home space exploration and seti Xenology: The Science of Asking Who's Out There

Xenology: The Science of Asking Who's Out There

by David Brin, Ph.D.

If self-destruction is the common fate of "civilized" species, then there might be no more than a handful of them in the Milky Way at a given moment, separated by vast tracts of silent starscape. If, on the other hand, a reasonable fraction of races live a long time, the galaxy might be teeming with life.

Xenology: The Science of Asking Who's Out There

In the early 1960s, while the world was entranced by the spectacle of human beings hurled into "outer space" in rocket ships, a series of philosophical earthquakes shook the sedate field of astronomy. Just when the skies were beginning to seem known and familiar, all at once things changed. Stellar astronomers suddenly faced unsettling data from new classes of objects called "quasars" and "radio galaxies." There were disturbing theories about so-called "black holes." Even those who had long studied planets now found their comfortable domains invaded by geologists and meteorologists, who weren't at all shy about moving into the new territory.

It was no coincidence that all of this happened just as the Space Race was getting under way. New instruments and techniques often lead to upheavals in a science.

Still, the greatest intellectual challenge to the worldview of modern astronomers came in the early sixties, not because of new space probes, telescopes, and computers but because of an idea.

Starting in 1959, with a classic paper by Cocconi and Morrison, a series of articles and books were published with titles like Interstellar Communication, Habitable Planets for Man and, in 1966, a major work entitled Intelligent Life in the Universe by Iosef Shmuelovich Shklovskii and Carl Sagan. With these studies astronomers began to concern themselves with life itself.

The early sixties were pivotal for the field of "exobiology" (extraterrestrial biology) and especially the sub-branch that dealt with intelligent life, "xenology." For the first time it was legitimate for leading scientists to publicly consider the possibility of contact with intelligent species off of the planet Earth.

Of course a lot of thought had gone into the subject previously, on the pages of science fiction novels and magazines. Many of the private discussions between authors such as Sagan, radio astronomer Frank Drake, and Rand Corporation scientist Stephen Dole grew out of ideas germinated by the likes of Clarke, Asimov, and Clement during the thirties and forties. But prior to the publication of Intelligent Life in the Universe, the number of "respectable" papers on the subject printed in the West could be counted on one's fingers and toes.

In the Soviet Union extraterrestrial intelligence was not only considered possible but was required by Leninist dogma. (It was assumed dialectically impossible that any advanced intelligence could be anything but socialist, of course.) When scientist I. S. Shklovskii wrote "Universe, Life, Mind" in 1962, his thoughts were widely popularized, and extracts were reprinted in major Soviet scientific journals.

In the West it took more time for scientific speculation about the distribution of life in the cosmos to become acceptable. A tradition of skepticism and rigor kept Western science relatively safe from scientio-religions such as Lysenkoism, which caused so much harm in Russia. But the same attitudes made it hard for those interested in the possibility of alien life-forms to bring up the subject in scientific gatherings without being criticized for "playing with science fiction."

The older scientists who dished out the ridicule shouldn't be blamed too harshly. In squelching early discussions of exobiology, they may have been overreacting to the excesses of earlier enthusiasts, such as Percival Lowell, the astronomer who convinced millions that there were living Martians and "networks of canals" on the red planet.

But with the arrival of the space age, resistance to "science fictional" ideas was dealt a fatal blow. Those who had declared that "spaceships" belonged only in comic books were caught flat footed. A new generation of scientists brought exobiological speculation out of the fringes and onto the pages of respectable journals. These men and women, who had proven their scientific credentials with solid research, came from an age group that didn't consider "science fiction" a dirty word. Most of them had cut their teeth on the stuff.

The first time I witnessed the subject of extraterrestrial intelligence brought up at a scientific seminar was at a Wednesday Cal Tech colloquium in 1968. The speaker remarked on the remote possibility that pulsars might be beacons of an advanced civilization. They were, after all, several thousand times more regular in their repetitive "beepings" than any other astronomical radio source ever discovered.

