The Drake Equation.
The Drake equation (sometimes called the Green Bank equation or the Green Bank Formula) is an equation used to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy. The equation was devised by Frank Drake, Emeritus Professor of Astronomy and Astrophysics at the University of California, Santa Cruz.
The Drake equation states that: N = N* fp ne fl fi fc fL
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 billion 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 is the number of planets per star that are capable of sustaining life
Question: For each star that does have a planetary system, how many planets are capable of sustaining life?
Answer: Current estimates range from 1 to 5.
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 range from 100% (where life can evolve it will) down to close to 0%.
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 range from 100% (intelligence is such a survival advantage that it will certainly evolve) down to near 0%.
fc is the fraction of fi that communicate
Question: What percentage of intelligent races have the means and the desire to communicate?
Answer: 10% to 20%
fL is the fraction of the planet's life during which the communicating civilizations live.
Question: For each civilization that does communicate, for what fraction of the planet's life does the civilization survive?
Answer: This is the toughest of the questions. If we take Earth as an example, the expected lifetime of our Sun and the Earth is roughly 10 billion years. So far we've been communicating with radio waves for less than 100 years. How long will our civilization survive? Will we destroy ourselves in a few years like some predict or will we overcome our problems and survive for millennia? If we were destroyed tomorrow the answer to this question would be 1/100,000,000th. If we survive for 10,000 years the answer will be 1/1,000,000th.
When all of these variables are multiplied together we get:
N, the number of communicating civilizations in the galaxy.
Using the current estimates for each variable, the Drake Equation predicts approximately 2000 communicating civilizations in the Milky Way galaxy.
Try the Drake Equation Yourself
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.