Extra-Terrestrial Life and the Drake Equation

1. Extra-Terrestrial Life and the Drake Equation Astronomy 311 Professor Lee Carkner Lecture 25

2. 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

3. 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

4. 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

5. 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 non-planetary settings (nebulae, smoke rings, etc.)

6. The H-R Diagram

7. Extra-Solar Planets

8. 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

9. The Carbonate-Silicate Cycle Atmosphere Water + CO2 (rain) CO2 Volcano Ocean CO2 + silicate (subvective melting) Carbonate + water (stream) Carbonate + silicate (Sea floor rock)

10. 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)

11. 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

12. The Miller-Urey Experiment

13. fl -- Life • Complex molecules containing carbon, (e.g. proteins and amino acids) • Organic material is also found in carbonaceous chondrites and comets

14. 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

15. 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

16. fL -- Lifetime • Lifetime of a star like the Sun = 10 billion years (1 X 1010) • How long does a civilization last for?

17. fL -- Destroying Civilization • What could destroy a civilization? • Environmental or technological disaster • Space colonization greatly reduces risk or extinction

18. 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 ~

19. 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