The speaker was only partly serious, but sides were quickly taken, and it was soon very clear that most of those with tenure didn't like this kind of talk at all.

Attitudes were changing very rapidly during those years. A few years later some of those who were the angriest in 1968 applauded the loudest when Carl Sagan unveiled the gold plaque that was to be placed upon Pioneer 10, the first human artifact to be launched on a trajectory out of the solar system.

Today that plaque is famous. It, and those that followed on Pioneer 11 and the Voyagers, depict the nude figures of a woman and a man, an arm raised in greeting, a schematic of the planets of our system, and a rayed pattern of lines and binary dots representing the most prominent pulsars detectable from Earth. The pulsar map should enable any distant beings who recover the spacecraft to trace its point of origin within a light-year in space, and its launch date to within six months.

Shortly thereafter respectable scientists were discussing not whether extraterrestrial intelligences exist but how to go about listening for signals from our nearest neighbors! Small (very small) amounts of public money were allocated to adapting radio telescopes for the search.

If the first revolution in the nascent field of xenology came on the pages of science fiction pulps of the thirties and forties, the Second Xenological Revolution took place in the sixties, when scientists in large numbers began asking, "Where are they?"

Anyone interested in the possibilities of life outside the Earth should certainly read Intelligent Life in the Universe. Although some of its science is dated, it remains the classic in the field. Still, to a veteran reader of SF, Intelligent Life may seem overly tame and conservative. For instance, the authors barely mentioned the possibility of travel between the stars. To investigators of that time, it seemed pointless to discuss interstellar colonization.

Science fiction has long used, as furniture, ships that bypass relativity. But to early xenologists it was dangerous enough talking about alien life-forms, without risking one's scientific reputation talking about "hyperspace warpdrive" and the like. Shortcuts may lend SF a lot of pizzazz and spawn stories about galactic empires, but Einstein's speed limit dominates serious talk about life in the universe.

The gulfs separating the stars are vast. And during the sixties it seemed unlikely that even the modest velocities allowed under Einstein's edicts could ever be reached economically.

Thus the first era of modern scientific xenology (from 1959 to about 1972) dealt with the possibility of intelligent life springing up in isolation (here and there on fertile planets scattered across the sky), islands of intelligence separated from one another by vast distances and for all time.

the age of innocence

What could early students of this new science say about extraterrestrial life-forms? No matter how daring, they were faced with one major limitation: a near total lack of data. The only known case of intelligent life is here on Earth. Until the Viking mission, some held on to Percival Lowell's dreams for Mars. Now the evidence seems to weigh against finding even microbes there. Putting aside, for the moment, speculations about dolphins, whales, and gorillas, it's pretty hard to extrapolate a graph from only one data point.

Still, three scientific discoveries and one useful philosophical tool gave researchers the courage to make crude estimates about the distribution of life among the stars.

The first discovery came when it was found almost ridiculously easy to make amino acids, and other precursors to living matter, from abundant molecules such as methane, ammonia and cyanogen. Stanley Miller subjected a water solution of these substances to electrical discharge and ultraviolet radiation and got an organic "soup" in short order. Leslie Orgel of the Salk Institute accomplished the same thing by a freezing process. The high pressures of ice formation not only gave up amino acids, but the purine adenine as well. (Adenine is one of the four building blocks of DNA, and is the core of ATP, adenosine tri-phosphate, which controls the energy economy of the living cell.

So many mechanisms have been found that can change crude precursors into "biological" molecules that today organic activity seems almost an automatic consequence of the distribution of chemical elements in the universe.

The second major discovery supports this point of view. During the last two decades, radio astronomers — listening to narrow emission lines from interstellar space — have discovered great clouds of complex molecules: ethylene, formaldehyde, ethyl alcohol; some even claim evidence for — you guessed it — adenine.

(Astronomer and science fiction author Sir Fred Hoyle, looking at starlight scattered from interstellar dust, thinks that the dust itself may actually be something akin to bacteria... living cells about a micron in size, in diffuse colonies spanning light-years and outmassing suns. It's an extravagant speculation, but fun to think about.)

