Download What about number of planets with life?

Extra-Terrestrial Intelligence:
How Common Is It?
Stephen Eikenberry
15 April 2013
AST 2037
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The Drake Equation
• First proposed by
Cornell astronomer
Frank Drake
• Actual mathematical
formula for estimating
number of intelligent
civilizations currently
in the Galaxy
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What about number of planets
with life?
• Nlife = N* fP ne fl
• So …
• N* = 3 x 1011 stars
• fP = 1/4 fraction of “suitable stars”
(an educated guess)
• ne =1 suitable planets per suitable stars
• fl = fraction of these planets where life forms
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What about number of planets
with life?
• Nlife = N* fP ne fl
• Nlife = 3 x 1011 stars * 1/4* 1 * fl
• = 75 billion * fl
• If fl =1 , then we have 75 billion life-bearing planets in
the Galaxy
• If fl =1 in a million, then we have 75,000 life-bearing
planets in the Galaxy
• If fl =1 in 75 billion, then we are on the ONLY lifebearing planet in the Galaxy
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Summary from Last Time
• Rarity/prevalence of life in the Galaxy depends on several
factors
• How many stars are there in the Galaxy?
• How many of them are suitable for Life?
• How many of those stars have suitable planets?
• How many of those suitable planets around suitable
stars produce Life?
• We can calculate an estimate of the number of civilizations
in the Galaxy using the Drake Equation
• A shorter version tells us how many planets have any life at
all
• Current estimates: as high as 75 billion, as low as 1 (us!)
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The Drake Equation
• Nciv = N* fP ne fl fi ft Lciv / Lgal
• Nciv = number of current civilizations in the Galaxy
• N* = 3 x 1011 stars; fP = ¼ fraction of “suitable stars”;
ne =1 suitable planets per suitable stars
• fl = fraction of these planets where life forms
• fi = fraction of life-bearing planets where intelligence
evolves
• ft = fraction of intelligence-bearing planets where
technology develops for communication
• Lciv = average lifetime of a civilization
• Lgal = lifetime of Galaxy
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Intelligence Fraction
• What fraction of life-bearing planets produce intelligent
life?
• Remember definition of intelligence: capable of
developing technology sufficient for interstellar
communication (i.e. radio)
• Earth suggests this fraction is about 100%
• But … is this right?
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Intelligence & Dinosaurs
• Dinosaurs were not
intelligent
• But they were still dominant
life on Earth … for about
200 Myr !!
• They were only wiped out by
a major extinction (K-T
asteroid impact)
• This allowed mammals and
(eventually) “intelligent” life
to dominate
• We would not survive that
impact either!!
• So, dinosaurs died from bad
luck, not stupidity (?)
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Intelligence & Dinosaurs
• So, dinosaurs died from bad luck, not stupidity (?)
• No real evidence that intelligence is fundamentally “better”
than other traits
• If dinos around today, would they “win”?
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Intelligence Fraction
• This is an endless debate
• But .. does the outcome REALLY matter?
• Even if intelligence is NOT inevitable, it happened once
out of two tries (us + dinosaurs)
• Would estimate fraction at ½ or 1/3 (Permian extinction
ended another “try”?)
• In short: 1 out of a few, so this is not a huge factor; it is
close to 1 (not 0.01, not 0.000001)
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Technology Fraction
• Again, Earth suggests this fraction is 1
• But, what about intelligent life without technology?
• Idyllic scene
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Human Population
• But … technology clearly provides an advantage in
numerical increase
• Human
population versus
time
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Technology & Malthus
• In early 1800s, Thomas Malthus predicted that the world
was rapidly approaching its maximum sustainable
population
• Why did it continue to grow? Technology – we now get
much more production per acre out of agriculture
(pesticides, irrigation, fertilizer)
• Corn yield versus
year
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Technology & Health
• Technology (i.e. medicine) also increases population
via survival rates
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Technology & Health
• Technology (i.e. medicine) also increases population
via survival rates
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Technology & Evolution
• So … evolutionary “success” is seen as improved
reproductive capability
• If have intelligence, sooner or later drive to survive results
in some technology development
• Even chimps do it!
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Technology & Evolution
• “Technological” life reproduces/survives preferentially
• Eventually leads to dominant life being technological
• The more technology, the more likely it is to survive (even
if it does not directly kill off the others – contrary to all of
human history!)
