NSCI 314

LIFE IN THE COSMOS14 -EXTRASOLAR PLANETS (CONTINUED)

AND THE DRAKE EQUATION

Dr. Karen KolehmainenDepartment of Physics, CSUSB

http://physics.csusb.edu/~karen/ 

METHODS FOR DETECTING EXTRASOLAR PLANETS DIRECT OBSERVATION

TRANSITS

GRAVITATIONAL LENSING

ASTROMETRY

DOPPLER EFFECT (MOST SUCCESSFUL)

DOPPLER EFFECT A SHIFT IN THE WAVELENGTH OF A WAVE DUE

TO RELATIVE MOTION OF THE SOURCE AND THE OBSERVER

IF THE SOURCE AND OBSERVER ARE MOVING TOWARDS EACH OTHER, THE WAVELENGTH IS SHORTENED.

IF THE SOURCE AND OBSERVER ARE MOVING AWAY FROM EACH OTHER, THE WAVELENGTH IS LENGTHENED.

THE FASTER THE RELATIVE MOTION, THE MORE THE WAVELENGTH CHANGES.

SEE DEMONSTRATION (JAVA APPLET) AT: http://lectureonline.cl.msu.edu/~mmp/applist/doppler/d.htm

DOPPLER EFFECT FOR SOUND WAVES, A CHANGE IN

WAVELENGTH IS A CHANGE IN PITCH.– THE SOUND IS HIGHER PITCHED IF THE

SOURCE AND OBSERVER ARE MOVING TOWARDS EACH OTHER.

– THE SOUND IS LOWER PITCHED IF THE SOURCE AND OBSERVER ARE MOVING AWAY FROM EACH OTHER.

EXAMPLE: SIREN ON A MOVING CAR

DOPPLER EFFECT FOR LIGHT WAVES, A CHANGE IN

WAVELENGTH IS A CHANGE IN COLOR.– THE LIGHT IS BLUER IF THE SOURCE AND

OBSERVER ARE MOVING TOWARDS EACH OTHER (BLUESHIFT).

– THE LIGHT IS REDDER IF THE SOURCE AND OBSERVER ARE MOVING AWAY FROM EACH OTHER (REDSHIFT).

EXAMPLE: LIGHT COMING FROM DISTANT GALAXIES IS REDSHIFTED DUE TO THE EXPANSION OF THE UNIVERSE.

STELLAR DOPPLER SHIFT DETECTION

Star Moves Toward Observer

LIGHT FROM STAR IS BLUE SHIFTED

Unseen Planet Moves Away From Observer

STELLAR DOPPLER SHIFT DETECTION

Star Moves Away From Observer

LIGHT FROM STAR IS RED SHIFTED

Unseen Planet Moves Towards Observer

DOPPLER EFFECT DETECTION OF PLANETS PLANET AND STAR ORBIT AROUND THEIR

COMMON CENTER OF MASS SINCE THE STAR IS MUCH HEAVIER, IT

MOVES IN A SMALLER CIRCLE (OR ELLIPSE)

THE PLANET IS UNSEEN, BUT LIGHT FROM STAR IS ALTERNATELY BLUESHIFTED AND REDSHIFTED DUE TO THE MOTION OF STAR

CYCLE REPEATS OVER AND OVER AGAIN

DOPPLER EFFECT DETECTION OF PLANETSWORKS ONLY IF ORBIT IS SEEN

NEARLY EDGE-ONEASIEST TO DETECT IF

–PLANET IS MORE MASSIVE–PLANET CLOSER TO STAR

CLOSE TO 300 PLANETS DISCOVERED SINCE 1995 VIA THIS TECHNIQUE

OVER 90% OF EXTRASOLAR PLANETS DISCOVERED THIS WAY

WHAT CAN WE DETERMINE? ORBITAL PERIOD (TIME NEEDED FOR

ONE ORBIT)

AVERAGE DISTANCE OF PLANET FROM STAR

ECCENTRICITY (SHAPE) OF ORBIT

LOWER LIMIT ON PLANET’S MASS

RESULTS OVER 300 EXTRASOLAR PLANETS HAVE BEEN

DISCOVERED SINCE 1995, MOST USING THE DOPPLER EFFECT TECHNIQUE.