It's clear, then, from basic chemistry and radio astronomy, that the basic materials for life are out there. What about the right environments? We have to assume, until we have reason to think otherwise, that complex life must grow and evolve to intelligence on planets orbiting stable stars. Are there other "nursery worlds" like the Earth?

There are plenty of stable, long-lived G-type dwarf stars like the Sun out there... about 6 percent of the galaxy's several hundred billion stars. Are there planets circling many of them?

The data are still poor. It's hoped that the Space telescope will tell us more about the companions of nearby stars. Some scientists think there is good evidence that at least one of our neighbors, Barnard's star, has possibly two dark companions a bit more massive than Jupiter.

We do know that F-, K-, and G-type dwarf stars rotate much more slowly than larger, hotter stars. The Sun contains 99.9 percent of the matter in the solar system, yet it has only 0.5 percent of the angular momentum. The rest is distributed among the planets of the solar system, especially Jupiter. Most astronomers believe that those slowly rotating stars that aren't members of multiple-star systems have to possess dark companions that were used to "dump off" excess angular momentum early during star formation. Recent models of gas-cloud condensation tend to support this belief.

We've covered three discoveries, then, that help us believe that it's reasonable to talk about life outside the Earth. What is that "philosophical tool" we mentioned that caps the legitimacy of xenology? It is sometimes called the cosmological principle, or the assumption of mediocrity.

Since Copernicus, astronomy has been a series of lessons in humility, all leading to the conclusion that "there is nothing special about where and when we are." First the Earth was displaced from the center of the solar system, then the Sun became a nondescript traveler in orbit about the rim of the galaxy. The galaxy became merely one island universe among billions, and the universe seems to have no "middle" at all.

The cosmological principle tells us we should avoid the temptation to think that there's anything unique about the Earth in space, time, or situation. It is the major philosophical underpinning for the new study of xenology. It forces even the most cantankerously conservative astronomer to admit that someone, somewhere, might be peering up at HIS stars, among which insignificant motes is our own Sun.

Drake Equation
[Drake Equation]

If xenology has some justification, then, where did the first generation of scientific xenologists get their numbers? How did they estimate the population of our galaxy... or the probably distance to our nearest neighbors?

The Drake Equation is the most popular way to guess at the possible distribution of technological species. It was invented by Frank Drake when he was at the Arecibo National Radio Observatory. It remains the most widely accepted tool for xenological speculation.

Let N = the current number of technological civilizations in the galaxy. Then,

N = R P n(e) f(1) f(i) f(c) L

Here R is the average rate of production of suitable stars since the formation of the galaxy, approximately one per year. (The current rate is slower. R is an average that includes the burst of star creation early in the galaxy's history.) P is the fraction of stars that are accompanied by stably orbiting planets. Factor n(e) is the average number of planets per system that have the requisite conditions to support life.

The other factors include f(1), the fraction of these congenial planets on which life actually occurs; f(i), the fraction of these on which "intelligence" appears; f(c), the fraction of intelligent species that attain technological civilizations, and L, the average lifespan of each species.

For what then seemed fairly good reasons, Sagan and others chose to assign P and n(e) each values near 1. These guesses, within an order of magnitude, don't seem to conflict with what we now know about planets.

For purposes of discussion it was assumed that congenial planets normally develop life, [f(1) = 1], that about a tenth of the planets with life evolve intelligence, [f(i) = 0.1],and that about a tenth of the latter will see technological civilizations [f(c) = 0.1]. In other words, a likely planet will contribute roughly 0.01 technological races during its history.

A complete discussion of the Drake Equation can be found in books and in many recent technical articles. (Some references for the interested reader are given at the end of this article.) There are reasons to believe that the equation is, in fact, short about three factors. But suffice it here to say that the best guesses, with plenty of up-and-down leeway in every parameter, led Sagan and others a decade ago to a rough estimate.

N = 0.01 L

This meant the average life span of technological races would determine the number present in the galaxy at any time. If self-destruction is the common fate of "civilized" species, then there might be no more than a handful of them in the Milky Way at a given moment, separated by vast tracts of silent starscape. If, on the other hand, a reasonable fraction of races live a long time, the galaxy might be teeming with life.

