Description

The Drake Equation was developed by Frank Drake in 1961 as a way to focus on the factors which determine how many intelligent, communicating civilizations there are in our galaxy. The Drake Equation is:

N = N* fp ne fl fi fc fL

The equation can really be looked at as a number of questions:

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 around 200 billion.

fp is the fraction of stars that have planets around them 

 

Question:	What percentage of stars have planetary systems?
Answer:	Thanks to advances in technology, new planets are being discovered every month. Within a few years we may have a reasonably accurate estimate. For now we'll say 20% (a standard estimate given my many including Frank Drake).

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:	If you base it on our solar system you might say 3 planets could possibly support life - Venus, Earth, and Mars. There is also the chance that one or more of Jupiter's moons could support life. If our system is typical the answer may be between 3 and 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 guesses 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:	Guesses 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:	Who knows? Let's guess 10% to 20%

fL is 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 another 10,000 years the answer will be 1/1,000,000th.

When all of these variables are multiplied together we come up with: 

N the number of communicating civilizations in the galaxy. 

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The real value of the Drake Equation is not in the answer itself, but the questions that are prompted when attempting to come up with an answer. Obviously there is a tremendous amount of guess work involved when filling in the variables. As we learn more from astronomy, biology, and other sciences, we'll be able to better estimate the answers to the above questions. Many of these questions will be addressed in depth in future issues of Enigma.

NOTE: Drake's original equation actually used the terms R in place of N* and L in place of fL where R stood for the rate of star creation in the galaxy (i.e.: how many new stars are created each year) and L stood for the length of the time a given communicating civilization survives (in years). In general you will get the same answer using either set of terms, however I feel the terms used on this page make the equation more understandable.

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