AT LEAST 20 STARS HAVE BEEN FOUND TO HAVE TWO OR MORE PLANETS.

MOST PLANET MASSES ARE IN JUPITER RANGE. (MANY ARE EVEN HEAVIER.) THE LIGHTEST PLANET FOUND SO FAR IS 4 EARTH MASSES.

MANY PLANETS ARE VERY CLOSE TO STAR .– HALF OF ALL DISCOVERED PLANETS ARE

CLOSER IN THAN 0.5 AU– MANY ARE CLOSER TO THEIR STARS THAN

MERCURY IS TO OUR SUN MOST ORBITS ARE VERY ECCENTRIC (HIGHLY

ELLIPTICAL - FAR FROM CIRCULAR).

DISTRIBUTION OF PLANETS

MERCURY VENUS EARTH

0.5 A.U. 1.0 A.U.

MARS

1.0 A.U. 2.0 A.U.

2.3 A.U.

2.5 A.U.

2.5 A.U.

3.3 A.U.

THE PROBLEM IN UNDERSTANDING THIS OUR MODELS OF SOLAR SYSTEM

FORMATION PREDICT SMALL ROCKY PLANETS CLOSE TO STAR AND MASSIVE GAS GIANTS FARTHER AWAY (>5 AU), AS IN OUR SOLAR SYSTEM

BUT MANY OBSERVED SOLAR SYSTEMS HAVE MASSIVE PLANETS (PROBABLY GAS GIANTS) CLOSE TO STAR

EXPLANATION??OBSERVED MASSIVE PLANETS WERE

FORMED FARTHER OUT FROM STAR (>5 AU), WHERE GAS GIANTS ARE EXPECTED TO FORM

AFTER FORMATION, THE PLANETS MIGRATED TO NEW ORBITS DUE TO GRAVITATIONAL INTERACTIONS WITH – OTHER PLANETS– MATERIAL IN THE SOLAR DISK (NEAR THE

END OF SOLAR SYSTEM FORMATION)– OTHER STARS PASSING NEARBY

MIGRATING PLANETSCOMPUTER MODELING INDICATES

– PLANETS ARE MORE LIKELY TO MIGRATE INWARD THAN OUTWARD

– NEW ORBIT IS USUALLY HIGHLY ECCENTRIC

– WHEN A LARGE PLANET MIGRATES, SMALLER PLANETS ARE PROBABLY THROWN INTO THE STAR OR OUT OF THE SOLAR SYSTEM BY GRAVITY OF MIGRATING MASSIVE PLANET

– HENCE THERE ARE PROBABLY NO SUITABLE PLANETS IN THE SYSTEM

EXTRASOLAR PLANETS DO MOST SOLAR SYSTEMS HAVE

MASSIVE PLANETS (GAS GIANTS) CLOSE TO THE STAR?

IF SO, PLANETS THAT ARE SUITABLE FOR LIFE MAY BE RARE.

BUT KEEP IN MIND THAT…– MASSIVE PLANETS CLOSE TO THEIR STARS

ARE EASIEST TO DETECT (LARGEST DOPPLER EFFECT).

– THEREFORE, “OBSERVATIONAL BIAS” IS PRESENT. OUR SAMPLE OF KNOWN EXTRASOLAR PLANETS IS NOT REPRESENTATIVE OR “TYPICAL.”

EXTRASOLAR PLANETS OUR CURRENT TECHNOLOGY CANNOT

DETECT EARTH-LIKE PLANETS. WE ARE JUST BEGINNING TO BE ABLE

TO DETECT JUPITER-LIKE PLANETS (AT JUPITER'S DISTANCE FROM THE STAR). A FEW SUCH PLANETS HAVE BEEN FOUND.

SOLAR SYSTEMS THAT CONTAIN JUPITER-LIKE PLANETS AT JUPITER-LIKE DISTANCES FROM THE STAR ARE MORE LIKELY TO HAVE EARTH-TYPE PLANETS CLOSER IN TO THE STAR.

EXTRASOLAR PLANETS WE HAVE FOUND EXTRASOLAR PLANETS

ORBITING ABOUT 10% OF STARS EXAMINED.

