Search for Extraterrestrial Intelligence

SETI

Michael L. Sitko

University of Cincinnati

June 29, 2004

Early Speculations

c. 400 BCE - Metrodorus of Chios - "It is unnatural in a large field to have only one shaft of wheat and in the infinite universe only one living world.”

c. 50 BCE - Lucretius - "Nothing in the universe is unique and alone, and therefor in other regions there must be other Earths inhabited by different tribes of men and breeds of beasts.”

Giordano Bruno - there must be "an infinite number of suns with planets and life around them". Burned at the stake in 1600 CE

“Attempts” at Communicationc. 1820 Karl Gauss - plant huge forest of trees in the form of a right triangle Look at Mars for same?

c. 1840 Joseph von Littrow - light trenches filled with kerosene

1899 - N. Tesla attempts to send a powerful burst of radio noise as a "message" and then listen for a reply.

1922 - G. Marconi, tries to listen for radio signals from a boat in a remote oceanic location.

OZMA - The first modern radio search

1960 - Frank Drake looks at 2 stars (Tau Ceti & Epsilon Eridani) at 1 wavelength (H I 21-cm line) with 85-ft NRAO dish for a few months.

SETI established as an experimental science

1959 - David Morrison & Giuseppi Cocconi propose radio search for signals from intelligent extraterrestrials (September 19 issue of Nature)

How Many Intelligent Civilizations Are There?

Number of intelligent civilizations

Nic

=Ric ⋅Lic

Ric =rateof formationof intelligent civilizations

Lic =lifetimeof intelligent civilizations

Nic =R* PpPeNePLPI Lic

R* rateof star formation

Pp probabilityof having planets

Pe probabilitythat ecozonelastslong enough

Ne number of planetsinecozone

PL probabilitythat lifewill arise

PI probabilitythat intelligencewill arise

Rate of Star Formation

The Milky Way Galaxy is about 14x109 years old

It contains roughly 200x109 stars

Mean rate of star formation:

But most of this occurred early on - the rate is probably about 1/10 of this, about 2 stars/year.

200x109 stars

14x109 years=14stars / year

Finding Extrasolar Planets

•Direct Imaging

•Proper Motion Studies

•Doppler Searches

•Transit Searches

Doppler Searches

Epsilon Eridani - an original Ozma target

HD 209458b

QuickTime™ and aGIF decompressor

are needed to see this picture.

Transit Searches

Ne

Most dynamic models of 1990s suggested ~1 terrestrial planet in CHZ.

Most KNOWN planetary systems dominated by HOT JUPITERS!

Ne may be small!!

Probability of Having Planets

Searches for extrasolar planets suggests that this number is 0.2 or greater. Current searches are still not complete.

This depends on the metallicity of the star!

Darwin (ESO) & TPF (NASA)

Ecozone Issues

L*4πdp

2 ×crosssection×(1−albedo)

4πR*2σT*

4

4πdp2 πRp

2 (1−a)

4πRp

2σTp4 (roughly...)

Tp

=1

2

R*

dp

(1−a)1/4T*

Energy absorbed by planet:

Written another way:

Energy emitted by planet:

Equilibrium T for planet:

Example: Our Solar System:

Planet dp a Predicted T Observed T

Mercury 5.8x1012 0.056 440 100-620Venus 1.1x1013 0.76 230 750Earth 1.5x1013 0.39 250 180-330 (290 avg.)Mars 2.3x1013 0.16 220 130-290 (SubSolEq)Jupiter 7.8x1013 0.51 104 160 (cloud tops)Saturn 1.4x1014 0.61 81 90 (cloud tops)

What Goes?

Simulate EvolutionRate of outgassing of volatiles (H, C, N, O) from the interiorCondensation of H2O vapor into oceansSolution of atmospheric gases into oceansPhotodissociation of H2O in the upper atmosphereEscape of H from the uppermost atmosphere (exosphere)Chemical reactions in atmospheric gasesPresence of life and variations in biomassPhotosynthesis and burial of organic sedimentsUrey reaction (CaSiO3+CO2CaCO3+SiO2)Oxidation of surface minerals (2FeO+OFe2O3)Variations in the luminosity of the SunVariations in the albedo of the EarthGreenhouse effect

Hart Models (1980)

Continuously Habitable ZonesMichael Hart (1980) Require liquid water for 0.8 Gyrs, etc.

