The Drake Equation

      Frank Drake devised the "Drake equation" - a formula to calculate how many civilisations might be broadcasting radio signals. This is the Drake equation.

N = R* x fp x ne x fl x fi x fc x L

R* is the rate starbirth.
The number of new stars formed in the Galaxy. This is widely accepted by astronomers to be 10 per year.
fp is stars with planets.
The percentage of those new stars that form planets.
ne is habitable planets.
The number of those planets that are the right distance from their star, and of the right size to be suitable for life.
fl is planets with life.
The percentage of those planets on which life has actually evolved.
fi is intelligent life.
Percentage of planets where life forms have evolved sufficiently for "intelligent" creature to exist.
fc is communicable life.
The percentage of those planets on which intelligent lifeforms have developed technologically so they can communicate with other civilisations.
L is life span of civilisation.
The length of time that a technologically advanced civilisation exists and in which it can send a potentially receive signals.