• Theory of Neanderthal extinction: Peaceful nontechnological Neanderthals were victims of Cro-Magnon
genocide
• So … technology fraction is about 1
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Technology & Evolution
• So … technology fraction is about 1
• But … does everyone with the technology to communicate
WANT to communicate?
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Civilization Lifetime
• Why is this critical?
• Do the math so far …
• Nciv = N* fP ne fl fi ft Lciv / Lgal
•
= 3x1011 * 0.25* 1 * 1 * 1 * 1 * Lciv / 1.3x1010 yrs
•
= 5 * Lciv
• Carl Sagan estimated this as 10 Lciv (in 1974)
• Pessimistic case would be Lciv / 10 billion (a BIG
difference)
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Civilization Lifetime
• So … Nciv depends an awful lot on Lciv
• Take L = 90 yrs (lifetime for which Earth has had a
civilization capable of communicating via radio, so far
…)
• Then expect N = 450 civilizations in the Galaxy right
now!
• On the other hand, could be as low as 450/1010  0
• Is L larger than this, though?
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What Limits Civilization Survival?
• Malthus says we are limited
by food/resources (i.e.
starvation)
• Malthus thought we would
all die of starvation by 1900
• But … history shows that
technology advances can
dramatically improve our
ability to generate food
from limited natural
resources
• Mmmm ….
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What Limits Civilization Survival?
• On the other hand, read Jared Diamond’s “Collapse”
• Many civilizations have in fact collapsed here on Earth
• This is often at the PEAK of their technological
advancement (i.e. the ancient Maya)
• Why?
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Collapse of Civilization
• Why?
• More tech, leads to
more people
• More people leads to
more demand
• Exponential population
growth means
technology needs to
KEEP evolving (faster
and faster!)
• If it fails to keep up 
COLLAPSE!
• So, L could be long or
could be short
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Natural Extinction?
• Asteroid impact,
implies L ~108 yrs
• Can technology avoid
even that?
• Gamma-ray bursts
can’t be avoided
• One heated atmosphere
already (!) March 3,
2003
• But … timescale is also
>108 yrs
• (If not, why not
previous extinctions …
or were there?)
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Self-Limiting Lifetime?
• For the last 50 years,
humanity has had the
ability to destroy itself
completely
• MAD = Mutually
Assured Destruction
• For last 20 years,
fingers have been off
the triggers
• But … still a worry
• Could imply a short L
(~100 years??)
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Number of Civilizations
• So … estimates range from L = 80 years to L = ??? (1000
years? 100,000 years? Longer? Mass extinctions seem
to happen every 50-100 million years)
• A “realistic” estimate is Nciv = 2* Lciv
• So … anywhere from 450 civilizations to 1000 to millions
(!)
• A pessimistic estimate is Lciv / 10 billion – which gives
ZERO even for long lifetimes (limited by mass extinction
timescale)
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Distance between Civilizations
• Draw on Galaxy on board
• Mean distance estimate:
• Galaxy “Habitable Zone” is a circle-ish shape,
about 10 kpc in radius
• Area around each civilization averages R2/N
• So … radius around each civilization is about R/N
• This corresponds to anywhere from 1 kpc (for 100
civilizations) to 10 pc (for 1 million civilizations)
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Distance & Communication
• We are not currently in regular radio contact with
extra-terrestrial civilizations
• Why not? Would we expect to be?
• Well … timescale for communication could be tough
• 1 pc = about 3 light years
• For “nominal” 100 civilizations, closest is about 1000 pc
away = 3000 light years
• If they sent us a message at the time of the Julius
Caesar … it is not yet here!!
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Distance & Communication
• Even for an “optimistic” case,
average distance is 10 pc away
• This is 30 light years
• Play the “Contact” game:
• We start broadcasts around 1936
• Message arrives there in 1966
• They send a return message to us
within 1 year
• Would only have reached us in
1997
• So … just barely possible in the
most optimistic scenario
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Summary
• Drake Equation guides estimate of number of civilizations
in the Galaxy right now
• Depends on many unknowns
• One of the most important is lifetime of civilizations
• We can estimate that this is at least 80 (100) years
• Could be as long as 100 million years (??), but depends on
ecological collapse, natural extinction, or violent selfdestruction
• “Realistic” estimates of number of civilizations ranges
from about 450 to as many as 1 million
• Pessimistic estimates would say “zero”
• Even for optimistic estimates, we would just BARELY be
at the threshold of being able to communicate with other
civilizations
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