Cameron von Hoerner, Shklovskii, and Sagan all guessed at L, allowing for various ways in which a culture might end. Generally, their results suggested that the number of civilizations in the galaxy might be on the order of one million, most of them long-lived and patient species. The numbers giving rise to this estimate were a bit arbitrary, but not unreasonable.

If the planets of a million stars held sophont races, then about 0.001 percent of all eligible stars in the galaxy would be inhabited by thinking beings. The average distance separating these islands of technology would be on the order of several hundred light-years — a gap that seemed unbridgeable corporeally, but easily crossed by radio waves.

This was the state of affairs in the early seventies. With interstellar travel virtually ruled out, the accepted model depicted isolated motes of intelligence separated by sterile tracts of space.

These speculations led to CYCLOPS, OZMA, SETI, and CETI. The search for extraterrestrial life was born. The radio astronomers who slapped together borrowed time and equipment to scan the sky were hopeful, and numerous articles about their endeavors came out on the pages of magazines.

We could take up several articles just talking about SETI. The early arguments over search strategy are fascinating reading. What kinds of antennae would be best suited for the job? Would extraterrestrial intelligent species (ETIS) transmit in the "water hole" frequencies? Should we transmit our own messages, or just wait and listen? If we wait, should we let our nearest neighbors get their first impressions of us from DEW-line radar and I Love Lucy?

Extraterrestrials might not use radio for long-range communication. If lasers carried their traffic, we might not be able to eavesdrop on interstellar conversations.

Even if we can't tape long-distance calls, though, we might still listen for leakage from a planet's commercial radio network... or search for a beacon... a signal meant to be picked up by new radio-using species like ourselves.

Many papers came out during the early seventies suggesting that advanced extrasolar sophonts would likely broadcast the interstellar equivalent of Sesame Street, to help younger species (like us) pass over their initial dilemma of survival or self-destruction. The reasoning went that it would be in the older species' interest to help its younger neighbors live long enough to get a decent conversation going.

The first formal search for extraterrestrial intelligence came when Frank Drake and his associates looked at the two nearest candidates, the two Sun-like stars that lie within twelve light years and are not members of multiple-star systems. Drake's team found nothing but star noise coming from the K2 dwarf, epsilon Eridani. Then they turned their telescope to the star Tau Ceti.

And lo! They hear something! For a brief instant they felt a thrill, as modulated signals came down the cable, obviously of intelligent origin! But then, as the telescope settled down, the "signal" faded away, never to return. They soon concluded that the signal was indeed coded noise from the nearest civilization — some commercial traffic in nearby Milford, Massachusetts!

Undaunted, Drake and others expanded the search. The telescopes turned and scanned. Nothing was found. The Russians joined the search, enthusiastically. They reported only negative results.

No problem, astronomers suggested. Any advanced species wouldn't waste energy broadcasting over the entire bandwidth of, say, the hydrogen 21-centimeter line. To conserve power, and to attain a high signal-to-noise ration, they would modulate over a very narrow band. Just wait, they suggested, until we can develop fineband simultaneous multichannel analyzers!

Yet the second and third generation of eavesdropping devices have come up with nothing.

True, still better instruments are planned. The money and time spent in the search has been insignificant compared with the potential rewards, which might include clues to the very survival of the human race. (There is a battle under way as this is being written, to restore the piddling two million-dollar appropriation for SETI, which recently was Proxmired to death.

Still, just one decade ago some of the radio-xenologists were talking as if they expected to be cracking codes in short order.

Now a few even glumly propose that no one is "out there" after all at least, not in our vicinity.

How can this be? If we've been at the search for less than fifteen years, using spare time and borrowed equipment, how could anyone expect success so soon? Sure, it'd be nice to find neighbors twelve or twenty light-years away; you could hold a "conversation" within one person's life span, for example. But according to most calculations using the Drake formula, the average distance between technological civilizations might be a few hundred light-years. There are well over a million stars in a sphere a hundred parsecs across. It would take some time to search even the most likely of these, choosing only those radio hands we guess to be the best (not knowing whether our idea of "best" is universal).

