The Drake Equation

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The Drake equation is a single line of multiplication whose product would be N: the number of civilizations in the Milky Way whose signals a radio telescope could, in principle, catch. It has stood in astronomy textbooks for sixty years, and it has never once been solved. The failure embarrasses no one, because solving it was never the point. Frank Drake wrote it out in 1961 as a meeting’s order of business — seven quantities, each a session topic, an intractable question cut into parts small enough to argue about.

The meeting had a prehistory of about a year. In 1960, spurred by Giuseppe Cocconi and Philip Morrison’s 1959 proposal that interstellar civilizations might signal by radio, Drake — a radio astronomer at the National Radio Astronomy Observatory in Green Bank, West Virginia — turned the observatory’s 85-foot Tatel telescope toward Tau Ceti and Epsilon Eridani, two nearby Sun-like stars, and listened for several weeks. Project Ozma, the first organized scientific search for extraterrestrial signals, logged one false alarm — a high-altitude plane — and no aliens; what it proved, in Drake’s own accounting, was that the search itself was workable and worth taking seriously. It also drew attention. The Space Science Board of the National Academy of Sciences asked its staff officer J. P. T. Pearman to assemble a meeting on whether listening was worth pursuing, and Pearman encouraged Drake to organize it. It convened at Green Bank in November 1961, deliberately unannounced; the subject was then fringe enough to endanger careers.

Ten people attended — some accounts say about a dozen. Around Drake and Pearman gathered Philip Morrison himself; the senior astronomer Otto Struve; the radio engineer Dana Atchley; the electronics pioneer Bernard Oliver, who had invited himself; Su-Shu Huang, who had coined the term habitable zone; the biochemist Melvin Calvin, whose Nobel Prize was announced midway through the meeting — champagne had been laid in against the news; John C. Lilly, who held the room with accounts of bottlenose dolphin intelligence and proto-language; and Carl Sagan, the youngest present. By the close the group had named itself the Order of the Dolphin, and Calvin later mailed silver dolphin pins to the members. “It was just a souvenir of the particular time together,” Morrison said afterward. The dolphin science aged worst: Lilly’s claims did not hold up, and Drake came to judge the work poor science.

The agenda itself went up as a line of algebra: N = R* · fp · ne · fl · fi · fc · L. Its terms, left to right: the rate at which suitable stars form; the fraction of those stars with planets; the number of planets per system fit for life; the fraction of fit planets where life in fact appears; the fraction of living worlds that evolve intelligence; the fraction of intelligent species that develop detectable technology; and L, the average years such a civilization remains detectable. The first six terms, multiplied, give the galaxy’s annual production of new transmitting societies; multiplied again by the years each stays on the air, they give the standing population. Drake’s analogy was a university’s enrollment — annual intake times years to a degree. The Green Bank company judged the production rate on the order of one society per year, contracting the whole question to its final term: N approximately equals L. Answers from the same equation run from one — ourselves, alone — to several million. Drake’s own worked example, one new society a year and an average lifetime of ten thousand years, gives ten thousand civilizations; it is sometimes reported as a finding, and it is an illustration.

In 1961, of the seven factors exactly one was known: the rate of star formation. The next two have since crossed from conjecture to measurement — and they are, as its keepers count it, the only terms that have. A 2015 meta-analysis puts the Milky Way’s output at 1.65 ± 0.19 solar masses of new stars per year. The planet fraction stopped being a guess in 1995 with 51 Pegasi b, the first confirmed planet of a Sun-like star; confirmed exoplanets number 6,298 as of mid-2026, 2,784 of them found by the Kepler telescope, and microlensing statistics suggest an average of about 1.6 planets per star — on the order of 160 billion worlds in this galaxy. The Green Bank guesses for the planet terms — nearly all stars, roughly one suitable world apiece — fell within a factor of two or three of the modern values. The count of suitable worlds is now a measurement too, though it disagrees with itself by definition as much as by data: Earth-size planets receiving one to four times Earth’s starlight circle 11 ± 4 percent of Sun-like stars in a 2013 Kepler analysis, while the final 2021 reduction of Kepler’s data finds 0.37 to 0.60 rocky planets per star in a conservatively drawn habitable zone, the nearest of them, statistically, within about twenty light-years. Many of the candidate worlds orbit red dwarfs, whose flares and tides may strip or sterilize them; “suitable” is doing quiet work.

