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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 6: The Drake Equation and the Search for Extraterrestrial Intelligence

Key Ideas

SETI=Search for Extraterrestrial Intelligence
'Skips' to the last step in the search for life.
Relies on the assumption that intelligent life is common
How common is intelligent life? We don't know
The Drake Equation parameterizes our ignorance
Number of intelligent civilizations is critically dependent on their lifetime and willingness to communicate.
Average distance to nearest civalization dictates:
Length of time for message to propogate
Number of possible 'conversations'

Jill Tarter (b1944)

Trained as an astronomer
PhD from Berkeley
Director of the Center for SETI Research
Not the inspiration for Contact
But consulted with Jodie Foster
One of the first "astrobiologists"
One of the first and principle SETI researchers

One approach...

Indirect detection of Earth-mass, habitable planets:
Kepler, other future missions
Direct detection of Earthlike planets
Measurement of light is very powerful
Determining the properties of the planets
Spectra allow detection of biomarkers
May show evidence of life

Another approach?

If one assumes that intelligent life is common, and those civilizations can and do communicate, then maybe
We can skip all these intermediate steps and look for signals from those civilizations directly.

Difficulty with 'Skipping to the End'

If nothing is found?
One doesn't know which of the many possible explanations is correct

How many communicating civilizations?

Drake Equation

How many extraterrestrial civilizations are there that we may come in contact with?
N_*= number of stars that form per year
f_planet - fraction that have planets
n_Earth - number of potentially habitable planets
f_life - fraction that give rise to life
f_intelligent - fraction of those that give rise to intelligent life
f_communicate - fraction of civilizations that can and do communicate
L - length of time that civilization that can communicate
Age = age of the Galaxy

Measurement and Conjecture

Only the first 3 terms of the Drake Equation can be measured:
Number of stars
Fraction of Stars with Planets
Number of Potentially Habitable Planets
The rest are purely conjectural

Number of stars in the Galaxy

N_*=100 billion

Fraction of Stars with Planets

Results of searches for exoplanets: Roughly ~15% of stars have planets
However, these are mostly massive planets
What is the frequency of smaller, rocky planets?
Don't know - but it appears as though it might be a lot higher
Also, only applies to FGKM stars. Ok?
Optimisitic: f_planet= 50%

Potentially Habitable Planets

Don't actually know.
Depends on distribution of rocky planets.
Optimistic: n_Earth=1

Fraction with life?

Don't actually know
What clues do we have from life on Earth?

Late heavy bombardment lasted ~1 billion years
An impact of ~500 km would sterilize the planet
Last such impact, 3.8-4.2 billion years ago
Life arose about as soon as it could.

--> f_life=1

Fraction with intelligent life?

Don't actually know
What clues do we have from life on Earth?
It took less than a few hundred million years for life to develop.
It took ~45 times longer for intelligent life to develop.

--> f_life=0.1?
Fraction that communicate?
Can communicate
Choose to communicate.

Rise of technology is predicated on the rise of science
--The ability (and willingness) to make sense of the world in terms of logic and physical principles

Science is:
--A largely cultural development
--Relatively recent phenomenon

Started with the ancient Greek scholars

Given that our civilization is a relatively recent development,
It is likely that other civilizations are much, much older and more developed, perhaps
alien anthropologists may not want to 'disturb' our progress, and so
even if they can communicate, they might not choose not to.

Shameless (and baseless) optimism

Lifetime of civilizations

Take ourselves as an example
Minimum value
--Only had radio communications technology for ~100 years.

Maximum value
--A few years from now?
--A century from now?
--When Earth moves out of the Habitable Zone (0.5-3 Gyr from now)?
--When the Sun runs out of fuel? (5 Gyr from now)?

A Wild Guess

N_*= 100 billion
f_planet - 0.5
n_Earth - 1
f_life - 1
f_intelligent - 0.1
f_communicate - 1
L - 100 years
Age = 10 billion years


Communicating Average Density

Number of Civalizations / Volume of Galaxy
Average Density = 2.5 times 10^{-10} per cubic parsec

Average Distance

~2kpc =6,500 lyr.
Takes 6,500 years from a signal to reach the nearest civilization!
But we assumed a lifetime of only 100 years!

Requirement for at least one "conversation" Lifetime > 1700 years


We can try to 'skip over' some steps in the search for life to find intelligent civilizations directly
Success depends on number of intelligent civilizations
Where do we look?
How do we look?
What are we looking for?

See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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