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Extra-Terrestrial Life and the
Drake Equation
Astronomy 311
Professor Lee Carkner
Lecture 26
Observing Project
Due Friday
Project should be neat, organized, labeled and
have all questions fully answered
Telescope objects:
 Venus, Uranus, Neptune, Saturn, Moon
We will try for the Sun on Friday
Meet in planetarium
We will try to observe tonight at 9pm
Check web page
Is There Anybody Out There?
People have long speculated about life on other
worlds

Modern observations indicate that the solar system is
uninhabited


How can we estimate the possibility of extraterrestrial life?
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



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 = The number of civilizations in the galaxy
R* = Number of stars in the galaxy
fp = Fraction of stars with planets
ne = Average number of suitable planets per star
fl = Fraction of suitable planets on which life
evolves
fi = Fraction on which intelligence develops
fc = Fraction that can communicate
fL = Lifetime of civilization / Lifetime of star
The Milky Way
R* -- Stars
We start with the number of stars in the
galaxy


We are ruling out life around neutron stars or
white dwarfs or in non-planetary settings
(nebulae, smoke rings, etc.)
The H-R Diagram
The Orion Star Forming Region
Protoplanetary Disk in Orion
Extra-Solar Planets
fp -- Planets
Very high mass stars go supernova before
planets can form


Need medium mass stars (stars like the Sun)

fp -- Finding Planets
Studies of star forming regions reveal
that circumstellar disks are common
around young stars


Only about 75 have been found, but we
can only find the most obvious ones
The Carbonate-Silicate Cycle
Atmosphere
Water
+
CO2
(rain)
CO2
Volcano
CO2
+ silicate
(subvective
melting)
Ocean
Carbonate + silicate
(Sea floor rock)
Carbonate
+ water
(stream)
Venus
Mars
ne -- Suitable Planets
What makes a planet suitable?

Must be in habitable zone


Heat may also come from another
source like tidal heating (Europa)
ne -- Unsuitable Planets
The Moon -Mars -Jupiter -Venus --
Earth at 2 AU -- CO2 builds up to try and warm
planet, clouds form, block sunlight
The Miller-Urey Experiment
Comet
fl -- Life
The building blocks of life on Earth are
organic compounds

The Miller-Urey experiment
demonstrates that organic material
could have formed from the material
available on the early Earth

The KT Impact
fi -- Intelligence
Life alone is not sufficient, intelligence is needed to
communicate

Many things could interfere with evolution in this
time


Life on Earth has gone through many disasters (e.g.
mass extinctions), but has survived
Europa
fc -- Communication
Even intelligent life may not be able to
communicate

What could keep intelligent life from building
radio telescopes?





O’Neill Colony
O’Neill Colony -- Interior
fL -- Lifetime
fL = Lifetime of civilization / Lifetime
of star


How long does a civilization last for?
fL -- Destroying Civilization
What could destroy a civilization?




Space colonization greatly reduces risk
or extinction
The Fermi Paradox
Physicist Enrico Fermi asked, “If there
are many civilizations in the galaxy
why haven’t they contacted us?”
Cosmic Zoo -Berserker Theory -The Gibson Continuum --
The Von Neumann Problem
Build a self replicating space probe (a Von Neumann
machine)



Even if it takes 100,000 years to get to the next star
and 1000 years to make a copy, in 100 million years
the galaxy is full of machines

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