Transcript Is Anyone Out There? Solving the Drake Equation

A Statistical Approach
•
Q: Is there life beyond the earth?
•
How many of these planets have intelligent
life?
•
How many are able to communicate with us?
– (have adequate technology to send signals into
space)
•
(How many of
them want to?)
?
Ns = number of stars in the Galaxy
fs-p = fraction of stars with planets
fp-e= fraction of planets that are “earthlike”
fp-l = fraction of “earthlike” planets that develop life
fl-i = fraction of above that develop intelligence
fi-c= fraction of above that develop communication
Tc = lifetime of communicative civilization
Tg = age of Galaxy
• Most of the terms in the Drake Equation are in
the form of fractions.
• f=1 implies something that always happens
• f=0 implies something that never happens
• Values in between are things that might happen
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f=0.5 means a 50/50 chance
f=0.1 means a 1 in 10 chance
f=10-3 is a 1/1000 chance
f=10-6 is one in a million
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This is well known to astronomers…
Ns = 200-400 billion = 2 to 4 × 1011
So far,
so good…
Lots of Potential Sites
M31, the Andromeda Galaxy
Astrophoto by Robert Gendler
• Q: Given one of the many stars in the
galaxy…
• What is the probability that it has planets?
•
Until recently no exoplanets were known
“explosion of
discovery”
Transit method now
becoming the
preferred method of
detection
100-150 new
systems detected
each year
•
Searches still have a lot of bias
–
–
Cannot “see” the planets directly, only their effect on the parent
star (gravitational or light blocking)
Hard to detect small (earth-size) planets
•
–
Only Jupiter/Saturn/Uranus/Neptune sized planets (mostly)
Biased towards Jupiter size objects  easiest to detect
We don’t yet have a decent unbiased sample.
And it’s nowhere near complete.
But at least its now large (about 1000 systems)

We now know that at least 10% of “typical” stars
have planets. (fs-p = 0.1)
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Infrared studies of discs around young stars
indicate fs-p ~ 0.2-0.5.

But we can only detect a limited subset of
planets…

So maybe they all do! (fs-p = 1)
•
•
Q: Given many solar systems, what fraction of
these have “earthlike” planets?
If 1 (or more) in the “typical” solar system:
–
•
fp-e = 1 (or more)
If typical systems do not have an earthlike planet:
–
fp-e << 1
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Star:
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Massive stars have short lifetimes…
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Low mass star:
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not long enough to develop life.
Not enough ionizing radiation,
“habitable zone” is very small,
Susceptible to outbursts (“flares”).
Distance from star:
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Defines
“habitable
zone”
Too close: TOO HOT!
Too far: TOO COLD!
Orbit too elliptical: Temperature varies too much!
Need a stable orbit over time!
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Planet’s composition:
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Need liquid H2O

(are NH3, CH4 etc. acceptable substitutes?)
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Need an atmosphere!
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Need organic (carbon) compounds
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(silicon based life?)
No acidic / corrosive environment
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Planet’s size

Too small -> less gravity ->
no atmosphere -> no liquid H2O
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Also, loses geothermal energy too fast
No magnetic field?
Too big – probably tend to be
“gas giants” like Jupiter.
No solid surface.
 (Floating life forms?)
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Other factors
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Moderate axial tilt
Moderate rotation rate
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No spin-orbit lock?
Large moon necessary for the above?
What about moons of gas giants?
“Good Jupiter”
In the Galactic Habitable Zone?
No nearby supernovae,
gamma emitters, etc.
?
• Our own solar system has fp-e = 1
• (Of course!!)
• Stretching the definition, maybe fp-e = 2 or more:
• Mars?
• Europa?
• Titan?
Probably “borderline”
Outside habitable zone
But tidal interactions…
• So far no truly “earthlike” planets have
been found outside the solar system.
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Gliese 581 c/d ?
And only a few come close…
Guess from current data…. ~few / 300 ~ 0.01 ?
But current searches are biased against “earthlike” planets!
May be much higher! (like close to one – habitable zone
probability)
• Q: Given an “earthlike” planet…
• What is the probability
that it will develop life?
• Simplest definition:
– A living organism is something
capable of replicating
• Bacteria
• Viruses
• Other one-celled organisms
– Need a self-assembling,
self-replicating genetic code!
• Earth-based life: DNA / RNA
• Are there other possibilities?
• If life always arises on “earthlike” planets,
then fp-l = 1
• Otherwise, fp-l < 1 (maybe << 1)
• Only one known example of a planet with
life!
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Two extreme possibilities
A:
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Even the simplest life is extremely complex!
Simplest organisms have about a million base pairs in
DNA/RNA
Lots of things have to go “just right”; overcoming
failure points
fp-l is “obviously” very small! Less than 10-6
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B:
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Building blocks of life are found in space and on other
planets
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Initial life on earth seems to have developed rather
quickly…
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Organic molecules
Water
fp-l might be large (possibly  1?)
But seems to have developed only once , not many
times…
 Life can survive under all sorts of conditions
▪ Extremophiles!
 If life were to be found on Mars…
▪ Implies fp-l is large!
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Q: Given a planet with simple life forms…
…things like bacteria…
…what’s the probability that intelligent life
will eventually develop?
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Simplest life forms: self-replicating organisms
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But “copies” are not exact
 Mutations
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Those variants best suited to survive,
best able to reproduce, are more
likely to pass on their genetic code
to the next generation
 Natural selection
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Over time those changes
progressively accumulate
 Evolution
 Given a planet with intelligent life…
 What is the probability that they develop
tools to communicate through space?
 Given a planet with intelligent life forms
that can communicate…
 How long do they remain that way?
 We only became able to communicate…
 Early 1900’s: <100 years ago!
 How much longer will we last?
 5 billion years: sun turns into a red giant
 Mass extinctions every ~100 million years
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Tc : once a civilization becomes able to
communicate, how long does it stay able to do so?
Are we Alone?
29
10
1
1
1
1/10
1/10
=1% of 1 Billion
10 Million
=1 million
Implications of N= 1 million
1 Civilization per 100,000 stars
 Nearest random one is 1000 light years away!
 Life (all life) is RARE!
 If intelligent life is UNIQUE to the Earth then
either:
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–
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F_i = 1 in a billion
L = 1000 (for everyone) !!
This seems unlikely and therefore, with proper
planetary management, one day we will be in
the club and know the answer.