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Searching for Alien Intelligence
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.
It contains roughly 200 billion stars.
Mass
Ncivil = N*  fp  np  fl  fi  fc  fL
Fraction of stars with rocky planets
in the habitable zone is unknown
Ncivil = N*  fp  np  fl  fi  fc  fL
Our solar system has 1 planet in the habitable zone right now
(np=1), but 2 others are just outside of it, and may have been
within the habitable zone in the past (np=3). Most stars probably
do not have np>3, otherwise the planets would be too close and
they would disrupt each other’s orbits.
Ncivil = N*  fp  np  fl  fi  fc  fL
There may be 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 an unstable
crusts due to tides.
Mars
0
2
4
6
Million Years ago
8
10
Ncivil = N*  fp  np  fl  fi  fc  fL
Luminosity
Type Mass (M)
Temperature
(L)
O
25
80,000
35,000
B
15
10,000
30,000
A
3
60
11,000
F
1.5
5
7,000
G
1
1
6,000
K
0.75
0.5
5,000
M
0.5
0.03
4,000
Lifetime
(Billion yrs)
0.003
0.015
0.5
3
10
15
200
Massive stars probably die before life can form. But most stars are M
stars, which have very long lives. So most habitable planets should have
billions of years to produce life. Whether that’s enough time depends
on how long it typically takes for life to arise, which is unknown.
Ncivil = N*  fp  np  fl  fi  fc  fL
What fraction of planets with life produce intelligent life?
Perhaps evolution inevitably leads to at least one intelligent
organism on a planet. If so, then fi=1.
But for 2.5 billion years, life on Earth did not evolve past
single-celled organisms. Perhaps the development of
complex (and intelligent) life is very rare. In that case, fi<<1.
Ncivil = N*  fp  np  fl  fi  fc  fL
What fraction of intelligent life communicates? Some may
not develop technology for it, or may not want to
communicate.
Ncivil = N*  fp  np  fl  fi  fc  fL
Our Sun spent its first 4.5 billion years without hosting a
civilization. We only achieved the technology to communicate
with radio transmissions ~ 70 years ago. How long will we
maintain this ability?
 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
Ncivil = N*  fp  np  fl  fi  fc  fL
Now make your best guess at each number and multiply them.
What do you get?
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
Now make your best guess at each number and multiply them.
What do you get?
N* = the number of stars in the Milky Way = 200,000,000,000
fp = the fraction of stars that have “habitable planets” = 0.5
np = the number of habitable planets per system = 2
fl = the fraction of habitable planets where life evolves = 0.5
fi = the fraction of life-planets that evolve intelligence = 0.01
fc = the fraction of civilizations that communicate = 0.01
fL = the fraction of the star’s life that the civilization exists
= 10,000 / 10,000,000,000 = 10-6
= 20 civilizations communicating in Milky Way now
Distance to the Nearest Civilization
2000 light years
50,000 light years
The volume of the Milky Way is V=1.5  1013 cubic light years
The density of communicating civilizations is Ncivil / V
The distance between civilizations is
3
V / N civil
If N = 20, then nearest civilization is 10,000 light years away
Extraterrestrial Communication
Radio waves are not blocked by interstellar dust, so they can
travel through the entire Milky Way. They also require the
least energy to transmit because they have low energy.
The atmosphere is
transparent to radio
waves. As a result, we
can easily receive radio
signals from space, and
our transmissions escape
into space as well.
Extraterrestrial Communication
Through our radio and TV broadcasts, we have been
transmitting radio signals into space for 70 years. Aliens
around other stars would see these transmissions increase
and decrease regularly because of the Earth’s rotation.
SETI: The Search for Extraterrestrial Intelligence
Rather than send a transmission into space and wait for a reply,
we can listen for other civilizations who are already broadcasting.
Which wavelength to listen to?
Somewhere between 21 cm (H I) and 18 cm (OH) might be a good
choice because the background static from the galaxy is low here,
and any intelligent civilization should know the wavelength of
hydrogen emission, the most common element in the universe.
Deep Space Explorers:
Pioneer 10 and 11
Launched in 1972-73
Visited outer planets in 1970’s
Beyond orbit of Pluto in 1980’s
Now 80 AU from Earth
Traveling at 10 km/s
= 3 light years in 100,000 years
Last contact in 2002
Deep Space Explorers:
Voyager 1 and 2
Launched in 1977
Visited outer planets in late
1970’s and 1980s
Beyond orbit of Pluto in 1990’s
Now 100 AU from Earth
Traveling at 15 km/s
= 5 light years in 100,000 years
Voyager’s Golden Records
Current Locations of Pioneer & Voyager
The Pale Blue Dot
On February 14, 1990,
Voyager 1 turned around
and photographed the
planets as it sped beyond
the orbit of Pluto. This
famous image of the Earth
from a distance of 4 billion
miles is known as the “Pale
Blue Dot”.