Astronomy 294
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'

Radio wavelengths may be the best place to look for extraterrestrial intelligence

1420 MHz (or some mathematical multiple) may be the most natural frequency to search

No detections so far, although searches are ongoing

We are sending out signals as well

--Some deliberate

--Some not-so-deliberate

Traveling to other systems is very difficult

--Stars are far away

--Speed of light is a fundamental limit

--Enormous energy required

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

f_communicate=1

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

N=50

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

Listening?

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?

How many neighbors?

An Optimistic Guess

N*=100 billion

fplanet=0.5

nEarth=1

flife=1

fintelligent=0.1

fcommunicate=1

L=5 billion years (Age of the Sun)

Age=10 billion years

N=2.5 billion

Average Density

~0.01 per cubic parsec

Average Distance

~5 parsecs

Talk is cheap!

If you really want to bridge interstellar distances, use light to send messages.

Messages travel at the speed of light

Very low energy cost per message

How do we look?

Where to look?

What wavelengths to use?

Microwaves: 1000-10,000 MHz is a region of relatively low cosmic background "noise"

--Relatively cheap

--Less sensitive to interstellar dust

Lasers at visible or IR wavelengths: very few natural lasers in the sky to cause confusion

Earth is already on-the-air

We have been inadvertently beaming radio signals into space for the last 80 years:

--Radio broadcasts from the 1920s onward

--Television broadcasts from the 1950s onward.

--We could detect these with current technology

Episodes of "I Love Lucy" will have already reached most solar neighborhood stars (~40 light years)

If our optimistic estimate is correct

There is at least one civilization within our 'horizon'

They may already be sending a return message

Very likely very advanced!

SETI

The Search?

SETI:

--Search for Extra-Terrestrial Intelligence

A relatively inexpensive search strategy to look for radio signals from extraterrestrial civilizations.

Various kinds:

--Targeted

--"Piggy back"

Potentially Habitable Planets

Don't actually know.

Depends on distribution of rocky planets.

Optimistic: n_Earth=1

Examples of Current Searches

SERENDIP

--UC Berkeley

--Arecibo

--'Piggy Back'

--SETI@home

Allen Telescope Array

--SETI Institute

--Targeted Survey

--350 Antennas

What are we looking for?

Signals that appear "artificial"

--Very narrow "bandwidth" (<300Hz, the narrowest natural sources)

--Pulsed signals (common way to encode information)

--Very little frequency "drift"

Where to look?

Natural Frequency

Hydrogen offers a natural frequency

"Spin-flip" occurs at 1420 MHz (21 cm)

Mathematical multiples

Hydrogen is the most common element in the Universe

Natural place to look

Might want to avoid this directly because of natural emission

Some mathematical multiple, i.e.

"Hydrogen times pi = 4461 MHz"

So far, no detections.

WOW signal

SETI project at the Big Ear radio telescope at Ohio State

On August 15, 1977, a strong, narrowband radio signal was detected

Had properties expected from a non-terrestrial and non-solar origin

Lasted 72 seconds

Has not been detected again.

The perils of long distances

Signals get much weaker with distance

Limits the range of stars we can probe.

Sending Signals?

Increasing radio silence

Earth's radio brightness has been decreasing:

--Introduction & spread of cable TV.

--Increased use of "directed" communication (e.g., fiber optics, beamed satellite, etc.).

Sufficiently advanced civilizations may emit less "waste radio" & become radio quiet:

--If a civilization wants to be found, it may have to deliberately broadcast its presence.

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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