Nadia Drake interviews the creator of the formula for predicting how many detectable civilizations exist in the Milky Way

By Nadia Drake

Web edition: November 1, 2011

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Astronomer Frank Drake and Science News astronomy writer Nadia Drake.

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On November 1, the formula for estimating the abundance of extraterrestrial life in our galaxy celebrates its 50th birthday. It’s known as the Drake equation for its creator, Frank Drake, who is also my father.

The equation grew out of my dad’s need to organize a meeting he’d convened at the Green Bank Observatory, in West Virginia. Then 31, he had been thinking for a while about the materials needed to build communicating, extraterrestrial life. He ended up crafting a formula that calculates the number of detectable, intelligent civilizations in the Milky Way galaxy. That equation is now found in most astronomy textbooks.

“It was just something that had been in my mind for months. It wasn’t 'ah-hah,' it was just — to me — obvious,” he recalled during a recent interview with me. [Subscribers to Science News can read the complete Q&A here. It will also be available in the November 7 issue of Science News Prime, the tablet version of Science News available on the iPad.]

To that meeting in Green Bank, my dad invited “everybody in the world” he could think of with a scientific interest in extraterrestrial intelligence — all 12 people. Carl Sagan was there. Otto Struve was there. Melvin Calvin was there.

At the time, looking for aliens still sat on the fringes of science. But in the intervening five decades, searching for extraterrestrial intelligence has moved inside, even taking up residence in people’s homes. As radio telescopes turned a hopeful ear to the stars and optical telescopes kept their eyes peeled for an illuminated alien pulse, a cavalry of citizens armed with personal computers sorted through jumbles of data. A new branch of science — astrobiology — even grew up and dedicated itself to the scientific search for little green men — or at least microbes — and the places they might call home...

But even though the Search for Extraterrestrial Intelligence still runs into roadblocks, I think it’s fair to say that many people eagerly await the tell-tale message hailing us from across the interstellar sea, the signal that will tell us that Earth isn’t an isolated inhabited island alone amidst endlessly quiet worlds.

The equation describing the number of potential callers has remained unchanged in the half century since it was written, but some of its seven terms have evolved slightly different definitions. My dad and I spoke about these more puzzling pieces of the equation, like the vexing “L” — the amount of time for which a civilization is detectable — and “n_e” or the fraction of habitable planets. Wait — planets, or any kind of habitable, celestial body? What about moons like Europa?

“When we talk about the number of habitable planets, what we really mean is the number of habitable bodies. There may be systems where there are more habitable moons than planets,” Dad said during our interview.

Now, each new dispatch from exoplanet hunters sends n_e tiptoeing closer to an answer, as its next-door factor “f_p” — the fraction of stars with planets — has already done.

Maybe in another 50 years, we’ll know how to fill in the values for each of the equation’s terms. And maybe, during those 50 years, we’ll even hear a whisper — or a shout — from across the galaxy.

Science News subscribers can click here to read the full text of Nadia Drake's Q&A with Frank Drake.