number of these have been in the Soviet Union, where a state scientific commission on extraterrestrial intelligence was organized in the 1960s, and where Party leaders are said to regard SETI as a corollary of dialectical materialism. In this country recently there have been several “parasitical” or “piggybacked” searches; that is, SETI researchers have simply listened in as radio astronomers have gone about their work. At the moment, only two full-time professional searches are in progress. One, at the Ohio State University Radio Observatory, is operated by the observatory’s assistant director, Robert Dixon, in a facility under constant threat of being razed to make room for a golf course. The other, known as Project Sentinel, is run by Paul Horowitz, a professor of physics at Harvard University; although Sentinel uses facilities borrowed from Harvard, it is funded entirely by the Planetary Society, a nonprofit group of some 130,000 astronomy buffs. In addition, at least three amateur radio astronomers arc scanning the skies wath garage-made equipment. (Astronomy being one of the few hard sciences to which amateurs bring important contributions—spotting comets, asteroids, and the like—few professionals seem inclined to scoff at the efforts of backyard SETI enthusiasts.)

Nobody is known to be going the other way—that is, trying to speak to aliens rather than just to overhear them—unless one counts commercial radio and television signals, which leak into space. These, however, are much feebler than signals deliberately broadcast on particular wavelengths and in specific directions would be. Thus there seems to be little danger that Star Irek reruns will ever become Earth’s de facto emissaries. The only formal attempt so far to make contact with extraterrestrials was a two-and-a-half-minute message beamed to star cluster M13, in the constellation Hercules, which happened to be overhead during the dedication, on November 16, 1974, of the world’s largest radio telescope, in Arecibo, Puerto Rico. Designed by Drake and the staff of the Arecibo observatory, the SETIgram, as one might call it, consisted of 1,679 binary pulses, which, when arranged into seventythree consecutive rows of twenty-three characters each, would take shape as a visual message. Thus decoded, the SETIgram would look something like a Navajo blanket, but Drake and his staff believed that anyone capable of receiving the message would be able to decipher from it a good deal of information about human beings and their solar system. The Arecibo transmission was more a symbolic than a serious attempt at communication, however. For one thing, the signal itself was short, and it was broadcast with little power. For another, it will take 24,000 years just to reach the Hercules star cluster. And if it is picked up and answered promptly, the world will have to wait another 24,000 years for the reply.

EVEN THE MOST SOBER ASTRONOMERS HAVE A SNEAKing fondness for the science-fiction aspects of their trade. As the years after Ozma went by, more and more came to believe that the chances of finding another solar system and hearing its inhabitants had been greatly improved by the past two decades’ worth of innovations in both optical and radio astronomy. Optical astronomers use telescopes that gather and focus light. The accuracy of these conventional devices has been augmented in recent years by the enhanced sensitivity of interferometers—instruments that can be used to pinpoint a source of light. Astronomers are now able to measure more precisely where the stars are in the heavens, and they may even be able to detect minute wobbles in a star’s path that would be caused by the orbit of a large planet. Several observatories have turned up preliminary indications of the existence of such wobbles in the paths of neighboring stars. And a year ago the orbiting Infrared Astronomical Satellite (IRAS), which scans infrared light, recorded rings of dust— which may include more substantial stuff, such as gravel and even planets—around a number of nearby stars.

The space shuttle’s schedule for 1986 calls for the craft to carry and jettison into orbit a large optical telescope. This will be the first time such a telescope has been used beyond the atmosphere, where it will be unhampered by the protective cloud of air and grit that shrouds this planet. Although the purpose of the space telescope is not to look for other planets, it will be so much more accurate than any telescope on earth that planets may be spotted all the same. A telescope mounted on a space station that NASA wants to build would be even more useful. Astronomers think that space telescopes will yield confirmed discoveries of other planetary systems within the first decade of operation—a development that David Black, a theoretical astrophysicist at NASA’s Ames Research Center, near Mountain View, California, says would be “quite literally a second Copernican revolution.”

The technology for radio-astronomical searches for life—not just planets—has improved because of the ubiquitous silicon chip. Using advanced electronics, scientists at Stanford University and Ames have invented a device called the multi-channel spectrum analyzer, or MCSA, that can pay attention to millions of separate frequencies at the same time. The types of MCSAs that these scientists are tinkering with can drink in a big gulp of the radio spectrum, divide it into eight million narrow channels of onewave per second each, and listen to all of them at once; in addition, they can scan for signals on wider bands that overlap the smaller segments. One such machine could perform an Ozma-sized survey in less than a second. Drake says, “These devices will improve SETI search programs as much as the two-hundred-inch Mount Palomar telescope improved optical astronomy over Galileo’s original telescope.”

Because of these developments, in 1980 a committee of the conservative National Academy of Sciences (NAS) startled even many SETI advocates by recommending that the U.S. government itself undertake a search. Harlan Smith, the head of the committee and the director of McDonald Observatory, at the University of Texas at Austin, says, “I always thought SETI was a good idea, but you couldn’t actually do it in a worthwhile manner until the spectrum analyzers started coming out.” In 1982 the NAS polled American astronomers and discovered, somewhat to the amusement of everyone involved, that they considered SETI to be one of their most important future tasks. At about the same time, the International Astronomical Union (IAU) ended two decades of official skepticism and established a permanent committee for SETI.

