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SETI - The Search for
Extraterrestrial Intelligence
Cosmic Evolution
• Cosmic evolution:
– Includes seven major evolutionary phases
in the history of the universe: particulate,
galactic, stellar, planetary, chemical,
biological, and cultural evolution.
– The continuous transformation of matter
and energy that has led to the appearance
of life and civilization on Earth.
What are some characteristics of life? In other
words, how do you decide if something is
What do we mean by life?
• Hard to define what we mean by life.
• Characteristics of living organisms:
– They can react to their environment and can often
heal themselves when damaged.
– They can grow by taking in nourishment from their
– They can reproduce, passing along some of their
own characteristics to their offspring.
– They have the capacity for genetic change and
can therefore evolve from generation to
generation to adapt to a changing environment.
Life in the Universe
• The general case in favor of extraterrestrial life is summed up in
what are sometimes called the assumptions of mediocrity:
– Because life on Earth depends on just a few basic
– Because the elements that make up these molecules are (to
a greater or lesser extent) common to all stars.
– If the laws of science we know apply to the entire universe
(which we assume), then, given sufficient time, life must
have originated elsewhere in the cosmos.
• The opposing view maintains that intelligent life on Earth is the
product of a series of extremely fortunate accidents
(astronomical, geological, chemical, and biological events
unlikely to have happened anywhere else in the universe).
Life on Earth
• Building blocks of life as we know it - amino acids and
nucleotide bases (organic, carbon-based, molecules).
– Amino acids build proteins and nucleotide bases form
• In 1953, the first scientist proved that you could make
amino acids and nucleotide bases from simpler
ingredients that would have existed on a young Earth
(water, methane, carbon dioxide, and ammonia).
– Can synthesize biological molecules through
nonbiological means.
– However, these experiments have yet to create a living
An Interstellar Origin
• Suggested that there wasn’t enough raw material on Earth for
the reactions to occur at a significant rate to form organic
• An alternate possibility - the organic material was produced in
interstellar space and arrived on Earth in the form of
interplanetary dust and meteors that didn’t burn up during their
descent through the atmosphere.
• Large amounts of organic material were detected on comets
Halley and Hale-Bopp.
How do you decide if
something alive is intelligent?
Diversity and Culture
• However it got here, we know life did appear.
• Anthropologists believe that intelligence is
strongly favored by natural selection.
• Perhaps most important was the
development of language. This allowed for
cultural evolution (the changes in the ideas
and behavior of society).
Life as We Know It
• Generally taken to mean carbon-based life
that originated in a liquid-water environment,
or life as it is on Earth.
• In our solar system, Europa and Titan both
hold the possibility of harboring life.
• Most likely planet to harbor life (or to have
had it in the past) is Mars.
• Need to keep in mind that life as we know it
can exist in extremely hostile environments
(and does on Earth as well).
The Drake Equation
• Statistical equation that gives the probability
of intelligent life existing elsewhere in the
• Several of the factors are a matter of opinion.
• Important as it divides a huge problem into
more workable chunks.
• We’ll go through each of the factors
Which of the following factors
would you expect to see in the
Drake equation?
A. Fraction of stars with planetary
B. Fraction of planets on which life arises
C. Average lifetime of a technologically
competent civilization
D. All of the above
E. A and B only
Rate of Star Formation
• Milky Way has roughly 100 billion stars
now shining and is 10 billion years old.
• Using the above figures, we have a star
formation rate of 10 stars per year.
• This is probably a fairly good average
rate, even though we know it’s varied
over time.
Fraction of Stars Having
Planetary Systems
• If condensation theory is correct, then
planetary systems are a natural result of the
star formation process.
• We assign a value near 1 to this factor - we
think essentially all stars have planetary
• We already have proof of other planetary
systems, and the number known will just
increase as technology improves, so we’re
not simply being overly optimistic here.
Number of Habitable Planets
per Planetary System
• Habitable zone - three-dimensional zone of
comfortable temperatures that surrounds every
• Have to exclude the majority of binary star
• Have to exclude all but 10% of the systems
we’ve found so far as large Jupiter-like planets
with interior orbits would destabilize any
terrestrial planets’ motion.
