Extra-Solar Planets

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Searching for Extraterrestrial Civilizations
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The Drake Equation
In 1961, Frank Drake synthesized an equation to
estimate the number of civilizations currently
communicating in our Galaxy.

• Ncivil N ? fp ? np ? fl ? fi ? fc ? fL
• where
• N the number of stars in the Milky Way
• fp the fraction of stars that have
  habitable planets
• np the number of habitable planets per system
• fl the fraction of habitable planets where
  life evolves
• fi the fraction of life-planets that evolve
  intelligence
• fc the fraction of civilizations that
  communicate
• fL the fraction of the stars life that the
  civilization exists

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Ncivil N ? fp ? np ? fl ? fi ? fc ? fL

• The number of stars in the Milky Way is
  relatively well known. We can measure the
  density of stars in the vicinity of the Sun and
  we can estimate how the light from stars changes
  with galactic radius. The Milky Way contains
  roughly 200,000,000,000 stars.

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Ncivil N ? fp ? np ? fl ? fi ? fc ? fL
In the past few years, we have detected planets
around many stars. But stable orbits about
binary stars are almost impossible. (Over time,
the planet would either be ejected into space,
crash into one of the stars, or be thrown into a
very eccentric orbit.)

• Unless the two stars are very far apart, binary
  stars cannot have planets. This eliminates
  perhaps half the stars in the sky.

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Ncivil N ? fp ? np ? fl ? fi ? fc ? fL

• During the first billion years of the Solar
  System, all the planets were constantly being
  bombarded from space by debris left over from the
  protostellar disk. Such planets are not
  habitable.

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Main Sequence Lifetimes
O, B, and A stars dont live long enough for life
to develop. Also, M stars are so faint that
their habitable zones are negligibly small.
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Ncivil N ? fp ? np ? fl ? fi ? fc ? fL

• Finally, most metal-poor stars dont have planets
  (or, at least, Jovian planets). This excludes
  most stars with metallicities less than the Sun.

1/3 1/2 1 1.8 3
Metallicity (compared to Sun)
After you eliminate binary stars, O,B,A, and M
stars, and metal-poor stars, only about 10 of
stars are left!
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Ncivil N ? fp ? np ? fl ? fi ? fc ? fL

• If Jupiter-size planets spiral in from the outer
  solar system, they will destroy all the habitable
  planets in their path.

But this doesnt mean that planets cant form
after the in-spiral. Also, their moons might be
habitable!
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Ncivil N ? fp ? np ? fl ? fi ? fc ? fL

• A star may have many planets orbiting it, but in
  order to support life, it must have planets in
  the habitable zone.

If the planet is too far from the Sun, there is
no energy to support life. If too close to the
Sun, the planet will not have any liquid water
(or similar compound) to move nutrients around.
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Ncivil N ? fp ? np ? fl ? fi ? fc ? fL

• In the Solar System, np ? 1, since Mars borders
  the habitable zone. But the size of this zone
  depends on the luminosity of the star the
  brighter the star, the larger the zone.

Note that this assumes solar heating. As we have
seen, there are other ways of heating a planet.
(For example, Europa is heated by tides, and
perhaps could have life in its oceans.)
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Ncivil N ? fp ? np ? fl ? fi ? fc ? fL

• There may other factors that limit the
  development of life. For example
• Planets without large moons may have the
  direction of their spin axis shift over time.
  This may produce long term climatic shifts.
• Planets with very large moons may have unstable
  crusts due to tides.

Mars
0 2 4
6 8
10 Million Years ago
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Ncivil N ? fp ? np ? fl ? fi ? fc ? fL

• This is out of the range of astronomy. Take a
  guess.

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Ncivil N ? fp ? np ? fl ? fi ? fc ? fL

• Not all intelligent life can or wants to
  communicate
• Maybe theyre dolphins
• Maybe they have a Congress

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Ncivil N ? fp ? np ? fl ? fi ? fc ? fL

• Our Sun spent the first 4.5 billion years of its
  life without hosting a civilization capable of
  communication. We only achieved this capability
  50 years ago. How long will we keep it??
• Extreme Optimistic Case we continue as a
  civilization for the rest of the lifetime of the
  Sun fL 1/2
• Extreme Pessimistic Case we destroy ourselves
  in the next 50 years fL 100 / 10,000,000,000
  0.00000001

Now multiply the numbers together. What do you
get???
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Distance to the Nearest Civilization

• Lets approximate the Milky Way as a large disk

2000 light years
50,000 light years
The volume of the Galaxy V 1.5 ? 1013 cubic
light years
The density of communicating civilizations is
Ncivil / V
The distance between civilizations is
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How Long to Say Hello?
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Extraterrestrial Communication

• Radio waves are least effected by interstellar
  extinction. They also require the least energy
  to transmit (each photon has very low energy) and
  are easy to detect. Weve been transmitting them
  for some time.

