One day in 1950, nuclear physicist Enrico Fermi posed a question to a few colleagues he was lunching with at Los Alamos National Laboratory that would become known as the Fermi Paradox: If the Milky Way is indeed teeming with alien civilizations, as many theories suggest, where are they? Shouldn't we see evidence of their existence? Nearly 60 years later, the question remains just as vexing. After all, decades of searching for extraterrestrial radio signals or evidence of alien civilizations have come up empty.

Nevertheless, search for extraterrestrial intelligence (SETI) programs soldier on. And the hunt for any alien life, even in microbial form, is ramping up quickly with instruments probing Mars and other likely nearby candidates in greater detail and with the regular detection of new planets outside our solar system. In the absence of hard evidence for intelligent extraterrestrial life, some researchers have set out to estimate just how much might be out there. The hope is that they can justify the continuation of SETI searches or even refine them and thus up the odds of finding ET, perhaps someday rendering the Fermi Paradox moot.

In a recent paper published online by the International Journal of Astrobiology, graduate student Duncan Forgan of the Royal Observatory, Edinburgh, in Scotland set up a numerical model of the universe under different scenarios of biogenesis. His model relies on current observational knowledge of stars and planetary systems, as well as some assumptions about the viability of life and its ability to evolve into an advanced, intelligent form. If life can only arise under a narrow set of initial conditions, Forgan estimates there should be 361 advanced, stable civilizations in the Milky Way. If life can spread from one planet to another through biological molecules embedded in asteroids, though, the number jumps to nearly 38,000. (Even given a densely populated galaxy, Forgan notes, there is no guarantee of immediate mutual contact.)

Forgan's model makes use of the Monte Carlo method, by which the starting variables in a system are randomized over repeated simulations to allow for uncertainties in their values. By averaging the results from 100 such simulations, Forgan's analysis yields an estimate that accounts for variations in inputs.

But some in the field argue that estimates of the extent of extraterrestrial intelligence cannot carry any degree of accuracy, given the gaps in our knowledge. Such numerical estimates are "still subject to all the other uncertainties and all the other imponderables" regarding the origins of life, says planetary scientist Ian Crawford of Birkbeck College at the University of London. "We have to admit that we're hugely ignorant of many of the pieces of information that we would need to know before we could realistically estimate the prevalence of intelligent life elsewhere in the galaxy."

Mark Burchell, a professor of space sciences at the University of Kent in England, says that astronomically speaking, our knowledge base is fairly refined. "But the biological and social aspects of the equation remain speculative," he said in an e-mail. "As Forgan points out, we are limited to single-event observations (life on Earth) to make sweeping general predictions (life elsewhere)."

Forgan acknowledges that the analysis suffers from some uncertainties, stemming in part from a small and somewhat biased data set on planets outside the solar system. Some 300 planetary systems have been found since 1995, when the first planet orbiting a normal star other than the sun was discovered. But the detection methods employed in this effort tend to find planets that are quite large and hot. The European COROT satellite and NASA's forthcoming Kepler spacecraft, however, should be able to locate more Earth-like worlds in the coming years with dedicated, sensitive monitoring of dips in stellar brightness that occur when a planet passes in front of a star. Forgan says that "Earth-mass, rocky planets are still the best bet for habitability," so such discoveries would significantly affect his conclusions.

He also notes that the numbers, subject as they are to uncertainties, should not be considered the sole outcome of his paper. Simply refining models of where and when life should arise, he says, might improve SETI searches. "Searching for life in the galaxy is the ultimate needle in the haystack," Forgan says, and any guidance as to where and when to search for that needle should be useful.

But Crawford thinks such analyses won't affect the status quo. "We've got no option but to keep looking; there's nothing else we can do," he says. "All the SETI searches can do is what they've been doing for the last 40 years and keep listening."