Is Anyone Out There Solving the Drake Equation

1. Is Anyone Out There?Solving the Drake Equation Jeremy P. Carlo Columbia University AAI Astronomy Day 5/10/2008

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. What this is not about: Aliens visiting the earth Alien abductions, UFOs, etc. Us going to other planets in search of life Justification: Traveling to other solar systems is hard. Much easier to use radio.

4. The Drake Equation Developed in 1960 by Frank Drake and others at SETI (SETI: Search for Extra-Terrestrial Intelligence) N = Ns*fs-p*fp-e*fp-l*fl-i*fi-c*Tc / Tg N = # of communicative civilizations in our galaxy, right now

5. 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

6. Mathematical Aside: Scientific Notation How to deal with really big or small (�astronomical�) numbers! 10,000,000,000,000 = big number. Count up the zeroes� 13 10,000,000,000,000 = 1013 (1E13 in the computer) 0.000000001 = small number. 0.000000001 = 1/1,000,000,000 = 1/109 = 10-9 (1E-9) 450,000,000 = 4.5�100,000,000 = 4.5�108 (4.5E8) multiplication: 1013 �1011 = 1024 division: 109/103 = 106

7. 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 etc.

8. Ns: # of stars in the galaxy This is well known to astronomers� Ns = 200-400 billion = 2 to 4 � 1011 So far, so good�

9. 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?

10. fs-p: fraction of stars having planets Until recently no exoplanets were known First discovery 1989, then� Today, almost 300 exoplanets known! 20 known multi-planet systems!

11. Detecting Exoplanets

12. 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 Hard to detect small (earth-size) planets Only Jupiter/Saturn/Uranus/Neptune sized planets (mostly) Favor �hot Jupiters� Also orbital inclination angle, parent star�s mass & brightness� Which stars do you choose for detailed study? We don�t yet have a decent unbiased sample. And it�s nowhere near complete. But we can estimate�

13. 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)

14. 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

15. fp-e : factors to consider 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!

16. 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 Need elements heavier than hydrogen / helium No �Population II� stars!

17. 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?)

18. fp-e : factors to consider Other factors Moderate axial tilt Moderate rotation rate No spin-orbit lock? Red dwarfs out? 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.

19. fp-e: fraction of solar systems with an �earthlike� planet 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! But limited if red dwarf planets aren�t allowed (must be <0.2 or so)

20. fp-l: fraction of �earthlike� planets that develop life Q: Given an �earthlike� planet� What is the probability that it will develop life?

21. 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?

22. 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! Not much hard data to go on here�

23. fp-l : factors to consider Two schools of thought: School 1: 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� fp-l is �obviously� very small!

24. fp-l : factors to consider School 2: 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� So it�s not just popping up everywhere!

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

26. fp-l : factors to consider If life were to be found on Mars� Implies fp-l is large!

27. 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?

28. 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

29. 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?

30. 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?

31. Tc / Tg : It�s all about the timing� Tg is the age of the galaxy Tg = 10 billion years = 1010 years Whew!

32. 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?

33. 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 But will we even last that long