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Recently, many residents of Euclid, Ohio, reported seeing a UFO night after night, hovering over the western horizon. Like any UFO sighting, this one got me really excited. Not because of all the potential probing going on, but because you just know that Phil Plait—an astronomer who worked on the Hubble Space Telescope, and who keeps a very active blog at Discover.com—was going to tear them a new one.

Mr. Plait hates, I repeat hates, when UFO sightings get credulously reported in the news. They make him explode, and this time, as always, it’s a treat. What riles a lot of people like him up is that, to borrow his example, “very few astronomers... report UFOs.” The experts, those who are in the know if anyone is, are often conveniently ignored to keep a story palatably one-sided. (His verdict, by the way: the people of Euclid—a town named for the genius who basically invented geometry—are looking at the planet Venus.)

Just because astronomers find no evidence of aliens, though, doesn’t mean there aren’t any. Humans just haven’t seen them yet. What are the odds that we will? To get a sense, we’ll examine a sliver from the history of statistics: the Drake Equation.

In 600 BC, Thales of Miletus—the so-called “father of Western philosophy”—argued that an infinite universe must contain infinite possible worlds. This, for all we know, is the first reference to extraterrestrials. The universe may not be infinite (I’m pretty sure that would imply infinite energy, which violates the laws of physics), but from then to now humans have often wondered: who is out there, and will we hear from them?

Enrico Fermi, a brilliant physicist and future Nobelist, turned the question around in 1951: IF someone is out there, why HAVEN'T we heard from them? This question is now called the “Fermi Paradox.” It weighs the apparent likelihood of extraterrestrial life somewhere in the universe against the total lack of evidence of any. Fermi was a very talented estimator; he famously ballparked the kilotonnage of the first nuclear blast by sprinkling shredded paper on the wind. Fermi considered alien life pretty likely, given the 70,000,000,000,000,000,000,000 stars in the universe, and assuming Earth is a common type of planet (which is debatable). But he never put a number on it.

In 1961, Frank Drake tried to. An astronomer and astrophysicist, he created an equation for a meeting of astronomers in West Virginia. The meeting inaugurated SETI, the Search for Extraterrestrial Intelligence (you can join the search from home).

This is his equation: N = R* x fp x ne x fℓ x fi x fc x L

N is what we want to know: the number of civilizations we might communicate with, or just get evidence of, in the Milky Way.

Everything else is what ‘N’ depends on. If any of these equal 0, ‘N’ must equal 0:

R* = The rate that new stars are created in our galaxy.

fp = The percent of those stars that have planets.

ne = The number of those planets that are habitable.

fℓ = The percent of those habitable planets that actually spawn some kind of life.

fi = The percent of those life-spawning planets that give rise to intelligent life, i.e. not just amoebae and protozoa.

fc = The percent of those intelligent life forms that discover ways to transmit signals between star systems.

L = The amount of time those signals are transmitted before they are voluntarily or involuntarily shut off.

Drake’s own estimate looked like this: 10 x (50%) x 2 x (100%) x (1%) x (1%) x 50,000 = 50 civilizations like ours. He never meant this number to be anything more than a reasonable estimate. But stop right there. With unknown, or maybe in some cases unknowable, quantities thrown into the mix, how reasonable can an estimate be?

Is that kind of estimation even possible? Let’s look. From left to right, Drake’s variables get more and more slippery. His estimate for ‘R*’ is solid enough—he said 10 stars are born yearly in the Milky Way, NASA now says 7. He said that half of those stars are likely to have planets, and again his estimate was good—around 40% of stars are thought to have planets, many of which are hot gas giants. With more sensitive instruments, like those on the recently launched Kepler space observatory, we might see more, particularly smaller, more Earth-like worlds.

But the next variable, ‘ne,’ is subject to more disagreement. Drake said probably two planets in an average solar system could sustain life. I don’t claim to be a scientist, but I disagree. Lots of exoplanets have elliptical orbits (and so, unstable temperatures). Their amounts of heavier elements, like carbon and silicon, vary quite a bit. This variable’s value should probably be a little lower. Maybe 1? Or, like, 0.2? But even this is more solid than...

The four remaining variables: they’re in the realm of near-total speculation. What percent of habitable planets are likely to spawn life? Intelligent life? Life aware of long-distance radio communication? Life that lasts long enough to transmit anything?

What?? In each of these slots, it’s almost unfair to pencil anything in except a big, red question mark. After all, we have nothing more than our own planet to go on. Plenty of estimates have been made anyway, high and low, but they’re anecdotal. If anything, the way someone fills in these slots reveals less about the cosmos than about his or her mind. High %s = optimism (about life’s ability to form, to evolve, and to last, or to reach out into the universe). Low %s = pessimism, or at least conservative estimation.

It’s tough, avoiding the second camp. There are just so so so many obstacles standing in the way of life. And even taking basic life as a given (microbes or vertebrates, take your pick), it’s absurd, I think, to estimate further and expect the output to mean something. Of all the species that ever lived on this planet—millions and millions of them—only one has evolved a society, complex technology, abstract thought, and scientific investigation. And pretty recently, at that. On this planet, intelligence and technology are exceptions in the extreme.

Another Zen-like question for you: what ratio of exceptional civilizations would be versed in interstellar communication? Um. Honestly, we can hardly count ourselves. Humans receive signals very skillfully, but transmitting them is a problem of energy. Our strongest radio signals degrade terribly as they travel further from our solar system. (It’s a myth that a shell of TV and radio waves is expanding from our solar system—they degrade to almost nothing in just a few light-years, and the nearest star is over four light-years away.) And then, how long does the “average” civilization last? The only way to begin to know would be to see ours end.

Above all, though, you have to wonder why anyone in the universe would bother trying a transmission. If we were to discover an alien civilization tomorrow, it might take hundreds of thousands or even millions of years for our hello-transmission to reach them. Then, we’d have to sit on our hands just as long to hear anything back from them. If they’re still there. Back-and-forth communication is, currently anyway, wildly impractical.

So what are we left looking for? Probably a “beacon” signal: some distant life’s beam of information about themselves, broadcast in every direction, in the face of crushing odds. Not to start a dialogue—just to prove they were here. It’s a chilling idea. After all, a signal like that is little more than a cosmic grave marker. What good would that do?

Then again, what harm?

It’s pleasurable and intellectually expanding to try looking for other beings, after all, and bored and defeatist not to. Thales and Euclid—mathematicians and reasoners first and foremost—probably would be proud to see us trying. Both were horribly unsatisfied with the then-current explanations for matter, existence, and the universe. And Thales was so obsessed with the heavens that he famously fell into a well while gazing at the stars.

The Drake Equation is a happy exercise in inquiry. Inquiry is, I daresay, always a good thing: even those Euclideans, bless their hearts, are at least using their brains in the spirit of inquiry, albeit in a bassackwards way. Good for them. What matters more than odds is the desire to know the odds. And even if ‘N’ = 0, there’s so much else above us waiting to be seen.

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