Name_________________________
STUDENT INSTRUCTION AND ANSWER
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Activity 3: Concept
ApplicationUsing 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 "Earthlike 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 Earthlike planets where life actually
develops 
Fl =


fi
 Fraction (percentage) of Earthlike planets with at least one species
of intelligent life 
Fi =


fc
 Fraction (percentage) of Earthlike 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 Sunlike
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 f_{p}, n_{e}, f_{1}, f_{i},
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 106.
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.
