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# A1 25 Life

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Miller's Astronomy 1 lecture notes on Life in the Universe

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### A1 25 Life

1. 1. Life LACC: §29.1, 29.2, 29.3 • The Drake Equation: How many communicating civilizations are in our galaxy? Could be just about any number, but 10 to 10,000 isn’t unreasonable. • The Fermi Paradox: Where is everybody? Since we don’t see anybody, we’re probably alone. • SETI: The Search for Extraterrestrial Intelligence--is it worth it? Even if they are out there, it could take millennia for signals to be passed back and forth. An attempt to answer the “big questions”: What is out there? How did we get here? Are we alone? Thursday, May 27, 2010 1
2. 2. The Drake Equation How can we estimate the number of technological civilizations that might exist among the stars? The Drake Equation, as it has become known, was ﬁrst presented by Dr. Frank Drake (now Chairman of the Board of the SETI Institute) in 1961 and identiﬁes speciﬁc factors thought to play a role in the development of such civilizations. The equation is usually written: N = R* • fp • ne • fl • fi • fc • L http://www.seti.org/Page.aspx?pid=336 Thursday, May 27, 2010 2
3. 3. The Drake Equation N = R* • fp • ne • fl • fi • fc • L Where, N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable. R* =The rate of formation of stars suitable for the development of intelligent life. fp = The fraction of those stars with planetary systems. ne = The number of planets, per solar system, with an environment suitable for life. fl = The fraction of suitable planets on which life actually appears. fi = The fraction of life bearing planets on which intelligent life emerges. fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space. L = The length of time such civilizations release detectable signals into space. http://www.seti.org/Page.aspx?pid=336 Thursday, May 27, 2010 3
4. 4. The Drake Equation N = R* • fp • ne • fl • fi • fc • L R* = The rate of formation of Suitable Stars in the Milky Way Galaxy Estimates for the number of stars in the Milky Way vary from a low of 100 billion to a high of 400 billion. Estimates for the age of the Milky Way also vary from a low of 800 million years [!!!] to a high of 13 billion years. [200 billion stars / 10 billion years = 15 stars/year] An important caveat to the above values is that the rate of star formation in the galaxy is not constant over time. In the galaxy's younger days, stars were being formed at a much higher rate. Today, estimates for the overall star formation rate range from 5 to 20. [But, which stars are “suitable”? O? B? A? F? G? K? M?] http://www.astrodigital.org/astronomy/drake_equation.html Thursday, May 27, 2010 4
5. 5. The Drake Equation N = R* • fp • ne • fl • fi • fc • L R* = The rate of formation of Suitable Stars in the Milky Way Galaxy However, the criteria that these stars be suitable means that they must be F, G or K stars, and these stars account for about 10% of the stars in the Galaxy. [15 stars/year x 10% F, G, or K = 1 suitable star per year] http://www.airynothing.com/smackerels/DrakeEquation.html ...astronomers have recently determined that stars formed at a higher rate several billion years ago, when the stars that might now bear intelligent life were being born. So a value of R = 3 is more realistic. http://www.skyandtelescope.com/resources/seti/3304541.html?page=2&c=y Thursday, May 27, 2010 5
6. 6. The Drake Equation N = R* • fp • ne • fl • fi • fc • L fp = the fraction of stars that have planetary systems Reasonable guesses might be 20 to nearly 100 percent. (A September 2003 paper by Charles Lineweaver and Daniel Grether delves into this.) At least half of the young stars seen in [the Orion Nebula] are surrounded by thick, dusty disks — excellent planet-forming material. The planet is thought to be one to two times as massive as Jupiter, according to the scientists who imaged it. It orbits a star similar to a young version of our Sun. http://www.skyandtelescope.com/resources/seti/ http://www.space.com/scienceastronomy/ 3304541.html?page=2&c=y 050401_ﬁrst_extrasolarplanet_pic.html Thursday, May 27, 2010 6
