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Copyright © 2009 Pearson Education, Inc.
Life in the Universe
Copyright © 2009 Pearson Education, Inc.
Life in the Universe
• The only place we know life exists is here on Earth
• One of humanity’s Big Questions is whether it exists elsewhere
• We can get some clues by considering life’s history here on Earth
• When we do, we can get an idea how likely life “as we know it” is
• We should keep in mind that life as we know it may not be the only kind possible
• But it is the kind that we will be best able to recognize, if it does exist…
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Copyright © 2009 Pearson Education, Inc.
When did life arise on Earth?
What do these events tell us about the possibility
that life exists elsewhere in the universe?
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When did life arise on Earth?
• The first evidence of life
appears ~150 million years
after it became possible
• It is not fossil evidence, but
trace chemical evidence
• The evidence is in the ratio of
carbon-12 to carbon-13
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Carbon isotope evidence for life
• In >3.8-billion-year-old
rocks like these in
Greenland there is a higher
than normal ratio of 12C:13C
• Living things incorporate 12C more easily than 13C
• So the higher ratio is taken
as indirect evidence for life
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When did life arise on Earth?
• The oldest fossils of living things
date to ~3.5 billion years ago.
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Earliest Fossils
• The oldest fossils of living things
date to ~3.5 billion years ago.
• Fossil stromatolite in 3.5-billion-
year-old rock
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Earliest Fossils
• The oldest fossils of living things
date to ~3.5 billion years ago.
• Fossil stromatolite in 3.5-billion-
year-old rock
• This is a living stromatolite
• Stromatolites are layered structures
formed by colonies of bacteria
• They still exist today, typically in
extreme environments like hyper-
salty lakes and lagoons
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Earliest Fossils
• The oldest fossils of living things
date to ~3.5 billion years ago.
• Fossil stromatolite in 3.5-billion-
year-old rock
• This is a living stromatolite
• Stromatolites are layered structures
formed by colonies of bacteria
• They still exist today, typically in
extreme environments like hyper-
salty lakes and lagoons
• Here are some in a lagoon in
Australia
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When did life arise on Earth?
• So fossil evidence shows that life
certainly existed on Earth by 500
million years after conditions
would permit it to survive
• And chemical evidence suggests
it probably existed much earlier
• But how did it come to be?
• We don’t know
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• We do know that all “life as we know
it” has an inside and an outside
• These are separated by a lipid
membrane (along with a cell wall in
plants)
• Vesicles made of lipids are easy to
make in the laboratory
How did life arise on Earth?
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How did life arise on Earth?
• Life as we know it also has a nucleic acid genome
containing instructions for building the organism
• And we know that all of that was accomplished here on
Earth surprisingly quickly
• So we believe that given similar conditions elsewhere, life
will also arise
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Necessities for Life As We Know It
• Nutrient source
• Energy (starlight, chemicals, heat)
• Liquid water (hardest to come by)…
• And can only exist if planet is in “habitable zone”
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Are habitable planets likely?
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Habitable Planets
Definition:
A habitable world contains the basic necessities for life as
we know it, including liquid water.
• It does not necessarily have life.
• There still needs to be sufficient time for life to evolve
• And certain other requirements as well
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Constraints on star systems:
1. Old enough to allow time for origin and evolution (rules
out high-mass stars — 1%)
2. Need to have stable orbits (might rule out
binary/multiple star systems — 50%)
3. Size of habitable zone: region in which a planet of the
right size could have liquid water on its surface
Even so… billions of stars in the Milky Way seem
at least to offer the possibility of habitable worlds.
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Finding them will be hard
• Looking for an Earth-like planet around a nearby star is
like standing on the East Coast of the United States and
looking for a pinhead on the West Coast—with a VERY
bright grapefruit nearby.
• But new technologies should soon show the way.
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• The Kepler Space Telescope
is monitoring 156,000 stars
for transit events for 4 years.
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Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
Planned orbiting
interferometers will obtain
spectra and crude images
of Earth-size planets.
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Spectral Signatures of Life
Earth
Venus
Mars
oxygen/ozone
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Are Earth-like planets rare or
common?
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Elements and Habitability
• Some scientists argue that
proportions of heavy
elements need to be just
right for the formation of
habitable planets.
• If so, then Earth-like
planets are restricted to a
galactic habitable zone.
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Impacts and Habitability
• Some scientists argue that
Jupiter-like planets are
necessary to reduce the
rate of impacts.
• If so, then Earth-like
planets are restricted to
star systems with Jupiter-
like planets.
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Climate and Habitability
• Some scientists argue that
plate tectonics and/or a
large moon are necessary
to keep the climate of an
Earth-like planet stable
enough for life.
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Jovian Moons
• An intriguing possibility is that
life as we know it could exist
on jovian moons
• If, that is, those moons are in
the HZ of their star
• Of the 542 confirmed
exoplanets, 124 of them are in
the HZ
• Only 2 of those are near Earth-
size, and they are close with
highly elliptical orbits
• A number of the jovian planets
are in more Earth-like orbits
• Good for jovian moon life
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The Bottom Line
• We don’t yet know how important or
negligible these concerns are.
• The general feeling among most scientists is
that microbial life is likely to be common
• But how common intelligent, technological
life like us humans is, is unknown
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How many civilizations are out there?
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The Drake Equation
Number of civilizations with whom we could potentially
communicate
= NHP flife fciv fnow
NHP = total number of habitable planets in galaxy
flife = fraction of habitable planets with life
fciv = fraction of life-bearing planets with civilization at
some time
fnow = fraction of civilizations around now
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We do not know the following values for the Drake
equation:
NHP : probably billions
flife : ??? Hard to say (near 0 or near 1)
fciv : ??? It took 4 billion years on Earth
fnow : ??? Can civilizations survive long-term?
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Are we “off-the-chart” smart?
• Humans have
comparatively large
brains.
• Does that mean our
level of intelligence
is improbably
high?
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How does SETI work?
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SETI experiments look for deliberate signals from E.T.
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We’ve even sent a few signals ourselves…
Earth to globular cluster M13: Hoping we’ll hear
back in about 42,000 years!
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Your computer can help! SETI @ Home: a screensaver
with a purpose
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Where are the aliens?
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Fermi’s Paradox
• Plausible arguments suggest that civilizations should be common. For example, even if only 1 in 1 million stars gets a civilization at some time 100,000 civilizations!
• So why haven’t we detected them?
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Possible solutions to the paradox
1. We are alone: life/civilizations much rarer than
we might have guessed
• Our own planet/civilization looks all the more
precious…
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2. Civilizations are common, but interstellar travel
is not, perhaps because:
• interstellar travel is more difficult than we think.
• the desire to explore is rare.
• civilizations destroy themselves before achieving
interstellar travel.
These are all possibilities, but they are not very
appealing.
Possible solutions to the paradox
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3. There IS a galactic civilization…
… and someday we’ll meet them.
Possible solutions to the paradox
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• If there are other civilizations as advanced as ours….
• What would they be like?
• What would the “people” look like?
• How would they think?
• Presumably they would have similar science to ours…
• What would their art be like?
• What would their philosophy be like?
• What would their religion be like?
Exocivilizations
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