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1. Is Anyone Out There?Solving the Drake Equation Jeremy P. Carlo
Columbia University
AAI Astronomy Day 5/10/2008
2. 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?)
3. 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.
4. The Drake Equation 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
5. The Drake Equation 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
6. Mathematical Aside: Scientific Notation 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 = 4.5100,000,000 = 4.5108 (4.5E8)
multiplication: 1013 1011 = 1024
division: 109/103 = 106
7. Mathematical Aside: Fraction s 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.
8. Ns: # of stars in the galaxy This is well known to astronomers
Ns = 200-400 billion = 2 to 4 1011
So far, so good
9. fs-p: fraction of stars having planets Q: Given one of the many stars in the galaxy
What is the probability that it has planets?
10. fs-p: fraction of stars having planets Until recently no exoplanets were known
First discovery 1989, then
Today, almost 300 exoplanets known!
20 known multi-planet systems!
11. Detecting Exoplanets
12. fs-p: fraction of stars having planets 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 stars mass & brightness
Which stars do you choose for detailed study?
We dont yet have a decent unbiased sample.
And its nowhere near complete.
But we can estimate
13. fs-p: fraction of stars having planets 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)
14. fp-e: fraction of solar systems with an earthlike planet 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
15. fp-e : factors to consider 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!
Orbit too elliptical: Temperature varies too much!
Need a stable orbit over time!
16. fp-e : factors to consider Planets 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!
17. fp-e : factors to consider Planets 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?)
18. fp-e : factors to consider 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.
19. fp-e: fraction of solar systems with an earthlike planet Our own solar system has fp-e = 1
(Of course!!)
Stretching the definition, maybe fp-e = 2 or more:
Mars?
Europa?
Titan?
So far no truly earthlike planets have been found outside the solar system.
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!
But limited if red dwarf planets arent allowed (must be <0.2 or so)
20. fp-l: fraction of earthlike planets that develop life Q: Given an earthlike planet
What is the probability that it will develop life?
21. fp-l: fraction of earthlike planets that 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?
22. fp-l: fraction of earthlike planets that develop life 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
23. fp-l : factors to consider
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!
24. fp-l : factors to consider 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 its not just popping up everywhere!
25. fp-l : factors to consider Life can survive under all sorts of conditions
Extremophiles!
26. fp-l : factors to consider If life were to be found on Mars
Implies fp-l is large!
27. fl-i: fraction of planets with lifethat develop intelligent life Q: Given a planet with simple life forms
things like bacteria
whats the probability that intelligent life will eventually develop?
28. fl-i: fraction of planets with lifethat develop intelligent life 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
29. fi-c: fraction of planets with intelligent life that develop communication Given a planet with intelligent life
What is the probability that they develop tools to communicate through space?
30. Tc / Tg : Its all about the timing Given a planet with intelligent life forms that can communicate
How long do they remain that way?
31. Tc / Tg : Its all about the timing Tg is the age of the galaxy
Tg = 10 billion years = 1010 years
Whew!
32. Tc / Tg : Its all about the timing Tc : once a civilization becomes able to communicate, how long does it stay able to do so?
33. Tc / Tg : Its all about the timing We only became able to communicate
Early 1900s: <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