7. 7. The Drake Equation N = R* • fp • ne • fl • fi • fc • L ne = is the average number of planets in a star's habitable zone. The terrestrial example suggests complex life requires water in the liquid state and its planet must therefore be at an appropriate distance. This is the core of the notion of the habitable zone or Goldilocks Principle . The habitable zone forms a ring around the central star. If a planet orbits its sun too closely or too far away, the surface temperature is incompatible with water being liquid (though sub-surface water, as suggested for Europa, Enceladus, and Ceres, may be possible at varying locations). http://www.absoluteastronomy.com/ http://physics.uoregon.edu/~jimbrau/ topics/Rare_Earth_hypothesis astr123/Notes/Chapter28.html Thursday, May 27, 2010 7
8. 8. The Drake Equation N = R* • fp • ne • fl • fi • fc • L fl= The fraction of life bearing planets on which life emerges. A deep ocean hydrothermal vent belching sulﬁde-rich hot water. A variety of life forms comprise a food web based on bacteria that live off of the energy provided by the sulﬁde-rich vent waters. It is possible that the Earth's earliest life forms evolved in an environment like this. Photo: NOAA/WHOI. http://www.nss.org/adastra/volume14/rothschild.html Thursday, May 27, 2010 8
9. 9. The Drake Equation N = R* • fp • ne • fl • fi • fc • L fl= The fraction of life bearing planets on which life emerges. The ﬁrst ecosystem ever found having only a single biological species has been discovered 2.8 kilometers (1.74 miles) beneath the surface of the earth in the Mponeng gold mine near Johannesburg, South Africa. There the rod-shaped bacterium Desulforudis audaxviator exists in complete isolation, total darkness, a lack of oxygen, and 60°C heat (140°F). D. audaxviator survives in a habitat where it gets its energy not from the sun but from hydrogen and sulfate produced by the radioactive decay of uranium. Living alone, D. audaxviator must build its organic molecules by itself out of water, inorganic carbon, and nitrogen from ammonia in the surrounding rocks and ﬂuid. http://esciencenews.com/articles/2008/10/09/ bold.travelers.journey.toward.center.earth# Thursday, May 27, 2010 9
10. 10. The Drake Equation N = R* • fp • ne • fl • fi • fc • L fi= The fraction of life bearing planets on which intelligent life emerges. It is also true that, although simple forms of life on Earth arose quickly, it took billions of years before complex life forms appeared and longer still before intelligent life arose. Thus even if evolution will eventually bring about intelligent life, it could well require a stable environment for a signiﬁcant fraction of the lifetime of a suitable star. http://www.lifeinuniverse.org/noﬂash/Drakeﬁ-07-02-06.html Top Six Most Intelligent Animals in the World 6. Pigs 3. Smaller Toothed Whales 5. Elephants 2. Octopus 4. Dolphins 1. Chimpanzees* *: including other monkeys that are related... http://www.scienceray.com/Biology/Zoology/Top-Six-Most-Intelligent-Animals-in-the- World.623017 Thursday, May 27, 2010 10
11. 11. The Drake Equation N = R* • fp • ne • fl • fi • fc • L fc = The fraction of communicative planets Though not a SETI League "hit," no discussion of SETI results would be complete without this one. The most famous of all SETI candidate signals (it was even mentioned on The X-Files), the Ohio State University "Wow!" signal was detected on 15 August 1977. Twenty years later, after more than 100 follow-on studies, it remains an intriguing unexplained phenomenon. These articles discuss the signal and its implications to The SETI League's present search. http://www.setileague.org/photos/hits.htm Thursday, May 27, 2010 11
