The Drake equation, also known as the Green Bank equation and the Sagan equation, is a famous equation devised by Frank Drake that makes an attempt to estimate the number of extraterrestrial civilizations we may come into contact with in the future.
This equation was devised by Frank Drake in 1960. The main purpose of the equation is to allow scientists to quantify the uncertainty of the factors which determine the number of such extraterrestrial civilizations. 
The equation is as follows:
- N is the number of civilizations in our galaxy with which communication might be possible;
- R* is the average rate of star formation in our galaxy
- fp is the fraction of those stars that have planets
- ne is the average number of planets that can potentially support life per star that has planets
- fℓ is the fraction of the above that actually go on to develop life at some point
- fi is the fraction of the above that actually go on to develop intelligent life
- fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
- L is the length of time such civilizations release detectable signals into space.
Other vairants of the equation exist, but that is the most common.