Math-phobes, fear not; it's not as hard as it looks. The idea, which was dreamed up by cosmologist Frank Drake back in 1961, is that you can estimate the number of civilizations in the universe with whom communication might be possible (Nb) by multiplying the probabilities of seven other independent variables, to wit:
R* = the average rate of star formation in our galaxySome of those (such as R*) are considered to be understood well enough that we can make a fairly sure estimate of its magnitude. Others -- such as fp and ne -- were complete guesses in Drake's time. How many stars had planets? Could be nearly 100%, or it could be the Solar System was some incredibly fortunate fluke, and we're one of the only planetary systems in existence. But now, with improvements in the techniques for surveying stars, we're finding planets everywhere we look -- most stars seem to have planets, and some research published just last month by a team of astronomers at the University of Witwatersrand (South Africa) has shown that planets could form stable orbits in multiple-star systems, something previously thought extremely unlikely.
fp = the fraction of those stars that have planets
ne = the fraction of those stars with planets whose planets are in the habitable zone
fl = the fraction of planets in the habitable zone that develop life
fi = the fraction of those planets which eventually develop intelligent life
fc = the fraction of those planets with intelligent life whose inhabitants develop the capability of communicating over interstellar distances
L = the average lifetime of those civilizations
That they can do so is fortunate not only for alien intelligence enthusiasts like myself -- as much as half of all stars are thought to be part of multiple-star systems -- but for this guy:
So the estimates keep being revised upward. The one we still have no real idea about is L -- how long civilizations tend to last. Carl Sagan, when he described the Drake Equation in his amazing series Cosmos, was pessimistic -- many civilizations, he suggested, lasted long enough to develop weapons of mass destruction, then proceed to blow themselves to smithereens.
But the fact is, we just don't know about L. But one that was complete speculation -- fl, the fraction of planets in the habitable zone that develop life -- just got a bit of a boost from a study done at the University of Bristol (England). The researchers, Holly C. Betts, Mark N. Puttick, James W. Clark, Tom A. Williams, Philip C. J. Donoghue, and Davide Pisani, published their results in Nature: Ecology and Evolution last week in a paper titled "Integrated Genomic and Fossil Evidence Illuminates Life's Early Evolution and Eukaryote Origin." And one of the points the team makes is that once the Earth's surface had cooled sufficiently that water was able to exist in liquid form, life appeared in a relative flash -- while it was still being clobbered every other day by meteorites.
The authors write:
Establishing a unified timescale for the early evolution of Earth and life is challenging and mired in controversy because of the paucity of fossil evidence, the difficulty of interpreting it and dispute over the deepest branching relationships in the tree of life. Surprisingly, it remains perhaps the only episode in the history of life where literal interpretations of the fossil record hold sway, revised with every new discovery and reinterpretation. We derive a timescale of life, combining a reappraisal of the fossil material with new molecular clock analyses. We find the last universal common ancestor of cellular life to have predated the end of late heavy bombardment (>3.9 billion years ago (Ga)).Besides being of obvious interest to evolutionary geneticists, this should get astronomers' blood pumping; it implies that life originated on Earth when the conditions were still nothing short of hostile, with the corollary that once a planet has conditions that allow liquid water, life probably follows soon thereafter.
The implication being that it's likely that every planet with water that sits in its star's habitable zone has some form of life.
So understandably enough, I think this is way cool. It doesn't give us any information about the remaining variables we have little information about, especially fi, fc, and L. There's no particular reason to believe that intelligence is a necessary outcome of evolution; it's tempting to think that the process always drives organisms to be bigger, better, stronger, and smarter, but that's not supported by the evidence. After all, it bears remembering that by far the dominant life-forms on Earth right now, both in terms of biodiversity and overall numbers, are... insects.
It might be that intelligence sufficient to communicate over interstellar distances is a very uncommon occurrence, which leads to the most likely scenario (in my opinion) being plentiful planets with huge diversity of life, but few that have anything like us.
Still, the galaxy is a big place, with billions of stars, so even if it's unlikely, intelligent life probably exists somewhere. Which segues into tomorrow's post, which is about the Fermi Paradox. When told about the Drake Equation, physicist Enrico Fermi famously shrugged his shoulders and said, "Then where is everybody?"
Tomorrow we'll look at a few possible answers -- some of which are considerably more cheerful than others.
This week's Skeptophilia book recommendation is from one of my favorite thinkers -- Irish science historian James Burke. Burke has made several documentaries, including Connections, The Day the Universe Changed, and After the Warming -- the last-mentioned an absolutely prescient investigation into climate change that came out in 1991 and predicted damn near everything that would happen, climate-wise, in the twenty-seven years since then.
I'm going to go back to Burke's first really popular book, the one that was the genesis of the TV series of the same name -- Connections. In this book, he looks at how one invention, one happenstance occurrence, one accidental discovery, leads to another, and finally results in something earthshattering. (One of my favorites is how the technology of hand-weaving led to the invention of the computer.) It's simply great fun to watch how Burke's mind works -- each of his little filigrees is only a few pages long, but you'll learn some fascinating ins and outs of history as he takes you on these journeys. It's an absolutely delightful read.
[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]