# Drake equation

In 1961, Frank Drake wrote down an equation for the chance of a contactable alien civilization from another planet in the Milky Way Galaxy. This is known as the Drake Equation (sometimes called the Green Bank equation or the Green Bank Formula). Carl Sagan mentioned the Drake equation often so it that has been mistaken for the 'Sagan equation'.

## The equation

The Drake equation states that:

$N = R^{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L \!$

where:

N = the number of civilizations in our galaxy with which communication might be possible;

and

R* = the average rate of star formation per year in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
f = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
fc = 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.[1][2]

## References

1. "PBS NOVA: Origins - The Drake Equation". Pbs.org. Retrieved 2010-03-07.
2. "Space: SETI: Frank Drake Biography". Retrieved November 17, 2011.