12. 12. The Drake Equation N = R* • fp • ne • fl • fi • fc • L L = Average lifetime of a technological civilization Among the cosmic phenomena that astronomers have identiﬁed as being of potential biological importance are: 1. Large-scale stellar ﬂares http://www.daviddarling.info/ 2. Nearby supernova explosions encyclopedia/C/coscatasbio.html 3. Gamma-ray bursters 4. Impacts by asteroids or comets Among these are the very hazards which humanity presently faces: 1. Large-scale nuclear or biological war http://www.daviddarling.info/ 2. Global epidemics of lethal disease resulting from encyclopedia/E/etcivhaz.html ◦ the emergence of antibiotic-resistant pathogens ◦ back-contamination 3. Environmental disasters stemming from a combination of industrial pollution, over- population, and destruction of natural habit, including ◦ global warming due to a gradual elevation in the level of greenhouse gases ◦ disintegration of the ozone layer 4. Unforeseen side-effects of new technologies, such as genetic engineering Thursday, May 27, 2010 12
13. 13. The Drake Equation Taking the historical values given by Drake and his colleagues in 1961: Drakes current values: R* = 10 suitable stars per year R* = 5 suitable stars per year fp = 50% of the stars have planets fp = 50% of the stars have planets ne = 2 planets on average are habitable ne = 2 planets on average are habitable fl = 100% of habitable planets produce life fl = 100% of habitable planets produce life fi = 1% of life bearing planets produce fi = 20% of life bearing planets produce intelligence intelligence fc = 1% of intelligences can communicate fc = 100% of intelligences can communicate L = 10,000 average lifetime of a civilization L = 10,000 average lifetime of a civilization N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 10,000 N = 5 × 0.5 × 2 × 1 × 0.2 × 1 × 10,000 = = 10 civilizations in our galaxy. 10,000 civilizations in our galaxy. http://www.fermisparadox.com/Fermi- http://www.pbs.org/wgbh/nova/ paradox.htm origins/drake.html Thursday, May 27, 2010 13
14. 14. The Fermi Paradox Fermi realized that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonize the entire Galaxy. Growth of the number of probes would occur exponentially and the Galaxy could be explored in 4 million years. While this time span seems long compared to the age of human civilization, remember the Galaxy is over 10 billion years old and any past extraterrestrial civilization could have explored the Galaxy 250 times over. Within a few million years, every star system could be brought under the wing of empire. http://abyss.uoregon.edu/~js/cosmo/lectures/lec28.html Thursday, May 27, 2010 14
15. 15. SETI: Should we bother? Let’s consider our Milky Way Galaxy as a ﬂat disk about 30 kpc in diameter, 0.5 kpc thick, and containing about 100 billions stars. The volume of our galaxy would then be πr2•h= 350 kpc3 100,000,000 stars / NDrake = about how many stars you could expect to search before ﬁnding one with a communicating civilization 350kpc3 / NDrake = typical volume surrounding each communicating civilization in kpc3 3√(typicalvolume surrounding each communicating civilization) = estimation of distance between communicating civilizations in kpc distance between communicating civilizations in kpc • 3260 ly/kpc • 2 = typical time between sending a signal and receiving a reply Thursday, May 27, 2010 15
16. 16. SETI: Should we bother? Let’s consider our Milky Way Galaxy as a ﬂat disk about 15 kpc in radius, 0.5 kpc thick, and containing about 100 billions stars. The volume of our galaxy would then be πr2•h= 350 kpc3 100,000,000 stars / 10,000 = 10,000 stars about how many stars you could expect to search before ﬁnding one with a communicating civilization 350 kpc3 / 10,000 = 0.035 kpc3 typical volume surrounding each communicating civilization in kpc3 3√(0.035kpc3) = 0.33 kpc = 330 pc estimation of distance between communicating civilizations in kpc 0.33 kpc • 3260 ly/kpc • 2 = 2100 years typical time between sending a signal and receiving a reply Thursday, May 27, 2010 16
17. 17. LACC Ch 29: Franknoi, Morrison, and Wolff, Voyages Through the Universe, 3rd ed. Study for you Final. Thursday, May 27, 2010 17