Transcript Is Anyone Out There? Solving the Drake Equation

Jeremy P. Carlo
Columbia University
AAI Astronomy Day
5/10/2008
•
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?)
?

What this is not about:
 Aliens visiting the earth
▪ Alien abductions, UFOs, etc.
 Us going to other planets in search of life

Justification: Traveling to other solar systems
is hard. Much easier to use radio.
SPEED
TRAVEL TIME
COST
SPACE TRAVEL
Sloooow…
Looooong….
$$$$$$$$
RADIO
COMMUNICATION
Fast! (c)
Long, but not as
much
Cheap!
•
Developed in 1960 by Frank Drake and others at
SETI
– (SETI: Search for Extra-Terrestrial Intelligence)
N = Ns*fs-p*fp-e*fp-l*fl-i*fi-c*Tc / Tg
N = # of communicative civilizations in our galaxy, right now
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
•
How to deal with really big or small (“astronomical”)
numbers!
•
10,000,000,000,000 = big number.
Count up the zeroes… 13
10,000,000,000,000 = 1013 (1E13 in the computer)
•
0.000000001 = small number.
0.000000001 = 1/1,000,000,000 = 1/109 = 10-9 (1E-9)
•
450,000,000 = 4.5×100,000,000 = 4.5×108 (4.5E8)
multiplication: 1013 ×1011 = 1024
• division: 109/103 = 106
•
•
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
•
•
•
•
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
etc.



This is well known to astronomers…
Ns = 200-400 billion = 2 to 4 × 1011
So far,
so good…
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
– First discovery 1989, then…
The
Snowball
Effect!
Today, almost 300 exoplanets known!
20 known multi-planet systems!
•
Searches still have a lot of bias
– Cannot “see” the planets directly, only their effect on the parent
star
– Hard to detect small (earth-size) planets
• Only Jupiter/Saturn/Uranus/Neptune sized planets (mostly)
– Favor “hot Jupiters”
– Also orbital inclination angle, parent star’s mass & brightness…
– Which stars do you choose for detailed study?
We don’t yet have a decent unbiased sample.
And it’s nowhere near complete.
But we can estimate…

We now know that at least 10% of “typical” stars
have planets. (fs-p = 0.1)

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

Star:
 Massive stars have short lifetimes…
▪ not long enough to develop life.
 Low mass star:
▪ Not enough ionizing radiation,
▪ “habitable zone” is very small,
▪ Susceptible to outbursts (“flares”).

Distance from star:
 Too close: TOO HOT!
 Too far: TOO COLD!
Defines
“habitable
zone”
 Orbit too elliptical: Temperature varies too much!
 Need a stable orbit over time!

Planet’s composition:
▪ Need liquid H2O
▪ (are NH3, CH4 etc. acceptable substitutes?)
▪ Need an atmosphere!
▪ Need organic (carbon) compounds
▪ (silicon based life?)
▪ No acidic / corrosive environment
▪ Need elements heavier than
hydrogen / helium
▪ No “Population II” stars!

Planet’s size
 Too small -> less gravity ->
no atmosphere -> no liquid H2O
▪ 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?)

Other factors
 Moderate axial tilt
 Moderate rotation rate
▪ No spin-orbit lock?
▪ Red dwarfs out?
 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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55 Cancri f ?
HD28185 b ?
And only a few come close…
Gliese 581 c/d ?
Guess from current data…. ~few / 300 ~ 0.01 ?
But current searches are biased against “earthlike” planets!
May be much higher!
But limited if red dwarf planets aren’t allowed (must be <0.2 or so)

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!

Not much hard data to go on here…

Two schools of thought:

School 1:
 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”
 fp-l is “obviously” very small!

School 2:
 Building blocks of life are found in space and on other
planets
▪ Organic molecules
▪ Water
 Initial life on earth seems to have developed rather
quickly…
▪ fp-l might be large (possibly  1?)
 But seems to have developed only once , not many
times…
▪ So it’s not just popping up everywhere!
 Life can survive under all sorts of conditions
▪ Extremophiles!
 If life were to be found on Mars…
▪ Implies fp-l is large!

Q: Given a planet with simple life forms…
…things like bacteria…
…what’s the probability that intelligent life
will eventually develop?

Simplest life forms: self-replicating organisms

But “copies” are not exact
 Mutations

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

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?

Tg is the age of the galaxy

Tg = 10 billion years = 1010 years

Whew!

Tc : once a civilization becomes able to
communicate, how long does it stay able to do so?
?

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

But will we even last that long…