08/01/2011 12:53 pm ET | Updated Oct 01, 2011

Life: Miraculous or Mundane?

Despite the preferred modus operandi of the classical Greeks, there are some science puzzles that can't be solved by just sitting around, sipping retsina and cranking up your brain.

Oh, sure, there are plenty of physicists torture-testing their cerebrums with string theory, despite any proof from the labs that this esoteric geometry really describes the nitty gritty of matter. Similarly, theoreticians enjoy wrestling with the idea that our cosmos is just one of many (parallel) universes. This postulate also looks good on blackboards, but until there's experimental proof, it's merely a seductive suggestion.

In other words, when it comes to science, brain power is not the end of the story. Clever arguments may produce interest and occasionally tenure, but only those ideas that survive the filter of experiment will make it into our grandkids' textbooks.

The truth of this situation is particularly obvious when it comes to searching for life beyond Earth. Is it out there? Is the universe bloated with biology, replete with inhabited planets and moons? Or is our world a magnificent exception, a lone instance of life in a cosmos of incomprehensible size -- a tiny fleck of gold in an endless vista of dross?

The researchers who hunt for extraterrestrial life will routinely brandish two arguments that favor the former point of view: namely, that life is both unremarkable and ubiquitous.

The first is that worlds that could support life (known, in the unimaginative nomenclature of scientists, as "habitable") seem to be commonplace. Exactly how "commonplace" is now being judged by NASA's Kepler telescope, and the jury's still out. But it seems clear that at least a few percent of all stars are orbited by worlds capable of hosting life as we know it. That would imply that our galaxy alone has billions of habitable worlds.

If you wish to claim that all of these are sterile with the exception of our own, then you need to insist that -- no matter how attractive the planet -- biology is very, very difficult to get underway.

Countering that proposition is the second frequent argument of researchers looking for extraterrestrial life, namely that biology appeared very soon in Earth's history. There is strong fossil evidence for single-celled life on this planet going back 3.5 billion years, and clues that life began even earlier. The appealing bottom line here is that life sprang up on our world almost as quickly as it could, suggesting that the genesis of life doesn't require very special circumstances. If you can do something quickly, it's probably not a difficult project.

But this latter argument has been challenged in a recent paper by cosmologists David Spiegel and Edwin Turner (both of Princeton University). They've applied what's called Bayesian statistics to evaluate the significance of the early appearance of life on Earth. The Bayesian approach is appropriate in evaluating situations where there's some sort of observer bias. In this case, the bias is that those trying to estimate the probability of life's origin (namely us) wouldn't be here to make the estimate if Earth hadn't spawned life quickly.

Spiegel and Turner's conclusion is that we shouldn't draw too much encouragement from biology's speedy appearance on terra firma. Life on other worlds, even habitable worlds, could be rare -- possibly extremely rare.

In some sense, you could liken this to a lottery grand prize winner trying to evaluate the import of his win. "I bought a lottery ticket and won first prize. So clearly, that's something that must happen frequently."

Well, maybe not. Similarly, the fact that Earth quickly spawned biology cannot be used to endorse the idea that, if I gave you a million other Earths, many of them would soon be paved with protoplasm.

Even without the sophisticated analysis used by Spiegel and Turner, the uncertainty they point out should be obvious. The problem is that we have only one example: Earth. Generalizing from one case of anything is always fraught with danger. Imagine having a café conversation with a bunch of Florentine friends in the year 1600, debating the following: "Do you think it's in the nature of planets to have moons?" On the one hand, you might say "well, the Earth has a natural satellite, so obviously moons are mundane." Conversely, it could be that there's something special about our world. Of course, as soon as Galileo discovered satellites around Jupiter, the whole argument ended -- moons were, as it turned out, as common as crickets.

The critical test was to have more than one example.

So this brings us inevitably back to the fundamental processes of science. While it's pleasant and often enlightening to consider what might be, only the lens of experiment will reveal what is.

Consequently, when it comes to life in space, we have to do more than simply consider what's on our blackboards. We have to do the hard work -- we have to search. If we find life in just one other place- - whether it be microbes on Mars or a signal from the stars -- that will solidify our argument that life is commonplace. We'll know that biology is flagrant and frequent.

To quote Fox Mulder, the truth is out there.