Intelligent Life in the Universe Lecture 33 APoD: Easter Island Eclipse In-Class Question 1) Do you think life exists elsewhere in the Universe? a) Yes b) No c) Don t know d) Don t care 2 33-1
Lecture opics Probabilities Rates and totals he Drake equation Computes the expected number of technical civilizations in the galaxy 3 Are we alone? Do other civilizations exist in the galaxy or elsewhere? How might we estimate this statistically and what are the uncertainties? We would like to quantify whether life and, in particular, other civilizations might exist in the galaxy. 33-2
Probabilities How many dates might a guy get in this class? N D N W f ask f accept N D = number of dates N W = number of women in the class f ask = fraction he asks out f accept = fraction that accept f show_up = fraction that show up f show _ up 5 Suppose N w = 100 Shy guy: f ask = 0.02 (2%) f accept = 0.50 (50%) f show_up = 1.00 (100%) N D = 100 x 0.02 x 0.5 x 1.0 = 1 date Outgoing guy: f ask = 0.20 (20%) f accept = 0.10 (10%) f show_up = 0.50 (50%) N D = 100 x 0.2 x 0.1 x 0.5 = 1 date 6 33-3
Rates and otals Suppose R * = Rate at which are born t l = Average lifetime of a star How many are alive at a given time? he number of is: N = R * x t l ( Rate times time ) 7 otal number of alive Death line Now 10 yrs Stars dead Stars not yet born ime Suppose: R * = 1 star/year (represented by spikes above). And live only 10 years. 10 would be alive at any given time. 8 33-4
Number of civilizations Suppose that each star developed a civilization. If the lifetime of the civilization is t l then the total number of civilizations alive is: N R t But this isn t the whole story... l 9 he Drake Equation Attempts to quantify the number of civilizations that might exist in the galaxy. Named after, Frank Drake pioneered this analysis while at Cornell 10 33-5
N R f f f f f t p h s i t l N = Number of technological civilizations in the galaxy. R * =Rateat which are born, averaged over the lifetime of the galaxy. (Stars/year) f p = Fraction having planetary systems. f h = Average number of life-suitable (habitable) planets within those systems having planets. 11 N R f f f f f t p h s i t l f s = f i = f t = t l = Fraction of habitable planets on which at least simple life arises. Fraction of life-bearing planets on which intelligence evolves. Fraction of those intelligent life planets that develop a technological society. Average lifetime of a technological civilization. (years) 12 33-6
N 10 R f ff ff ff ff ft t yearp ph hs si i t tl l R * = Rate at which are born, averaged over the lifetime of the galaxy. (Stars/year) here are ~100 billion in the galaxy today. And the galaxy is about 10 billion years old. R * ~ 10 /year 13 N 10 1f f f f f f f f f t t year p h h s s i i t t l l f p = Fraction having planetary systems. If our understanding of star formation is correct, then planets are a natural consequence. All could have planets, so we take f p ~ 1 However, only ~5% of nearby sun-like have giant planets (depends highly on metallicity). 14 33-7
N 10 1 f 1 f f f t year 10h ss ii tt ll f h = Average number of life-suitable (habitable) planets within those systems having planets. he ecosphere size varies with stellar type, but we might expect the odds to be similar to our solar system, so we choose f h ~ 1/10 Accept only F, G and K. 15 Caveats: Galactic Habitable Zone Region in the Galaxy over which life and life bearing worlds are likely to exist Requirements Available material to build planets High enough metallicity to produce terrestrial planets Right mix of heavy elements to radioactively heat core of planet (drives plate tectonics which regulate CO 2 in the atmosphere) Seclusion from cosmic threats Impacts by asteroids (depends on Jupiter) and comets (affected by galactic tides, GMCs, and passing ) Blasts of radiation (active galactic nucleus outbursts, supernovae, and gamma ray bursts) Orbit near co-rotation circle place where orbital period of star equals rotation period of spiral arm pattern. 16 33-8
Metallicity In the outer parts of the galaxy, the metallicity will be too low for giant planet formation Galactic Hazards Supernovae and stellar encounters are much more frequent in the interior of the galaxy 17 Galactic Habitable Zone 18 33-9