Two hundred light-years makes "conversation" a little more difficult. But a Sesame Street beacon would be just as useful as ever at that range. Just knowing extraterrestrials existed might profoundly boost Homo sap's sagging morale.

It seems like we are presenting an argument to restore that appropriation from Congress, not laying out a case for doom and gloom. It only appears to be a matter of time and effort. Success, in the long run, seems assured to the persistent.

What has changed, then? What has caused this spreading anxiety?

It's not the sort of thing one would expect to be a cause for pessimism. At first hearing it sounds like very good news.

Starships are possible

continue reading... is it time to launch starships?


Xenology: The Science of Asking Who's Out There

about this article

"Xenology: The Science of Asking Who's Out There" (published in full here) was first published in the 1980s in Analog Magazine as a popular adaptation of the much deeper and more scholarly 'classic' review of the field — "The Great Silence: The Controversy Concerning Extraterrestrial Intelligent Life" — which appeared in the Quarterly Journal of Royal Astronomical Society, Fall 1983, v.24, pp 283-309.

Copyright © 1983 by David Brin. All rights reserved.

join David Brin's discussions

David Brin blogs at Contrary Brin and posts social media comments on Facebook, Twitter, Quora, and MeWe specifically to discuss the political and scientific issues he raises in these articles. If you come and argue rationally, you're voting, implicitly, for a civilization that values open minds and discussions among equals.

cited in this article

John Billingham, ed., Life in the Universe (book #ad)

Giuseppe Cocconi and Philip Morrison, "Searching for Interstellar Communications" (pdf)

Stephen H. Dole, "Habitable Planets for Man" (pdf)

Cyril Ponnamperuma and A. G. W. Cameron, Interstellar Communication: Scientific Perspectives (book #ad)

I.S. Shklovskii and Carl Sagan, Intelligent Life In the Universe (book #ad)

on interstellar travel technology:

R.N. Bracewell, Nature (London) 186 (1960): 670

Robert Forward, "Interstellar Flight Systems" AIAA Paper No. 80-0823 (1980)

A.R. Martin, ed., "Project Daedalus: Final Report of the British Interplanetary Society Starship Study" BIS (1978)

Gerard K. O'Neill, Physics Today 27 (1976): 32

on possible dispersal of intelligent life:

John A. Ball, Icarus 19 (1973) 347

John Billingham, ed., Life in the Universe. Cambridge, MA: MIT Press, 1981

A.G.W. Cameron, ed., Interstellar Communication, New York: W. A. Benjamin Inc., 1963, 1970

Michael Hart, Q.J.R.A.S. 16 (1975): 128

Eric Jones, Icarus 46 1981: 328

T.B.H. Kuiper and M. Morris, Science 196 (1977): 616

William I. Newman and Carl Sagan, Icarus 46 (1981): 293

I.S. Shklovskii and C. Sagan, Intelligent Life in the Universe. San Francisco: Holden Day, 1966

Matthew O'Dowd, Fermi's Paradox and the Psychology of Galactic Empires (TED Talk, 2017)

on the cretaceous catastrophe:

L.W. Alvarez, W. Alvarez, F. Asaro, and H. V. Michel. Science 208 (1980): 1095

who's looking at you?

letting others have their say

Michael Wall, Out There

Elizabeth Tasker, The Planet Factory

Michael Summers and James Trefil, Exoplanets: Diamond Worlds, Super Earths, Pulsar Planets, and the New Search for Life Beyond Our Solar System

