The Drake Equation


In this homework you will apply many aspects of your astronomy knowledge as well as your ability to form educated guesses, design experiments, and understand uncertainty to estimate the number of advanced civilizations in the Milky Way that we might be able to detect.

Suggested Reading
Section 24.4 "The Search For Extraterrestrial Intelligence"

Many versions of the Drake Equation exist. The version in this homework differs from the one presented in the book but shares a couple of terms in common. Three of the terms on the right side of the equation have units; R* is in units of stars per year, nE is in units of number per star, and L is in units of years. All other terms on the right side of the equation are fractions and should all be between zero and one.


The Drake Equation is written as

     N = R* • fP • nE • fL • fI • fC • L

N = The number of civilizations in the Milky Way Galaxy whose electromagnetic emissions are detectable.
R* = The rate of formation of stars suitable for the development of intelligent life.
fP = The fraction of those stars with planetary systems.
nE = The number of planets, per solar system, with an environment suitable for life.
fL = The fraction of suitable planets on which life actually appears.
fI = The fraction of life bearing planets on which intelligent life emerges.
fC = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
L = The length of time such civilizations release detectable signals into space.
  1. For each term of this equation, make an educated guess about what the number value is or look up a professional astronomer's estimate that you agree with (a 'google' search for "Drake Equation" will turn up a plethora of opinions). Write a sentence or two about each term explaining your reasoning for choosing your values. If you want, you can also make "optimistic" and "pessimistic" estimates. If you are truly clueless about some of these terms, just choose a value that is not absurd and support why it is not absurd.
  2. What is your value for the number of detectable civilizations?
  3. Which of the terms in your Drake Equation carries the most uncertainty? Explain.
  4. Is the uncertainty from the last question enough to get a radically different answer for N?
  5. Scientists have been debating the possibility of intelligent extraterrestrial life for a long time. Why don't they all agree?
  6. Use your estimate to explain your opinion about whether or not the search for intelligent life is worthwhile.
  7. For any two of the first four terms in the Drake Equation (R*, fP, nE, fL), come up with a way astronomers can measure the values through observations.