N 10 1 1 f f f t year 10 s i t l f s = Fraction of habitable planets on which at least simple life arises. How likely is it life will form? Is life rare? It is certainly complex! Laboratory experiments show that complex organic molecules can be formed in an atmosphere similar to that expected on the early earth. 19 he Urey-Miller Experiment Harold Urey and Stanley Miller (1953) Made primordial soup mixture water, methane, carbon dioxide, ammonia Passed simulated lightning through it. Produced gunk containing many of the amino acids found in life today. 20 33-10
Cyril Ponnamperuma About a decade later constructed nucleotide bases in a similar manner. Both experiments did not closely resemble the early atmosphere. But showed biological molecules can be synthesized by nonbiological means. Astrobiology Studies the origin, evolution, and possible future of life in the Universe his is an area of active research 21 Primordial Soup 33-11
Creating Organics is easy Using better knowledge of the primordial ocean and atmosphere. Various energy sources can produce amino acids and nucleotide bases. Energy sources such as: solar UV radiation, lightning, volcanic heat, natural radioactivity, and atmospheric shock waves produced by meteorites. 23 1 N 10 1 1f ff ff t t year 10 s i i t t l l f s = Fraction of habitable planets on which at least simple life arises. Making organics is easy, but creating life may not be. Some might argue that under the right conditions life has to happen. Most optimistic case: f s ~ 1 24 33-12
In-Class Question 1) What is the galactic habitable zone of the Milky Way? a) Sufficient metals the build planets b) Seclusion from cosmic threats c) Inner regions of the galaxy d) a and b e) b and c 25 1 N 10 1 1 1f ff t t year 10 i t t l l f i = Fraction of life-bearing planets on which intelligence evolves. he appearance of a well-developed brain might not happen if left to random chance. But natural selection tends to single out the more adaptable, more intelligent species. he optimistic view takes intelligence as inevitable: f i ~ 1 26 33-13
Dinosaurs and extinction Dinosaurs ruled the world for ~ 100 million years, but were pretty stupid (technically). Was the mass extinction (due to an asteroid impact) of the dinosaurs necessary for Homo Sapiens to evolve? 27 Other influences? What role did Jupiter and Saturn have in allowing life to form on Earth. Cleared out cometary objects! But also deflects them too he Moon Stabilizes the orientation of the Earth s spin axis Otherwise we could have days that last a whole year! 28 33-14
N 10 1 1 111 1 f t t year 10 t l l f t = Fraction of those intelligent life planets that develop a technological society. It is hard to imagine an intelligent species avoiding technology. echnical civilizations arose independently in many areas of the world. aking technological development as inevitable: f t ~ 1 29 N 10 1 1 111 t year 10 l t l = Average lifetime of a technological civilization. (years) How long does a technical civilization last? We ve had one for ~100 years. here are many unknowns to our own future, let alone predicting how long another civilization might last. 30 33-15
N N 1 10 1 11110 t year 10 l 6 years t l = Average lifetime of a technological civilization. (years) Suppose the average lifetime of a technical civilization is 1 millions years 1% of the reign of the dinosaurs 100 times longer than human civilization has existed! 1 million civilizations in our galaxy. 31 Uncertainties! Important - each term in the Drake equation (probably) gets more uncertain when proceeding from left to right. For lack of a better example we have adopted an Earth/human bias when estimating various terms. We do not know the uncertainties. 32 33-16
How far to our neighbors? For 1,000,000 civilizations in the galaxy the average distance between them will be ~ 150 ly!!! two-way communication will take at least 300 years! But this is a large over prediction since the Galactic Habitable Zone has much, much less than 10 11 33 How far? (cont d) If the lifetime of a technical civilization is less than 3000 years Average distance is so large that civilizations will die, on average, before two-way communications can be established! 34 33-17