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Lec. 33: Intelligent Life in the Universe
Intelligent Life in the Universe
APoD: Easter Island Eclipse
Lecture 33
In-Class Question
1) Do you think life exists elsewhere in the
Universe?
a) Yes
b) No
c) Don’t know
d) Don’t care
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Lecture Topics
Probabilities
 Rates and totals
 The Drake equation


Computes the expected number of
technical civilizations in the galaxy
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Are we alone?
Do other civilizations exist in the galaxy
or elsewhere?
 How might we estimate this statistically
and what are the uncertainties?
 We would like to quantify whether life
and, in particular, other civilizations
might exist in the galaxy.

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Probabilities

How many dates might a guy get in this
class?
N D  N W  f ask  f accept  f show _ up
ND = number of dates
NW = number of women in the class
fask = fraction he asks out
faccept = fraction that accept
fshow_up = fraction that show up
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Suppose Nw = 100

Shy guy:
fask = 0.02 (2%)
 faccept
= 0.50 (50%)
 fshow_up = 1.00 (100%)
 ND = 100 x 0.02 x 0.5 x 1.0 = 1 date
 Outgoing guy:
fask = 0.20 (20%)

 faccept
= 0.10 (10%)
 fshow_up = 0.50 (50%)

 ND = 100 x 0.2 x 0.1 x 0.5 = 1 date
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Rates and Totals

Suppose
R* = Rate at which stars are born
tl = Average lifetime of a star

How many stars are alive at a given time?

The number of stars is:
N = R* x tl ( Rate times time )
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Total number of stars alive
Now
Death line
10 yrs
Stars not yet born
Stars dead
Time

Suppose: R* = 1 star/year (represented by
spikes above). And stars live only 10 years.

10 stars would be alive at any given time.
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Number of civilizations
Suppose that each star developed a
civilization.
 If the lifetime of the civilization is tl then
the total number of civilizations alive is:

N T  R  tl

But this isn’t the whole story ....
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The Drake Equation
Attempts to quantify the number of
civilizations that might exist in the
galaxy.
 Named after, Frank Drake

pioneered this analysis
 while at Cornell

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N T  R  f p  f h  f s  f i  f t  t l
NT = Number of technological civilizations in the
galaxy.
R* = Rate at which stars are born, averaged
over the lifetime of the galaxy.
(Stars/year)
fp = Fraction having planetary systems.
fh = Average number of life-suitable (habitable)
planets within those systems having
planets.
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N T  R  f p  f h  f s  f i  f t  t l
fs = Fraction of habitable planets on which at
least simple life arises.
fi = Fraction of life-bearing planets on which
intelligence evolves.
ft = Fraction of those intelligent life planets that
develop a technological society.
tl = Average lifetime of a technological
civilization. (years)
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N T  10
R stars
 f p  ffph  ffh s  f fs i  ffi t ftt l  tl
year
R* = Rate at which stars are born, averaged
over the lifetime of the galaxy. (Stars/year)


There are ~100 billion stars in the galaxy
today.
And the galaxy is about 10 billion years old.
 R* ~ 10 stars/year
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N T  10 stars
 1f p fhfh f s fs fi fi ft fttl tl
year
fp = Fraction having planetary systems.


If our understanding of star formation is
correct, then planets are a natural
consequence.
All stars could have planets, so we take
fp ~ 1

However, only ~5% of nearby sun-like stars
have giant planets (depends highly on
metallicity).
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Lec. 33: Intelligent Life in the Universe
f1  f  f  f  t
N T  10 stars
year  1  10h  f ss  f i i  f t t  t l l
fh = Average number of life-suitable (habitable)
planets within those systems having planets.


The ecosphere size varies with stellar type,
but we might expect the odds to be similar to
our solar system, so we choose
fh ~ 1/10
Accept only F, G and K stars.
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Caveats: Galactic Habitable Zone


Region in the Galaxy over which life and life
bearing worlds are likely to exist
Requirements

Available material to build planets



High enough metallicity to produce terrestrial planets
Right mix of “heavy elements” to radioactively heat core
of planet (drives plate tectonics which regulate CO2 in the
atmosphere)
Seclusion from cosmic threats



Impacts by asteroids (depends on Jupiter) and comets
(affected by galactic tides, GMCs, and passing stars)
Blasts of radiation (active galactic nucleus outbursts,
supernovae, and gamma ray bursts)
Orbit near “co-rotation” circle – place where orbital period
of star equals rotation period of spiral arm pattern.
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Metallicity
In the outer parts of the galaxy,
the metallicity will be too low for
giant planet formation
Galactic Hazards
Supernovae and stellar
encounters are much more
frequent in the interior of the
galaxy
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Galactic Habitable Zone
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N T  10 stars
 1  101  f s  f i  f t  tl
year
fs = Fraction of habitable planets on which at
least simple life arises.



