Transcript 25drake6s
Extra-Terrestrial Life and the Drake Equation Astronomy 311 Professor Lee Carkner Lecture 25 Final Exam Monday, 3 pm, SC102 Two hours long Bring pencil and calculator Same format as other tests Matching, multiple choice, short answer About 50% longer Covers entire course The Drake Equation In 1961, astronomer Frank Drake developed a formula to predict the number of intelligent species in our galaxy that we could communicate with right now No one agrees on what the right values are Solving the Drake equation helps us to think about the important factors for intelligent life The Drake Equation N=R* X fp X ne X fl X fi X fc X fL N = R* = Number of stars in the galaxy fp = ne = Average number of suitable planets per star fl = Fraction of suitable planets on which life evolves fi = fc = Fraction that can communicate fL = Lifetime of civilization / Lifetime of star R* -- Stars Our best current estimate: R*=3 X 1011 (300 billion) We are ruling out life around neutron stars or white dwarfs or in nonplanetary settings (nebulae, smoke rings, etc.) The H-R Diagram Extra-Solar Planets fp -- Planets What kind of stars do we need? High mass stars may become a giant before intelligent life can develop Need medium mass stars (stars like the Sun) Can we find planets? Circumstellar disks that produce planets are common Exoplanets have now been found We have just begun the search for planets The Carbonate-Silicate Cycle Atmosphere Water + CO2 (rain) CO2 Volcano CO2 + silicate (subvective melting) Ocean Carbonate + silicate (Sea floor rock) Carbonate + water (stream) ne -- Suitable Planets What makes a planet suitable? Must be in habitable zone Simulations of inner planet formation produce a planet in the habitable zone much of the time Heat may also come from another source like tidal heating (Europa) ne -- Unsuitable Planets The Moon -Mars -- Has atmosphere but too small to have plate tectonics Jupiter -- Too large, has no surface Venus -Earth at 2 AU -- CO2 builds up to try and warm planet, clouds form, block sunlight The Miller-Urey Experiment fl -- Life Complex molecules containing carbon, (e.g. proteins and amino acids) Organic material is also found in carbonaceous chondrites and comets fi -- Intelligence On Earth life evolved from simple to complex over a long period of time (~3-4 billion years) Impacts (e.g. KT impact) Climate Change (e.g. Mars drying up) Life on Earth has gone through many disasters (e.g. mass extinctions), but has survived fc -- Communication Even intelligent life may not be able to communicate What could keep intelligent life from building radio telescopes? Airworld (floating gasbags can’t build things) Social, cultural or religious reasons Lack of curiosity or resources fL -- Lifetime Lifetime of a star like the Sun = 10 billion years (1 X 1010) How long does a civilization last for? fL -- Destroying Civilization What could destroy a civilization? Environmental or technological disaster Space colonization greatly reduces risk or extinction N Multiply these factors together to get N The galaxy is a disk 100,000 light years across If you evenly distribute the civilizations across the galaxy, how close is the nearest one? N ~ 1 N ~ 10 D ~ 15000 light years N ~ 1000 D~ N ~ 100,000 D ~ 590 ly N ~10,000,000 D~ Summary: Life in the Galaxy Medium size, medium luminosity star with a planetary system A planet of moderate mass in the habitable zone Organic compounds reacting to form simple life Life evolving over billions of years with no unrecoverable catastrophe Intelligent life building and using radio telescopes A long lived civilization