Mankind is, at least for the moment, listening for its neighbors again. The decades-old search for extraterrestrial intelligence (SETI) was interrupted last year when funding to operate the 42 radio telescopes in the Allen Telescope Array, with which the SETI Institute of Mountain View, Calif., listens to emissions from space. Private funds and a deal with the U.S. Air Force resurrected the program in December, which is news that gladdened the hearts of many of us hopeful of finding intelligent alien life out there somewhere. How long the SETI Institute can keep the lights on is still unclear.

People will no doubt continue to argue about whether SETI is worthwhile or a waste of money. Even its staunchest proponents acknowledge that the chances of imminent success are low — though the impact would be huge on science and humanity's perception of its place in the cosmos. Meanwhile, its fiercest opponents would have to admit that the money spent on SETI is trivial: the program costs about $2.5 million a year, about 1/30th the cost of a small Learjet or about 1/3,000th of what the world spends on iPads.

What makes the debate particularly curious is that notwithstanding a great many numbers that get thrown around on the topic, estimates of the odds of SETI's eventual success are almost meaningless and beside the point. And in their pursuit of alien intelligence, SETI researchers can find that they have strange bedfellows in certain creationists, who try to make similar arguments — but with less scientific merit.

The odds of E.T.

One of the arguments frequently deployed in defense of SETI research is based on the Drake equation, which was introduced in 1961 and describes the number of alien civilizations with which we might communicate as the product of a variety of factors.

The weakness of the Drake equation, however, is that it's not really an equation at all: it's a loose, hand-waving argument in mathematical form. For most of the time that the equation has been around, no one has known the values for most of those terms — the fraction of stars with planets, the fraction of planets that can support life, and so on — nor has it been certain that the equation accounted for all the relevant variables.

The Drake equation is by no means an essential justification for SETI. Nevertheless, abundant discoveries made in planetary science and astrophysics have started filling some values into the equation, and they at least seem highly favorable for the odds of life being out. In January alone, NASA's Kepler space telescope confirmed the existence of 26 exoplanets in 11 star systems, raising the number of known planets to well more than 700, in addition to thousands of other candidate worlds. The 100 billion stars in our Milky Way galaxy seem to have on average two or more planets, a great many of which are at least loosely earthlike in terms of their size and orbital position within a "habitable zone" neither too close to nor too far from their sun. (The guidelines for scoring planets as "earthlike" are generous, however, because we know nothing about their atmospheres, composition, or water content. Venus and Mars count as highly earthlike, too.) Easily, tens of millions of planets in our galaxy might be capable of supporting life as we know it.

Moreover, even just within our solar system, discoveries about Europa, Titan, and some of the other moons of the giant planets have given scientists reason to think that they might be able to support at least primitive forms of life — perhaps cells living in iced-over seas warmed by volcanic heat. That possibility didn't even figure into the Drake equation, so maybe the potential abodes of life are even more numerous than calculated.

The counterargument from biology

Then again, compelling as the math from astronomy might be, biology might be less forgiving. Science can't even yet set meaningful numbers against how likely or not different stages in the evolution of intelligent life are. For example, as biochemist and science writer Nick Lane of University College London explained recently in a brilliantly lucid lecture, evidence suggests that the establishment of complex cells may have been an improbable "freak accident" even on this planet, given that it seems to have evolved only once in the past four billion years. (Lane's talk can be watched here on YouTube.)

As Lane says, the universe might be chock full of bacteria, with earth as the only oasis of complexity and intelligence. We just don't know.

Creationists like to spin their own version of the argument about life's improbability. (Just to head off any misunderstandings, I hasten to add that Nick Lane is most emphatically not a creationist in any way.) The creationists sometimes say that life's existence on earth can only be the result of "intelligent design" that created it. Yet they typically get the math wrong by ignoring how much natural selection can sometimes make long-shot outcomes almost inevitable. In effect, evolutionary biologists like Lane are weighing the odds that life as we know it could have evolved more than once; creationists deny that it could have evolved even once (despite the obvious evidence that it did).

