8/9/2007, 12:26 PM

Are we alone here in the unimaginably vast cosmos?

Unimaginable as it may be, there is a remarkable and reasonable way to take a crack at imagining an answer to this question. In fact, there's even a well-established scientific discipline to study the origin, evolution, and distribution of life in the universe--it's called exobiology. But closer to home, as recently as November 2006, we may have found a very reasonable place to go looking for life on our neighbor world--Mars, right down the cosmic alley, if you will. We'll get to that a little further down. First ... How to calculate the likelihood of extraterrestrial life A well-known astronomer named Frank Drake proposed a series of approximations back in 1961 to give us a reasonable estimate of the chances of having an extraterrestrial civilization "out there." It's simple. Just use the calculation N = R*fp nefl fi fcL. Easy, right? Here's what it means: Called the Drake equation, this calculation starts with total number of stars in a given part of space, say, our galaxy. At any time in the cosmos, some stars are forming. Others are using up their fuel and dying. Still others create more fusion energy than their gravity can contain; they’re exploding. So, astronomers often estimate the total number of stars using a term associated with a rate of star formation. We call it “R*” (R-star). For the Milky Way, it's around 400 billion; hence the expression, "billions and billions" of stars. (This phrase was used by the extraordinary talk show host Johnny Carson, when he parodied the extraordinary astronomer Carl Sagan.) Then we can consider the percentage or fraction of stars that would also have planets orbiting them. That's fp. Of those fp planets, what portion of them have conditions that could sustain life, akin to the conditions on our own planet? That's ne, ("n sub-e" for "earthlike"). Now, the fraction of planets on which self-aware, or "intelligent" life has evolved: fi. Life on Saturn's moon?How about the fraction of self-aware-ians (sic) that feel like communicating with other civilizations?: fc. Then, an important and perhaps sobering one, the lifetime of such a civilization: L So how many planets could have intelligent life? We write that total number with a capital N. Again, the calculation N = R*fp nefl fi fcL gives us the likelihood of intelligent life "out there." Exobiologists have good estimates for just one of these terms: R* (representing the total number of stars, if you've forgotten). Because all the rest of the terms are still pretty unknown, you can go wild with this equation. Let's say there are and were about 400 billion stars in the Milky Way Galaxy when ours came along. Then, let's say only a tenth of them have planets (which is a low estimate). Of those, let's say only a millionth of them are Earth-like. Looking around, life may not be that unlikely, so let's say of these Earth-like planets, the chances of some kind of life happening are about one in ten. Then only a 10,000th of them have intelligent life. Of those, perhaps a 100th stumble on to radio-wave telescopes. Then let's say, once a civilization comes into being, it doesn't blow itself up, catastrophically disrupt its planet's climate, or lose interest in radio astronomy, so it lasts about 10,000 years. So, let's see: N = (400 billion)(1/10)(1/1,000,000)(1/10)(1/10,000)(1/100)(10,000) = 40 civilizations. That's not that many, seeing as how there is so much space in space. On the other hand, what if these crazy-wild guesses are way off? What if life is very likely? Using clever techniques and telescopes, we have discovered hundreds of planets orbiting other stars, and the search for these planets is in its infancy. What if intelligent civilizations are common? It's easy to use different guesses for each (fudge) factor, recompute this, and estimate that civilizations number not in the dozens, but in the millions. It's quite an exercise. So get this: If there is liquid water on Mars rig