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

2. Is There Anybody Out There?  People have long speculated about life on other worlds   Modern observations indicate that the solar system is uninhabited   These searches have only covered a tiny part of the galaxy, however  How can we estimate the possibility of extra- terrestrial life?

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 f p X n e X f l X f i X f c X f L  N =  R * = Number of stars in the galaxy  f p =  n e = Average number of suitable planets per star  f l = Fraction of suitable planets on which life evolves  f i =  f c = Fraction that can communicate  f L = Lifetime of civilization / Lifetime of star

5. The Milky Way

6. R * -- Stars  We start with the number of stars in the galaxy    We are ruling out life around neutron stars or white dwarfs or in non- planetary settings (nebulae, smoke rings, etc.)

7. The H-R Diagram

8. The Orion Star Forming Region

9. Protoplanetary Disk in Orion

10. Extra-Solar Planets

11. f p -- 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   We have just begun the search for planets

12. The Carbonate-Silicate Cycle Water + CO 2 (rain) Ocean Carbonate + silicate (Sea floor rock) CO 2 Volcano Atmosphere Carbonate + water (stream) CO 2 + silicate (subvective melting)

13. Venus

14. Mars

15. n e -- Suitable Planets  What makes a planet suitable?   Must be in habitable zone  0.95-1.37 AU for the Sun   Heat may also come from another source like tidal heating (Europa)

16. n e -- Unsuitable Planets  The Moon -- Too small to have an atmosphere  Mars --  Jupiter -- Too large, has no surface  Venus --  Earth at 2 AU -- CO 2 builds up to try and warm planet, clouds form, block sunlight

17. The Miller-Urey Experiment

18. Comet

19. f l -- Life   Complex molecules containing carbon, (e.g. proteins and amino acids)   Organic material is also found in carbonaceous chondrites and comets

20. The KT Impact

21. f i -- Intelligence  Life alone is not sufficient, intelligence is needed to communicate   Many things could interfere with evolution in this time    Life on Earth has gone through many disasters (e.g. mass extinctions), but has survived

22. Water Worlds - Intelligent Life?

23. Europa

24. f c -- Communication  Even intelligent life may not be able to communicate   What could keep intelligent life from building radio telescopes?  Waterworld (can’t smelt metals underwater)   Wrong biology (no hands, no eyes, etc.)   Lack of curiosity or resources

25. O’Neill Colony

26. O’Neill Colony -- Interior

27. f L -- Lifetime  f L = Lifetime of civilization / Lifetime of star   Beginning of civilization defined as when radio telescopes are invented 

28. f L -- Destroying Civilization  What could destroy a civilization?  Nuclear or biological war   Impact  Civilization may be able to rebuild 

29. N  Multiply these factors together to get N  The number of intelligent civilizations in our galaxy that we could communicate with right now   If you evenly distribute the civilizations across the galaxy, how close is the nearest one?  N ~ 1  N ~ 10D ~  N ~ 1000D ~  N ~ 100,000D ~  N ~10,000,000D ~ 

30. 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