In five minutes: the Drake equation
The Drake equation is a way of asking how many civilizations in the Milky Way could be sending signals that we might pick up. Frank Drake, a radio astronomer, wrote it in 1961 as an agenda for a small meeting at the National Radio Astronomy Observatory in Green Bank, West Virginia. About ten scientists attended, including a young Carl Sagan.
Drake had already tried the search itself. In 1960 he pointed a radio telescope at two nearby Sun-like stars, Tau Ceti and Epsilon Eridani, in a project called Ozma. It heard nothing artificial. So the question for 1961 was how to think about that silence: how likely is it that anyone is out there at all?
His answer was not a number but a chain. Multiply the number of stars born each year, the share with planets, the number of those planets that could hold life, and so on, until you reach N: the number of civilizations in the galaxy that are communicating right now. Every link is a separate question that a different kind of expert can work on, from astronomers and chemists to biologists and historians.
The short version: the equation is a map of what we do not know. The links about stars and planets are now measured fairly well. The links about life, intelligence and how long a civilization lasts are still guesses, so N can come out anywhere from far below one to millions. The question it frames has been a favourite of writers too, and it runs through the history of science fiction.
The seven terms, one by one
The equation is N = R* × fp × ne × fl × fi × fc × L. In plain words:
- R*: the rate at which new stars form in our galaxy, in stars per year. Current estimates are a few stars a year.
- fp: the fraction of stars that have planets. Planet surveys now suggest it is close to one: most stars have at least one.
- ne: the average number of planets per planetary system that could support life, for example rocky worlds at a distance where water can stay liquid.
- fl: the fraction of those planets where life actually begins. We know one example, Earth, so this one is still a guess.
- fi: the fraction of life-bearing planets where intelligent life appears.
- fc: the fraction of intelligent species that build technology able to send a detectable signal into space.
- L: how many years such a civilization keeps sending signals we could detect.
The last term is the most sobering. N counts only civilizations that are active at the same time as we are. If a technological society lasts a few centuries before it collapses or goes quiet, there may be very few overlapping with us, even if many have existed. For comparison, humans have leaked radio signals into space for roughly a century.
Try it with numbers
These inputs are our own illustration, not published estimates. First, a cautious set: two stars a year, nearly all with planets (1), 0.2 habitable planets per system, a 10% chance of life, a 1% chance of intelligence, a 10% chance of detectable technology, and a lifetime of 1,000 years. Multiply them and N is 0.04. Most likely, we would be alone.
Now a generous set: three stars a year, planets everywhere (1), 0.4 habitable planets per system, life always appears (1), intelligence half the time (0.5), technology half the time (0.5), and a lifetime of 100,000 years. N is 30,000.
Both sets use values a scientist could defend today, yet the answers differ by a factor of about 750,000. That gap is the point of the equation. It shows that the debate is not about the formula, but about three or four numbers nobody can measure yet.
The real science
When Drake wrote the equation, no planet outside our Solar System was known. The first one around a Sun-like star, 51 Pegasi b, was found in 1995 by Michel Mayor and Didier Queloz, who shared the 2019 Nobel Prize in Physics for it. NASA's Kepler telescope, launched in 2009, showed that planets are common, and the count of confirmed exoplanets passed 5,000 in 2022. How many Sun-like stars host an Earth-sized planet in the habitable zone is still debated: published estimates run from a few percent to a few tens of percent, depending on the method. So R*, fp and part of ne have moved from guesses to measurements.
The biological terms have not. Earth is about 4.5 billion years old, and life appeared fairly early in that time. Some researchers read this as a hint that life starts easily. But one example cannot fix a rate, and the step from simple life to technology took billions of years here. We have no second case to compare.
Two modern papers changed how scientists use the equation. In 2018, Anders Sandberg, Eric Drexler and Toby Ord at Oxford's Future of Humanity Institute argued that we should not plug in single best guesses. Instead, feed in the full range of uncertainty for each term. When they did, a large share of the probability landed on a galaxy with no other civilization in it, and they argued this dissolves much of the puzzle of why we see no one. In 2016, Adam Frank and Woodruff Sullivan turned the question around: skip the early terms and ask how rare technological life would have to be per habitable planet. Their answer is that unless the chance is below roughly one in a trillion trillion, humans are not the only technological species that has ever arisen in the universe. This is a statement about history, not about who is out there now.
These ideas connect directly to the Fermi paradox, which asks why we hear nothing if the equation allows many civilizations, and to the Great Filter, which proposes that one of the steps in the chain is extremely hard.
Where it shows up in fiction
Carl Sagan's novel Contact (1985), later filmed in 1997, is built around the kind of radio search that Drake began, and Sagan was among the people who brought the equation to a wide audience. Liu Cixin's The Three-Body Problem gives one famous answer to the L term: civilizations may stay silent because broadcasting is dangerous. That idea, the dark forest, is explained further in our other sci-fi guides, and we avoid its plot here.
Andy Weir's Project Hail Mary (2021) treats the meeting of two very different intelligences as a practical problem to be solved, which is a good picture of what a fc and L conversation might actually look like. Mild spoiler: the novel is about first contact. You can find our guide at Project Hail Mary.
Common misconceptions
- "It predicts how many aliens exist." No. N counts only civilizations that are sending detectable signals now. Microbes, silent societies and extinct ones are not included.
- "It is a law of physics." No. It is a bookkeeping device. Only some of its terms have been measured, and the formula itself makes no prediction until you choose the inputs.
- "With so many stars, aliens must exist." Not necessarily. If the chance of life starting, or of intelligence evolving, is tiny enough, even hundreds of billions of stars can add up to zero.
- "We have heard nothing, so no one is there." Not yet a safe conclusion. Searches so far have covered only a small part of the sky, frequencies and time that a thorough search would need.
Read next
Start with the Fermi paradox, which asks the same question from the other side: if the equation allows many civilizations, where are they? Then read the Great Filter for one explanation of the silence. For a story that puts first contact on the page, try our guide to Project Hail Mary.
