A Statistical ApproachA 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• 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

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

Lots of Potential Sites

• 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 Searches still have a lot of biasbias– Cannot “see” the planets directly, only their effect on the parent Cannot “see” the planets directly, only their effect on the parent

star (gravitational or light blocking)star (gravitational or light blocking)– Hard to detect small (earth-size) planetsHard to detect small (earth-size) planets

• Only Jupiter/Saturn/Uranus/Neptune sized planets (mostly)Only Jupiter/Saturn/Uranus/Neptune sized planets (mostly)– Biased towards Jupiter size objects Biased towards Jupiter size objects easiest to detect easiest to detect

We don’t yet have a decent We don’t yet have a decent unbiasedunbiased sample. sample.And it’s nowhere near And it’s nowhere near completecomplete..

But at least its now large (about 1000 systems)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)

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 Q: Given many solar systems, what fraction of these have “earthlike” planets?these have “earthlike” planets?

• If 1 (or more) in the “typical” solar system: If 1 (or more) in the “typical” solar system: – ffp-e p-e = 1 = 1 (or more)(or more)

• If typical systems do not have an earthlike planet:If typical systems do not have an earthlike planet:– ffp-e p-e << 1 << 1

Star:Star: Massive stars have short lifetimes… Massive stars have short lifetimes…

not long enough to develop life.not long enough to develop life.

Low mass star: Low mass star: Not enough ionizing radiation, Not enough ionizing radiation, ““habitable zone” is very small,habitable zone” is very small, Susceptible to outbursts (“flares”).Susceptible to outbursts (“flares”).

Distance from star:Distance from star: Too close: TOO HOT!Too close: TOO HOT! Too far: TOO COLD!Too far: TOO COLD! Orbit too elliptical: Temperature varies too much!Orbit too elliptical: Temperature varies too much! Need a stable orbit over time!Need a stable orbit over time!

Defines “habitable

zone”

Planet’s composition:Planet’s composition: Need liquid HNeed liquid H22O O

(are NH(are NH3,3, CH CH44 etc. acceptable substitutes?) etc. acceptable substitutes?)

Need an atmosphere!Need an atmosphere!

Need organic (carbon) compounds Need organic (carbon) compounds (silicon based life?)(silicon based life?)

No acidic / corrosive environmentNo acidic / corrosive environment

Planet’s sizePlanet’s size Too small Too small -> less gravity -> -> less gravity ->

no atmosphere -> no liquid H no atmosphere -> no liquid H22OO Also, loses geothermal energy too fastAlso, loses geothermal energy too fast No magnetic field?No magnetic field?

Too big Too big – probably tend to be – probably tend to be “gas giants” like Jupiter. “gas giants” like Jupiter. No solid surface.No solid surface.

(Floating life forms?)(Floating life forms?)

Other factorsOther factors Moderate axial tilt Moderate axial tilt Moderate rotation rateModerate rotation rate

No spin-orbit lock? No spin-orbit lock? Large moon necessary for the above?Large moon necessary for the above? What about moons of gas giants?What about moons of gas giants? ““Good Jupiter”Good Jupiter” In the Galactic Habitable Zone?In the Galactic Habitable Zone? No nearby supernovae, No nearby supernovae,

gamma emitters, etc.gamma emitters, etc.

?

• Our own solar system has Our own solar system has ffp-ep-e = 1 = 1• (Of course!!)(Of course!!)

• Stretching the definition, maybe Stretching the definition, maybe ffp-ep-e = 2 = 2 or more:or more:• Mars? Mars? • Europa? Europa? • Titan?Titan?

• So far no truly “earthlike” planets have So far no truly “earthlike” planets have been found outside the solar system.been found outside the solar system.• And only a few come close… And only a few come close… • Guess from current data…. ~few / 300 ~ 0.01 ?Guess from current data…. ~few / 300 ~ 0.01 ?• But current searches are biased against “earthlike” But current searches are biased against “earthlike”

planets!planets!• May be May be muchmuch higher! (like close to one – habitable zone higher! (like close to one – habitable zone

probability)probability)

Probably “borderline”

Outside habitable zoneBut tidal interactions…

Gliese 581 c/d ?

• Q: Given an “earthlike” planet…Q: Given an “earthlike” planet…

• What is the probability What is the probability that it will develop life?that it will develop life?

• Simplest definition:Simplest definition:– A living organism is something A living organism is something

capable of replicatingcapable of replicating•Bacteria Bacteria •VirusesViruses•Other one-celled organismsOther one-celled organisms

– Need a self-assembling, Need a self-assembling, self-replicating genetic code!self-replicating genetic code!•Earth-based life: DNA / RNAEarth-based life: DNA / RNA•Are there other possibilities?Are there other possibilities?

• If life If life alwaysalways arises on “earthlike” arises on “earthlike” planets, then planets, then ffp-l p-l = 1= 1

• Otherwise, Otherwise, ffp-l p-l < 1 < 1 (maybe << 1)(maybe << 1)

• Only one known example of a planet Only one known example of a planet with life!with life!

Two extreme possibilities A:

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

B: 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…

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…Given a planet with intelligent life…

What is the probability that they develop What is the probability that they develop tools to communicate through space?tools to communicate through space?

Given a planet with intelligent life forms Given a planet with intelligent life forms that can communicate…that can communicate…

How long do they remain that way?How long do they remain that way?

We only became able to communicate…We only became able to communicate…

Early 1900’s: <100 years ago!Early 1900’s: <100 years ago!

How much longer will we last?How much longer will we last?

5 billion years: sun turns into a red giant5 billion years: sun turns into a red giant

Mass extinctions every ~100 million yearsMass extinctions every ~100 million years

Tc : once a civilization becomes able to communicate, how long does it stay able to do so?

Are we Alone?Are we Alone?29

10 1 1 1 1/10

1/10

=1% of 1 Billion10 Million

=1 million

Implications of N= 1 million

1 Civilization per 100,000 starsNearest random one is 1000 light years away!Life (all life) is RARE!If intelligent life is UNIQUE to the Earth then either:

– 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.