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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.
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