by Carl Sagan
Almost all the stuff of the stars and the interstellar matter between the stars is hydrogen and helium, the two simplest atoms. All other atoms are impurities, trace constituents. This is also true for the massive outer planets of our Solar System, like Jupiter. But it is not true for the comparatively tiny hunks of rock and metal in the inner part of the Solar System, like our planet Earth. This is because the small terrestrial planets have gravities too weak to hold their original hydrogen and helium atmospheres, which have slowly leaked away to space.
The next most abundant atoms in the universe turn out to be oxygen, carbon, nitrogen, and neon. These are atoms everyone has heard of. Why are the cosmically most abundant elements those that are reasonably common on Earth–rather than, say, yttrium or praseodymium?
The theory of the evolution of stars is sufficiently advanced that astronomers are able to understand the various kinds of stars and their relations–how a star is born from the interstellar gas and dust, how it shines and evolves by thermonuclear reactions in its hot interior, and how it dies. These thermonuclear reactions are of the same sort as the reactions that underlie thermonuclear weapons (hydrogen bombs): The conversion of four atoms of hydrogen into one of helium.
But in the later stages of stellar evolution, higher temperatures are reached in the insides of stars, and elements heavier than helium are generated by thermonuclear processes. Nuclear astrophysics indicates that the most abundant atoms produced in such hot red giant stars are precisely the most abundant atoms on Earth and elsewhere in the universe. The heavy atoms generated in the insides of red giants are spewed out into the interstellar medium, by slow leakage from the star’s atmosphere like our own solar wind, or by mighty stellar explosions, some of which can make a star a billion times brighter than our Sun.
Recent infrared spectroscopy of hot stars has discovered that they are blowing off silicates into space–rock powder spewed out into the interstellar medium. Carbon stars probably expel graphite particles into surrounding cosmic space. Other stars shed ice. In their early histories, stars like the Sun probably propelled large quantities’ of organic compounds into interstellar space; indeed, simple organic molecules are found by radio astronomical methods to be filling the space between the stars. The brightest planetary nebula known (a planetary nebula is an expanding cloud usually surrounding an exploding star called a nova) seems to contain particles of magnesium carbonate: Dolomite, the stuff of the European mountains of the same name, expelled by a star into interstellar space.
These heavy atoms–carbon, nitrogen, oxygen, silicon, and the rest–then float about in the interstellar medium until, at some later time, a local gravitational condensation occurs and a new sun and new planets are formed. This secondgeneration solar system is enriched in heavy elements.
The fate of individual human beings may not now be connected in a deep way with the rest of the universe, but the matter out of which each of us is made is intimately tied to processes that occurred immense intervals of time and enormous distances in space away from us. Our Sun is a second- or thirdgeneration star. All of the rocky and metallic material we stand on, the iron in our blood, the calcium in our teeth, the carbon in our genes were produced billions of years ago in the interior of a red giant star. We are made of star-stuff.
Our atomic and molecular connection with the rest of the universe is a real and unfanciful cosmic hookup. As we explore our surroundings by telescope and space vehicle, other hookups may emerge. There may be a network of intercommunicating extraterrestrial civilizations to which we may link up tomorrow, for all we know. The undelivered promise of astrology–that the stars impel our individual characters–will not be satisfied by modern astronomy. But the deep human need to seek and understand our connection with the universe is a goal well within our grasp.
27. Extraterrestrial Life: An Idea Whose Time Has Come
Thousands of years ago, the idea that the planets were populated by intelligent beings was uncommon. The idea was that the planets themselves were intelligent beings. Mars was the god of war, Venus was the goddess of beauty, Jupiter was the king of the gods.
In early Roman times a few writers, for example Lucian of Samasota, conceived that at least the Moon was a place that was populated as the Earth was. His science-fiction story describing travel to the Moon was called the “True History.” It was, of course, false.
The idea of the planets as an elegant celestial clockwork created by the Deity for the amazement and utility of men emerged in the Renaissance. In the year 1600 Giordano Bruno was burned to death at the stake, in part for uttering and publishing the heresy that there were other worlds and other beings inhabiting them.
The pendulum swung far in the other direction in subsequent centuries. Writers such as Bernard de Fontenelle, Emanuel Swedenborg, and even Immanuel Kant and Johannes Kepler could safely imagine that perhaps all the planets were inhabited. Indeed, the idea was expressed that the name of the planet gave some hint to the character of its inhabitants. The denizens of Venus were amorous; those of Mars, warlike or martial; the inhabitants of Mercury, fickle or mercurial; those of Jupiter, jolly or jovial. And so on. The great British astronomer William Herschel even supposed that the Sun was inhabited.
But as the extremes of the physical environments in the Solar System became clearer and the exquisite adaptation to the environment of organisms on Earth became more apparent, skeptics arose. Perhaps Mars and Venus were inhabited, but surely not Mercury, not the Moon, not Jupiter. And so on.
