Drake Equation
Estimate the number of detectable civilizations in the Milky Way.
A framework for thinking about extraterrestrial life.
The Formula
N = R* × fp × ne × fl × fi × fc × L
The Drake equation estimates how many communicating civilizations might exist in our galaxy. Each factor narrows the estimate from total stars down to detectable civilizations.
Variables
| Symbol | Meaning |
|---|---|
| N | Number of detectable civilizations in the Milky Way |
| R* | Rate of star formation per year |
| fp | Fraction of stars with planetary systems |
| ne | Number of habitable planets per system |
| fl | Fraction of habitable planets where life develops |
| fi | Fraction of life-bearing planets with intelligent life |
| fc | Fraction of intelligent civilizations that develop detectable technology |
| L | Length of time such civilizations emit detectable signals (years) |
Example 1
Using optimistic estimates
R* = 1.5, fp = 1, ne = 0.4, fl = 0.3, fi = 0.1, fc = 0.1, L = 10,000
N = 1.5 × 1 × 0.4 × 0.3 × 0.1 × 0.1 × 10,000
N = 18 civilizations
Example 2
Using conservative estimates
R* = 1.5, fp = 0.5, ne = 0.2, fl = 0.1, fi = 0.01, fc = 0.01, L = 1,000
N = 1.5 × 0.5 × 0.2 × 0.1 × 0.01 × 0.01 × 1,000
N = 0.0015 (suggesting we may be alone in the galaxy)
When to Use It
Use the Drake equation when:
- Framing discussions about the probability of extraterrestrial life
- Understanding the key factors that affect the existence of civilizations
- Exploring how changes in individual factors affect the overall estimate
- Teaching probability and estimation thinking