Drake Equation

The Drake Equation is a probability (possibility) measurement of the existence of extraterrestrial life

formulated in 1961 by Dr. Frank Drake

N = R_* • ƒ_p • n_e • ƒ₁ • ƒ_i • ƒ_c • L
R_* = the average rate of star formation in our Galaxy
ƒ_p = the fraction of those stars that have planets
n_e = the average number of planets that can potentially support life per star that has planets
ƒ₁ = the fraction of planets that could support life that actually develop life at some point
ƒ_i = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
ƒ_c = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space
from Drake Equation (wikipedia)

Drakes estimates:
R_∗ = 1 yr⁻¹ (1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative)
f_p = 0.2 to 0.5 (one fifth to one half of all stars formed will have planets)
n_e = 1 to 5 (stars with planets will have between 1 and 5 planets capable of developing life)
f_l = 1 (100% of these planets will develop life)
f_i = 1 (100% of which will develop intelligent life)
f_c = 0.1 to 0.2 (10–20% of which will be able to communicate)
L = 1000 to 100,000,000 communicative civilizations (which will last somewhere between 1000 and 100,000,000 years)
= between 1000 and 100,000,000 planets with civilizations in the Milky Way Galaxy