Jeremy P. Carlo Columbia University AAI Astronomy Day 5/10/2008

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?) ?

 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. SPEEDTRAVEL TIMECOST SPACE TRAVELSloooow…Looooong….$$$$$$$$ RADIO COMMUNICATION Fast! (c)Long, but not as much Cheap!

Developed in 1960 by Frank Drake and others at SETI – (SETI: Search for Extra-Terrestrial Intelligence) N = N s *f s-p *f p-e *f p-l *f l-i *f i-c *T c / T g N = # of communicative civilizations in our galaxy, right now

N s = number of stars in the Galaxy f s-p = fraction of stars with planets f p-e = fraction of planets that are “earthlike” f p-l = fraction of “earthlike” planets that develop life f l-i = fraction of above that develop intelligence f i-c = fraction of above that develop communication T c = lifetime of communicative civilization T g = age of Galaxy

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 = (1E13 in the computer) = small number = 1 / 1,000,000,000 = 1 / 10 9 = (1E-9) 450,000,000 = 4.5×100,000,000 = 4.5×10 8 (4.5E8) multiplication: ×10 11 = division: 10 9 /10 3 = 10 6

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.

 This is well known to astronomers…  N s = billion = 2 to 4 ×  So far, so good… M31, the Andromeda Galaxy Astrophoto by Robert Gendler

Q: Given one of the many stars in the galaxy… What is the probability that it has planets?

Until recently no exoplanets were known – First discovery 1989, then… Today, almost 300 exoplanets known! 20 known multi-planet systems! The Snowball Effect!

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…

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

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

 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! Defines “habitable zone”

 Planet’s composition: ▪ Need liquid H 2 O ▪ (are NH 3, CH 4 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!

 Planet’s size  Too small -> less gravity -> no atmosphere -> no liquid H 2 O ▪ 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?)

 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. ?

Our own solar system has f p-e = 1 (Of course!!) Stretching the definition, maybe f p-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) Probably “borderline” Outside habitable zone But tidal interactions… Gliese 581 c/d ? 55 Cancri f ? HD28185 b ?

 Q: Given an “earthlike” planet…  What is the probability that it will 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?

 If life always arises on “earthlike” planets, then f p-l = 1  Otherwise, f p-l < 1 (maybe << 1)  Only one known example of a planet with life!  Not much hard data to go on here…

 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”  f p-l is “obviously” very small!

 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… ▪ f p-l might be large (possibly  1?)  But seems to have developed only once, not many times… ▪ So it’s not just popping up everywhere!

 Life can survive under all sorts of conditions ▪ Extremophiles!

 If life were to be found on Mars… ▪ Implies f p-l is large! X

 Q: Given a planet with simple life forms… …things like bacteria… …what’s the probability that intelligent life will eventually develop?

 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

 Given a planet with intelligent life…  What is the probability that they develop tools to communicate through space?

 Given a planet with intelligent life forms that can communicate…  How long do they remain that way?

 T g is the age of the galaxy  T g = 10 billion years = years  Whew!

 T c : once a civilization becomes able to communicate, how long does it stay able to do so? ?

 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…