MAYBE THE OTHER 90% OF STARS (OR MANY OF THEM, AT LEAST) MAY HAVE PLANETARY SYSTEMS MORE LIKE OURS, WHICH WE CANNOT YET DETECT.

IMPROVED TECHNOLOGY WILL ANSWER THIS, PROBABLY WITHIN THE NEXT DECADE.– NASA IS PLANNING A “TERRESTRIAL PLANET FINDER.”

STELLAR/PLANETARY HIERARCHYSTARS 0.08 TO 20 SOLAR MASSES

BROWN DWARFS0.013 TO 0.08 SOLAR MASSES13 - 80 JUPITER MASSESMASSES IN BETWEEN THOSE OF

PLANETS AND STARS

GAS GIANT PLANETS 0.04(?) - 13 JUPITER MASSES

ROCKY (TERRESTRIAL) PLANETS< 0.04 JUPITER MASSES OR < 13 EARTH MASSES (?)

(1 EARTH MASS ~ 0.003 JUPITER MASSES)

THE DRAKE EQUATION THIS EQUATION IS USED TO ESTIMATE THE

NUMBER OF “TECHNOLOGICAL” CIVILIZATIONS IN THE MILKY WAY GALAXY. – WE DEFINE A “TECHNOLOGICAL” CIVILIZATION AS

ONE THAT IS CAPABLE OF (AND INTERESTED IN) ENGAGING IN INTERSTELLAR COMMUNICATIONS WITH OTHER CIVILIZATIONS.

– NOTE: WE ARE ONLY MAKING THIS ESTIMATE FOR OUR GALAXY, BUT THE NUMBER SHOULD BE ABOUT THE SAME FOR ANY SIMILAR SPIRAL GALAXY.

THIS IS THE NUMBER OF CIVILIZATIONS THAT COULD BE SENDING OUT RADIO (OR OTHER) SIGNALS THAT WE MIGHT BE ABLE TO RECEIVE.

THE DRAKE EQUATION WHY TRY TO ESTIMATE THE NUMBER OF

TECHNOLOGICAL CIVILIZATIONS?– IF THE ESTIMATED NUMBER IS VERY SMALL, SEARCHES

FOR SIGNALS FROM ALIEN CIVILIZATIONS MIGHT NOT BE WORTH THE TIME, EFFORT, AND EXPENSE.

– IF THE ESTIMATED NUMBER IS LARGE, SEARCHES ARE MORE LIKELY TO BE SUCCESSFUL. THEREFORE IT’S EASIER TO ARGUE THAT THE TIME, MONEY, AND EFFORT ARE WORTH IT.

KEEP IN MIND THAT:– WE CAN’T MAKE AN EXACT CALCULATION OF THE

NUMBER OF CIVILIZATIONS, ONLY A VERY ROUGH ESTIMATE.

– OUR ESTIMATE WILL APPLY ONLY TO LIFE THAT IS SIMILAR TO TERRESTRIAL LIFE. IF EXOTIC LIFE EXISTS, CIVILIZATIONS MAY BE MORE COMMON.

DRAKE EQUATIONN = N* fs np fl fi fc fL

N = Number of civilizations in the MW galaxy capable of communication(what we'd like to find)

N* = Number of stars in the MW galaxy

fs = fraction of stars that are suitable stars

(so the result of N*fs is number of

suitable stars in MW galaxy)

DRAKE EQUATIONN = N* fs np fl fi fc fL

np = average number of planets that are suitable for life per each suitable star(result of N*fsnp is number of suitable

planets in MW galaxy)

fl = fraction of suitable planets on which life actually originates(result of N*fsnpfl is number of planets

with life in MW galaxy)

DRAKE EQUATIONN = N* fs np fl fi fc fL

fi = fraction of those planets with life on which intelligent life evolves

(result of N*fsnpflfi is number of planets

with intelligent life in MW galaxy)

NOTE: By “intelligent,” we mean of roughly human intelligence.

DRAKE EQUATIONN = N* fs np fl fi fc fL

fc = fraction of planets with intelligent life on which technology sufficient for interstellar communication develops

(result of N*fsnpflfi is number of planets

with technological life in MW galaxy)

You might think we're done, but there is one more factor!

