Life on Other Worlds?

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Transcript Life on Other Worlds?

Life on Other Worlds?
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The chemistry of all known life forms on
Earth is based on which essential element?
1.
2.
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5.
Hydrogen
Helium
Carbon
Oxygen
Iron
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The Physical Basis of Life
All life forms on Earth, from viruses to complex mammals
(including humans) are based on carbon
(C) chemistry.
Carbon-based DNA and RNA molecule strands are the basic
carriers of genetic information in all life forms on Earth.
The Tobacco
Mosaic Virus
contains a
single strand
of RNA,
about 0.1
mm long
This
complex
mammal
contains
about 30 AU
of DNA.
Do we find evidence for carbon-based
compounds anywhere else than on Earth?
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2.
No
Yes, but only in the atmospheres of
other terrestrial planets
Yes: in the atmospheres of most
planets (and moons) which have an
atmosphere.
Yes: On the surfaces of other
planets and moons throughout the
solar system.
3 and 4.
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The basic building blocks of life
• Simple carbon-based (organic)
compounds are found not only in the
solar system (on planets and moons),
but also in interstellar space:
• Methane
• Ethane
• Formaldehyde
• …
The Miller Experiment
Miller Experiment in 1952: Simulating conditions
on Earth when life began ~ 4 billion years ago:
Water (oceans), primitive atmosphere gases (hydrogen, ammonia,
methane), and energy from electric discharges (lightning).
Experiment produced some of the
fundamental building blocks of life:
amino acids, fatty acids, ...
Requirements of Life
• Liquid water (for chemical reactions and
as transport medium).
• Atmosphere (to avoid rapid vaporization of
water; gases 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.
Geologic
Time
In geologic
terms, higher life
forms, in
particular
mammals and
humans, have
evolved only
very recently.
Humans have
existed for only ~
3 million years.
Do we have evidence for
planets around other stars?
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No.
Yes, but only very few
might have planets.
Yes, many (single) stars
might have planets.
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Requirements for Life in Other
Planetary Systems
• Planetary systems are probably common.
Do you expect that virtually all stars will be
single, rather isolated stars like our sun?
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Yes. Most stars should be single
stars.
No. Stars form in clusters, so many
stars could still be bound in small
groups (double-, triple-star systems).
No. Stars form in clusters, so virtually
all stars should still be bound in large
groups of several hundreds to
thousands of stars.
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Requirements for Life in Other
Planetary Systems
• Planetary systems are probably common.
About 50 % of all stars in our Milky Way are
bound in double- or triple-star systems.
• Stable orbit around the star → consider only single stars.
If we have a planet with a reasonable
atmosphere, water, and carbon-based
compounds, could life always evolve on them?
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No, it needs to have seasons.
No, it needs to be at an appropriate
distance from the star to provide
reasonable temperatures.
No, it needs to be around a star that
lives at least several billion years.
All of the above
2. and 3.
Yes.
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Requirements for Life in Other
Planetary Systems
• Planetary systems are probably common.
• Stable orbit around the star → consider only single stars.
• Time for evolution → consider only stars that live for
several billion years.
• Moderate temperatures → Life zone around the star
Too cold
Orbit of Mars
Orbit of Earth
Life zone
Orbit of Venus
Too hot
Sun
Are we alone?
The Drake Equation
Let’s try to estimate the number of civilizations (Nc) in
our Milky Way that are currently in a state of their
development that they are intelligent, advanced
enough, and willing to communicate with us.
Factors to consider :
1) Number of stars in the Milky Way:
N* ≈ 2*1011
2) Fraction of stars with planets:
fp ≈ 0.01 – 0.5
Factors to consider (contd.)
3) Number of planets per star that lie in the
life zone for longer than 4 billion years:
nLZ ~ 0.01 - 1
Too cold
Orbit of Mars
Orbit of Earth
Life zone
Orbit of Venus
Too hot
Sun
Factors to consider (contd.)
4) Fraction of suitable planets on
whith life actually begins:
fL ~ 0.01 - 1
5) Fraction of eco-systems in
which a life form evolves
to inteligence
fI ~ 0.01 - 1
6) Fraction a star’s life during
which there is a
communicative civilization
FS ~ 10-8 – 10-4
The Drake Equation
The number of technologically advanced
civilizations per galaxy that are currently able and
willing to communicating with others:
Nc = N* · fp · nLZ · fL · fl · FS
Most of the factors are highly uncertain.
Nc ~ 2*10-5 – 2*107
What does it mean if Nc ~ 2*10-5?
There are currently 2*10-5 communicating
civilizations in the Milky Way.
There are currently 50,000 (= 1/[2*10-5])
communicating civilizations in the Milky Way.
If you take 2*10-5 galaxies, you can expect to
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find one communicating civilization in them.
If you take 50,000 galaxies, you can expect to
find one communicating civilization in them.
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The Drake Equation
Nc ~ 2*10-5 – 2*107
Possible results range from
1 communicative civilization within a few
dozen light years
to
us being the only communicative
civilization in the Milky Way.
So, what’s the final word on the question:
“Are we alone in the Milky Way?”
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There is most likely no other
communicative civilization in the Milky
Way at this time.
There is a reasonable chance that there
25%
are a few other communicative
civilizations in the Milky Way at this time.
There are probably thousands or even
millions of other communicative
civilizations in the Milky Way at this time.
We have no clue!
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