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Life in the Universe
Life in the Universe
Life on Earth
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Is there life beyond Earth?
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Many astronomers of the past have suggested
that life existed elsewhere.
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Great for science fiction plots, but at present there is
no undeniable evidence that aliens have been here.
Kepler – thought there were inhabitants on the Moon.
Herschel – claimed life existed on nearly all the
planets.
Lowell – thought he saw canals on Mars.
Before searching for life elsewhere in the
universe we must first look at what conditions are
necessary for life on Earth.
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Life in the Universe
Life on Earth
What is necessary for Life to exist?
 While animals need moderate temperatures and
abundant oxygen, simpler life can live under
much more extreme conditions and locations
(extremophiles).
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Underground, high and low temperatures.
There seem to be 3 basic requirements.
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A source of nutrients
Energy to fuel the activities of life
Liquid water (the biggest constraint)
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Life in the Universe
Life in the Solar System
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To look for life elsewhere, we need to
search for places where the basic
necessities of life exist – the habitable
worlds.
This eliminates most of worlds in our solar
system.
 Moon and Mercury are barren and dry.
 Venus too hot for liquid water.
 Jovian planets are gaseous.
 This leaves Mars and a few of the moons
orbiting the Jovian planets, notably Europa.
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Life in the Universe
Life in the Solar System
Mars
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Percival Lowell thought he saw canals on
Mars, but we are quite confident now that
there are no civilizations on Mars.
Nevertheless, we have good evidence the
liquid water once flowed on the Martian
surface.
Today it contains subsurface ice which
could be heated to form areas of liquid
water underground.
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Life in the Universe
Life in the Solar System
Missions to Mars, looking for life
 The Viking missions took soil samples and
looked for chemical changes that could be
attributed to biological processes.
3 experiments suggested that life may be
present, but also ordinary chemical reactions
could have caused the same results.
 A fourth experiment found little organic
material, the opposite of what one would
expect if life were present.
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Life in the Universe
Life in the Solar System
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The Mars Phoenix mission detected water under
the surface, though soil was basic and may have
trouble harboring life (perchrolates)
Pathfinder, Spirit and Opportunity studied the
Martian conditions to see if life might have
existed.
The Mars Express orbiter detected methane gas.
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Methane should disappear within a few centuries due
to chemical reactions.
So, something is supplying Mars with methane.
It could come from comet impacts, volcanoes, or life.
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Volcanism seems to be the most likely candidate.
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Life in the Universe
Life in the Solar System
Martian Meteorites
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One meteorite which landed in Antarctica 13,000
years ago and found in 1984 was clearly of
Martian origin.
Inside the meteorite were complex organic
materials and structures which looked like
nanobacteria , very small bacteria which have
been discovered on Earth.
These structures can also be made by chemical
and geological means.
Contamination from being on the Earth may also
explain the presence of organic materials.
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Life in the Universe
Life in the Solar System
Life on Europa
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Europa has enough tidal heating to possibly form
a subsurface ocean underneath its icy crust.
Life there could form like the “black smokers” on
Earth.
Larger life forms could exist in the vast oceans,
but energy sources are limited and this would
tend to limit the size of any life there.
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Life in the Universe
Life in the Solar System
Life on Ganymede, Callisto and Titan
 Ganymede and Callisto might have
subsurface oceans, but their internal heat
is small and liquid water would not be
terribly abundant.
 Titan has no native liquid water, but an
abundance of organic materials.
Could life evolve from the lakes of methane?
 Water might be brought in from comets, but
this would eventually freeze.
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Life in the Universe
Life Around Other Stars
Beyond the Solar System
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Where in the Galaxy might we find life?
Since technology might allow us to obtain
surface pictures or spectra, we restrict ourselves
to considering extrasolar planets with habitable
surfaces.
So far all detected extrasolar planets (except
maybe one or two) are gaseous giants and are
unlikely to have surface life.
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However, they may be surrounded by moons
which may support life.
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Life in the Universe
Life Around Other Stars
Constraints on Star Systems
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A star must be stable and live long enough
to allow a planet to develop life.
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Stars greater than a few solar masses are
ruled out (but this is only about 1% of all
stars).
A star must allow stable planet orbits.
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Binary and multiple star systems are much
less likely to have this – about 50% of all star
systems.
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Life in the Universe
Life Around Other Stars
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A third constraint is a planet must form in
the habitable zone.
This is a region where a terrestrial type planet
would have the right surface temperature for
liquid water to exist.
 Stars less massive than the Sun have smaller
zones.
 A star like the Sun (or more massive) would
have the largest zone.
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Even if we restricted our search to Sun-like stars,
we would still have to consider billions of stars in
our Galaxy.
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Life in the Universe
Life Around Other Stars
Finding Habitable Planets
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Two upcoming missions may be able spot Earth
sized planets.
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Kepler will look for transits of planets across other
stars.
The Space Interferometer Mission (SIM) may be
able to detect Earth sized planets.
A decade or so from now, the Terrestrial Planet
Finder (TPF) or something like it may be able to
image extrasolar planets.
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Infrared spectra from future telescopes can look for
signatures of life
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Life in the Universe
Life Around Other Stars
Rare Earth?
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Some feel that an Earth type planet (with its
complex type of life) is rare:
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Galactic constraints
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Too close to the galaxy’s center and the rate of supernovae
are too great.
Too far from the center and “metal” content is too low.
This leaves about 10% of the galaxy’s disk that might be
habitable.
A stellar system needs a Jupiter-like planet to sweepout and deflect meteors that might wipe out life on
Earth.
Climate stability
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Plate tectonics and the carbon dioxide cycle.
Earth’s large Moon keeps axial tilt relatively stable.
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Life in the Universe
Life Around Other Stars
Rare Earth?
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Counterarguments to the Rare Earth
Hypothesis
The above conditions may not affect the
creation and advancement of complex life as
much as we think.
 There may be other overlooked conditions and
processes that could assist the creation and
advancement of complex life.
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Life in the Universe
The Search for Extraterrestrial Intelligence
What About “Intelligent” Life beyond the Solar System?
 SETI (Search for Extraterrestrial Intelligence) is trying to
find signs of alien communication
 How many civilizations out there?
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The (modified) Drake Equation suggests the number of
civilizations we might be able to contact:
# of civilizations = NHP × flife × fciv × fnow
where NHP is the number of habitable worlds
flife is the fraction of these world which actually have life
fciv is the fraction of these worlds which have
interstellar communications
fnow is the fraction of these worlds which have
a civilization at the present time
It is hard to know exactly what any of these numbers are at the
present time.
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Life in the Universe
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Interstellar Travel
If one is restricted to going no faster than
the speed of light, then interstellar travel
will be difficult.
In any event, vast new energy sources
must be used to propel a ship.
Hydrogen scoopers.
 Nuclear bombs or nuclear power
 Matter-antimater.
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Life in the Universe
Interstellar Travel
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If there is advanced alien life out there, why
haven’t we seen them (Fermi Paradox)?
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We are alone and there is no other advanced life out
there.
Civilizations are common, but no one has colonized
the galaxy because
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Technology prevents a widespread travel.
The desire to explore is unusual.
Civilizations destroy themselves before they can colonize the
stars.
There is a galactic civilization, but it has not revealed
itself!!!
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THE END