The atmosphere is transparent to radio waves.
Light at these wavelengths can not only enter the
Earth from space, but it can also depart the
Earth for space.
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SETI The Search for Extraterrestrial
Intelligence

• Rather than transmit (and wait for a reply), we
  can listen for other civilizations. (But what
  frequency? Will it sound like noise?)

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The Von Neumann Machine

• Before we start, consider
• The age of the universe 13,700,000,000 yrs
• The age of our solar system 4,500,000,000
  yrs
• The age of our written history 5,000
  yrs
• The age of our technology 100 yrs

Where will our technology be in another 100 (or
200 or 1000) years?
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N 1?

• Suppose there is at least 1 extra-terrestrial
  civilization out there that is more advanced than
  us by at least a few hundred years. Suppose at
  least one person in that civilization wants to be
  famous. He/She/It could
• Build a spaceship that could go to another star.

In 1977, NASA did this. They built two Voyager
spacecrafts, which flew by Jupiter, Saturn,
Uranus, and Neptune. In the 1990s, these
satellites passed the orbit of Pluto, and both
will eventually reach nearby stars (in 25,000
years).
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N 1?

• Suppose there is at least 1 extra-terrestrial
  civilization out there that is more advanced than
  us by at least a few hundred years. Suppose at
  least one person in that civilization wants to be
  famous. He/She/It could
• Build a spaceship that could go to another star.
• Program the spaceship/robots to look for a planet
  or an asteroid around that star

Voyager did this as well. While passing by the
outer planets, it found many new moons.
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N 1?

• Suppose there is at least 1 extra-terrestrial
  civilization out there that is more advanced than
  us by at least a few hundred years. Suppose at
  least one person in that civilization wants to be
  famous. He/She/It could
• Build a spaceship that could go to another star.
• Program the spaceship/robots to look for a planet
  or an asteroid around that star
• Program the spaceship/robots to land and explore
  the object

Weve been doing this for the past 40 years
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N 1?

• Suppose there is at least 1 extra-terrestrial
  civilization out there that is more advanced than
  us by at least a few hundred years. Suppose at
  least one person in that civilization wants to be
  famous. He/She/It could
• Build a spaceship that could go to another star.
• Program the spaceship/robots to look for a planet
  or an asteroid around that star
• Program the spaceship/robots to land and explore
  the object
• Program the spaceship/robots to build an
  unmistakable sign, indicating the existence of
  the person
• Program the spaceship/robots to use the materials
  on the planet to duplicate itself twice. The
  duplicates would then fly off to other stars and
  repeat the process.

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N 1?

• If someone could build a Von Neumann machine
  (even one moving as slow as Voyager), then
• After 70,000 yr, there would be a sign up in
  1 star system
• After 140,000 yr, there would be a sign up in
  3 star systems
• After 210,000 yr, there would be a sign up in
  7 star systems
• After 280,000 yr, there would be a sign up in
  15 star systems
• After 350,000 yr, there would be a sign up in
  31 star systems
• After 420,000 yr, there would be a sign up in
  63 star systems
• " " " " "
  " " " " " " "
• After 2,590,000 years, there would be a sign up
  in over 200,000,000,000 star systems. There
  would be a sign around every star in the Galaxy!
  And remember, the Galaxy is about 13,000,000,000
  years old.

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N 1?

• Even at the speed of the Voyager spacecraft, it
  would only take only 2,590,000 years to populate
  the entire Milky Way Galaxy with signs announcing
  your presence. This is 0.02 the age of the
  Galaxy.
• Why hasnt someone/thing done this already.
  Where is the sign in our Solar System???

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Final Exam

• Final exam in
• 108 Forum (Section 2) and
• 111 Forum (Section 4)
• at 800 a.m.(!) on Thursday, December 18
• See you then!