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Life in the Solar System
... and Beyond
• While Earth is the only confirmed body with
life in the Solar System, we have found many
other bodies which may support life
• Today, we will talk about some of these
worlds and the life that may exist there
• Knowledge of life in our Solar System also
lets us estimate what life may exist beyond
Best
Solar
Candidates
System Inventory
for Life?
1 star
Sun
4 terrestrial planets
Mercury, Venus, Earth, Mars
4 gas giant planets
Jupiter, Saturn, Uranus, Neptune
7 large moons
Moon, Io, Europa, Ganymede,
Callisto, Titan, Triton
small moons, asteroids,
comets, KBOs, dust
Phobos, Ceres, Amalthea,
Rhea, Enceladus, Titania, Proteus,
Pluto, Charon
Mars
• No hard evidence for any
lifeform on the surface,
though only Viking landers
looked
• Much more likely in the
distant past when Mars
was warmer and wetter
• May still be very small
microbial life near water ice
deposits around the poles
or a few feet below the
surface
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Jupiter?
• The idea is a bit far fetched,
but there are layers in
Jupiter's atmosphere where
temperatures and pressures
would be right for liquid
water
• Any life that developed in
the atmosphere would float
through the different layers,
much like life in Earth's
oceans
• Very unlikely, but possible
Europa and Enceledus
• Of all the gas giant moons,
Europa and Enceledus offer
the best possibilities
• Each moon, especially
Europa, shows evidence for
a large amount of liquid
water
• Life systems much like the
deep sea vents on Earth
may exist on both worlds
• It will take quite a bit of time
and effort to know for sure
Titan
• Titan seems to have all the
proper ingredients for life, but
not high enough temperatures
• A large amount of
hydrocarbons reside in the
atmosphere and surface
• Some amino acids are also
detected
• If Titan were warmer, it may be
an ideal location for life
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Beyond the Solar System
The Habitable Zone
• When looking outside our
Solar System, it is easiest
to think about zones
where life could survive
• The habitable zone
usually corresponds to
locations where liquid
water can exist
• The habitable zone is 'not
too hot' and 'not too cold',
but should be just right for
life
The Habitable Zone
• Since different stars have different brightness and
temperatures, the size and location of the habitable zone
can change
Problems with the Habitable Zone
• There has been some
dispute of the accuracy
of the habitable zone
• After all, there are
locations in our Solar
System well outside the
HZ where liquid water
may exist
• Despite the argument,
the habitable zone is the
most favorable location
for life to occur in a star
system
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Finding the Right Star
• While it is easiest to think
about life around a Sun-like
star, that doesn't have to be
the case
• However, there are some
stars which make their
surroundings quite inhabitable
• Some small, red M dwarfs
have violent flaring activity
• Large, massive O/B stars
have strong winds and put out
more ultraviolet and X-ray
radiation
Artist concept of a flaring M dwarf
The Galactic Habitable Zone
• The HZ concept can
be expanded to the
galaxy as a whole
• The interior of the
galaxy is a very
energetic, very
unstable place
• The extremities of the
galaxy contain too few
heavy elements for
Earth-like planets to
form
What About Intelligent Life?
• One of the most
interesting and
controversial branches
of astronomy is the
Search for ExtraTerrestrial Intelligence
• The SETI Institute is
basically a large
listening program, trying
to find signals from
aliens
• The search continues...
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Can They Hear Us?
• Radio and television signals
have been leaving Earth for over
50 years
• If anyone within 50 light-years is
listening, then they could have
detected us
• Odds of an intelligent civilization
being that close is very slim
• Also, our signals are not
concentrated - instead they
spread out thinly in all directions
Can We Hear Them?
• Under current technology, it
would be hard to hear them
unless the signal was directed
straight at us
• Unfortunately, they have no
reason to send signals, since
most of them would not know we
exist
• Improvements in tech and
facilities over the next decade
will let us here the slightest
whispers of civilization
SETI
• The SETI Institute is a nonprofit, privately funded
organization
• They use radio data which
comes primarily from the
Arecibo dish in Puerto Rico
and the 100m radio
telescope in Greenbank,
West Virginia
• A new array of telescopes is
being built in California
called the Allen Telescope
Array, which will be fully
devoted to SETI
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Are They Even Out There?
• One of the ultimate
scientific/philosophical/
religious questions is 'Are
We Alone?'
• From a sheer numbers
standpoint, the odds of only
Earth possessing intelligent
life are essentially
impossible
• Each galaxy contains 100's
of billions of stars and there
are 100's of billions of
galaxies out there
The Drake Equation
• One quick and easy way
to estimate the number of
civilizations in the galaxy
is the Drake equation
• Frank Drake developed
the equation in the 1960's
• Based on a set of
astronomical and
biological factors, the
equation will tell you the
number of intelligent,
communicating
civilizations in the galaxy
The Drake Equation
N  R *  f p  ne  f l  f i  f c  L
N
# civilizations with detectable electromagnetic emissions
R*
fp
ne
rate of star formation (stars/year)
fraction of stars that are stable with planetary systems
# of worlds/system with environment suitable for life
fl
fi
fc
L
fraction of planets on which life develops
fraction of life-bearing planets on which intelligent life arises
fraction of intelligent civilizations revealing existence
length of time civilizations release detectable signals (years)
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Rate of Star Formation
• This factor we have a pretty
good estimate for
• Stars are constantly being
born in the galaxy, and the
rate at which they are born
will tell you how many are
available
• The galaxy has between
100-200 billion stars, and is
about 10 billion years old
• Therefore, the average rate
is about 10-20 stars per year
Dusty star formation regions
(REAL PICTURE!)
Fraction of Stable Stars with Planets
• Not all of the stars in the
galaxy will be stable and not
all of the stars will have
planets
• We have only begun to find
planets around stars, but the
results are quite encouraging
• It is most likely a natural
process for stars to have
planets
• Earth-like planets are still
waiting to be found
Number of Worlds with
Conditions Suitable for Life
• This factor is the number
of planets or moons in a
solar system that may be
able to support life
• Habitable zone
considerations come into
play with this factor
• As we already
discussed, there may be
five or six other places
that could support life
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Fraction of Worlds with Life
• Of all the planets or
moons in the star
system, this is the
fraction which actually
develop life
• We know of ONE planet
with life in our Solar
System
• If you include all the
planets and large moons,
then this number is
something like 1/15
Fraction with Intelligent Life
• Of those planets which have
developed life, this is the
fraction which has developed
'intelligent' life
• For our Solar System, Earth is
the only case with life and it
developed intelligence
• The definition of 'intelligence'
is somewhat hazy - some
would argue the case for
humanity
Fraction with Communicating Life
• Of the worlds with
intelligence, what fraction will
produce a signal that can
travel through space?
• Our civilization has been
around thousands of years,
but we have only been
communicating for ~50 years
• The development of
technology will likely lead to
some sort of signal into
space
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Length of Time Civilization is
Communicating
• This is one of the hardest factors
to estimate, since we have no
good reference
• Our civilization has been
communicating for ~50 years, but
how much longer will we be
sending signals?
• This factor depends on how long
the civilization survives, and how
long its signal leaks into space
So How Many?
N  R *  f p  ne  f l  f i  f c  L
• Even conservative estimates for each of these
factors produce a sizeable number of intelligent
civilizations in the galaxy
• With advances in technology, it is likely that IF
there is a signal to be found, it will be found in our
lifetime
• We will do an exercise on Friday involving the
Drake equation and the possibilities of alien life
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