In the last few decades of the nineteenth century the observations of the planet Mars by Giovanni Schiaparelli and Percival Lowell quickened public excitement about the possibility of intelligence on our planetary neighbor. Lowell’s passion for the idea of intelligent beings on Mars, his articulateness, and the wide publication of his books did much to bring this idea to the public attention, as did sciencefiction writers who followed the Lowellian scenario.
But as the evidence for intelligent life on Mars withered, and as the environment of Mars was perceived to be more and more inclement by terrestrial standards, popular enthusiasm for the idea waned.
By then, scientific interest in extraterrestrial life had reached a nadir. The very enthusiasm with which Lowell pursued the idea of intelligent beings on Mars and the attention that these ideas received from the man in the street repelled many scientists. In addition, a new astronomical field, astrophysics, the application of physics to the surfaces and interiors of stars, had achieved phenomenal success, and the brightest and most enthusiastic young astronomers went into stellar astronomy rather than planetary studies. The pendulum had swung so far that in the period just after the Second World War, there was–in all of the United States–only one astronomer doing serious physical investigations of the planets, G. P. Kuiper, then of the University of Chicago. Not only had astronomers been turned off extraterrestrial life, they had been turned off planetary studies in general.
Since 1950, the situation has slowly reversed again; the pendulum is once more swinging. The development of new measuring instruments (a by-product of World War II), at first ground-based and then, more important, space-borne, has produced a massive infusion of basic new knowledge about the physical environments of the Moon and planets. Young scientists have again been attracted to planetary studies, not only astronomers, but also geologists, chemists, physicists, and biologists. The discipline needs them all.
We now know that the building blocks for the origin of life are in the cards of physics and chemistry; whenever standard primitive atmospheres are exposed to common energy sources, the building blocks of life on Earth drop out of the atmosphere in times of days or weeks. Organic compounds have been found in meteorites and in interstellar space. Small quantities have been found even in such an inhospitable environment as the Moon. They are suspected to exist in Jupiter, in the outer planets of the Solar System, as well as on Titan, the largest moon of Saturn. Both theory and observation now suggest that planets are a common, if not
invariable, accompaniment of stars, rather than an exceedingly rare occurrence, as was fashionable to believe in the first decades of this century (see pages 192 and 193).
We now have, for the first time, the tools to make contact with civilizations on planets of other stars. It is an astonishing fact that the great one-thousand-footdiameter radio telescope of the National Astronomy and Ionosphere Center, run by Cornell University in Arecibo, Puerto Rico, would be able to communicate with an identical copy of itself anywhere in the Milky Way Galaxy. We have at our command the means to communicate not merely over distances of hundreds or thousands of light-years; we can communicate over tens of thousands of lightyears, into a volume containing hundreds of billions of stars. The hypothesis that advanced technical civilizations exist on planets of other stars is amenable to experimental testing. It has been removed from the arena of pure speculation. It is now in the arena of experiment.
Our first attempt to listen to broadcasts from extraterrestrial societies was Project Ozma. Organized by Frank Drake in 1960 at the National Radio Astronomy Observatory (NRAO), it looked at two stars at one frequency for two weeks. The results were negative. Slightly more ambitious projects are, at the time of writing, being performed at the Gorky Radiophysical Institute in the Soviet Union and at NRAO in the United States. All in all, perhaps a few hundred nearby stars will be examined at one or two frequencies. But even the most optimistic calculations on the distances to the nearest stars suggest that hundreds of thousands to millions of stars must be examined before an intelligible signal from one of them will be received. This requires a large effort covering a sizable period of time. But it is well within our resources, our abilities, and our interests.
The change in the climate of opinion about extraterrestrial life was reflected in 1971 by a scientific conference held in Byurakan, Soviet Armenia, and sponsored jointly by the Soviet Academy of Sciences of the U.S.S.R. and the National Academy of Sciences of the United States. I had the privilege of chairing the U.S. delegation to this meeting. The participants represented astronomy, physics, mathematics, biology, chemistry, archaeology, anthropology, history, electronics, computer technology, and cryptography. The group, which included two skeptical Nobel laureates, was marked for its crossing of national as well as disciplinary boundaries. The conference concluded that the chances of there being extraterrestrial communicative societies and our present technological ability to contact them were both sufficiently high that a serious search was warranted. Some of the specific conclusions that were reached were these:
1. The striking discoveries of recent years in the fields of astronomy, biology, computer science and radiophysics have transferred some of the problems of extraterrestrial civilizations and their detection from the realm of speculation to a new realm of experiment and observation. For the first time in human history, it has become possible to make serious and detailed experimental investigations of this fundamental and important problem.
2. This problem may prove to be of profound significance for the future development of Mankind. If extraterrestrial civilizations are ever discovered, the affect on human scientific and technological capabilities will be immense, and the discovery can positively influence the whole future of Man. The practical and philosophical significance of a successful contact with an extraterrestrial civilization would be so enormous as to justify the expenditure of substantial efforts. The consequences of such a discovery would greatly add to the total of human knowledge.
