The first time I ever heard of the Drake equation was when I scored a research internship the summer before my freshman year at Cornell's Space Sciences department, then home to Frank Drake, Carl Sagan, and Hans Bethe, all at the same time. I was nominally working for professor Steve Ostro, who had been funded by the US Air Force to apply the Aracebo radio telescope in detecting potentially earth-impacting asteroids. It was an interesting project, as the Cold War was still a going concern, and there was the distinct possibility that any significant asteroid impact could have been mistaken for a nuclear missile attack and so trigger our mutually assured destruction. And who wouldn't want to look into space using a 100 meter diameter detector stuck in a Puerto Rican crater?
But there was another project in the department that caught my attention at the time. Over the course of the summer, I ended up spending more and more time thinking about Professor Drake's Primordial Soup project, a recreation of the famous Miller and Urey experiment from the 50's. At first glance, with all the smelly beakers and custom glass-ware and Tessla-coil type apparatus, it seemed an odd project to be housed in the Space Sciences department. I remember thinking that some renegade chemists must have exceeded their lab space allocation across the arts and sciences quad and started squatting in Space Sciences.
Unable to bear my curiosity any longer, I managed to corner one of the grad students involved to discover that Professor Drake was trying to understand if it was possible to begin with a primordial soup of fundamental elements and basic compounds known to be likely components of a newly coalesced planet, and zap it with the right combination of water, heat, and electrical activity to end up with amino acids. In other words, he was trying to see how likely it was for the building blocks of life to evolve from the building blocks of the early solar system.
That seemed pretty profound to my barely post-pubescent brain, but the corollary that really captured me was when he went on to say, "...and then we can plug that likelihood into the Drake equation to see how many other sentient species exist in our galaxy!" Having been molded from an early age by the original Star Trek series and Star Wars epics, there was simply no way I could resist. So I ended up spending most of my idle hours that summer worrying about far flung civilizations across the galaxy.
The Drake equation came up again when I was taking the first serious course in the Space Sciences department that was intended to separate the real astrophysics candidates from the technical folks looking to satisfy a course distribution requirement. It was a great course that applied some rather advanced mathematics to solve interesting problems ranging from stellar formation, dynamics of shocked gas fronts, and emission from interstellar nebulae, all the way to black holes and space-time curvature. It quickly became my favorite time sink. My final term paper on Theories on Super-luminal Quasar Ejection (or explanations for how luminous jets of matter ejected from Quasars could possibly appear to be moving faster than the speed of light) was even published in the school science journal.
But curiously enough, the one theory that prompted the most intense debate was ultimately described by the simplest equation we looked at all semester, the old Drake equation. (For those of you stricken with Math Anxiety, fear not. This one won't hurt a bit.) No calculus or other advanced mathematics is necessary. Simple elementary school-grade multiplication of a few parameters is the only skill necessary to find an estimate of the likelihood that we can find another intelligent species somewhere in our Galaxy. What caused the debate was the proper selection of all the parameters. For those of you who would like to relive the great debate, here's the (simple) math that Frank Drake first wrote on the blackboard in 1961, and some recent thinking on each of the parameters to get you started:
Nc= N* fp ne fl fi fc fL
N* represents the number of stars in the Milky Way GalaxyCurrent estimates are around 100 billion. fp is the fraction of stars that have planets around themOLD estimates: 20% to 50%. (see below for update)
ne is the number of planets per star that are capable of sustaining life
OLD estimates: from 1 to 5. (see below for update)
fl is the fraction of planets in ne where life evolvesCurrent estimates: 100% (where life can evolve, it will) to near 0%. fi is the fraction of fl where intelligent life evolvesCurrent estimates: 100% (intelligence is a principle survival advantage that it will certainly evolve) to near 0%. fc is the fraction of fi that develop interstellar-scale communications technologyCurrent Estimates: 10% to 20% fL is fraction of the planet's life during which the civilizations communicate.This is the most difficult parameter to estimate. The expected lifetime of our Sun and the Earth is roughly 10 billion years. We've been communicating with radio waves for less than 100 years. How long will our civilization survive? When might we fully transition to non-radiative technologies like cable and fiber-optics and give up most radio emissions? If we were destroyed tomorrow the answer to this question would be 1/100,000,000th. If we and our current radio infrastructure survive for 10,000 years the answer will be 1/1,000,000th.
Rather than spoil all the fun and tell you what I think is a best guess result, here is a link to an automated online calculator that will let you amateur scientists form your own hypotheses, use your own mouse to twiddle the knobs, and astound yourself with almost inescapable conclusions as to our uniqueness (or lack thereof) in the Universe.
The first bit emerged in the latest September 8 issue of Science, where a paper entitled "Exotic Earths: Forming Habitable Worlds with Giant Planet Migration," was authored by Dr. Sean Raymond of the University of Colorado at Boulder, Avi Mandell of Pennsylvania State University and NASA's Goddard Space Flight Center in Greenbelt, Md., and Dr. Steinn Sigurdsson of Pennsylvania State University.
After running over eight months of computer simulations of proto-planetary system formation, the team discovered that over one third of their simulations resulted in star systems with water-laden planets residing within the "Habitable Zone" of orbital radii that could support life as we know it. (This tells us that the result of multiplying the two parameters of fp*ne from the Drake equation above should be around 0.33, where the earlier range of estimates spanned from 0.2 all the way up to 2.5)!)
The key result of the study was that large hot proto-planetary gas giants that form near the star tend to gravitationally fling matter around the system in such a way that the Earth-like planets in the Habitable Zone are much more stable than originally suspected.
The actual Science article is available online but it requires a subscription, so here is a link to the key image from the article from Pharyngula.
"These gas giants completely shake up the system as they migrate, but eventually things settle down," said Mandell. "We now think there is a new class of ocean- covered and possibly habitable planets in solar systems very different from our own."
The diagram shows the final configuration of four simulations, with our solar system shown for scale. Each simulation plotted horizontally shows the relative size of each body, though gas giant planets shown in black are not to-scale. The eccentricity (the radial excursion over an orbit) of each body's orbit is shown via the brackets beneath it. The color of each body corresponds to its water content, and the inner dark region to the relative size of its iron core.
The second bit to keep in mind is that Nc above describes the number of detectable civilizations in our galaxy. Don't be too depressed if your result was rather small. Given that there are over 125 billion galaxies in the known universe, it is almost certain that we are not alone, even if we are unlikely to detect our more distant neighbors.
When I say very tiny, I mean a slice about the size of the period at the end of this sentence held at arms length. Really miniscule. And guess what? The sky looks pretty darn similar no matter which way you happen to look. When you start to count across the entire sky, 125 billion is our best guess at the total. (It's a guess because it took the camera about eleven days to collect enough light to make this one image. So we haven't actually mapped the entire sky at this resolution, but we have looked in enough places and seen enough commonality to be confident in the projection. Imagine the precision and control necessary to keep a large satellite pointed this accurately while using a digital camera with an 11-day shutter speed!)
So how many civilizations do you think exist across the over 125 billion galaxies that might have as many as 100 billion stars each? Astronomical numbers indeed.
Here is a link to another calculator that extends Drake's equation to answer all sorts of other questions.
The Drake Equation on Wikipedia.