DRAKE EQUATIONN = N* fs np fl fi fc fL

fL = fraction of those civilizations that exist NOW (as opposed to ones that existed in the past, but don’t exist any more)

We find fL via fL = L/tL = average lifetime of a technological civilizationt = age of Milky Way galaxy

(This assumes that the probability of a civilization arising has remained constant over the lifetime of our galaxy.)

DRAKE EQUATIONN = N* fs np fl fi fc fL

The Drake equation is sometimes written in a form that is different than that above. All of these other forms are equivalent.

For example, the textbook uses the form:

N = NHP flife fciv fnow , withNHP = N* fs np = number of suitable planets in MW,flife = fl = fraction of suitable planets on which life

actually develops,fciv = fi fc = fraction of those planets with life on which

a technological civilization develops, andfnow = fL = fraction of civilizations that exist now.

FACTORS IN THE DRAKE EQUATIONN = N* fs np fl fi fc fL

N = Number of technological civilizations in the Milky Way galaxy

To calculate an estimated value of N, we must first estimate the other factors in the Drake Equation. Let’s go through these one by one. Many of these factors are not very well known.

N* = Number of stars in the MW galaxy

N* = 400 billion stars (may be off by ~30%)

SUITABLE STARS Drake Equation: N = N* fs np fl fi fc fL

fs= fraction of stars that are suitable Recall that properties of a suitable star are:

- main sequence - long enough main sequence lifetime - reasonable sized habitable zone - enough heavy elements (younger star) - not too close to center of galaxy - not in a binary or multiple star system?

SUITABLE STARS

Drake Equation: N = N* fs np fl fi fc fL

fs= fraction of stars that are suitable

fs = 0.1 = 1/10 (optimistic case)

fs = 0.001 = 1/1000 (pessimistic case)

fs = 0.05 = 1/20 (my best estimate)

SUITABLE PLANETSDrake Equation: N = N* fs np fl fi fc fL

np = average number of suitable planets per suitable star

Recall that properties of a suitable planet are:- in habitable zone - reasonably circular orbit- rocky planet (not a gas giant)- massive enough to keep an atmosphere- has a large moon??- giant planets found in desirable locations within solar system??

-

SUITABLE PLANETSDrake Equation: N = N* fs np fl fi fc fL

np = average number of suitable planets per suitable star

- Our solar system has one for sure (Earth), and several others that are almost but not quite suitable (Mars and Venus).

- If we consider Europa-type planets or moons (with an internal source of heat replacing the sun), the number could be higher.

- If solar systems like those containing known extrasolar planets are common, the number could be lower.

SUITABLE PLANETSDrake Equation: N = N* fs np fl fi fc fL

np = average number of planets that are suitable for life per each suitable star

np = 2 (optimistic case)

np = 0.1 = 1/10 (pessimistic case)

np = 0.5 = 1/2 (my best estimate)

DEVELOPMENT OF LIFEDrake Equation: N = N* fs np fl fi fc fL

fl = fraction of suitable planets on which life actually originates

Problem: we know of only one suitable planet (Earth), so we have little information on this.

But… Life got started very early on the earth, basically as soon as the earth cooled off sufficiently. This suggests that it is “easy” for life to originate.

DEVELOPMENT OF LIFEDrake Equation: N = N* fs np fl fi fc fL

fl = fraction of suitable planets on which life actually originates

fl = 1 (optimistic case - life will always arise if the planet is suitable)

fl = 0.005 = 1/200 (pessimistic case)

fl = 1 (my best estimate)

DEVELOPMENT OF INTELLIGENCEDrake Equation: N = N* fs np fl fi fc fL

fi = fraction of those planets with life on which intelligent life evolves

There are actually two (at least) steps here: first the evolution of “complex” life forms (e.g., multicellular life), and then the evolution of intelligent life.

We don’t know how likely these developments are, but let’s examine some “pro and con” arguments.

DEVELOPMENT OF INTELLIGENCE

ARGUMENTS WHY INTELLIGENCE SHOULD ARISE EASILY

- Evolution produces a wide diversity of life forms, so perhaps it is inevitable that mutations leading to intelligence will eventually arise.