There the movement stops. The fraction of suitable planets where life begins has no measured value; there is no second example of life to measure. The intelligence and technology fractions are guesses about a sample of one. The classic textbook tabulation set life at certainty and the intelligence-and-technology pair at one in a hundred — round numbers chosen, not found — and the observatory that hosted the meeting notes in its own gallery text that the equation mixes “physical and political variables.” The uncertainty swells left to right by construction: the equation runs in order from what astronomy can weigh to what nothing yet can.

It ends at L, and everything hangs there. In the classic parameterization N works out to L divided by ten: if civilizations destroy themselves within a decade of acquiring radio, N is one and the galaxy is in practice empty; if one in a hundred learns to live with its own weapons, N is a million and the nearest neighbor lies a few hundred light-years off. The equation and the astronomy do not move — the answer swings six orders of magnitude on a term no telescope can touch, because L is a claim about conduct, not physics. The one data point in hand is short: humanity has been transmitting at scale for less than a century. The astrophysicist Adam Frank’s 2016 verdict: L remains “still completely unknown.”

The equation’s standing has always been double-edged, and so is its best summary. Jill Tarter, who spent a career searching, called it “a wonderful way to organize our ignorance” — a compliment containing the serious critique: structure is all it supplies. Even its keepers concede it cannot be computed; the observatory where it was written tells inquirers that only estimates exist. It is also narrower than its neighbors: it counts who is transmitting now, while the Fermi paradox asks why, on such expectations, nothing has ever been seen, and the Kardashev scale grades hypothetical civilizations by energy and enters this arithmetic nowhere. The sharpest modern attack arrived in 2018, when Sandberg, Drexler and Ord observed that Drake-style point estimates “implicitly assume certainty regarding highly uncertain parameters.” Multiplying single best guesses manufactures a confidently crowded galaxy; propagating the honest ranges — several spanning many orders of magnitude — leaves substantial probability that humanity is alone in the observable universe, with consequences for the great silence announced in their title, “Dissolving the Fermi Paradox.” Drake, for his part, had always declined proposed amendments, distributions included: “none of these refinements is necessary nor do they alter the equation in any essential and substantive way.” The two positions have not been reconciled. The field holds both.

What the equation has lost as a calculation it keeps as a template. Sara Seager’s version — a deliberate take-off, by her own description — drops radio and intelligence altogether and counts planets with detectable signs of any life: stars surveyed, the fraction quiet, the fraction with rocky habitable-zone planets, the fraction observable, the fraction with life, the fraction whose life exhales a gas a spectrograph could catch. The last two terms, she grants, are “just guesses”; her optimistic worked example yields about two inhabited planets detectable within roughly a decade of TESS and the James Webb telescope. Adam Frank and Woodruff Sullivan’s 2016 reformulation removes L by changing the tense of the question — not how many civilizations exist now, but whether ours is the first ever — at which point exoplanet data pin every astrophysical term, and pessimism acquires a price: humanity is the only technological species ever to have arisen only if the odds per habitable-zone planet fall below about one in 10²⁴. And in 2020 Westby and Conselice ran the logic the other way, calibrating the free fractions to Earth’s own timeline; their headline figure of at least 36 communicating civilizations in the galaxy was widely reported as a discovery and is in fact a conditional lower limit — contingent on Earth-like development wherever conditions allow and a lifetime of exactly one century, with the nearest such world up to seventeen thousand light-years off and undetectable for the foreseeable future.

Outside the journals the equation leads a second life. A plaque marks the wall of the Green Bank room that held the blackboard where it was first written; the SETI Institute, which built its research program on the equation’s frame, likes to claim that only E = mc² outranks it for fame — a boast with no survey behind it. The comparison that follows is editorial: the seven factors are a chain of being recast as arithmetic — star, planet, life, mind, craft, and, last, conduct. The order is a descent from the measurable to what only history decides, and sixty years of measurement have lit the chain strictly from its stellar end. Two terms have crossed from guess to measurement since the meeting; one was known all along; all three are astronomy. The four nearest the human are still dark, and the last of them is not a fact about the universe at all, but about how long anyone, anywhere, manages to stay on the air.

→ Related: Fermi Paradox · Kardashev Scale · Dyson Sphere · Anthropic Principle