Activity 3: Concept Application-Using The Drake Equation

N = R x fp x ne x f1 x fi x fc x L

In Activity 2, we estimated the number of students that had particular characteristics. In this activity, we will use the same estimation techniques to discover the number of existing extraterrestrial civilizations that possess the technology to communicate beyond their home planet. Your task is to complete the table below and use those values to solve the Drake Equation in order to estimate the number of intelligent civilizations in the Milky Way. You might wish to review the Drake Equation Background Information Sheet before making your estimation. After you make the calculation, answer the reflection questions.

R - Number of target stars in the galaxy that:

  • are second generation stars with heavy elements
  • are hot enough to have a large habitable zone
  • have a long enough lifetimes for life to develop
R =
fp - Fraction (percentage) of those stars with planets or planet systems.
Fp =
ne -Number of "Earth-like planets" in a planetary system that are at the right temperature for liquid water to exist (in the habitable zone).
Ne =
fl - Fraction (percentage) of Earth-like planets where life actually develops
Fl =
fi - Fraction (percentage) of Earth-like planets with at least one species of intelligent life
Fi =
fc - Fraction (percentage) of Earth-like planets where the technology to communicate beyond their planet develops
Fc =
L - "Lifetime" of communicating civilizations (years) - Note: This number must be divided by the age of the galaxy, 10 billion years, when you make your final calculation.
L =
N - Number of communicative civilizations
N =

Reflection Questions about the Drake Equation
N = R x fp x ne x f1 x fi x fc x L

A. What value did you get for the number of civilizations?

B. How does the value change if you double the lifetime of communicating civilizations?


C. How does the estimate change if we discover that only 1/3 of Sun-like target stars have planets?


D. How would you change your estimate if we discovered that early life developed on both Venus and Mars?


E. Determine the most reasonable maximum and minimum values that your group believes the terms fp, ne, f1, fi, and fc could have. Record your values for each term below.




F. Calculate the range of values for N that result from using the maximum and minimum values that your group recorded in the previous question.



G. Do the maximum and minimum values that you calculated make sense to your group? Explain why you think they might be too large, too small, or just right.



H. How many intelligent, communicating species in the galaxy do we actually know about? What then is the actual minimum value for N. (Hint it is not zero.) Explain your reasoning.



In this paragraph we will offer some values for several of the terms in the Drake
Equation that are often used by scientists when making these estimates. If we think that all stars that are like our Sun have planets, then we could estimate fp = 1 to represent 100%. If we use our solar system as a model then there is only one planet in the habitable zone that we know has liquid water on its surface (Earth) so we could imagine setting ne =1. Since Earth is the only planet in our solar system that we know to have developed life, it seems reasonable to set fl = 0.1 to represent that about one out of every 10 planets has life. It is essentially impossible to know the fraction of species that develop on a planet that turn out to be intelligent and able to communicate so a conservative estimate for fi and fc that we might use is 0.1 for each term. As a rough guess we might imagine that across the galaxy intelligent communicating civilizations last for about 20,000 years out of the 10 billion year existence of the galaxy, which sets L = 2 x 10-6.

I. What value do you get if you use the estimates provided in the preceding paragraph? How does this value compare to your original estimate and your estimate for a maximum value or your estimate for a minimum value?







CHALLENGE PROBLEM: Scientists recently discovered a massive gas giant planet orbiting the star 51 Peg. This planet orbits in the star's habitable zone (where liquid water can exist). Describe how this finding might change your estimate.