- Intelligence bestows a tremendous selective advantage on organisms possessing it:- Better at finding food- Better at escaping from predators- Better at attracting a mate

- Based on terrestrial fossil evidence over the last few tens of million years, it appears that there has been an increase in intelligence over time for many types of mammals and birds.

DEVELOPMENT OF INTELLIGENCEARGUMENTS WHY INTELLIGENCE MAY NOT

ARISE EASILY - The development of intelligence is not the “goal”

or “purpose” of evolution. - Life on earth existed for a long time before

multicellular life evolved.- Multicellular life on earth existed for a long time

before intelligent life (humans) evolved.- A lot of organisms on earth have been highly

successful without developing intelligence.- Perhaps the evolution of multicellularity and/or

intelligence wouldn’t have happened without special circumstances (e.g., specific climate changes) that might not be common on other planets.

DEVELOPMENT OF INTELLIGENCEDrake Equation: N = N* fs np fl fi fc fL

fi = fraction of planets with life on which intelligent life evolves

fi = 1 (optimistic case)

fi = 0.001 = 1/1000 (pessimistic case)

fi = 0.01 = 1/100 (my best estimate)

DEVELOPMENT OF TECHNOLOGYDrake Equation: N = N* fs np fl fi fc fL

fc= fraction of planets with intelligent life on which technology sufficient for interstellar communication develops

- Is technology a natural consequence of intelligence?

Again, in the absence of any information about what happened on other planets, let’s examine life on earth as a guide.

DEVELOPMENT OF TECHNOLOGY- Is technology a natural consequence of intelligence,

or are other things besides intelligence also necessary in order for technology to develop?

- Dolphins are probably the second smartest species on Earth (after humans). If dolphins were a little smarter, could they have developed technology?

- Possible reasons why they might not:- They have no hands with which to manipulate

objects.- A creature that lives in water might not be likely

to develop certain types of technology (e.g., fire). - A creature that lives in water might not

develop an understanding of astronomy.

DEVELOPMENT OF TECHNOLOGY Some human civilizations on Earth have

developed technology and others have not. Why?

Unfortunately, some people believe in racist explanations, i.e., innate superiority of some groups of people.

More likely explanation – some locations on Earth are more conducive to the development of technology than others, due to: – Better or more varied climates– Differences in the availability of natural resources – Availability of animals that can be domesticated– See “Guns, Germs, and Steel” by Jared Diamond

DEVELOPMENT OF TECHNOLOGY Therefore, in order to develop a

technological civilization, other things besides intelligence might be necessary, such as:–Hands (or similar organs)–Living on dry land–Better or more varied climates–Certain natural resources –Creatures that can be domesticated

DEVELOPMENT OF TECHNOLOGYDrake Equation: N = N* fs np fl fi fc fL

fc= fraction of planets with intelligent life on which technology sufficient for interstellar communication develops

fc = 1 (optimistic case)

fc = 0.01 = 1/100 (pessimistic case)

fc = 0.5 = 1/2 (my best estimate)

DO THEY EXIST NOW?Drake Equation: N = N* fs np fl fi fc fL

fL= Probability that they’re around NOW (as opposed to civilizations that existed in the past, but don’t exist any more)

fL = L/t

t = Age of MW galaxy = 10 billion years

L = Average lifetime of a technological civilization (in years) = Average lifetime of civilization with ability and desire to communicate

LIFETIMES OF CIVILIZATIONS

L = Average lifetime of a technological civilization

L = 10 billion years (optimistic case) = Age of galaxy

L = 100 years (pessimistic case) Civilizations destroy themselves quickly

or lose interest in communication!

NOTE: L is the least well-known factor in the Drake equation!

