The Drake Equation

This page has been viewed and approved by Dr. Frank D. Drake
of the SETI and Extrasolar Planetary Foundations


The Drake Equation was developed in 1961 by Dr. Frank Drake.
It can determine how many possible intelligent, communicating civilizations there are in our galaxy.

Nc = N* fp ne fl fi fc fL

The equation represents the following:

N* represents the number of stars in the Milky Way Galaxy
Question: How many stars are in the Milky Way Galaxy?
Answer: Current estimates are 200 to 400 billion.
fp is the fraction of stars that have planets around them
Question: What percentage of stars have planetary systems?
Answer: Current estimates range from 20% to 50%.
ne number of planets per star that are capable of sustaining life
Question: For each star that has a planetary system, how many planets are capable of sustaining life?
Answer: estimates range from 1 to 2.
fl is the fraction of planets in ne where life evolves
Question: On what percentage of the planets that are capable of sustaining life does life actually evolve?
Answer: Current estimates are 100% (where life can evolve it will) .
fi is the fraction of fl where intelligent life evolves
Question: On the planets where life does evolve, what percentage evolves intelligent life?
Answer: Estimates are 50% .
fc is the fraction of fi that communicate
Question: What percentage of intelligent life forms have the means and the desire to communicate?
Answer: 10% to 20%
fL is fraction of the planet's life during which the communicating civilizations may survive
Question: For each civilization that does communicate, for what
fraction of the planet's life does the civilization survive?
Answer: This is the most vague question. Using the Earth as our model, the expected lifetime
of our Solar System is approximately 10 billion years. Already communication by radio has
been for less than 100 years. How long can our civilization survive without destroy ourselves
as some predict or will we overcome our problems and survive indefinately? If doomsday came
today this figure would be 1/100,000,000th. If we survive for 10,000 more years this figure
would be 1/1,000,000th.
When all of these variables are multiplied, we get:
Nc, the number of communicating civilizations in the galaxy.


Use the Drake Equation Yourself

Calculate the possible number of communicating civilizations within our own galaxy using the Drake Equation.

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:

Nc = the possible number of communicating civilizations in the galaxy =


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