Is Anyone Out There? Solving the Drake Equation

1. Is Anyone Out There?Solving the Drake Equation A Statistical Approach

2. Q: Is there life beyond the earth? • How many of these planets have intelligent life? • How many are able to communicate with us? • (have adequate technology to send signals into space) • (How many of them want to?) ?

3. The Drake Equation Ns= number of stars in the Galaxy fs-p= fraction of stars with planets fp-e= fraction of planets that are “earthlike” fp-l = fraction of “earthlike” planets that develop life fl-i = fraction of above that develop intelligence fi-c= fraction of above that develop communication Tc = lifetime of communicative civilization Tg = age of Galaxy

4. Mathematical Aside: Fraction s • Most of the terms in the Drake Equation are in the form of fractions. • f=1 implies something that always happens • f=0 implies something that never happens • Values in between are things that might happen • f=0.5 means a 50/50 chance • f=0.1 means a 1 in 10 chance • f=10-3 is a 1/1000 chance • f=10-6 is one in a million

5. Ns: # of stars in the galaxy • This is well known to astronomers… • Ns = 200-400 billion = 2 to 4 × 1011 • So far, so good… Lots of Potential Sites M31, the Andromeda Galaxy Astrophoto by Robert Gendler

6. fs-p: fraction of stars having planets • Q: Given one of the many stars in the galaxy… • What is the probability that it has planets?

7. fs-p: fraction of stars having planets • Until recently no exoplanets were known “explosion of discovery” Transit method now becoming the preferred method of detection 100-150 new systems detected each year

8. fs-p: fraction of stars having planets • Searches still have a lot of bias • Cannot “see” the planets directly, only their effect on the parent star (gravitational or light blocking) • Hard to detect small (earth-size) planets • Only Jupiter/Saturn/Uranus/Neptune sized planets (mostly) • Biased towards Jupiter size objects  easiest to detect We don’t yet have a decent unbiased sample. And it’s nowhere near complete. But at least its now large (about 1000 systems)

9. fs-p: fraction of stars having planets We now know that at least 10% of “typical” stars have planets. (fs-p = 0.1) Infrared studies of discs around young stars indicate fs-p ~ 0.2-0.5. But we can only detect a limited subset of planets… So maybe they all do! (fs-p = 1)

10. fp-e: fraction of solar systems with an “earthlike” planet • Q: Given many solar systems, what fraction of these have “earthlike” planets? • If 1 (or more) in the “typical” solar system: • fp-e = 1 (or more) • If typical systems do not have an earthlike planet: • fp-e << 1

11. fp-e : factors to consider Defines “habitable zone” • Star: • Massive stars have short lifetimes… • not long enough to develop life. • Low mass star: • Not enough ionizing radiation, • “habitable zone” is very small, • Susceptible to outbursts (“flares”). • Distance from star: • Too close: TOO HOT! • Too far: TOO COLD! • Orbit too elliptical: Temperature varies too much! • Need a stable orbit over time!

12. fp-e : factors to consider • Planet’s composition: • Need liquid H2O • (are NH3, CH4 etc. acceptable substitutes?) • Need an atmosphere! • Need organic (carbon) compounds • (silicon based life?) • No acidic / corrosive environment

13. fp-e : factors to consider • Planet’s size • Too small -> less gravity -> no atmosphere -> no liquid H2O • Also, loses geothermal energy too fast • No magnetic field? • Too big – probably tend to be “gas giants” like Jupiter. No solid surface. • (Floating life forms?)

14. fp-e : factors to consider ? • Other factors • Moderate axial tilt • Moderate rotation rate • No spin-orbit lock? • Large moon necessary for the above? • What about moons of gas giants? • “Good Jupiter” • In the Galactic Habitable Zone? • No nearby supernovae, gamma emitters, etc.

15. fp-e: fraction of solar systems with an “earthlike” planet Probably “borderline” Outside habitable zone But tidal interactions… Gliese 581 c/d ? • Our own solar system has fp-e = 1 • (Of course!!) • Stretching the definition, maybe fp-e = 2 or more: • Mars? • Europa? • Titan? • So far no truly “earthlike” planets have been found outside the solar system. • And only a few come close… • Guess from current data…. ~few / 300 ~ 0.01 ? • But current searches are biased against “earthlike” planets! • May be much higher! (like close to one – habitable zone probability)

16. fp-l: fraction of “earthlike” planets that develop life • Q: Given an “earthlike” planet… • What is the probability that it will develop life?

17. fp-l: fraction of “earthlike” planets that develop life • Simplest definition: • A living organism is something capable of replicating • Bacteria • Viruses • Other one-celled organisms • Need a self-assembling, self-replicating genetic code! • Earth-based life: DNA / RNA • Are there other possibilities?

18. fp-l: fraction of “earthlike” planets that develop life • If life always arises on “earthlike” planets, then fp-l = 1 • Otherwise, fp-l < 1 (maybe << 1) • Only one known example of a planet with life!

19. fp-l : factors to consider • Two extreme possibilities • A: • Even the simplest life is extremely complex! • Simplest organisms have about a million base pairs in DNA/RNA • Lots of things have to go “just right”; overcoming failure points • fp-l is “obviously” very small! Less than 10-6

20. fp-l : factors to consider • B: • Building blocks of life are found in space and on other planets • Organic molecules • Water • Initial life on earth seems to have developed rather quickly… • fp-l might be large (possibly  1?) • But seems to have developed only once , not many times…

21. fp-l : factors to consider • Life can survive under all sorts of conditions • Extremophiles!

22. fp-l : factors to consider X • If life were to be found on Mars… • Implies fp-l is large!

23. fl-i: fraction of planets with lifethat develop intelligent life • Q: Given a planet with simple life forms… …things like bacteria… …what’s the probability that intelligent life will eventually develop?

24. fl-i: fraction of planets with lifethat develop intelligent life • Simplest life forms: self-replicating organisms • But “copies” are not exact • Mutations • Those variants best suited to survive, best able to reproduce, are more likely to pass on their genetic code to the next generation • Natural selection • Over time those changes progressively accumulate • Evolution

25. fi-c: fraction of planets with intelligent life that develop communication Given a planet with intelligent life… What is the probability that they develop tools to communicate through space?

26. Tc / Tg: It’s all about the timing… Given a planet with intelligent life forms that can communicate… How long do they remain that way?

27. Tc / Tg: It’s all about the timing… We only became able to communicate… Early 1900’s: <100 years ago! How much longer will we last? 5 billion years: sun turns into a red giant Mass extinctions every ~100 million years

28. Tc / Tg: It’s all about the timing… • Tc : once a civilization becomes able to communicate, how long does it stay able to do so?

29. Are we Alone? 29 =1% of 1 Billion 10 Million 1/10 10 1 1 1 1/10 =1 million

30. Implications of N= 1 million • 1 Civilization per 100,000 stars • Nearest random one is 1000 light years away! • Life (all life) is RARE! • If intelligent life is UNIQUE to the Earth then either: • F_i = 1 in a billion • L = 1000 (for everyone) !! • This seems unlikely and therefore, with proper planetary management, one day we will be in the club and know the answer.