Transcript Extra-Solar Planets

Searching for
Extraterrestrial
Civilizations
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* =
fp =
np =
fl =
fi =
fc =
fL =
the number of stars in the Milky Way
the fraction of stars that have “habitable planets”
the number of habitable planets per system
the fraction of habitable planets where life evolves
the fraction of life-planets that evolve intelligence
the fraction of civilizations that communicate
the fraction of the star’s life that the civilization exists
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.
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.)
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Unless the two stars are very far apart, binary stars cannot have
planets. This eliminates perhaps half the stars in the sky.
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.
Main Sequence Lifetimes
Type
Mass
(M)
Luminosity
(L)
Temperature
Lifetime
(Billion yrs)
O
25
80,000
35,000
0.003
B
15
10,000
30,000
0.015
A
3
60
11,000
0.5
F
1.5
5
7,000
3
G
1
1
6,000
10
K
0.75
0.5
5,000
15
M
0.5
0.03
4,000
200
O, B, and A stars don’t live long enough for life to develop. Also,
M stars are so faint that their habitable zones are negligibly small.
Ncivil = N*  fp  np  fl  fi  fc  fL
Finally, most metal-poor
stars don’t have planets
(or, at least, Jovian
planets). This excludes
most stars with
metallicities less than
the Sun.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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!
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 doesn’t mean that
planets can’t form after the inspiral. Also, their moons
might be habitable!
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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.
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.)
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
Million Years ago
8
10
Ncivil = N*  fp  np  fl  fi  fc  fL
This is out of the range of astronomy. Take a guess.
Ncivil = N*  fp  np  fl  fi  fc  fL
Not all intelligent life can or wants to communicate
 Maybe they’re dolphins
 Maybe they have a Congress
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???
Distance to the Nearest Civilization
Let’s 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
3
V / N civil
How Long to Say Hello?
Number of
Distance
Number of
Distance
Civilizations (light years) Civilizations (light years)
2
19,500
5,000
1,400
5
14,400
10,000
1,100
10
11,500
50,000
670
50
6,700
100,000
530
100
5,300
500,000
310
500
3,100
1,000,000
250
1000
2,500
5,000,000
140
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. We’ve 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.
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?)
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?
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 1990’s, these
satellites passed the orbit of Pluto,
and both will eventually reach nearby
stars (in ~ 25,000 years).
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.
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
We’ve been
doing this
for the past
~ 40 years
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.
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
 "
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 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.
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 hasn’t someone/thing done this already. Where is
the sign in our Solar System???
Final Exam
Final exam in
108 Forum (Section 2) and
111 Forum (Section 4)
at 8:00 a.m.(!) on Thursday, December 18
•See you then!