Are we alone?
Life elsewhere in the universe?
Are we alone????
Rate the following statements
from 1 (absolutely not) to 10 (definitely)
and be ready to explain why you think that way

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1. Other than the solar system, there are other planets in the milky
way and/or universe.
2. There are rocky earth-like planets in the milky way and/or
universe.
3. There are earth-like planets with liquid water in the milky way
and/or universe.
4. There is single-celled life somewhere else in the solar system.
5. There is single-celled life somewhere else in the universe.
6. There is multi-cellular life somewhere else in the universe.
7. There is intelligent life somewhere else in the universe.
Which of these has been scientifically confirmed?
Which of these do you think will be confirmed in your lifetime?
(Keep in mind, science looks for evidence…it’s not a poll like this.)
What does life have in common?
(a.k.a.—How much biology do you remember?)
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Cells
Responsiveness to stimuli
Growth
Reproduction
Energy transformation
Life forms on Earth are
carbon-based
• Life forms on Earth require
water (but water itself is not
carbon based…H2O)
Some Requirements of Life
• Liquid water (for chemical reactions and as
transport medium).
• Atmosphere (to avoid rapid vaporization
of water; gasses needed for organic
compounds)
• Moderate temperatures (keep water liquid;
avoid disintegration of organic compounds;
activate complex chemical reactions)
• Time for life to evolve from simple organic
compounds into higher life forms: several
billion years.
Simple to complex
Life on Earth is varied, but all coded in
DNA. Chains of billions of atoms did not
drift together spontaneously, organized, and
related.
Miller-Urey Experiment (1952) : glass
container was sealed with water (like the
oceans), hydrogen, methane, and
ammonia (similar to the gases present on a
volcanic, young Earth), and an electric
spark (like lightning from a thunderstorm).
Life was not produced, but the building
blocks, were. Organic molecules were
made from inorganic compounds by
physical means.
How did life begin?
Life began as single celled prokaryotes (no
nucleus) like bacteria.
We have fossil evidence of cyanobacteria
and stromatolites that date to 3.5 billion
years.
Prokaryotes engulf other prokaryotes to
create eukaryotes (with a nucleus).
Earth’s early atmosphere changes over to
contain more oxygen due to
photosynthesis of these early organisms.
More complex eukaryotic cells evolve over
billions of years.
Modern stromatolites
What do we look for in stars?
Habitable zones have….
• Stable orbit around the star  consider only single stars.
• Time for evolution  consider only class F5 or less massive stars.
• Moderate temperatures  Habitable zone around the star
Life in Our Solar System?
Most promising candidate:
Mars.
Mars rovers Spirit
and Opportunity
found evidence for
past water on Mars.
Life in Our Solar System?
Europa (moon of Jupiter)
or Enceledus (moon of Saturn)
Why are we curious about these
moons?
Cracks and smoothness of the surface
might indicate liquid water beneath the
surface.
Tidal flexing could be the source of heat
the keeps water in liquid form under the
surface.
Signatures of life?
What do we look for on an exoplanet?
We look at the spectral
lines reflecting off the
planets’ atmospheres
Water
Oxygen
Carbon dioxide
Ozone (and reduced
gases like nitrous oxide
or methane)
water
Ozone
Carbon
dioxide
water
What have we found so far?
Evidence of water on Mars in
the past
Glycine, an amino acid, found
in comet
~700 planets outside our
solar system (and counting)
Planet inside Habitable zone
around a star like our Sun
…..but no life
…..yet.