Download Where to Look: Habitable Zones

The formal definition of habitable zone is the distance
from the star at which temperatures allow for liquid
surface water. Why should we expand this definition?
SETI
Karen J. Meech, Astronomer
Institute for Astronomy
A.  Habitability can be subsurface, enabled by geothermal heat
B.  Liquid water is possible on bodies without substantial atmospheres if it
is under pressure (sub-surface)
C.  There are energy sources that allow for liquid water other than the sun
D.  Habitable zones change with time as a star ages
E.  A, B and C
Overview of Today: Searching for
Extraterrestrial Intelligence
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Where and How to Look?
Where to look for life
How to look for ETI
Chances of Success
Past, current & future
search programs
SETI is a Search for ET Intelligence " technology
Where to Look –
Stellar Lifetimes
L/LS
Lifetime
[yrs]
Class
Temp [K]
107
O
35,000
108 – 107
B
20,000 –
30,000
109 – 108
HR diagram: is a T sequence
Mass (gravity) supported by pressure
109 – 1010
More Mass " more P " higher T "
higher energy (L)
# 1010
Massive stars " shorter lives
A
10,000 –
15,000
F, G
6,000 –
10,000
K, M
3,000 –
6,000
White
dwarfs
T [K]
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Astronomy review
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Where to Look: Habitable Zones
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Region near star where
liquid H2O can exist
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Hot stars – wider zone
Hot stars – HZ far from star
Class
Temp [K]
L (Lsun)
H 2O
Freeze
H 2O
Boil
B1
23,000
13,000
173.7
93
A1
9,700
61
11.9
6.4
G2V
5,800
1
1.5
0.8
M5
3,000
0.015
0.19
0.1
The Galactic Habitable Zone
Habitable Zone
An extension of the concept of the HZ
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The region(s) of the Milky Way that allow complex
biological life to emerge
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Massive stars " supernovae
Release of dangerous radiation (not habitable) – red
Early on – not enough heavy elements for planets – blue
Green – optimal conditions for life
Evolution of the HZ
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Stellar luminosity increases
HZ moves outward as star’s T rises
Planet atmosphere composition changes
Some requirements for complex life in the Galaxy:
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Located in a region with few transient radiation events,
such as supernovae
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HZ of Stars
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Depletion of ozone
Low-mass planets in the HZ are tidally-locked to their host stars
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Enough metallicity, or building blocks for terrestrial
planets
Enough time for biological evolution
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Like the moon around the Earth
One side always hot, the other always cold
A planet orbiting a medium mass star in the HZ is not tidally
locked
Binary Star Systems
The Galactic Habitable Zone
Two predictions
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Stellar center of mass
Lineweaver et al. (2004)
Majority of habitable planets between 7-9 kpc,
expanding outwards with
time.
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Gowanlock et al. (2011)
Majority of habitable planets in the inner Galaxy,
expanding with time, but
entire disk suitable.
Schematic of a planet in a binary star system
a)  The planet orbits one component of the binary
b)  The planet is in a circumbinary orbit about both stellar components
From: Perryman 2011
Image: Astronomy Now magazine, Dec. 2011
Where to Look –
Binaries??
Where to Look – Binaries?
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Planets have been detected orbiting in binary star systems
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~50% of stars in the Milky Way are binary systems
Factors influencing habitability
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Eccentricity
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Temperature
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Dynamic stability
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! Cephei – First binary system with a planet
1.59MSun (K1IV), 0.4MSun (M1)
Period = 74 yr, e = 0.44
!  Planet 1.76MJup, e = 0.2
!  Stable if in resonance
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How circular are the orbits of planets?
How does the temperature of planets change with time?
How do the orbits of planets change with time?
Will planets be retained in binary systems over long periods?
Free floating planets have been detected
Class Discussion
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Why do you think that planets in binary star systems could
have habitable planets?
Where to Look: Extrasolar Planets
To date, there have been > 1000 confirmed exoplanets
Including the Kepler candidates, there are over 3000 exoplanet
detections
Statistics show that maybe 70% of stars might have planets
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! 185
stars with planets out to 150 LY
How: Manifestations of
Advanced Civilizations
Where to Look – Summary
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Rule out hot stars: OBA
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Rule out cool stars?? M
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Best F, G, K Stars
Lifetime too short
HZ too small
!  But . . . These are the most numerous
stars . . .
