Is SETI—the Search for Extraterrestrial Intelligence—a religion? This is one of the topics that Jill Tarter, Director of the Center for SETI Research at the SETI Institute, and I discussed on "Are We Alone?", the SETI Institute's weekly radio program on Wednesday May 17.

The discussion by Jill and I was in response to a claim made by George Basalla (professor emeritus of history at the University of Delaware) in his book Civilized Life in the Universe (Oxford University Press: 2006) that SETI is more of a faith-based enterprise than a genuine science. He points to SETI's failure to make "contact" after more than forty years of trying and its continuing efforts in the absence of any positive evidence as a sign that it relies more on a kind of religious zeal than anything else. (Incidentally, Basalla was invited to appear on the show but declined.)

Needless to say, Jill Tarter is less than impressed by this argument, as indeed am I. Firstly we know that there's intelligence in the universe. As I pointed out on the show there are dolphins and great apes. And you might even throw Homo sapiens into that mix on the rare occasions when we live up to our self-proclaimed species name. It isn't an unreasonable hypothesis that if intelligence has come about on one planet that it may also have arisen elsewhere, especially given the vast number of stars in this and other galaxies. SETI serves as a test of that hypothesis. But beyond that it's one of our noblest and most exciting scientific quests: to discover if we are alone and represent the high-water mark of intelligence and technology in the cosmos or, alternatively, if we're simply one member of a community of minded races, many of them perhaps vastly more ancient and advanced than ourselves.

Religions are characterized by two factors: worship—in other words, some system of devotion directed toward one or more omniscient and supranatural beings—and faith in the absence of material evidence. SETI qualifies as a religion on neither of these counts. Unless I'm very much mistaken no SETI researcher offers prayers to the subject of his or her quest (although it would be fascinating to know what spiritual traditions might have grown up among the civilizations of other stars). And any faith that's involved in SETI is only the kind of non-religious "faith" that any scientist adheres to—faith in the scientific method, the equipment she uses, the all-important peer review process, and so on. As I've mentioned, we already have material evidence for intelligence in the universe: it consists of the brains you're using right now to assimilate these thoughts. Unlike a religion which relies on pure faith that a god exists, we don't need faith that intelligence and technology exist.

To address Basalla's argument, that it's time for SETI advocates to lower their expectations and even admit they may be on a wild goose chase, I'd like to point to a parallel with the search for extrasolar planets - worlds that are in orbit around other stars. Until quite recently we had no evidence for planets beyond our own solar system; it was simply a hypothesis, like the hypothesis that there may be ETI. The practical search for extrasolar worlds kicked off back in the 1930s with the pioneering work of the Dutch-American astronomer Peter Van der Kamp. Although he collected data that seemed to suggest there were worlds in orbit around Barnard's Star and a few other nearby stars, this evidence proved to be unfounded (some of it due to tiny systematic wobbles in the telescope he was using). Only in the 1990s, sixty years after Van der Kamp began his investigations, did scientists find conclusive proof that there are other planets out there. Over the past decade or so, more than 180 extrasolar planets have been found.

If we were to follow Basalla's line of reasoning, the search for extrasolar planets also qualifies as a kind of religion. Shouldn't we simply have given up after four decades of looking? Surely that's enough time to have found something if it really existed? Isn't continuing beyond that a sign of misplaced faith and over-optimism? Fortunately the quest did go on and we're now reaping the rewards—new planets by the bucket-load.

Historically, the question of whether extrasolar planets existed and, if they did, how common they were and what they might be like, finds an interesting parallel with the central issues in SETI. There used to be two big theories about the origin of the planets in the solar system. One of these was called the catastrophic hypothesis.

It suggested that the planets had formed in the aftermath of a near collision between the Sun and another star from a swathe of gas ripped out of the Sun by the stellar intruder. If this were the case then planetary systems could be expected to be very rare because such close encounters between stars almost never happen. The rival theory of planet formation was the nebular hypothesis which argued that the planets of the solar system coalesced from a cloud of gas and dust left over from when the Sun was formed. The nebular hypothesis suggested that the birth of planets might be a routine business throughout the universe. Of course, this is the theory, in updated form, that astronomers believe in today and the discovery of numerous other planets is good confirmation of it.

The parallel debate going on in SETI and astrobiology concerns how often primitive life, such as bacteria, serves as the precursor of complex, multicellular life, and, ultimately, advanced intelligence. Supporters of the "Rare Earth" hypothesis think that it happens only very, very rarely. Others, including myself, think that intelligence offers a big survival advantage and that it will come about whenever it's given a reasonable chance. SETI is a first step towards resolving this issue. But it still has a very long way to go. Forecasting how intelligence will evolve is a hazardous business. We don't have much to go on. What we do know is that as soon as high technology takes hold, evolution is fantastically rapid and virtually unpredictable. Does anyone have a clue how the Internet or genetic engineering are going to develop over the next 10, 20, or 50 years? How about the next million years?

SETI researchers know their limitations. They're restricted at present to searching for radio and optical signals—our own best, fastest means of getting messages across interstellar distance. Who knows what our galactic elders, if they exist, may be using to communicate with? We have no idea what is out there or what forms alien intelligence may take. We are, as Seth Shostak pointed out during the radio interview, like Columbus sailing into uncharted waters. We don't know what we'll find. But the quest is extraordinary, exciting, abundantly worthwhile, and true to the methodology and spirit of science.

• VIDEO: Figuring Out the Odds of ET Existing
• VIDEO: Reflections of Fermi's Paradox
• VIDEO: Listening for Life
• GALLERY: Scenes from SETI at Arecibo