DRAKE EQUATION

EXTREME OPTIMISTIC CASE

(Use optimistic values of all factors except L)

N = 400 billion x 0.1 x 2 x 1 x 1 x 1 x L/10 billion

RESULT: N = 8 L

Now look at different values of L:

IF L = 100 YEARS (pessimistic case for L), THEN N = 800

IF L = 10 BILLION YRS (optimistic case for L), THEN N = 80 BILLION

DRAKE EQUATION

MY BEST ESTIMATE

N = 400 billion x 0.05 x 0.5 x 1 x 0.01 x 0.5 x

L/10 billion

RESULT: N = 0.005 L = L/200

Now look at different values of L:

IF L = 100 YEARS (pessimistic case for L), THEN N = 0.5

IF L = 10 BILLION YRS (optimistic case for L), THEN N = 50 MILLION

DRAKE EQUATIONEXTREME PESSIMISTIC CASE

(Use pessimistic values of all factors except L)

N = 400 billion x 0.001 x 0.1 x 0.005 x 0.001 x 0.01 x L/10 billion

RESULT: N = 0.0000000002 L = 2 X 10-10 L

Now look at different values of L:

IF L = 100 YEARS (pessimistic case for L), THEN N = 0.00000002

IF L = 10 BILLION YRS (optimistic case for L) THEN N = 2

DRAKE EQUATIONWE KNOW THAT N MUST BE AT LEAST 1

BECAUSE WE EXIST!

THEREFORE:

IF THE EXTREME PESSIMISTIC CASE IS CORRECT (N = 2 X 10-10 L),

WE WOULD CONCLUDE THAT

L > 5 BILLION YEARS.

THIS WOULD MEAN THAT CIVILIZATIONS ARE LONG-LIVED!

DRAKE EQUATIONWE KNOW THAT N MUST BE AT LEAST 1

BECAUSE WE EXIST!

THEREFORE:

IF THE EXTREME OPTIMISTIC CASE IS CORRECT (N = 8L),

WE CONCLUDE THAT L > 1/8 YEAR.

BUT WE ALREADY KNOW THIS!

(WE’VE HAD THE RELEVANT TECHNOLOGY FOR ABOUT 50 YEARS SO FAR.)

DRAKE EQUATION

WE KNOW THAT N MUST BE AT LEAST 1 BECAUSE WE EXIST!

THEREFORE:

IF MY BEST ESTIMATE IS CORRECT

(N = 0.005 L),

WE CONCLUDE THAT L > 200 YEARS.

DRAKE EQUATIONCONCLUSIONS BASED ON THE FACT THAT

WE EXIST

• EITHER N = L IS VERY ROUGHLY CORRECT (TO WITHIN A FACTOR OF A FEW HUNDRED OR A FEW THOUSAND), AS IN THE EXTREMELY OPTIMISTIC CASE OR MY BEST ESTIMATE

OR

2. IF THE EXTREMELY PESSIMISTIC VALUES OF VARIOUS FACTORS ARE CLOSE TO CORRECT, THEN L MUST BE VERY LARGE

DRAKE EQUATIONBUT

WE SUSPECT FROM HUMAN EXPERIENCE THAT L COULD EASILY BE SMALL! (MORE ON THIS LATER)

THEREFORE WE CAN PROBABLY EXCLUDE THE EXTREMELY PESSIMISTIC CASE.

REALITY IS PROBABLY CLOSER TO THE OPTIMISTIC CASE (N ~ L) OR TO MY BEST ESTIMATE (N ~ L/200).

DRAKE EQUATIONN = # OF CIVILIZATIONS IN MW GALAXY CAPABLE OF

INTERSTELLAR COMMUNICATION

L = AVERAGE LIFETIME OF SUCH A CIVILIZATION IN YEARS

RESULT: N ~ LVERY ROUGHLY,

(TO WITHIN A FACTOR OF A FEW 100 OR FEW 1000)

BUT HOW LARGE IS L??(BIGGEST SOURCE OF UNCERTAINTY)

DRAKE EQUATION We will examine factors that affect L (the

average lifetime of a “technological” civilization) later.

For now, let's examine how the value of L (and therefore N, the number of “technological” civilizations) affects the possibility of interstellar communication.– The more civilizations there are, the closer together

they'll be, on the average.– The distance between civilizations determines how

long it takes for messages to pass back and forth.– Messages can't travel faster than the speed of light

(one light year per year).

MILKY WAY GALAXY

N=1

*

THIS IS US

MILKY WAY GALAXY

DISTRIBUTION OF CIVILIZATIONS

N=10

*

THIS IS US

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WHY ISN’T THIS REALISTIC?