Signals from stars, messages on the moon

The only way to get clarity on what life might be out there is to look for it. In principle, it should be possible to construct telescopes (and similar instruments) large enough to resolve details about the atmospheres around extrasolar planets — such as whether they hold an abundance of free oxygen, which might be indicative of a process like photosynthesis. But they would still offer at best indirect evidence of life. If we want to find intelligent life, we have to look for it specifically (or hope that it finds us).

The radio telescope-based SETI effort currently represents the best shot at doing so. Notwithstanding its tenuous funding, SETI has been able to mobilize tremendous resources within the big community of enthusiasts among the public: programs such as SETI@home and setiQuest allow ordinary citizens to help process the vast amounts of data pulled in by the telescopes to determine whether any artificial, extraterrestrial signals hide within it.

But sifting through radio signals isn't the only way to search for evidence of E.T. Some scientists have suggested, for example, that extraterrestrials interested in reaching out to us would be more likely to use focused lasers or spacecraft to carry their messages rather than broadcasting them in all directions.

A particularly interesting recent proposal along these lines came last November from Paul Davies and Robert V. Wagner at Arizona Statue University.  In the online journal Acta Astronautica, they noted that any aliens who had visited earth might have left their calling cards on the moon, where they could endure for millions of years. As with the SETI@home program, they suggested, the public could be enlisted to pore through the hundreds of thousands of detailed photos of the moon's surface returning from NASA's Lunar Reconnaissance Orbiter for signs of alien artifacts, beacons, or messages.

Part of the challenge in such a project is likely to be the difficulty of recognizing the product of an alien mind. Hypothetical aliens with radically unusual body structures, unfamiliar senses, and completely nonhuman minds might have very different ideas of what constitutes a conspicuous message, and their notions of design might be unrecognizable to us. SETI researchers often trust that extraterrestrials would open a conversation with us by using universal mathematical signals, and that does seem like a reasonable assumption. Nevertheless, it is ultimately just an assumption, and actual aliens could surprise us.

Conversely, because we humans are virtually hardwired to perceive patterns, we can fool ourselves into finding evidence of design and intention in random phenomena. (The Face on Mars, anyone?) Davies and Wagner's idea sounds intriguing, but anyone organizing such a crowd-sourced SETI effort should be prepared to get a lot of false positive reports.

The watchmaker and the astronomers

The problem of distinguishing natural phenomena from artificial ones has always been dear to the hearts of intelligent design creationists. Their key argument, after all, goes back to the "watchmaker analogy" first posed in 1802 by William Paley in his book Natural Theology. Anyone coming upon a pocket watch in a field, he wrote, would recognize it as a made object from its complex features rather than a naturally occurring one like a stone. The existence of a watchmaker was therefore implicit. Paley argued that the complex features of life similarly implied the existence of a divine creator.

Modern I.D. creationists point to various intricate features of life, such as bacterial flagella, and make the same argument. Orthodox science rejects their conclusion because evolution through natural selection provides an alternative way for complexity and order to emerge without any directing intelligence. Some I.D. creationists nevertheless consider science's tolerance for SETI as evidence of hypocrisy: how can scientists simultaneously say that certain radio signals or design features on objects would be evidence of aliens while dismissing the idea that even more elaborate sets of features in living things are evidence of some other creator?

The answer is that in any situation, the purported evidence needs to be tested against alternative theories for its origins. The theory of evolution provides mechanisms for living things — which have heritable traits that affect their reproductive fitness — to develop complex adaptive traits without guidance. SETI researchers look for radio signals from space that would contain meaningful mathematical patterns that no known natural phenomenon would be likely to produce. And if natural phenomenon capable of producing them were found to exist, scientists would drop it as an unambiguous indicator of intelligence. Similarly, the lunar environment has no known processes that could explain the existence of, say, a slablike monolith out of 2001: A Space Odyssey or a mathematically revealing pattern cut into a crater floor. Moreover, the identification of such things as evidence of aliens would be provisional, pending further attempts to disprove them. I.D. creationists generally show less interest in trying to disprove their own discoveries than in insisting that the rest of the edifice of biology needs to be torn down to accommodate them.

Addendum: In the unlikely event that SETI research succeeds soon, let us all hope for more than The Onion foresees: "Intelligent, Condescending Life Discovered In Distant Galaxy."

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Image: Allen Telescope Array. (Credit: Seth Shostack, SETI Institute)