The Drake Equation is used to estimate the number of communicating civilizations in the known universe (though more often locally within the galaxy), or more simply put, the odds of finding intelligent life in the universe.
First proposed by radio astronomer Dr. Frank Drake in 1961, the equation calculates the number of communicating civilizations through multiplying several variables. It's usually written, according to the Search for Extraterrestrial Intelligence (SETI), as:
N = R* • fp • ne • fl • fi • fc • L
- N = The number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable.
- R* =The rate of formation of stars suitable for the development of intelligent life.
- fp = The fraction of those stars with planetary systems.
- ne = The number of planets, per solar system, with an environment suitable for life.
- fl = The fraction of suitable planets on which life actually appears.
- fi = The fraction of life bearing planets on which intelligent life emerges.
- fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
- L = The length of time such civilizations release detectable signals into space.
When calculated out, for the galaxy, we can find that at its lowest estimate the number of intelligent and communicating alien species is:
N = 1.5 × 10−5 × 10−9 × 0.2 × 304 = 0.000000000091
While at its highest it is calculated to be:
N = 3 × 1 × 0.2 × 0.13 × 1 × 0.2 × 109 = 156,000,000
With the lowest result suggesting that we are almost certainly alone in the galaxy, as well as perhaps the universe, and the highest suggesting we live in a very busy galaxy indeed.
This is perhaps were the Drake Equation draws its biggest critique, that with such a wide variation in results, due to its hard to define components, how can we be certain on anyone result?
With the lowest we find we are alone which is the one we have the most evidence for, even if it is disappointing.
The highest result however suggests we should be running into aliens all the time, thus raising the question:
Where is everybody?
In the end the Drake Equation is a fantastic method of estimating, over a broad spectrum, what the probability of intelligent life is, though for actually aiding in finding them it is near useless.
Further Info: https://en.m.wikipedia.org/wiki/Drake_equation