Download Life - Physics

Goal: To find life in our
universe.
Objectives:
1) To understand what to look for
2) To learn about What type of stars and planets
to look for if we want to find life
Looking for life outside our solar
system:
• To find life we first need to be able to
image planets directly.
• We need a spectrum
Which molecule, if found in some
abundance, would indicate that
there was some form of life on the
planet?
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A) Carbon Dioxide
B) Nitrogen
C) Water
D) Ozone
What determines the makeup of
the atmosphere?
• There are 3 processes:
• 1) geological – volcanoes mostly.
• Volcanoes spew water, Carbon Dioxide,
Nitrogen, and Sulfur Dioxide into the
atmosphere
Interactions with the sun
• Two ways here:
• 1) UV rays can break apart molecules.
• This will form some oxygen in an atmosphere for
example, but only trace amounts.
• This can break apart water.
• This is how Carbon 14 is formed from Nitrogen
14.
Solar wind
• If a planet has no sizable magnetic field
certain gasses (such as water vapor) will
be removed from the atmosphere.
Biological
• This is the one we want to search for.
• If there are molecules that are a result of
biological processes, are short lived, and
do not occur much naturally, if we find
them, we have found life!
• Note this will be life in general, like
bacterial and plant life, not intelligent life.
• So, what do we look for?
Smoking guns for life?
• Nitrogen can be useful.
• However, it is difficult to detect, and many
atmospheres have it naturally (Venus +
Mars have 3%, and Titan has mostly
Nitrogen).
• How about molecular Oxygen (O2)?
• Well, it is even more difficult to observe.
• Very trace amounts are produced
naturally, so you would have to show a lot
of it (like our 26%) to be able to say it was
life induced, but we still can’t detect it…
Guns?
• Methane and Nitrous Oxide
• Methane does not survive long in an atmosphere as it
gets destroyed by UV rays.
• NO tends to react with Oxygen or goes to molecular
Nitrogen.
• Either way both are too trace to be seen with the
instruments coming out.
The true gun
• However OZONE is the key!
• To have significant amounts of Ozone you
need a lot of free Oxygen,
• To have a lot of free Oxygen you need life!
• Also, Ozone is fairly easy to detect!
Star type
• Red dwarf – lack of magnetic field for
tidally locked planets. Min mass 0.4 solar.
• Big stars – die too quickly. Lifetime is:
10 billion / Mass squared
So, max mass for life in general is about 4
solar masses.
40 billion stars like this
Metallicity
• Stars with too little metals will have smaller
planets and will lack gas giants to prevent
massive asteroid impacts.
• Stars with too much metals will be more
likely to have larger gas giants and hot
Jupiters and less terrestrial planets.
• So the range is probably 1/3rd solar to 3X
solar.
Galactic Goldilocks
• Since there is a slight increase in
metallicity as you get to the center of the
galaxy you expect life within about 10 kpc
of the center.
• This would increase with time.
• Develop inner boundary as metals get
high.
Planet sizes
• Too small – loose mag field, loose
atmosphere (like Mars).
• Min radius maybe 2/3s Earth (about ¼
Earth mass).
• Too big – too much atmosphere, runaway
greenhouse.
• Max, tough to tell, maybe 5 Earth masses.
So that leaves
• About 20% of stars like our sun are
estimated to have an earthlike planet in
the habitable zone.
• So that means we could have 8 billion
planets suitable for life.
Wildcard:
• A large moon.
• Produced by large impact during solar
system formation.
• No idea how common. Maybe 1 in 10???
• Still 800 mil planets with life potential.
How far?
• Volume of galaxy is ~16 trillion cubic light
years.
• One planet per 2000 cubic light years.
• One per 13 light years (27 if need moon).
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Stars that close to us:
Tau Seti
Alpha Centauri system
Epsilon Eridani
Conclusion
• We have found what a planet needs to be
capable of supporting life.
• We have found what to look for to
determine if a planet has life.