Transcript Habitable Planets

Extraterrestrial Life and the Drake
Equation
Basic Ideas
• Number of Civilizations in our Galaxy
– Product of Number of stars and fractions
• N(ever) = N x A x B
– Multiplying by t/T gives the number now
• N(now) = N(ever) (t/T)
• Assumes “steady state” between birth and
death of civilizations
Usual Drake Equation:
N = R* f p n e f l f i f c L
N
=
number of communicable civilizations in our galaxy
R
=
Rate at which stars form
fp
=
Fraction of stars which have planetary systems
ne
=
Number of planets, per planetary system,
which are suitable for life
fl
=
Fraction of life bearing planets where intelligence develops
fc
=
Fraction of planets with intelligent life which develop a
technological phase during which there is a capacity
for and interest in interstellar communication
L
=
Average of lifetime of communicable civilizations
r
=
Average distance to nearest civilization
*
Harrison’s Drake Equation:
N (now) = N x A x B x (t/T)
N(now) =
Number of communicable civilizations in our Galaxy now
N
=
Number of stars in our Galaxy
A
=
Astronomical factors
B
=
Biological factors
t
=
Average lifetime of communicable civilizations
T
=
Age of the Galaxy
Astronomical Factors
• A = p 1 x p2 x p 3
– p1 = fraction of stars suitable
– p2 = fraction of suitable stars with earth-like
planet
– p3 = fraction with that planet in habitable
zone
Number of Stars in Galaxy
• We focus on our own Galaxy
– Light travel time from others > 106 yr
• Estimates of number of stars around 1011
– (probably 4 x 1011 more accurate)
– Calculated from mass of Galaxy and mass
of average star.
Fraction of stars suitable for life (p1)
• If star is much more massive than Sun
– It lives on main sequence for too short a time
• It took 3-4 x 109 yr for intelligent life to evolve on Earth
• Requires M < 1.25 Msun
• If star is not massive enough
– There MAY be problems with tidal lock
• If star in binary (2/3 of all stars)
– About half of these unlikely to allow planets in
stable orbits.
• Net result: p1 about 0.1 to 0.5
• Harrison takes p1 = 0.1
planets
Forbidden Zone
planets
Stable orbits around a Binary Star
Fraction of stars with Earth-like planet (p2)
•
•
•
•
About 150 known planets at this time
But all are much more massive than Earth
But those are the only ones we can detect yet
Fraction of stars with some planet probably
rather high
• Fraction with Earth-like totally unknown
• Searches planned with telescope in space
• Harrison takes 0.1
- with about 80 extrasolar planet candidates identified:
- more than 1000 stars examined.
Successful Doppler planet search programs:
ELODIE/CORALIE (H.P./La Silla) Mayor, Queloz, Udry, et al. (North/South)
Hamilton/HIRES (Lick/Keck) Marcy, Butler, Fischer, et al. (North)
Cs23 (McDonald 2.7m) Cochran, Hatzes (North)
AFOE (Whipple) Noyes, Brown, et al. (North)
ESO CES (La Silla) Kurster, Hatzes, Endl, et al. (South)
UCLES (AAT) Butler, Tinney, et al. (South)
Fraction with Earth-like planet in HZ (p3)
• Habitable zone (require liquid, probably
water)
• Fixes range of temperatures
• Fixes range of distances from star
• Also totally unknown
• Estimates often about 0.5
• Harrison takes 0.1
• Note: related to suitable star factor
Biological Factors
• B = p 4 x p5 x p 6 x p7
– p4 = fraction of habitable planets where
unicellular life arises
– p5 = fraction of those that evolve
multicellular life
– p6 = fraction of those that evolve humanlevel intelligence
– p7 = fraction of those that develop
advanced technological civilization
Fraction where life arises (p4)
• Life arose on Earth within about 109 yr
• We don’t understand the origin of life
– Lab experiments can explain first steps
– But formation of self-replicating molecule is
hard to understand
• p4 is totally unknown
• Harrison takes 0.1
Miller -Urey Experiment
Fraction with life that develop multicellular
life (p5)
• This took about 1.5 x 109 yr on Earth
• Key development is origin of eukaryotes
– Nucleus and organelle
• p5 is totally unknown
• Harrison takes 1 (optimist) or 0.1 (pessimist)
• Note: this is usually not a separate factor
Eukaryotes and Prokaryotes
First appeared ~ 1.5 - 2  109 years ago
complex structure, ~ 104 - 105 genes
First appeared
~ 3 - 4 109 years ago
Few thousand genes
Fraction with multicellular life that develop
intelligent life (p6)
• This took about 3.5 x 109 yr on Earth
– Comparable to age of Galaxy (10 x 109 yr)
• Key development is rapid growth of brain
– Primates, anthropoid apes, hominids
• p6 is totally unknown
• Harrison takes 1 (optimist) or 0.1 (pessimist)
Various Family Trees
Fraction with intelligent life that develop
technological civilization (p7)
• This happened very fast compared to others
– Human level intelligence has existed for
perhaps 105 yr
– Technological civilization has existed for
perhaps 50 yr
• Key developments
– Agriculture, language, science
• p7 is totally unknown
• Harrison takes 1 (optimist) or 0.1 (pessimist)
Connections
Time
2 Myr ago
??
