The Drake Equation: How Scientists Estimate Alien Civilizations
Science · 6 min read · By Dario Seretin · Updated
How many civilizations in our galaxy might be able to communicate with us? Nobody knows, but in 1961 astronomer Frank Drake wrote down a simple equation that breaks the question into smaller, more answerable pieces. The Drake equation is not a prediction. It is a framework that shows what we need to learn, and it is the natural companion to the puzzle described in The Fermi Paradox Explained.
Where did it come from?
Frank Drake, then at the National Radio Astronomy Observatory in Green Bank, West Virginia, had carried out the first modern radio search for extraterrestrial signals in 1960 (Project Ozma). In November 1961 he organized a small meeting at Green Bank on the subject, and wrote the equation as an agenda for it. Participants included astronomer Carl Sagan and other pioneers of the field. The group's own rough estimates at the time suggested the number of communicating civilizations in the galaxy could range from a handful to many millions, which shows how loose the inputs were.
The equation
In its usual form:
N = R* × fp × ne × fl × fi × fc × L
where N is the number of civilizations in our galaxy whose signals we could detect. Each factor is one link in the chain:
R*: the average rate of star formation in the galaxy, in stars per year.
fp: the fraction of those stars that have planets.
ne: the average number of planets per such star that could support life (for example, within the habitable zone).
fl: the fraction of those planets where life actually appears.
fi: the fraction of life-bearing planets where intelligence develops.
fc: the fraction of intelligent species that develop technology that releases detectable signals into space.
L: how long, in years, such a civilization keeps sending detectable signals.
What do we know about each term?
The first factors are astronomical, and science has made real progress on them:
R* is estimated at a few solar masses of new stars per year, which is roughly one to a few stars annually in the Milky Way.
fp was almost unknown in 1961. Since the first exoplanet around a Sun-like star was found in 1995, surveys such as NASA's Kepler mission have shown that planets are very common. More than 5,000 exoplanets are now confirmed.
ne is being measured. Kepler data suggest that a meaningful fraction of Sun-like stars host rocky planets in the habitable zone, though the exact percentage is still debated.
The remaining factors are biological and social, and here we have no data beyond one example: Earth.
fl: Life appeared on Earth relatively early, which some take as a hint that it may be common, but one case cannot establish a rate.
fi and fc: We do not know how likely intelligence or technology is to evolve.
L: This is the most uncertain of all. A civilization might last a few centuries or millions of years. Human radio technology has existed for only about a century.
Why the answer can be anything
Plug in optimistic values and N comes out in the thousands or millions. Plug in pessimistic ones and N can drop below one, meaning we are alone in the galaxy. Because the biological terms are unknown by many orders of magnitude, the equation cannot give a trustworthy single number today, and scientists do not claim that it can. Its value is in showing where the uncertainty lies and which measurements would narrow it.
Updates and criticism
As exoplanet research matured, some astronomers proposed focusing on the part of the problem that is now measurable. The "Drake-like" approach by astrophysicist Adam Frank and Woodruff Sullivan, for example, asks a different question: instead of asking how many civilizations exist now, it asks how small the chance per habitable planet would have to be for humanity to be the only technological species that has ever arisen. Their analysis suggests that the odds per planet would need to be extremely low for us to be the first, which is an interesting reframing, though it still rests on assumptions.
What would change the picture?
Finding evidence of independent life, even microbes, on Mars, Europa or Enceladus would raise our estimate of fl dramatically.
Detecting biosignature gases in the atmospheres of exoplanets would tell us more about how common life is.
A confirmed signal from a technological source would transform the equation, but nothing like that has been verified. The famous 1977 "Wow!" signal was never repeated and remains unexplained.
Key takeaways
The Drake equation multiplies seven factors to estimate the number of detectable civilizations in the Milky Way.
Astronomy has largely filled in the first three factors; the biological and social ones remain unknown.
The result can be anything from zero to millions, so it is a thinking tool, not a forecast.
Sources and further reading
Facts in this article are drawn from public scientific sources. For primary information, see: