NSCI 314 LIFE IN THE COSMOS 14 - EXTRASOLAR PLANETS (CONTINUED) AND THE DRAKE EQUATION Dr. Karen Kolehmainen Department of Physics, CSUSB http://physics.csusb.edu/~karen/
METHODS FOR DETECTING EXTRASOLAR PLANETS (PLANETS ORBITING OTHER STARS) DIRECT OBSERVATION TRANSITS GRAVITATIONAL LENSING ASTROMETRY DOPPLER EFFECT (MOST SUCCESSFUL)
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 200 EXTRASOLAR PLANETS HAVE BEEN DISCOVERED SINCE 1995, MOST USING THE DOPPLER EFFECT TECHNIQUE. SEVERAL 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 5.5 EARTH MASSES. MOST 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. 2.3 A.U. 2.5 A.U. MARS 2.5 A.U. 3.3 A.U. 1.0 A.U. 2.0 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 MOST OBSERVED SOLAR SYSTEMS HAVE MASSIVE PLANETS (PROBABLY GAS GIANTS) CLOSE TO STAR
EXPLANATION?? OBSERVED MASSIVE PLANETS WERE FORMED FARTHER OUT FROM STAR (>5 AU) AFTER FORMATION, THE PLANETS MIGRATED TO NEW ORBITS DUE TO GRAVITATIONAL INTERACTIONS WITH OTHER PLANETS MATERIAL IN THE PROTOPLANETARY DISK OTHER STARS PASSING NEARBY
MIGRATING PLANETS COMPUTER 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
ARE MIGRATING PLANETS COMMON? IF THEY ARE THE NORM, 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. 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). THERE ARE REPORTS OF A FEW SUCH PLANETS. SOLAR SYSTEMS CONTAINING JUPITER-LIKE PLANETS FARTHER OUT ARE MORE LIKELY TO HAVE EARTH-TYPE PLANETS CLOSER IN TO THE STAR. 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 HIERARCHY STARS 0.08 TO 20 SOLAR MASSES BROWN DWARFS 0.013 TO 0.08 SOLAR MASSES 13-80 JUPITER MASSES MASSES IN BETWEEN THOSE OF PLANETS AND STARS GAS GIANT PLANETS 0.04(?) - 13 JUPITER MASSES ROCKY (TERRESTRIAL) PLANETS <0.04(?) JUPITER 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 EQUATION N = N * f L N = Number oivilizations in the MW galaxy capable oommunication (what we'd like to find) N * = Number otars in the MW galaxy = fraction otars that are suitable stars (so the result of N * is number of suitable stars in MW galaxy)
DRAKE EQUATION N = N * f L = average number of planets that are suitable for life per each suitable star (result of N * is number ouitable planets in MW galaxy) = fraction ouitable planets on which life actually originates (result of N * is number of planets with life in MW galaxy)
DRAKE EQUATION N = N * f L = fraction of those planets with life on which intelligent life evolves (result of N * is number of planets with intelligent life in MW galaxy) NOTE: By intelligent, we mean of roughly human intelligence.
DRAKE EQUATION N = N * f L = fraction of planets with intelligent life on which technology sufficient for interstellar communication develops (result of N * is number of planets with technological life in MW galaxy) You might think we're done, but there is one more factor!
We find f L via f L = L/t L = average lifetime of a technological civilization t = age of Milky Way galaxy (This assumes that the probability of a civilization arising has remained constant over the lifetime of our galaxy.) DRAKE EQUATION N = N * f L f L = fraction of those civilizations that exist NOW (as opposed to ones that existed in the past, but don t exist any more)
FACTORS IN THE DRAKE EQUATION N = N * f L 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 otars in the MW galaxy N * = 400 billion stars (may be off by ~30%)
SUITABLE STARS Drake Equation: N = N * f L = fraction otars 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 near center of galaxy - not in a binary or multiple star system?
SUITABLE STARS Drake Equation: N = N * f L = fraction otars that are suitable = 0.1 = 1/10 (optimistic case) = 0.001 = 1/1000 (pessimistic case) = 0.05 = 1/20 (my best estimate)
SUITABLE PLANETS Drake Equation: N = N * f L = average number ouitable planets per suitable star - in habitable zone - reasonably circular orbit - massive enough to keep an atmosphere - has a large moon?? - giant planets found in desirable locations within solar system?? - 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, the number could be higher. - Iolar systems like those containing known extrasolar planets are common, the number could be lower.
SUITABLE PLANETS Drake Equation: N = N * f L = average number of planets that are suitable for life per each suitable star = 2 (optimistic case) = 0.1 = 1/10 (pessimistic case) = 0.5 = 1/2 (my best estimate)
DEVELOPMENT OF LIFE = Drake Equation: N = N * f L fraction ouitable 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 ofufficiently. This suggests that it is easy for life to originate.
DEVELOPMENT OF LIFE = Drake Equation: N = N * f L fraction ouitable planets on which life actually originates = 1 (optimistic case - life will always arise if the planet is suitable) = 0.005 = 1/200 (pessimistic case) = 1 (my best estimate)
DEVELOPMENT OF INTELLIGENCE Drake Equation: N = N * f L = 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 ontelligent 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 - Evolutioroduces a wide diversity oife 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 million years, it appears that there has been an increase in intelligence over time for many types of mammals and birds.
DEVELOPMENT OF INTELLIGENCE ARGUMENTS WHY INTELLIGENCE MAY NOT ARISE EASILY - The development ontelligence 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 INTELLIGENCE Drake Equation: N = N * f L = fraction of planets with life on which intelligent life evolves = 1 (optimistic case) = 0.001 = 1/1000 (pessimistic case) = 0.01 = 1/100 (my best estimate)
DEVELOPMENT OF TECHNOLOGY Drake Equation: N = N * f L = 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.
- 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 - A creature that lives in water might not develop an understanding of astronomy 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?
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 oome 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 Animals that can be domesticated See Guns, Germs, and Steel by Jared Diamond
DEVELOPMENT OF TECHNOLOGY = Drake Equation: N = N * f L fraction of planets with intelligent life on which technology sufficient for interstellar communication develops = 1 (optimistic case) = 0.01 = 1/100 (pessimistic case) = 0.5 = 1/2 (my best estimate)
DO THEY EXIST NOW? Drake Equation: N = N * f L f L = Probability that they re around NOW (as opposed to civilizations that existed in the past, but don t exist any more) = L/t t = Age of MW galaxy = 10 billion years L = Average lifetime of a technological civilization (measured in years) = Average lifetime oivilization 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 EQUATION EXTREME 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 EQUATION WE 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 EQUATION WE 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 EQUATION CONCLUSIONS BASED ON THE FACT THAT WE EXIST 2. 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 EQUATION BUT 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 EQUATION N = # OF CIVILIZATIONS IN MW GALAXY CAPABLE OF INTERSTELLAR COMMUNICATION L = AVERAGE LIFETIME OF SUCH A CIVILIZATION IN YEARS RESULT: N ~ L VERY ROUGHLY, (TO WITHIN A FACTOR OF A FEW 100 OR FEW 1000) BUT HOW LARGE IS L?? (BIGGEST SOURCE OF UNCERTAINTY)