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Binaries?
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Possible, but not first choice
We ve Already Sent Messages
# stars
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Invention of TV – 1940 s (~70 yrs ago)
EM radiation travels out to 70 LY
50% of power is narrow (0.1Hz) carrier wave
50% of power is the picture
! 1 AU
How far has our TV
signal travelled?
= 1.5 LY
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Our stellar neighborhood
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! Oort
Dyson Spheres
Cloud
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Oort cloud ~ 100,000 AU = 1.5 LY
Local stellar density ~ 0.004 stars / cubic LY
In 2013 the signal has passed ~ 6500 stars
http://workshop.chromeexperiments.com/stars/
Builds structure
surrounding star at HZ
distance
!  Collects all solar energy
!  Only IR / radio escape
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! 77
! 25
LY
Interstellar Travel
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80 years
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Lack technology
10 years
Fuel costs ~ $1015 (106 billion = quadrillion)
Cooling requires 2x103 km array
Scoop of 100 km radius
Not invented!
Characteristics of Effective
Communication
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Expensive ( millennia of the world GNP)
Uninvented Technologies??
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Smaller cost per probe
Send 106 probes (to get big enough star system sample)
Launching 1/day " 3000 years!
Cost $1013
Info sent by radio " many 100m class radio telescopes
Why send a probe? Just listen!
Ramjets – collect fuel from ISM
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Photon (matter-antimatter)
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Eavesdrop on another civilization
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v=30 km/s - to " Cen – 40,000 yr!
Cryogenic suspension? Generations?
Nuclear
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Space Probes – Cheaper Alternative?
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Chemical rockets
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A variant – Ringworld
LY
Types of Searches –
Interstellar Travel
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Advanced civilization
Requires minimum E over
background noise
Travel at the speed of light
Not deflected by stellar
magnetic fields
Easy to generate, beam &
detect
Not be absorbed by ISM
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1-10 GHz – low noise
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Recall c = #f
H radiates at 1.42 GHz
(# = 21 cm)
!  OH at 1.6-1.72 GHz
!  Cosmic Water Hole
!. But millions of frequencies are still possible . . . . !
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EM Radiation
No Mass
No Charge
Specific Wavelengths
Where to Search in the EM
Spectrum – The Water Hole
Probability of Success?
The Drake Equation
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Nciv
N*
fp
np
fl
fi
fc
Lc
Number of detectable civilizations
Number of stars now
Fraction forming planets
Fraction suitable for life
Fraction where life exists
Fraction with intelligence
Fraction with communication
Probability of survival
! Number
of stars now
R = 10 yr-1
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Stars form out of clouds of gas & dust
Process ~ well understood
We have a good observed number for
local rate of star formation (R*) over time
At least 100 billion stars in the Milky Way
Number of stars is related to the star
formation rate
N* = Sum of R* over time
Nciv = N* x fp x np x fb x fc x fi x fc x Lc
Illingworth, et al. (UCLO)
Fraction Forming
Planets
! How
Many are
Habitable?
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By Star type
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fp = 0.2-0.9
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Rate of hot Jupiters – 0.75%
Rate of Jupiter-mass objects inside 3.5 AU
– 7%
1-1.5% sunlike stars have gas giants
Mostly finding Jupiter-mass to date " must
have smaller planets too
True rate must be much higher!
!  40% of stars may have low mass
planets
New Kepler discoveries " 1200 planets!
! Fraction
With Life
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Organics in space are ubiquitous
Miller-Urey Experiments
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Hydrothermal vents
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Life occurred early on the Earth
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Extrasolar Planet Searches
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Organics + water + energy " amino acids
Oceans + internal planetary heat
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Rocky planet occurrence
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np = 0.01-0.5
M stars: small zones, but very abundant
F stars: much larger HZ
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23% of stars host super Earths with P < 50 days
(Howard et al. 2010)
17.4% of stars have planets with M < 10 Mearth
(Wittenmyer et al. 2011)
! Fraction
with
Intelligence?
