Kepler informs Drake: Plugging Real Numbers into the Equation

The question, “Are we alone in the universe?” penetrates to our very cores. When we look up at the night sky, is there anyone gazing back? Or are we set adrift in the cosmic ocean all by our lonesomes? If there are others beyond our sight, wondering the same thing, how many are there? Are they close enough to hold a conversation, or would the interstellar service provider drop the call? Can we ever know the answers to these humbling and vexing questions?

In 1961, astronomer Frank Drake aimed to reduce the uncertainty by putting numbers on the important planetary parameters. In preparation for a meeting commissioned by the National Academy of Sciences, a gathering that would set the stage for the now-famous radio wave investigation, the Search for Extraterrestrial Intelligence (SETI), Drake jotted down a list of things you’d need to know in order to determine how many potential pen pals we could expect in our galactic neighborhood. The eponymous Drake Equation was defined thusly:

Drake equation ,

where “N” is the number of alien civilizations within communication range (AKA “the thing we wanna know”) and the other factors are as follows:

R* = the average rate of star formation per year in our galaxy
fp= the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fl = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space

Again… this was in 1961. Four years before the best evidence of the Big Bang was stumbled upon, eight years prior to men walking on the moon, and more than a decade preceding the first conclusive identification of a black hole. Where was Frank Drake getting estimates for these values? Why did he assume that technologically advanced civilizations eventually die out? What if they colonized other planets; how would that affect the numbers? The lack of precision and utter guesswork inherent in this formulation did not go unmentioned, leading some, such as “xkcd” cartoonist Randall Monroe to proffer cheekier versions.


Drake’s equation may have taken form solely as a way to kick off discussion, but ya just can’t help but try to fill it out, can ya? With their limited knowledge, Drake and his colleagues plugged in numbers and chugged out a range of values between 1,000 and 100,000,000 communicating civilizations in our galaxy. So yeah, lots of uncertainty still, especially when the numbers you’re picking are plucked out of pure speculation to begin with. But what happens when you apply 50 additional years of astronomical knowledge? Can modern discoveries help refine Drake’s bullshit equation?

The original (conservative, they thought) estimate for R*, the average rate of star formation in the Milky Way, was put at about 1 per year. Using some tricky techniques of the INTEGRAL gamma-ray observation satellite in 2006, NASA and the European Space Agency were able to jack that number up to 7. Later studies with the Spitzer Space Telescope further pare that down to a single star like our sun annually. Hey, not a bad guess on that one. When you consider that the rate of stellar formation was higher in the past (back when a civilization would have to start out to get to a communication stage by now), things might look even better than initially expected!

The next term, fn, the fraction of stars that have planets, is something that couldn’t be honestly addressed until very recently. The original shot-in-the-dark supposals were between one fifth (0.2) and one half (0.5). The first “exoplanet” was confirmed in 1995, but it was only with the launching of the Kepler Space Telescope in 2009, however, that we gained the ability to identify smaller, rocky planets (like our own) by observing the dip in starlight from their parent stars as they cross in front. Kepler has provided a flood of planetary candidates, almost 3,000 of ‘em, leading to some bold predictions, such as that of Harvard-Smithsonian Center for Astrophysics researcher Francois Fressin, that nearly every sun-like star should have at least one planet. Uh, wow. Drake’ll take that, for sure.

Moving further down the chain… is still the realm of speculation. We await the confirmation of Kepler’s current 461 “maybes” that seem to exist within their host star’s so-called “habitable zone,” which would assist in constraining ne, the possible “life-supporting” planets each particular star boasts. A true Earth twin hasn’t been discovered yet, but as Kepler casts its net wider in 2013, many believe that moment is mere months away.


Frank Drake should be a happy man! And the rest of us, too, as we slowly trudge through the components of the equation with continuing data, bringing us closer to answering our fundamental questions. The value of fp, the fraction of habitable planets that do develop life, will be harder to put a figure on. Spectroscopic techniques can determine the components of far-off atmospheres, but none with the telltale markers of respiration have yet been identified. And once you get observational data for fi, the fraction of those planets that develop intelligent civilizations, it’s pretty much game over, right? One would be enough to make us greatly rethink our place in the universe.

But how likely is it? Proponents of the Rare Earth Hypothesis, such as Peter Ward and Donald E. Brownlee, suggest there are other factors affecting the possibility of intelligent life that aren’t accounted for in the Drake Equation and other estimates. Still, the numbers we’ve compiled over the last half-century have to be encouraging to those who dream of the ultimate foreign correspondence.

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