jeremy p. carlo columbia university aai astronomy day 5/10/2008
TRANSCRIPT
• 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
TIMECOST
SPACE TRAVEL Sloooow… Looooong…. $$$$$$$$
RADIO COMMUNICATI
ON
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… 1310,000,000,000,000 = 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 GalaxyAstrophoto by Robert Gendler
• Until recently no exoplanets were known– First discovery 1989, then…
Today, almost 300 exoplanets known!
20 known multi-planet systems!
The Snowball
Effect!
• 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! Orbit too elliptical: Temperature varies too
much! Need a stable orbit over time!
Defines “habitable
zone”
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?
• 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 aren’t allowed (must be
<0.2 or so)
Probably “borderline”
Outside habitable zoneBut tidal interactions…
Gliese 581 c/d ?
55 Cancri f ?
HD28185 b ?
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!
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?