• We assign a value of 1/10 to this factor (or 10%).
Fraction of Habitable Planets
on which Life Arises
• If the chemical reactions that led to the
complex molecules that make up living
organisms are completely random, then this
factor is probably close to 0.
• Lab experiments indicate that these reactions
aren’t completely random (some are more
favored than others), so maybe life isn’t so
• We’ll be optimistic and go with a value near 1
for this factor.
Fraction of Life-Bearing Planets
on which Intelligence Arises
• One school of thought sees natural selection
as a universal process and the development
of intelligence as inevitable, making this
factor nearly 1.
• The opposing side says that intelligent life
has existed on Earth a relatively short period
of time compared to simple life, so it’s
probably rare, making this factor very small.
• We’ll be optimistic and assume the value is
nearly 1.
Fraction of Planets on which
Intelligent Life Develops and
Uses Technology
• If the rise of technology is inevitable, given enough
time, then this factor is close to 1.
• If it is not inevitable, then this factor could be much
less than 1.
• Based on the fact that several tool-using societies
arose independently at several places on Earth, we’ll
go with technology being inevitable and take this
factor to be close to 1.
Average Lifetime of a
Technological Civilization
• Combining our factors thus far in the Drake equation
(10 x 1 x 1/10 x 1 x1 x 1 = 1), we can say that:
– The number of technologically intelligent
civilizations now present in the Milky Way galaxy =
lifetime of a technologically competent civilization
in years.
• So if average lifetime is 1000 years, then there are
1000 civilizations present.
• If the average age is less than a few thousand years,
then civilizations are unlikely to have the time to
communicate with even their nearest neighbors.
Dr. Frank Drake is the Director of the SETI Institute's Center for
the Study of Life in the Universe and also serves on the Board of
Trustees of the SETI Institute as Chairman Emeritus.
In 1960, as a staff member of the National Radio Astronomy
Observatory, he conducted the first radio search for extraterrestrial
He is a member of the National Academy of Sciences where he
chaired the Board of Physics and Astronomy of the National
Research Council (1989-92). Frank also served as President of
the Astronomical Society of the Pacific. He was a Professor of
Astronomy at Cornell University (1964-84) and served as the
Director of the Arecibo Observatory.
He is Emeritus Professor of Astronomy and Astrophysics at the
University of California at Santa Cruz where he also served as
Dean of Natural Sciences (1984-88).
In his spare time Frank enjoys cutting gem stones and growing
orchids.Frank has three grown sons and two daughters in college.
Both daughters are superb ballet dancers.
Meeting Our Neighbors
• Let’s assume the average lifetime of a technological civilization
is 1 million years.
• The Drake equation tells us that there are 1 million such
civilizations and we estimate their distances to be about 100
light-years apart from one another.
• Two-way communication would then take 200 years.
• Could we ever meet them? The speed of the fastest space
probes is 50 km/s. It would take us 50,000 years just to reach
Alpha Centauri (one of the closest stars). A distance of 100
light-years would take a million years to travel.
Radio Searches
• A cheap way to make contact across interstellar
space is to use electromagnetic radiation - fastest
means of transferring information from one place to
• Radio is the best bet as its least affected by
interstellar dust, etc.
• Possible to detect “waste” radio emissions as well
(like the TV and radio emissions from Earth).
The Water Hole
• Suppose that a civilization has decided to assist searchers by
actively broadcasting its presence to the rest of the galaxy. At
what frequency should we listen for such an extraterrestrial
• The constituents of water (which is thought to be necessary for
life), H atoms and OH molecules, radiate near 18 and 20 cm
(radio wavelengths). Radio wavelengths interact the least with
gas and dust, making the galaxy largely transparent to them.
This region of wavelengths is called the “water hole.”
• In the same region of wavelengths, there’s the least “static” or
noise from other sources (stars and interstellar clouds).
• Commonly known by the acronym SETI (search for
extraterrestrial intelligence), radio searches have been
underway since the late 1990s.
• No signals of extraterrestrial life have been detected.