Extra-Terrestrial Life and the Drake Equation

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 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. 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. 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 • We have just begun the search for planets

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

13. Venus

14. Mars

15. ne -- 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. ne -- Unsuitable Planets • The Moon -- Too small to have an atmosphere • Mars -- • Jupiter -- Too large, has no surface • Venus -- • Earth at 2 AU -- CO2 builds up to try and warm planet, clouds form, block sunlight

17. The Miller-Urey Experiment

18. Comet

19. fl -- 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. fi -- 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. fc -- 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. fL -- Lifetime • fL = Lifetime of civilization / Lifetime of star • Beginning of civilization defined as when radio telescopes are invented

28. fL -- 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 ~ 10 D ~ • N ~ 1000 D ~ • N ~ 100,000 D ~ • N ~10,000,000 D ~

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