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Extra-Terrestrial Life and the
Drake Equation
Astronomy 311
Professor Lee Carkner
Lecture 25
Final Exam
Monday, 3 pm, SC102
Two hours long
Bring pencil and calculator
Same format as other tests
Matching, multiple choice, short answer
About 50% longer
Covers entire course
The Drake Equation
In 1961, astronomer Frank Drake developed a
formula to predict the number of intelligent species
in our galaxy that we could communicate with right
now

No one agrees on what the right values are

Solving the Drake equation helps us to think about
the important factors for intelligent life
The Drake Equation
N=R* X fp X ne X fl X fi X fc X fL
N =
R* = Number of stars in the galaxy
fp =
ne = Average number of suitable planets per star
fl = Fraction of suitable planets on which life
evolves
fi =
fc = Fraction that can communicate
fL = Lifetime of civilization / Lifetime of star

R* -- Stars
Our best current estimate: R*=3 X 1011 (300
billion)

We are ruling out life around neutron
stars or white dwarfs or in nonplanetary settings (nebulae, smoke
rings, etc.)
The H-R Diagram
Extra-Solar Planets
fp -- Planets
 What kind of stars do we need?

 High mass stars may become a giant before
intelligent life can develop

 Need medium mass stars (stars like the Sun)

 Can we find planets?
 Circumstellar disks that produce planets are common
 Exoplanets have now been found
 We have just begun the search for planets
The Carbonate-Silicate Cycle
Atmosphere
Water
+
CO2
(rain)
CO2
Volcano
CO2
+ silicate
(subvective
melting)
Ocean
Carbonate + silicate
(Sea floor rock)
Carbonate
+ water
(stream)
ne -- Suitable Planets
What makes a planet suitable?

Must be in habitable zone

Simulations of inner planet formation
produce a planet in the habitable zone
much of the time
Heat may also come from another
source like tidal heating (Europa)
ne -- Unsuitable Planets
The Moon -Mars -- Has atmosphere but too small to
have plate tectonics
Jupiter -- Too large, has no surface
Venus -Earth at 2 AU -- CO2 builds up to try and
warm planet, clouds form, block sunlight
The Miller-Urey Experiment
fl -- Life

Complex molecules containing carbon,
(e.g. proteins and amino acids)

Organic material is also found in
carbonaceous chondrites and comets
fi -- Intelligence

On Earth life evolved from simple to complex over a
long period of time (~3-4 billion years)

Impacts (e.g. KT impact)
Climate Change (e.g. Mars drying up)
Life on Earth has gone through many disasters (e.g.
mass extinctions), but has survived
fc -- Communication
Even intelligent life may not be able to
communicate

What could keep intelligent life from
building radio telescopes?

Airworld (floating gasbags can’t build things)

Social, cultural or religious reasons
Lack of curiosity or resources
fL -- Lifetime

Lifetime of a star like the Sun = 10 billion
years (1 X 1010)

How long does a civilization last for?
fL -- Destroying Civilization
What could destroy a civilization?

Environmental or technological disaster


Space colonization greatly reduces risk
or extinction
N
 Multiply these factors together to get N

 The galaxy is a disk 100,000 light years across
 If you evenly distribute the civilizations across the
galaxy, how close is the nearest one?
N ~ 1
 N ~ 10
D ~ 15000 light years
 N ~ 1000
D~
 N ~ 100,000
D ~ 590 ly
 N ~10,000,000
D~

Summary: Life in the Galaxy
Medium size, medium luminosity star with
a planetary system
A planet of moderate mass in the habitable
zone
Organic compounds reacting to form simple
life
Life evolving over billions of years with no
unrecoverable catastrophe
Intelligent life building and using radio
telescopes
A long lived civilization