3. The technological and scientific resources of our planet are already large enough to permit us to begin investigations directed towards the search for extraterrestrial intelligence. As a rule, such studies should provide important scientific results even when specific searches for extraterrestrial intelligence do not succeed. At present, these investigations can be carried out effectively in the various countries by their own scientific institutions. Even at this early stage, however, it would be useful to discuss and coordinate specific programs of research and to exchange scientific information. In the future, it would be desirable to combine the efforts of investigators in various countries to achieve the experimental and observational objectives. It seems to us appropriate that the search for extraterrestrial intelligence should be made by representatives of the whole of mankind.
4. Various modes of search for extraterrestrial intelligence were discussed in detail at the Conference. The realization of the most elaborate of these proposals would require considerable time and effort and an expenditure of funds comparable to the funds devoted to space and nuclear research. Useful searches can, however, also be initiated at a very modest scale.
5. The Conference participants consider highly valuable present and forthcoming space-vehicle experiments directed towards searching for life on the other planets of our solar system. They recommend the continuation and strengthening of work in such areas as prebiological organic chemistry, searches for extrasolar planetary systems, and evolutionary biology, which bear sharply on the problem.
6. The Conference recommends the initiation of specific new investigations directed towards modes of search for signals.
(The complete Proceedings of the conference are published as Communication with Extraterrestrial Intelligence, Carl Sagan, ed., Cambridge, Massachusetts, The M.I.T. Press, 1973.)
Another sign of the increasing acceptability of the search for extraterrestrial intelligence is the recommendations of the Astronomy Survey Committee of the U. S. National Academy of Sciences, which had been asked to summarize the needs of astronomy in the decade of the 1970s. The Committee’s report was the first such national report on the future of astronomy to lay stress on the search for extraterrestrial intelligence–as a possibly important by-product of astronomical research in the near future and as a justification for the construction of large radio telescopes.
Nearer to home, there is an accelerating set of laboratory studies of the origin of life on Earth. If the origin of life on Earth turns out to have been exceedingly “easy,” the chances of life elsewhere are correspondingly high.
There is also a concerted effort in the United States–Project Viking–to land instrumented payloads on the surface of Mars to search for indigenous life forms.
The idea of extraterrestrial life is an idea whose time has come.
28. Has the Earth Been Visited?
By far the cheapest way of communicating with the Earth, if you’re a representative of an advanced extraterrestrial civilization, is by radio. A single bit of radio information, sent winging across space to the Earth, would cost far less than a penny. A radio search for extraterrestrial intelligence seems, therefore, a very reasonable place for us to begin. But should we not examine other possibilities closer to home? Wouldn’t we look silly if we expended a major effort listening for radio messages or searching for life on Mars if, all the while, there was here on Earth evidence of extraterrestrial life?
There are two hypotheses of this sort that have gained a following in the popular literature. The first postulates that the Earth is today being visited by spacecraft from other worlds–this is the extraterrestrial flying saucer or unidentified flying object (UFO) hypothesis. The second also postulates that the Earth has been visited by such spacecraft, but in the past, before written history.
The extraterrestrial hypothesis of UFO origins is a complex subject, powerfully dependent on the reliability of witnesses. A comprehensive discussion of this problem has recently been published in UFO’s: A Scientific Debate (Carl Sagan and Thornton Page, editors, Ithaca, N.Y., Cornell University Press, 1972), in which all sides of the subject have been aired. My own view is that there are no cases that are simultaneously very reliable (reported independently by a large number of witnesses) and very exotic (not explicable in terms of reasonably postulated phenomena–as a strange moving light could be a searchlight from a weather airplane or a military aerial refueling operation). There are no reliably reported cases of strange machines landing and taking off, for example.
Now, there is a range of hypotheses that can be examined in such a way. Let me give a simple example: Consider the Santa Claus hypothesis, which maintains that, in a period of eight hours or so on December 24-25 of each year, an out-sized elf visits one hundred million homes in the United States. This is an interesting and widely discussed hypothesis. Some strong emotions ride on it, and it is argued that at least it does no harm.
We can do some calculations. Suppose that the elf in question spends one second per house. This isn’t quite the usual picture–“Ho, Ho, Ho,” and so on–but imagine that he is terribly efficient and very speedy; that would explain why nobody ever sees him very much–only one second per house, after all. With a hundred million houses he has to spend three years just filling stockings. I have assumed he spends no time at all in going from house to house. Even with relativistic reindeer, the time spent in a hundred million houses is three years and not eight hours. This is an example of hypothesis-testing independent of reindeer propulsion mechanisms or debates on the origins of elves. We examine the hypothesis itself, making very straightforward assumptions, and derive a result inconsistent with the hypothesis by many orders of magnitude. We would then suggest that the hypothesis is untenable.
We can make a similar examination, but with greater uncertainty, of the extraterrestrial hypothesis that holds that a wide range of UFOs viewed on the planet Earth are space vehicles from planets of other stars. The report rates, at least in recent years, have been several per day, at the very least. I will not make that assumption. I will make the much more conservative assumption that one such report per year corresponds to a true interstellar visitation. Let’s see what this implies.