SETI requires a skeptical reappraisal: early SETI efforts were marked by overly optimistic estimates of the probable number of extraterrestrial civilizations in our galaxy. In light of new findings and insights, it seems appropriate to put excessive euphoria to rest and to take a more down-to-earth view. Earth may be more special, and intelligence much rarer, in the universe than previously thought

The possible existence of extraterrestrial intelligence (ETI) has always stirred the imagination of man. Greek philosophers speculated about it. Giordano Bruno was burnt on the stake in Rome in 1600, mainly because positing the likelihood of other inhabited worlds in the universe. Kant and Laplace were also convinced of the multiplicity of worlds similar to ours. In the latter part of the nineteenth-century Flammarion charmed vast circles with his books on the plurality of habitable worlds. But all these ideas were mainly philosophical considerations or pure speculations. It was only in the second half of the twentieth-century that the Search for Extraterrestrial Intelligence (SETI) became a scientifically underpinned endeavor. Since the late 1950s distinguished scientists have conducted research, attempting to receive intelligent signals or messages from space via radio-telescopes. Hundreds of amateur astronomers, members of the SETI-League in dozens of countries, are scanning the sky, trying to detect evidence of intelligent life elsewhere in our galaxy. SETI pioneers, such as Frank Drake and Carl Sagan, held the stance that the Milky Way is teeming with a large number of advanced civilizations. However, the many search projects to date have not succeeded, and this daring prediction remains unverified. New scientific insights suggest the need for a more cautious approach and a revision of the overly optimistic considerations.

The standard argument for the existence of a multiplicity of intelligent life runs like this: There are about 200 to 300 billion stars in our galaxy and probably hundreds of millions, maybe even billions of planets in our galaxy. Many of these planets are likely to be located in the so-called "habitable zone" in relation with their star, enjoying--as Earth--favorable conditions for the evolution of life. The physical laws, known to us, apply also to the cosmos, and far-away stellar formations are composed of the same elements as our solar system. Therefore, it is assumed, many should possess water and a stable atmosphere, considered to be basic requisites for the development of life. Such planets must have experienced geological and biological processes similar to those on Earth, leading to the development of primitive life organisms. Then, in the course of time, following a similar course of Darwin's theory of natural selection, these evolved into more complex forms, some eventually developing cognitive capacities and--as in our case--higher intelligence.

In other words, it is maintained, our solar system, Earth, and its evolution are not exceptional cases, but something very common in our Milky Way galaxy. Consequently it must be populated by a huge number of extraterrestrial civilizations, many of them older and more advanced than ours.

Considering the enormous number of stars and planets, these seem like fair and legitimate assumptions. It indeed appears unlikely that intelligence should have evolved only on our planet. If many of these civilizations are scientifically and technologically superior to us, contact with them would give mankind a boost in many ways.

These optimistic views are based mainly on the famous Drake formula N=[Rf.sub.p][n.sub.e][f.sub.i][[f.sub.j][f.sub.e]L. It considers the formation of stars in the galaxy, the fraction of stars with planetary systems, the number of planets ecologically suited for life, the fraction of these planets, on which life and intelligent life evolves, and those reaching a communicative stage and the length of time of technical civilizations. On the basis of this formula it was estimated that a million advanced civilizations probably exist in the galaxy. The nearest one should be at a distance of about 200 to 300 light-years from Earth. German astronomer Sebastian von Hoerner estimated a number between ten thousand and ten million such civilizations.

But because of many new insights and results of research in a number of scientific fields, ranging from paleontology, geology, biology to astronomy, I believe this formula is incomplete and must be revised. The early optimistic estimates are no longer tenable. A more realistic and sober view is required.

I by no means intend to discredit SETI; the search for extraterrestrial intelligent life is a legitimate scientific endeavor. But it seems prudent to demystify this interesting subject, and to reformulate its claims on a new level, free of the romantic flair that adorns it.

Years ago, I readily admit, I myself was quite taken in by the allegations that intelligence is a very common phenomenon in the galaxy. In books, articles, and on radio and television I advocated the idea that our world, beset by problems, could learn a lot from a civilization more advanced than ours. But, in the meantime, I became convinced that a more skeptical attitude would do reality better justice. There are probably only a few such civilizations in the galaxy, if any at all. The following considerations buttress this rather pessimistic appraisal.