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Studies of language " markers of intelligence
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Information theory
Complexity of communication
Ability to make tools / problem solve
Brain to body mass ratio
Many factors may be indicative of intelligence
fl = 0.001-0.01
fi = 0.001-0.01
Animal Communication Complexity
Fraction with Technology
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Species
Communication Entropy Repertoire
Size
Monkey
Social behavior
6.9
120
Whistles
6.67
102
Alphabet "
char (english)
4.8
26
Chinese
11.6
3000
Bottlenose
dolphin
Homosapien
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fc = 0.1-0.9
Survival?
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Homo sapiens has been around for
~ 3 million yrs (Australopithicine)
Technology – 100 yrs
Inevitable?
! Survival?
Mean lifetime of a civilization or
fraction which endures a long
enough time to be detected.
“The receipt of a message from an advanced civilization will show that
there are advanced civilizations, that there are methods of avoiding the
self-destruction that seems so real a danger of our present
technological adolescence. Finding a solution to a problem is helped
enormously by the certain knowledge that a solution exists. This is one
of many curious connections between the existence of intelligent life
elsewhere and the existence of intelligent life on Earth.”
Carl Sagan
Lc = 10-5-0.01
Meanings
Evaluating the Drake Equation
Variable Pessimist
N*
Optimist
Description
100 billion 100 billion Num of stars in MW
fp
0.2
0.9
Fraction of stars form planets
np
0.01
0.5
Fraction w/ habitable planets
fl
0.001
0.01
Existence of Life
fi
0.001
0.01
Rise of Intelligence
fc
0.001
0.9
Rise of technology
Lc
0.00001
0.01
Survival/duration now
Total
0.000002
50500
Communicating civilizations now
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Pessimist: 106 LY
between civilizations
Optimist: 16 LY " lots
of neighbors!
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Optimist
!  Curiosity is by product of
intelligence
!  Laws of physics same
everywhere
Pessimist
!  Consider development 104-109
yrs ago! No communication!
Frank Tipler
Proof why there are
no other civilizations
Optimistic vs. Pessimistic Views
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Optimist
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Curiosity is the by product of intelligence
Laws of physics same everywhere: might give rise to intelligent
life everywhere
Life occurred early on the Earth and lead to intelligent life
Many planets detected in the Milky Way
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Pessimist
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Tipler Argument
We’ve only been able to send signals for ~100 years
It is not clear that technology does not lead to self-destruction
(e.g. the cold war)
Fred Hoyle: “The chance that higher life forms might have
emerged in this way is comparable to the chance that a tornado
sweeping through a junkyard might assemble a Boeing 747 from
the materials therein.”
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Arguments Against Tipler
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Pre-Copernican comparison
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The Zoo Hypothesis
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We took a long time to learn we are not the center of the solar
system, why the only ones in the galaxy?
There are advanced civilizations
They leave us alone to develop ( prime directive )
Seti scientist N. Cohen
How do we know Frank Tipler exists? Have you ever
seen him? There are 4 billion people on this planet,
surely an intelligent creature would find some direct way
of making his presence known to a sizeable fraction of
the planet. . .
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1600 Feb 17, Rome " burned
at stake for heresy
There are an infinite number of
other worlds, and life exists on
them
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Mathematician Gauss
!  1820 s
!  Communication scheme
for inhabitants on Moon
!  Tree arrangement in
Siberia " pythagorean
theorem.
They do not exist
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Von Neumann machines
Pre-Copernican comparison
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We took a long time to learn we are not the center of the solar
system, why the only ones in the galaxy?
The Zoo Hypothesis
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There are advanced civilizations
They leave us alone to develop ( prime directive )
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Rare Earth Hypothesis
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Astrophysical explanation: major catastrophes
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Complex life is rare, microbial life may be abundant
The timescales for life extinguishing events like gamma ray bursts is
roughly the same timescale that it takes for intelligent life to arise
Threats from supernovae and close planetary system encounters
Project Ozma
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Giordano Bruno
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The Great Silence
Early SETI
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Will explore galaxy in 300 x
106 yr
No evidence on Earth
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Take the optimistic path for #
civilizations
No technology "
sophisticated ~ 100 yr
Older societies " more
advanced
Advanced societies will have
interstellar travel
1959 Phil Morrison " First
SETI article
It is difficult to estimate the chances of
success, but if we don t try the
chances are zero
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85 ft NRAO dish in West VA
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! 