How likely is it life will form? Is life rare?
It is certainly complex!
Laboratory experiments show that complex
organic molecules can be formed in an
atmosphere similar to that expected on the
early earth.
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The Urey-Miller Experiment
Harold Urey and Stanley Miller (1953)
 Made “primordial soup” mixture


water, methane, carbon dioxide, ammonia
Passed simulated lightning through it.
 Produced “gunk” containing many of the
amino acids found in life today.

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Lec. 33: Intelligent Life in the Universe
Cyril Ponnamperuma

About a decade later constructed nucleotide
bases in a similar manner.



Both experiments did not closely resemble the
early atmosphere.
But showed biological molecules can be
synthesized by nonbiological means.
Astrobiology


Studies the origin, evolution, and possible future of
life in the Universe
This is an area of active research
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Primordial Soup
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Creating Organics is easy
Using better knowledge of the
primordial ocean and atmosphere.
 Various energy sources can produce
amino acids and nucleotide bases.
 Energy sources such as: solar UV
radiation, lightning, volcanic heat,
natural radioactivity, and atmospheric
shock waves produced by meteorites.

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1
N T  10 stars

1

 1f s f ifi f tft tl tl
year
10
fs = Fraction of habitable planets on which at
least simple life arises.


Making organics is easy, but creating life may
not be. Some might argue that under the
right conditions life has to happen.
Most optimistic case:
fs ~ 1
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In-Class Question
1) What is the galactic habitable zone of the
Milky Way?
a) Sufficient metals the build planets
b) Seclusion from cosmic threats
 c) Inner regions of the galaxy
d) a and b
e) b and c
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N T  10 stars
 1  101  1  1f i f tf t tltl
year
fi = Fraction of life-bearing planets on which
intelligence evolves.
 The appearance of a well-developed brain
might not happen if left to random chance.
 But natural selection tends to single out the
more adaptable, more intelligent species.
 The optimistic view takes intelligence as
inevitable:
fi ~ 1
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Dinosaurs and extinction


Dinosaurs “ruled” the world for ~ 100 million
years, but were pretty stupid (technically).
Was the mass extinction (due to an asteroid
impact) of the dinosaurs necessary for Homo
Sapiens to evolve?
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Other influences?

What role did Jupiter and Saturn have in
allowing life to form on Earth.



Cleared out cometary objects!
But also deflects them too
The Moon


Stabilizes the orientation of the Earth’s spin axis
Otherwise we could have “days” that last a whole
year!
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N T  10 stars

1

 1  1  1ft tltl
year
10
ft = Fraction of those intelligent life planets that
develop a technological society.
 It is hard to imagine an intelligent species
avoiding technology.
 Technical civilizations arose independently in
many areas of the world.
 Taking technological development as
inevitable:
ft ~ 1
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1
N T  10 stars

1

 1  1  1  tl
year
10
tl = Average lifetime of a technological
civilization. (years)



How long does a technical civilization last?
We’ve had one for ~100 years.
There are many unknowns to our own future,
let alone predicting how long another
civilization might last.
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1

1


1

1

1

t
years
N T  10 stars
10
l
year
10
tl = Average lifetime of a technological
civilization. (years)
 Suppose the average lifetime of a technical
civilization is 1 millions years
 1% of the reign of the dinosaurs
 100 times longer than human civilization
has existed!

1 million civilizations in our galaxy.
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Uncertainties!
Important - each term in the Drake
equation (probably) gets more uncertain
when proceeding from left to right.
 For lack of a better example we have
adopted an Earth/human bias when
estimating various terms.
 We do not know the uncertainties.

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How far to our neighbors?
For 1,000,000 civilizations in the galaxy
the average distance between them
will be ~ 150 ly!!!
 two-way communication will take at
least 300 years!
 But this is a large over prediction since
the Galactic Habitable Zone has much,
much less than 1011 stars

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How far? (cont’d)
If the lifetime of a technical civilization is
less than 3000 years
 Average distance is so large that
civilizations will die, on average, before
two-way communications can be
established!

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