6500 B.C.
6500 B.C.
4000 B.C.
3000 B.C.
3000 B.C.
2800 B.C.
1500 B.C.
500 B.C.
200
1456
1540
1610
1665
1700s
1859
1895
1924
1936
1950s
1960
1990s
Information
Oral Language
Clay tokens
Technology
Stone tools
Collective hunting
Agriculture, cities
Wheel
Copper tools
Clay tablets
Syllabic alphabet
Letter alphabet
World View
Oyster World
Bronze tools
Iron tools
Natural Philosophy
Ptolemaic Model
Printing Press
Telescope
Copernican Model
Kepler, Galileo
Newton
Industrial Revolution
Darwin
Radio
Other galaxies
First TV Broadcast
Computers
Internet
Transistors, microchips
Miller-Urey
First Search for Signals
Harrison’s Results for N(ever)
• Astronomical factors: N x A = 100 x 106
planets suitable for life
• Biological factors: B = 0.1 (optimist); 10–4
(pessimist)
• N(ever) = 10 million (optimist); 10,000
(pessimist)
Harrison’s Results for N(now)
• Harrison assumes t = 1 x 106 yr; T = 10 x 109 yr
– N(now) = 10–4 N(ever)
– Note that t is completely unknown
• According to Harrison:
– N(now) = 1000 (optimist); 1 (pessimist)
• But, if t = 1000 years, N(now) = 10–3
– Never two at once
– We are alone
Distance to Nearest Neighbor
1. Assume civilizations spread uniformly
but randomly through galaxy
Galaxy
r
Ne are st
*
civiliz ation
us
r = radius of imaginary sphere centered on us
that touches nearest civilizaztion
search vol  r3
 r = 104 ly
N1/3
Distance to Nearest Neighbor
Ne are st
civiliz ation *
If N < 8000,
r from previous formula is 500 ly
About equal to thickness of Galaxy
Use cylinder for search vol  r 2 h
so
r
r = 5 104 ly
N1/2
h
Happy Feller
R
fp
ne
fl
fi
fc
L
N
r
Estimate
50
1
1
1
1
1
5  109
2.5  1011
1.6 ly
Birthrate
50
50
50
50
50
50
2.5 out of 4 stars
If N > 8000,
If N <8000,
r=
104 light years
N1/3
r=
5  104 light years
N1/2
Angela Angst
R
fp
ne
fl
fi
fc
L
N
r
Estimate
5
0.1
0.1
0.01
0.01
0.01
100
5  10–6
---
Birthrate
5
0.5
0.05
5 x 10–4
5  10–6
5  10–8
Never two civilizations
at same time
If N > 8000,
If N < 8000,
r=
104 light years
N1/3
r=
5  104 light years
N1/2
Mr. Average Guy
R
fp
ne
fl
fi
fc
L
N
r
Estimate
10
0.5
0.89
0.5
0.7
0.6
1  106
9.4  105
100
Birthrate
10
5
4.45
2.23
1.56
0.94
1 out of
4  105 stars
If N > 8000,
If N < 8000,
r=
104 light years
N1/3
r=
5  104 light years
N1/2
10  105 = 106
Points to bear in mind
• r is based on assuming spread uniformly
– Could be less in closer to center of MW
• r is based on averages
– Could be closer but unlikely
• r is less uncertain than N
• Since signals travel at c, time = distance in ly
• If L < 2r, no two way messages
For More Info
• Website for my class on Extraterrestrial Life
– http://www.as.utexas.edu/astronomy/educa
tion/spring05/evans/309l.php
– Has links to factors in Drake Equation
(usual form)
– Has link to a calculator
• You can try your own values