1. Drake’s Equation FYOS Lecture 10

2. Exam2 Main transits : planet blocks light from the star Main transits : planet blocks light from the star 2ndary transits : star blocks light from the planet 2ndary transits : star blocks light from the planet Curvature : phase of the planet Curvature : phase of the planet

3. Drake Equation Frank Drake Frank Drake o currently at SETI institute Berkeley o In 1961, at a meeting of about a dozen scholars at Green Bank, WV. o about the number of radio(?) transmitting civilizations Drake Equation o estimating the probability of communicable ET o at the moment, we only focus on our Galaxy

4. Drake Equation (Carl Sagan’s version) N number of transmitting civilizations N * number of stars in our Galaxy f planet fraction of stars with planets n E number of habitable planets per star f life fraction of planets with life f intell fraction of worlds with intelligent life f civ fraction of intelligent worlds capable of interstellar communication f L the fraction of a planetary lifetime with a technological civilization N = N* N* N* N* × f planet f planet × n E n E × f life f life × f intell f intell × f civ f civ × fLfLfLfL

5. Drake Equation (Carl Sagan’s version) N number of transmitting civilizations N = N * × f planet × f E × f life × f intell × f civ × f L ×××× ××= N N*N* f planet f Earth f life f intell f civ f long

6. Drake Equation (original version) R * : average star formation rate There are ~200 billion stars in our Galaxy. Our Galaxy is about 10 billion years old.  about 20 stars are born per year R * ≈ 20 N = R * × f planet × n E × f life × f intell × f civ × L

7. Drake Equation (original version) f planet : average fraction of stars with planets Planet formation process is universal (angular momentum conservation) Planet formation process is universal (angular momentum conservation) Exo-planets are being discovered nowadays  Doppler result indicates that at least ~20% of stars have planets. Exo-planets are being discovered nowadays  Doppler result indicates that at least ~20% of stars have planets. f planet ≈ 1 N = 20 × f planet × n E × f life × f intell × f civ × L

8. Drake Equation (original version) n E : average number of Earth-like planets per star system Planet formation process is universal (angular momentum conservation) Planet formation process is universal (angular momentum conservation) Rocky planets are formed closer to the central star. Rocky planets are formed closer to the central star. Close to a unity?? Close to a unity?? n E ≈ 0.5? N = 20 × 1 × n E × f life × f intell × f civ × L Or n E > 1 (Cassan et al. 2012, Nature, 481, 167)

9. Drake Equation (original version) f life : average fraction of Earth-like planets with life Uncertain. One of the main goals of astrobiology. Uncertain. One of the main goals of astrobiology. Life on Earth arose very early on Life on Earth arose very early on  implying that this fraction not so small? f life ≈ 50% N = 20 × 1 × 0.5 × f life × f intell × f civ × L

10. Drake Equation (original version) f intell : average fraction of life-bearing planets with intelligent species Uncertain. One of the main goals of astrobiology. Uncertain. One of the main goals of astrobiology. Intelligence is an advantageous evolutionary niche (E.Q. evolution) Intelligence is an advantageous evolutionary niche (E.Q. evolution) f intell ≈ 50% N = 20 × 1 × 0.5 × 0.5 × f intell × f civ × L

11. Drake Equation (original version) f civ : average fraction of civilizations capable of interstellar communication Have to use some sort of symbolic languages. Have to use some sort of symbolic languages. Will intelligent life want to communicate to others? Will intelligent life want to communicate to others? Inputs from anthropologists, psychologists, philosophers, and theologians Inputs from anthropologists, psychologists, philosophers, and theologians Quite uncertain. Quite uncertain. f civ ≈ 50% N = 20 × 1 × 0.5 × 0.5 × 0.5 × f civ × L

12. Drake Equation (original version) N = 20 × 1 × 0.5 × 0.5 × 0.5 × 0.5 × L ~ 1 N ≈ L Frank Drake’s California license plate

13. Drake Equation (original version) N ≈ L Laverage lifetime (in years) that a civilization remains technologically active How long will the civilization use radio communication? How long will the civilization use radio communication? Will they be around long enough to send messages and get a reply? Will they be around long enough to send messages and get a reply? We leaked radio communications from our TV/Radio broadcasts We leaked radio communications from our TV/Radio broadcasts o nowadays, mostly via cable o but, telephone communications through a cable now became wireless… At least for us, L is about 50 yrs At least for us, L is about 50 yrs

14. Average Distance between Civilization T R

15. T R Volume of our Galaxy = πR 2 × T Total number of Radio civilizations now = N Volume occupied by each civilization = πR 2 × T / N = d 3 Average distance b/w civilizations = d d d d

16. Average Distance between Civilizations T R If N=10,000 and with R= 50,000 light-years, T= 1,000 light-years… First Radio broadcasting December 24, 1906 from Brant Rock, Massachusetts. First major TV broadcasting : 1963.  barely reached ~100 Light-years from Earth… d

17. Most Optimistic Estimate N  40,000,000 civilizations d  58 Light-years … 5 nearest stars to Earth Proxima Centauri4.24 Ly α Centauri A4.35 Ly α Centauri B4.35 Ly Banard’s Star5.98 Ly Wolf 3597.78 Ly If true, we should have already detected or been contacted or visited by them… R*R* 20 stars/yr f planet 1 nEnE 2 f life 1 f intell 1 f civ 1 L1 million yrs

18. Pessimistic Estimate N  about 10 civilizations This few civilizations, we can no longer approximate each civilization as a cube… d  9,000 Light-years … ?how? If true, we may practically be the only one in our neightborhood.  Should we set out a bold journey to the infinity and beyond?  Should we set out a bold journey to the infinity and beyond? R*R* 20 stars/yr f planet 0.5 nEnE f life 0.5 f intell 0.5 f civ 0.01?? L100 yrs

19. Galactic Colonization Speed Example 1: speed 0.1c speed 0.1c settling time 150 yrs settling time 150 yrs  expansion speed of 0.01c.  It takes only 10 Myrs! Example 2: speed 0.01c speed 0.01c settling time 5000 yrs settling time 5000 yrs  expansion speed of 0.001c  takes only 100 Myrs! Coral Model of Galactic colonization

20. Shouldn’t a cosmic exploration be dependent on rocket speeds? “ Interstellar distances are no barrier to a species which has millions of years at its disposal ” Freeman Dyson In “Disturbing the Universe” 1979

21. Which estimate do you like better? N≈10  d≈9000 lyrs N≈10  d≈9000 lyrs N≈10,000  d≈900 lyrs N≈10,000  d≈900 lyrs N≈40,000,000  d≈60 lyrs N≈40,000,000  d≈60 lyrs If some of these alien civilizations can live long, then they should have enough time to colonize a good fraction of the Galaxy. Where are they? Fermi Paradox

22. Next week : possible solutions to the Fermi Paradox 1.We are the only one! o Rare Earth Hypothesis 2.They are here already! o Zoo Hypothesis, Sentinel Hypothesis 3.They exist (or existed) but incommunicable o unwillingness, short-lived, hostile, technical difficulties, etc. R. Shrestha  Summarize “Fermi Paradox” again (including von Neumann probe argument) R. Shrestha  Summarize “Fermi Paradox” again (including von Neumann probe argument) C. Barmore  #1 C. Barmore  #1 Z. Rindik  #1 Z. Rindik  #1 T. West  #2 T. West  #2 G. Chant  #2 G. Chant  #2 J. Hedley  #3 J. Hedley  #3 Y. Lee  #3 Y. Lee  #3