The Drake equation
In 1961, astronomer Frank Drake wrote an equation to organize the discussion at the first meeting on the search for extraterrestrial intelligence (SETI). It estimates N, the number of civilizations in our galaxy whose signals we might detect:
N = R × fp × ne × fl × fi × fc × L*
- R*: the average rate of star formation in the Milky Way (stars per year);
- fp: the fraction of stars with planets;
- ne: the number of potentially habitable planets per planetary system;
- fl: the fraction of those where life actually appears;
- fi: the fraction of those where intelligence evolves;
- fc: the fraction of those that develop detectable technology;
- L: how long such civilizations keep emitting detectable signals, in years.
What we know and what we don’t
The first terms are now measured: the Milky Way forms roughly 1.5 to 3 stars per year, and exoplanet surveys such as Kepler show that most stars have planets, with perhaps 10 to 50% of Sun-like stars hosting a rocky planet in the habitable zone. The last four terms are guesses: we have exactly one example of life, intelligence and technology. Reasonable estimates for N span from far less than 1 to millions.
Example
With R* = 1.5, fp = 100%, ne = 0.4, fl = fi = fc = 10% and L = 10,000 years:
- N = 1.5 × 1 × 0.4 × 0.1 × 0.1 × 0.1 × 10,000 = 6 civilizations;
- spread across a galactic disk 100,000 light-years wide and 1,000 thick, they would be roughly 11,000 light-years apart;
- a radio conversation with the nearest would take about 22,000 years per round trip.
The role of L
The result is proportional to L. If technological civilizations last only a few centuries, N is likely below 1; if they last millions of years, the galaxy could be busy. Some see this as the most important and most sobering term.
The Fermi paradox
If N were large, why have we seen no sign of anyone? This is the Fermi paradox. Proposed answers include a rare step somewhere in the chain (a “Great Filter”), short-lived civilizations, the difficulty of interstellar travel, or the fact that we have only searched a tiny fraction of the sky and radio spectrum.
How to use the calculator
Try optimistic and pessimistic values for each term. The spread of results, often many orders of magnitude, is the real lesson of the Drake equation.