By all accounts NASA has always been a hothed of SETI sympathizers. In 1978, when the agency first requested money to start a search, Senator William Proxmire, of Wisconsin, gave it one of his famous Golden Fleece awards. In 1981 Proxmire told the Senate that approving NASA’s request would be a “ridiculous waste of the taxpayers’ dollars.” That year he succeeded in attaching an amendment to the space budget that specifically prohibited any spending on SETI. More than one scientist appealed to Proxmire to relent. Carl Sagan, an early and prominent advocate of things interstellar, argued that the philosophical ramifications of the search would more than compensate for the modest cost involved.

Proxmire’s supplicants were motivated to some extent by apprehension that the coming decade or so might well be the last chance to have a search at all. Today an international convention keeps portions of the microwave spectrum free of most terrestrial broadcasts so that radio astronomers can do their work. But with the ever-expanding electronics revolution, more and more people covet those restricted frequencies. It seems likely that within fifty years broadcasts from this planet will fill the skies. Then, according to Drake, SETI, and perhaps even radio astronomy altogether, will be possible only from an observatory free of terrestrial interference—say, on the far side of the moon.

Apparently, the astronomers’ arguments were persuasive, because in the budget deliberations for 1983 Proxmire reversed his position and did not try to prevent Congress from allocating money for SETI.

As of now, NASA is planning to use the appropriation— $1.5 million a year for the next five years, with the amount of funds thereafter still to be determined—to prepare for a search that will rely on the spectrum analyzer. The agency plans to sweep the entire sky—both hemispheres—by cutting up the heavens into small sectors and listening to each for periods ranging from three tenths of a second to three seconds. Planners think that such short periods will be sufficient for the detection of continuously broadcast signals. All frequencies between one billion and ten billion waves per second will be heard—a wide swath of the microwave band that includes the waterhole. The NASA search also involves compiling a list of sunlike stars no more than eighty light years away and examining eight hundred of them for fifteen minutes per frequency band per star, in the range of one billion to three billion waves per second.

Large-scale though the program is, SETI specialists regard it as only a short step. Harlan Smith says, “There are few questions more important than whether the human race is alone in the universe. But the answer is going to be incredibly difficult to come by. It’s worth a modest investment every year for the foreseeable future by techniques that will doubtless improve as time goes on. By great good luck, we might succeed in learning something in the next few decades. But, for what it’s worth, I would not be surprised if the search requires centuries, or even millennia, before we conclude that at least our part of the galaxy is sterile with respect to intelligent life.”

THE REASON THE SEARCH WILL TAKE SO LONG IS SIMply that the universe is big, and examining every corner of it is a forbidding task, even with the most sophisticated technology. There are 200 billion stars in our galaxy, astronomers say, and just as many galaxies in the cosmos. For a search to be possible, criteria must be devised for selecting what regions of the sky to listen to and for how long; a set of such criteria is called, in SETI-speak, a search strategy. NASA’s plan to cover the entire sky is by no means universally favored. Many astronomers believe that the agency should examine only stars in our neighborhood of the galaxy; others think that the search should be concentrated near the galactic center, which is far away but has many more stars.

The more a message has to say, the more diffuse—and therefore the weaker—its signal will be. For this reason many scientists, Drake included, think that an extraterrestrial civilization making a deliberate attempt to communicate would break its message into two parts. Drake says, “A message with a high information content is more difficult to detect. That’s due to the laws of physics—it’s not something we can overcome with technology. So if a civilization wants to enrich the galaxy with its knowledge, the communication will probably involve two separate messages. The first is called the beacon, and it tells you where to tune in to get the second message. The beacon is a sort of signpost, telling you where the public library is. T he second message—the library—you could call the information channel.”

A comprehensive search strategy must come to terms not only with the disheartening immensity of the cosmos but also with a dizzying variety of possibilities within that vastness. Even if a civilization broadcasts in the waterhole, the planet’s motion will cause a change in the signal’s frequency (that is, a “Doppler shift”), in much the same manner that the motion of a passing train will cause bystanders to hear a change in the train whistle’s pitch. Secondary Doppler shifts will be created by the planet’s orbit around its star, the movement of that star around the galaxy, and the peregrinations of the galaxy itself—not to mention the motions of this planet, its sun, and its galaxy. Thus listening even at the hydrogen line is no easy task, for terrestrial eavesdroppers must guess which, if any, Doppler effects their targets would have compensated for, and must shift their receiving frequencies accordingly.

In addition to such natural problems inherent in the task, SETI is beset by more outre, epistemological difficulties. For instance, there is no guarantee that advanced civilizations would take radio waves seriously as a medium for communication. Some astronomers and physicists have speculated that advanced civilizations would use neutrinos (fast-moving subatomic particles so light that they may have no mass) or gravity waves (slight, wavelike undulations in the curvature of space) for interstellar chitchat. In the nineteenth century the German mathematician Karl Friedrich Gauss suggested that his contemporaries signal the existence of life on Earth by planting a forest in Siberia in a geometric configuration illustrative of the Pythagorean theorem. To some future civilization, our confidence that extraterrestrials would use radio waves to signal their existence to us may seem only slightly less naive.

It is also uncertain whether we could recognize a deliberate signal, even if one happened to trickle into our receivers. Human beings are adept at filtering signals of human origin from the noise; it is, of course, not yet known if this talent extends to signals of nonhuman origin. Even a transmission with a regular pattern would not necessarily be attributable to the manipulations of intelligence; certain natural radio emitters called pulsars send out radio signals at periodic intervals as well.

Philip Morrison, who is now a professor of physics at the Massachusetts Institute of Technology, says, “The main thing is to find a pattern that is unusual. The simplest criterion is to look for a channel that has a lot more energy in it than nearby channels; this is what Paul Horowitz does in the Sentinel search. We have no knowledge of any natural phenomenon that is much sharper than the immediate channels around it. The possibility that even that kind of signal is natural is not excluded, of course. But the natural phenomena we have found seem to spread over hundreds or thousands of channels. Horowitz’s idea seems to be a good one to me. It is also advantageous from the economic point of view. But there are other strategies. The Russians, for instance, didn’t do that at all. They first looked for pulses—fast pulses over broad bands. There are some people who talk about [computer] programs for pattern recognition. But that’s unnecessarily sophisticated for the present state of affairs.”

If we ever do come upon a deliberate signal and recognize it as such, there is no particular reason to suppose that anyone will be able to understand it. Not only may there be no common denominator of intelligence but also there may be none for comprehension. Some scientists believe that mathematics can be the source of a universal and convenient language for communication with anyone or anything, but there is no evidence to prove this comforting idea. Philosophers since Leibniz’s time have attempted to construct such a language, always unsuccessfully. The Arecibo transmission did not even cross the Atlantic without confusion; when the decoded version appeared in Nature, the picture was upside down. Scientists have argued over how likely it is that an alien civilization would decipher our messages correctly. Michael Arbib, a professor of computer and information science at the University of Massachusetts at Amherst, decoded the upside-down SETIgram in such a way that it showed the sender to be a sixlegged, large-brained creature with a tail.

IT IS DIFFICULT TO IMAGINE A SCIENTIFIC FIELD THAT has had fewer returns than SETI, or in which the prospect of any return is as unknown and portentous. In the research community, therefore, SETI attracts a special type of researcher. “It’s not a subject for young scientists,” Drake says. (He was a professor of astronomy at Cornell University from 1964 until this year, when he became the dean of natural sciences at the University of California at Santa Cruz.) “At first it’s exciting. But even after only a few days of looking it dawns on you that it’s going to take a long, long time to find anything. If you do it continuously, it can be curtains for your career. Young scientists have to get results.” With no new real data, Drake says, “the basic concepts of SETI have not changed since 1959.”

People who do not need results include, unhappily, cranks, and SETI has been plagued by them throughout its short life. An IAU-sponsored conference in Boston last June—that organization’s first officially sanctioned SETI meeting—was dotted with daffy, formidably unselfconscious proponents of “universal alphabets” and “preferred evolutionary pathways.” But they were greatly outnumbered by scientists—biologists, paleontologists, and organic chemists, as well as astronomers—who attended the conference in the belief that the formation of our solar system or the origin of life will never be fully understood until we discover other instances of these phenomena. “It is essential to understanding the origin of our solar system to find another example,” Black says. “Theories of planetary formation must be tested. And even one other solar system would provide constraints for our models. It would be an immense and pivotal discovery.” A surprisingly large part of the scientific community, eager to solve such mysteries as the nature of star formation, the origin of complex organic molecules, and the early course of life on Earth, considers SETI the only means to do so.

Among the life scientists who are professionally interested in SETI is Joshua Lederberg, a geneticist at Stanford University and a Nobel Prize winner, who coined the name “exobiology” for the study of extraterrestrial life. Although skeptics call exobiology “a science without a subject matter,” some people think that the very existence of the field has had a valuable and liberating effect on the biological sciences. According to Sagan, “The mere design of exobiological experiments forces man to examine critically the generality of his assumptions of life on Earth.”

But for nonspecialists, the strongest rationale for SETI may be one that Sagan has often discussed: L, the variable in Drake’s equation for the lifetime of technological civilizations. The survival of other cultures on other worlds implies that advanced cultures do not inevitably incinerate themselves in nuclear fires. If some civilization out there has made its way beyond weapons, knowledge of its success would offer hope to a species in danger of destroying itself. If the money turns out to be “wasted”—that is, if we look and listen, and are forced to conclude that we are alone after all—that newly disclosed solitude should give us pause. If in all the great emptiness of the universe there is only one flicker of consciousness, then scientists will have shown that the gift of life is more priceless than anyone ever wished.