Adam Frank, Light of the Stars: Alien Worlds and the Fate of the Earth

Michio Kaku, The Future of Humanity

John Willis, All These Worlds Are Yours

Dirk Schulze-Makuch and William Bains, The Cosmic Zoo


a brief intro to author David Brin

DAVID BRIN scientist


David Brin's science fiction novels have been New York Times Bestsellers, winning multiple Hugo, Nebula and other awards. At least a dozen have been translated into more than twenty languages. They range from bold and prophetic explorations of our near-future to Brin's Uplift series, envisioning galactic issues of sapience and destiny (and star-faring dolphins!).
Learn More

shorter fiction

Short stories and novellas have different rhythms and artistic flavor, and Brin's short stories and novellas, several of which earned Hugo and other awards, exploit that difference to explore a wider range of real and vividly speculative ideas. Many have been selected for anthologies and reprints, and most have been published in anthology form.
Learn More

Contrary Brin blog

Since 2004, David Brin has maintained a blog about science, technology, science fiction, books, and the future — themes his science fiction and nonfiction writings continue to explore.
Learn More

social media influencer

Who could've predicted that social media — indeed, all of our online society — would play such an important role in the 21st Century — restoring the voices of advisors and influencers! Lively and intelligent comments spill over onto Brin's social media pages.
Learn More


David Brin's Ph.D in Physics from the University of California at San Diego (the lab of nobelist Hannes Alfven) followed a masters in optics and an undergraduate degree in astrophysics from Caltech. Every science show that depicts a comet now portrays the model developed in Brin's PhD research.
Learn More

transparency expert

Brin's non-fiction book, The Transparent Society: Will Technology Force Us to Choose Between Freedom and Privacy?, continues to receive acclaim for its accuracy in predicting 21st Century concerns about online security, secrecy, accountability and privacy.
Learn More

speaker & consultant

Brin speaks plausibly and entertainingly about trends in technology and society to audiences willing to confront the challenges that our rambunctious civilization will face in the decades ahead. He also talks about the field of science fiction, especially in relation to his own novels and stories. To date he has presented at more than 200 meetings, conferences, corporate retreats and other gatherings.
Learn More

future/tech advisor

Brin advises corporations and governmental and private defense- and security-related agencies about information-age issues, scientific trends, future social and political trends, and education. Urban Developer Magazine named him one of four World's Best Futurists, and he was appraised as "#1 influencer" in Onalytica's Top 100 report of Artificial Intelligence influencers, brands & publications. Past consultations include Google, Microsoft, Procter & Gamble, and many others.
Learn More

Contacting BRIN

All the Ways in the World to Reach David Brin

an ornery, contrary BLOG, and other insightful wormholes!

Do not enter if you want a standard "Party" line! Contrary Brin's incendiary posts on science, sci-fi and politics and its engaged, opinionated community poke at too-rigid orthodoxies, proposing ideas and topics that fascinate — and infuriate. See for yourself, and if you like — subscribe for more.

Questions? Concerns? Email DAVID BRIN


facebook followers and fans

  join me on Facebook


twitter followers

  join me on Twitter


quora followers

  join me on Quora

get on the Brin newsletter!

DAVID BRIN newsletter sign-up

subscribe to David Brin's newsletter and keep up to date on his books, signings and appearances

pinterest boards

DAVID BRIN Pinterest

share David Brin's pins from these Pinterest boards and share the word about science and science fiction

other points of departure

visit other pages on this website

pages about DAVID BRIN

  • latest news and activities
  • information about DAVID BRIN
  • public speaking and consulting & popular topics
  • speaking/consulting references and testimonials & a list of past appearances
  • print and podcast interviews
  • video interviews and talks
  • Brin's presskit and complete biography
  • traditional media and social media
  • Brin quotes and frequently asked questions
  • pages about BRIN's science fiction

  • Brin's novels and books
  • Brin's short stories and novellas
  • all about Brin's uplift universe
  • a selection of book reviews
  • Brin's special-order books for sale
  • Brin's advice for new writers
  • Brin reviews sci fi films — including The Postman
  • a compilation of great sf books to read
  • recommended sf films
  • science fiction that teaches
  • BRIN's nonfiction explorations

  • privacy, security, accountability and transparency
  • designing and crafting our amazing 21st Century
  • predicting and projecting our near and far future
  • leading and following our politics and economy
  • keeping track of changes in science and technology
  • scanning our sky for habitable (inhabited?) worlds