## The Truth is Out There

Are we alone? It is, perhaps, one of the most significant questions human beings have ever asked—right up there with “Why are we here?” and “How did it all begin?”

Indeed, as soon as we understood that the universe was not circumscribed by the Earth’s horizons, but extended outward for unfathomable distances and contained within its compass innumerable worlds like our own, we began to wonder whether we are unique and alone, or if there might not be others out there—like us, and yet very unlike us.

The famous “Drake equation,” formulated by astronomer Frank Drake in 1961, sought to establish a mathematical, probabilistic framework to understand the question of whether or not humanity is really alone in the cosmos; it used a number of ingenious terms to estimate the number of technological civilizations in our galaxy.

The problem with Drake’s formulation was that three of those terms, in particular, were just too uncertain to permit a reliable estimate.

According to Adam Frank, astronomy and physics professor at the University of Rochester, and a coauthor of the paper (appearing in the journal *Astrobiology*): "We’ve known for a long time approximately how many stars exist. We didn’t know how many of those stars had planets that could potentially harbor life, how often life might evolve and lead to intelligent beings, and how long any civilizations might last before becoming extinct."

## Setting Some Limits

Recent exoplanet research, particularly with the Kepler Space Telescope, has constrained the first of these terms, determining that about one-fifth of stars possess planets within their habitable zones.

That leaves the second and third terms—the probability for advanced life to evolve, and the longevity of technological civilizations—but Frank and his colleague, Woodruff Sullivan of the University of Washington, simply altered the math a little. They eliminated the second term by calculating the odds against humankind being the only advanced civilization in the universe; and they dismissed the third by formulating a “cosmic archaeological question”—how often does intelligent life evolve throughout cosmic history?

Their new equation, which they call the “Archaeological form” of the Drake equation, looks like this—N* _{ast}* x f

*.*

_{bt}N* _{ast}*, the number of habitable planets, is defined as N

*= N* x f*

_{ast }*x n*

_{p}*, where N* is the total number of stars, f*

_{p}*is the fraction that form planets, and n*

_{p}*is the average of those planets circling in the habitable zones of their parent stars. The second term of the Archaeological form equation, f*

_{p}*, is defined as the likelihood of an advanced technological civilization arising on one of these habitable planets.*

_{bt}The results suggest that humankind is only likely to be unique if the odds of another civilization developing on a habitable world are less than one in 10^{22}.

That’s a very—some might say improbably—small number.

“To me, this implies that other intelligent, technology producing species very likely have evolved before us,” says Frank. “Think of it this way: before our result you’d be considered a pessimist if you imagined the probability of evolving a civilization on a habitable planet were, say, one in a trillion. But even that guess, one chance in a trillion, implies that what has happened here on Earth with humanity has in fact happened about 10 billion other times over cosmic history!”