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Download Is Anyone Out There? Solving the Drake Equation
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A Statistical Approach • 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?) ? 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 • 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 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 • Q: Given one of the many stars in the galaxy… • What is the probability that it has 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 • 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) 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) • • 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 Star: Massive stars have short lifetimes… Low mass star: not long enough to develop life. Not enough ionizing radiation, “habitable zone” is very small, Susceptible to outbursts (“flares”). Distance from star: Defines “habitable zone” Too close: TOO HOT! Too far: TOO COLD! Orbit too elliptical: Temperature varies too much! Need a stable orbit over time! 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 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?) 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. ? • Our own solar system has fp-e = 1 • (Of course!!) • Stretching the definition, maybe fp-e = 2 or more: • Mars? • Europa? • Titan? Probably “borderline” Outside habitable zone But tidal interactions… • So far no truly “earthlike” planets have been found outside the solar system. • • • • Gliese 581 c/d ? 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) • 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 fp-l = 1 • Otherwise, fp-l < 1 (maybe << 1) • Only one known example of a planet with life! 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 B: Building blocks of life are found in space and on other planets Initial life on earth seems to have developed rather quickly… Organic molecules Water fp-l might be large (possibly 1?) But seems to have developed only once , not many times… Life can survive under all sorts of conditions ▪ Extremophiles! If life were to be found on Mars… ▪ Implies fp-l is large! 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? 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 Tc : once a civilization becomes able to communicate, how long does it stay able to do so? Are we Alone? 29 10 1 1 1 1/10 1/10 =1% of 1 Billion 10 Million =1 million 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.