The title of yesterday’s post — ‘The Odds on Centauri’ — would fit well with today’s musings. Alpha Centauri makes us ponder the odds not just in terms of interstellar bets and future space probes, but also in terms of the likelihood of life around these stars. And after all, 2008 saw significant work on this question, including the contributions of Philippe Thébault (Stockholm Observatory) and colleagues, whose studies of Centauri A and B show that while stable planetary orbits exist there, the odds on those planets forming in the first place are long.


Greg Laughlin (UC-Santa Cruz) isn’t necessarily daunted by this work (he explains why here), but the planet-hunter extraordinaire is realistic about life-bearing planets in this environment, and even more judicious about the possibility of a technological society making its home in the system. The question rises naturally out of recent publicity given the 20th Century Fox film The Day the Earth Stood Still, in which it was announced that the entire movie had been beamed in the direction of Alpha Centauri as a publicity stunt.

Image: Centauri A and B hanging over the horizon of Saturn. This Cassini image was captured captured from about 66 degrees above the ringplane and faces southward on Saturn. Ring shadows mask the planet’s northern latitudes at bottom. Credit: NASA/JPL/Space Science Institute.

The question: If somehow this transmission made it to Centauri, would there be anyone there to watch it? We’ll leave aside the issues that plague the original transmission, factors such as the fact that Alpha Centauri is barely above the horizon in Florida, and at the time of the transmission, according to Laughlin, “Alpha Cen (RA 14h:39m, DEC -60deg:50min) was below the horizon as viewed from 28 35 06N, 80 39 04W.” Forget that for a moment. Let’s just ask whether there might be, around one of the Centauri stars, a planet that could house a technological civilization.

Here’s what Laughlin came up with in a recent systemic post:

fp = Chance of a habitable planet orbiting Alpha Cen B = 0.6

fl = Chance that life evolved on that planet = 0.01

fi = Chance that life developed intelligence = 0.1

fr = Chance that intelligence understood Maxwell’s Equations = 0.01

fn = Chance that Maxwell’s Equations are currently understood on Alpha Cen Bb = 64,000 / 3×109 = 0.0000213.

This gives (fp)x(fl)x(fi)x(fr)x(fn) = one in eight billion, with Alpha Cen Ab kicking in an additional one in a trillion chance.

Of course, we’re plugging in most of these parameters without being able to do more than guess at their true value. Laughlin asked students in his classes, for example, to choose a value for fn based on their own estimate of how long a society will build radios. We also have to approximate values like fr, re an intelligent being’s capacity to understand Maxwell’s equations, and so on. There is not, in other words, a reason to abandon interest if you are one of those who hope for great things around Centauri, but it’s sobering to see that as things stand now, one in eight billion is a reasonable figure.

The exciting thing about our era of exoplanet discovery is that we learn game-changing things on a frequent basis. I, for one, would be delighted with the discovery of a small, rocky world around either of the major Centauri stars, one from which future study might extract spectroscopic evidence of life of whatever kind. But I’ll also be delighted at whatever we learn about planetary formation in the close binary environment, because that will help us understand how likely habitable planets may be around many stars. And if it turns out there is no one to watch The Day the Earth Stood Still wherever its stray signal may turn up, I can’t say that our hypothetical extraterrestrials will have missed much.

Addendum: Anyone who enjoyed the 1951 original of The Day the Earth Stood Still will want to read Brett Holman’s comments on the film in his superb Airminded blog. I particularly like his last line: “I do wonder just how credible a threat is a fleet of flying saucers flown by robots who can be pacified simply by speaking the words ‘Klaatu barada nikto‘?”