Stellar Mass Spectral Type Inner r of CHZ Outer r of CHZ Thickness(solar masses) (AU) (AU) (AU)

>1.20 Red Giant Too Soon1.20 F7 1.616 1.668 0.0541.15 F8 1.420 1.481 0.0611.10 F9 1.240 1.310 0.0691.05 G0 1.086 1.150 0.0641.00 G2 0.958 1.004 0.0460.95 G5 0.837 0.867 0.0300.90 G8 0.728 0.743 0.0150.85 K0 0.628 0.629 0.0010.835 K1 0.598 0.598 0.000

In all cases, thickness is less than 0.1 AU Pe: 0.01

Recent Models - Kasting et al.

Newer models handle CH3 and NH3 more realistically.

Suggests

Pe>> 0.01

Life IssuesProbability that Life Evolves - Life appeared on Earth as soon as it was climatologically possible. Existence of “extremophiles” suggests PL~1 . But we may be fooling ourselves - only 1 example.

Probability that Life Develops “Intelligence” - again only 1 example. PI~1??

Lifetime of Intelligent Communicative Life - We have had the ability to “communicate” for ~50 years. Might have been as small as 10 years (Cuban Missile Crisis), but we or others might survive for the lifetime of the Sun. Lic~10-1010 yrs??

Put It All Together

Nic

=R* PpPeNePLPI Lic = 2yr( ) 1

5( ) 110( ) 1

10( ) 1( ) 1( )Lic

=0.044Lic ~0.44 −BIG

If Nic =106 ⇒ separation=100pc

If Nic =50 ⇒ separation=2000pc

Obviously very uncertain!

The only way to know for sure is to LOOK!

Search Strategies

Directed - use a major facility for short periods of time.Target Search - look at specific, promising starsSky Survey - look everywhere

Shared - use observations from other projects. You can analyze old data, or make "parasitic" observations.

Dedicated - use a facility exclusively (or at least primarily) for SETI work (in Target Search or Sky Survey)

Signal Types - Radio

Programs - Past, Present, and Future

1960 - OzmaFrank Drake

1973-1997 - OSU

Megachannel Extraterrestrial Assay (META)

Paul Horowitz

META Processors

Harvard 26-m radio telescope

NASA SETISky Survey

16 million channels - 1-10 GHz - equivalent to 10 billion “Ozmas”

Target Search

14 million channels - 1-3 GHz - looking at 1000 stars

Project cancelled by US Congress after 2 megachannel receiver/processor began.

BETA

Main BETA rack - containing 63 FFT boards (each processing 4 million channels)BETA

SETI Institute (founded in 1984) &

Project PhoenixContinuation of Target Search under Private Funding!

Parkes 210-ft, Green Bank 140-ft, Arecibo 1000-ft telescopes

Allen Telescope Array

Optical SETIProposed in 1961 by R. Schwartz & C. Townes

First carried out in 1990 by Stuart Kingsley in Columbus, OH

High-power narrow-band lasers can outshine the Sun at the emitting wavelengths

Look for short pulses with coincidence counters

Message Transmitted from Arecibo - 1974

What Would a Message Look Like?

Binary (or Morse Code) strings of “special” numbers - primes, etc.

What if We Hear “Them”? - SETI Protocol 1. Is it really extraterrestrial?

2. Get confirmation from other astronomers. (If extraterrestrial, tell your government about it).

3. If convincing: announce to International Astronomical Union, Secretary General of the UN, inform SETI groups.

4. Make the first public announcement

5. Make data available to all.

6. Everyone carefully record & disseminate signals

7. Protect frequencies.

8. Don't broadcast back to the ETs! Requires Debate.

9. Study signals. The SETI Committee of the International Academy of Astronautics keeps a list of experts to call on.