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MILKY WAY GALAXY

DISTRIBUTION OF CIVILIZATIONS

N=10

THIS IS US

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RANDOM DISTRIBUTION, MORE REALISTIC

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MILKY WAY GALAXY

DISTRIBUTION OF CIVILIZATIONS

N=50

*

THIS IS US

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*

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HOW HAVE THE DISTANCES BETWEEN CIVILIZATIONS CHANGED FROM N=10?

*

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*

*

*

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*

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*

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MILKY WAY GALAXYDISTRIBUTION OF CIVILIZATIONS

THE LARGER THE NUMBER OF CIVILIZATIONS,

THE SMALLER THE AVERAGE DISTANCE BETWEEN THEM,

THE MORE FEASIBLE INTERSTELLAR COMMUNICATION BECOMES.

ABUNDANCE OF LIFE IN THE GALAXY (ASSUMING N = L)

CASE

ABUNDANT

SCARCE

RARE

L(YEARS)

1 billion

2 million

2000

N

1 billion

2 million

2000

CASE

ABUNDANT

SCARCE

RARE

Average Distance

15 LY

100 LY

1000 LY

Number of 2-Way Conversations

30 million

10,000

1

NUMBER OF CONVERSATIONS: NUMBER POSSIBLE WITHIN TIME L, BASED ON THE ASSUMPTION THAT SIGNALS TRAVEL BACK AND FORTH

AT THE SPEED OF LIGHT

SOLVING THE DRAKE EQUATION“ONCE SETI FINDS THE FIRST ONE, IT’S JUST STATISTICS.”

PHILLIP MORRISON

SETI = SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE (METHODS TO BE DISCUSSED NEXT TIME)

WHAT DOES THIS STATEMENT MEAN?ONCE WE FIND THE FIRST EXTRATERRESTRIAL CIVILIZATION, WE'LL KNOW AN APPROXIMATE VALUE FOR N, AND THUS WE'LL HAVE A BETTER ESTIMATE OF L ALSO. LET'S SEE HOW THIS WORKS.

SOLVING THE DRAKE EQUATION

SUPPOSE SETI FINDS A SIGNAL, AND ASTRONOMERS DETERMINE THE DISTANCE TO THAT CIVILIZATION TO BE 100 LY.

ASSUMING THIS DISTANCE IS ALSO THE AVERAGE DISTANCE BETWEEN NEAREST-NEIGHBOR CIVILIZATIONS (STATISTICALLY A GOOD ASSUMPTION),

THEN N = 2 MILLION.

SOLVING THE DRAKE EQUATIONWITH N = 2 MILLION:

IF N ~ 10 L, THEN L ~ 200,000 YEARS, TIME FOR 100 ROUND-TRIP CONVERSATIONS.

IF N ~ 0.001 L, THEN L ~ 2 BILLION YEARS, TIME FOR 10 MILLION ROUND-TRIP CONVERSATIONS.

THEN WE CAN FEEL CONFIDENT THAT THERE ARE A LOT OF LONG-LIVED CIVILIZATIONS, AND THAT COMMUNICATION WITH THEM IS FEASIBLE.

WE CAN ALSO FEEL CONFIDENT THAT HUMAN CIVILIZATION IS LIKELY TO SURVIVE FOR A LONG TIME.

SOLVING THE DRAKE EQUATIONWHAT IF SETI DOES NOT FIND ANY EVIDENCE OF EXTRATERRESTRIAL CIVILIZATIONS WITHIN 1,000 LY? THEN N < 2000.

IF N ~ 10 L, THEN L < 200 YEARS, NO TIME FOR ANY CONVERSATIONS.

IF N ~ 0.001 L, THEN L < 2,000,000 YEARS, TIME FOR <1000 ROUND TRIP CONVERSATIONS.

THEN DEPENDING ON WHAT WE ASSUME ABOUT THE DRAKE EQUATION, CIVILIZATIONS MAY BE SO FEW AND FAR BETWEEN THAT COMMUNICATION WITH THEM MAY NOT BE FEASIBLE.

WE ALSO MAY FEEL MORE PESSIMISTIC ABOUT THE LIKELY LIFETIME OF HUMAN CIVILIZATION.