2 nearby stars
Strange noise pulses "
terrestrial interference
Star
Dist
[pc]
Type PM [ /yr]
$ Ceti
3.46
G8V
1.92
% Eri
3.28
K2V
0.98
Unraveling the Mystery
1967 Galactic Radio Survey
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Mysterious repeated pulsed
ratio signals
Repeat every 1.33730115 sec
Pulse duration 0.001 s
Regular LGM signals
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A New physical phenomenon
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A new physical phenomenon
" the Discovery of Pulsars
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Small Source Size: New object!
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J. Bell discovered the signal
(grad student)
A. Hewitt got the nobel prize
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Finite speed of light " cannot have
infinitely thin pulse
Small source
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Energy spread over many f "
expensive
No doppler effect " not from
orbiting planet
Knew distance of object
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Example:
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1
2
Source 3 x 105 km diam
Turn off instantaneously
Light from 1 takes longer than from 2
to reach earth
Signal strength " 109 more than
Earth s E output " not artificial!!
vel x time = dist; d / v = t
3x105 / 3x105 = 1 sec
Drake Puzzle Solution
What is the Message?
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First US SETI
meeting (11/1961)
Drake passed out a
test to participants
Language would
not be sent " picture
Encode a picture in
1 s and 0 s
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551 numbers
Divisible by prime #s
29 x 19 or 19 x 29
Other Early Efforts
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1974 Cornell
Arecibo Antenna
!  Message to Hercules cluster
!  25,000 LY away
!  300,000 stars
! 
Binary
!  001
!  010
!  011
!  100
!  101
!  110
!  111
!  1000
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1
2
3
4
5
6
7
8
! 
Left column
Sun & planets
Bottom Center
Human
R: Next to planets – populations
#2 has 11 (expedition?)
#3 has 3000 (colony?)
!  #4 has 7 billion
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Top: C and O (C-based life)
Other Early Efforts
Adenine
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Cytocene
Voyagers 1 & 2
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Guanine
Thymine
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1983 6/13 Pioneer 10
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Solar System
Gold disks with greetings,
music
First to leave Solar System
Gold Plaque
Scale of spacecraft & humans
Solar system & Earth
!  3D pulsar map of location
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Arecibo Antenna
Human Genome
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SETI Search Space
A Variety of Searches
Date
Observer
Observatory
# or Freq
Targets
! 
Drake
NRAO
1.42 GHz
% Eri, $ Tau
1968
Troitsky
Gorky
21, 30 cm
12 solar stars
1972
Verschuur
NRAO
1.42 GHz
10 nearby stars
! 
1972
Troitsky
Gorky
16, 30, 50 cm
All sky
! 
1972-6 Palmer, Zuckerman
NRAO
21 cm
670 nearby stars
1972
Kardashev
Europe
Several
All sky
1973-
Dixon, Cole
Ohio
21 cm
All sky
1974
Bridle, Feldman
Algonquin
1.3 cm
70 nearby stars
1975-6 Drake, Sagan
Arecibo
12.5, 18, 21 cm Nearby galaxies
1977
Tarter
NRAO
18 cm
200 nearby stars
1978
Horowitz
Arecibo
21 cm
185 nearby stars
1983
Horowitz
Harvard
1.42 GHz
All Sky
1985
Horowitz
META
1.42 GHz
All Sky
Fraction of Sky covered
Sensitivity of Search
!  Number of frequencies
Planetary Society & SETI
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Begun in 1981 – Targeted & All Sky
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131,000 channels 1.42 GHz
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META (1985)
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131,000 channels
200 stars
Sentinal – continuation of Suitcase SETI
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The NASA Search
Suitcase SETI – Horowitz (1983)
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Billion-channel Extra-Terrestrial Assay
109 channels
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The NASA Search
Targeted Program
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300-1000s per star
107 frequencies
Centered on 1.42 GHz, 1-3 GHz
770 Solar type stars
Within 22 LY
Good candidates for planets
Use Arecibo
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All Sky Survey
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Use 34-m DSN dishes
Scan entire sky (3-5 yrs)
107 channels, 1-10 GHz
If they existed, they would be here by now
Sagan – talked him out of objections (Drake Eqn)
Senate budget approved SETI in 1982
Renamed from SETI to HRMS (High Resolution Microwave Survey)
In 1993
! 
