Jeremy P. CarloColumbia University

AAI Astronomy Day5/10/2008

• 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

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

• 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…

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 ?

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!

X

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…