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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. N civil = N *  f p  n p  f l  f i  f c  f L where

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the drake equation
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 star’s life that the civilization exists

n civil n f p n p f l f i f c f l
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.

n civil n f p n p f l f i f c f l4
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.

n civil n f p n p f l f i f c f l5
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
Main Sequence Lifetimes

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.

n civil n f p n p f l f i f c f l7
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.

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!

n civil n f p n p f l f i f c f l8
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 in-spiral. Also, their moons might be habitable!

n civil n f p n p f l f i f c f l9
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.

n civil n f p n p f l f i f c f l10
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.)

n civil n f p n p f l f i f c f l11
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

n civil n f p n p f l f i f c f l12
Ncivil = N* fp  np  fl  fi  fc  fL

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

n civil n f p n p f l f i f c f l13
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
n civil n f p n p f l f i f c f l14
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
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

extraterrestrial communication
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
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
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?

slide20
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).

slide21
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.

slide22
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

slide23
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.
slide24
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.
slide25
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

Final exam in

108 Forum (Section 2) and

111 Forum (Section 4)

at 8:00 a.m.(!) on Thursday, December 18

  • See you then!