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"What do we need to know about to discover life in space?"
-Frank Drake, 1961

How can we estimate the number of technological civilizations that might exist among the stars? While working as a radio astronomer at the National Radio Astronomy Observatory in Green Bank, West Virginia, Dr. Frank Drake (now President of the SETI Institute) conceived an approach to bound the terms involved in estimating the number of technological civilizations that may exist in our galaxy. The Drake Equation, as it has come to be known, was first presented by Drake in 1961 and identifies specific factors thought to play a role in the development of such civilizations. Although there is no unique solution to this equation, it is a generally accepted tool used by the scientific community to examine these factors.
N = R_* × f_p × n_e × f_l × f_i × f_c × L

Where,

N = The number of communicative civilizations
The number of civilizations in the Milky Way Galaxy whose radio emissions are detectable.

R_* = The rate of formation of suitable stars
The rate of formation of stars with a large enough "habitable zone" and long enough lifetime to be suitable for the development of intelligent life.

f_p = The fraction of those stars with planets
The fraction of Sun-like stars with planets is currently unknown, but evidence indicates that planetary systems may be common for stars like the Sun. more info

n_e = The number of "earths" per planetary system
All stars have a habitable zone where a planet would be able to maintain a temperature that would allow liquid water. A planet in the habitable zone could have the basic conditions for life as we know it. more info

f_l = The fraction of those planets where life develops
Although a planet orbits in the habitable zone of a suitable star, other factors are necessary for life to arise. Thus, only a fraction of suitable planets will actually develop life.

f_i = The fraction life sites where intelligence develops
Life on Earth began over 3.5 billion years ago. Intelligence took a long time to develop. On other life-bearing planets it may happen faster, it may take longer, or it may not develop at all. For more information, please visit Dr. William Calvin's "The Drake Equation's fi"

f_c = The fraction of planets where technology develops
The fraction of planets with intelligent life that develop technological civilizations, i.e., technology that releases detectable signs of their existence into space.

L = The "Lifetime" of communicating civilizations
The length of time such civilizations release detectable signals into space.

For each variable choose what you think is the best answer from the combo box. After you've chosen all your answers press the calculate button and see how many communicating civilizations you think there are in the galaxy.

N* = the number of stars in the Milky Way galaxy:

fp = fraction of stars with planets around them:

ne = number of planets per star ecologically able to sustain life:

fl = fraction of those planets where life actually evolves:

fi = the fraction of fl that evolves intelligent life:

fc = the fraction of fi that communicates:

fL = the fraction of the planet's life during which the communicating civilizations survives:

N = the number of communicating civilizations in the galaxy =

All right, so it's all pretty bogus, BUT IT'S FUN TO THINK ABOUT!

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