! 
Budget of $2,000,000/ yr
Senator Proxmire – cut funding in 1981
! 
Megachannel Extra-Terrestrial Assay
8x106 channels
Very sensitive – Energy of Earth could be detected out to 1000 LY
BETA (1995)
! 
! 
Each survey has been
limited
1960
Richard Bryan - Nevada senator eliminated all NASA funding for SETI
SETI was only 0.1% of NASA’s annual budget, or a nickel per taxpayer
So, SETI Is Not !
! 
! 
! 
An investigation of UFO s or alien
abductions
A religion, or a cult
Politically correct
Dawn of public-private partnerships within Astrobiology?
! Project
Phoenix
! SETI
Institute: Allen Telescope Array
•  1000 nearby sun-like stars from 1995-2004
•  200 stars in Southern hemisphere
•  11000 hours; 240 light years
!  "#$%"$&$'()*$%
!  +,-%'./01%2.'#$'%
!  3456&%7)%8-9%1:%
! 
•  1 – 3 GHz
•  Realtime signal detection and follow up
•  Pioneer 10: signal beyond Solar System
;)(67$2%67%<67%=>$$?@%=A%
!  B5C&.(0*>.D67$%*6>7E$>'#.*%%
!  F$'.GE%H%()E'7>5(7.)E%I5E2'%>6.'$2%
*>.D67$&J%
!  B65&%A&&$E%#6'%2)E67$2%K+-%1.&&.)E%
!  A(($''%I)>%7#$%G$E$>6&%'(.$E7.L(%
()115E.7J%
! Berkeley
! Optical
SETI
! Enhanced
•  “Alien” Laser signals: continuous or pulse
! SKA
•  More efficient, narrower beam, bright
! Phoenix
•  Several searches: Harvard, Berkeley, Lick..
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We Transmit?
Not yet?
Could we be the youngest technology in the galaxy?
We are currently leaking
Transmission is a long term strategy
We are culturally too immature
Who will speak for Earth?
What will they say?
Organizations:
!  U
" nited Nations Committee on the Peaceful Uses of Outer Space
! 
! 
International Institute of Space Law
International Academy of Astronautics
! 
! Timelines
ATA
Allen Telescope Array
! 
! ATA
•  No “waterhole”; unique absorption lines ?
! Should
Exploration Of The Galaxy
! 
SKA
Square kilometer array
!  collaboration between ~10
countries
! 
for Active SETI
! If
we transmit a signal to all known stars, how many could we
receive a response within the following time durations
! 
! 
! 
10 years
!  A signal could travel to and from a star within a distance of 5 light yrs
!  There are only 3 stars within 5 light years
40 years
!  The volume is (43) 64 times larger than that of 10 years
!  There are ~100 stars within 20 light years
100 years
!  ~0.00323 stars per cubic light year in the solar neighborhood
!  Distance is 50 light years
!  Volume of a sphere: (4/3)*pi*503
!  ~1700 stars or 1 out of every 100 million stars in the Galaxy
! Timelines
! 
for Active SETI
1000 years:
! 
! 
! Citizen
! 
SETI efforts can be helped using the
SETI@Home distributed computing platform
! 
The platform uses your unused CPU cycles to
process radio telescope data
1.7 million stars within 500 light years
About 1 in every 100,000 stars in the Milky Way
! 
! 
! "#$%*>)C6C.&.7J%)I%'5(($''%
! %.'%2.M(5&7%7)%$'7.167$@%C57%.I%N$%
E$D$>%'$6>(#%7#$%(#6E($%)I%'5(($''%.'%
O$>)P%
! 0%=)(()E.%H%Q)>>.')E%
Science
With cheap multicore machines, there is a lot of
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Currently has over 3 million users