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SciFi Round Seven: Alpha Centauri

2012/10/25 10 comments

There’s a planet at Alpha Centauri.  And that’s just too cool not to follow up.  (For those of you who may want more technical information than the first link, the full Nature article is here… though it may take a subscription to see the whole thing.)

In honor of that discovery, let’s see how well some fictional accounts of Alpha Centauri stack up.  It’s a popular system to consider, since the stars at Alpha Centauri are the closest to Earth (other than Sol – aka “The Sun”) at a mere 4.4 light-years.  I’ll only be hitting a few examples, but oddly enough, Wikipedia has an extensive listing if you want to see them all…

Alpha Centauri Is More Than One Star

I was shocked to see this mentioned on the aforementioned Wikipedia page, but apparently some authors think that Alpha Centauri is only one star.  I have the good fortune to not have read any of these; I would have been very upset by them.

While not obvious to the naked eye (or authors from the northern hemisphere who don’t see it at all), Alpha Centauri is composed of two stars.  The larger, Alpha Centauri A, is a spectral type G2V, the same as the sun, has a slightly larger mass and slightly brighter.  Alpha Centauri B is type K2V, and is noticeably cooler and dimmer than the Sun, and has about 90% of the Sun’s mass.  The two stars are close enough in mass that the system’s center of mass is well between the two stars, not near the center of one or the other.  Their orbit is eccentric, with the distance between the two stars varying between roughly 10 and 50 AU.  For scale, that closest pass is about the distance between the Sun and Saturn.  There’s a nice animation of this, along with other information, which also shows an estimate for where the habitable zones of the stars may lie.

On top of the bright binary, there’s a third, even dimmer star called Proxima Centauri (or sometimes Alpha Centauri C).  It orbits around Alpha Centauri at a distance of about 15,000 AU.  Its spectral type is M5.5Ve — at a bit over a tenth of the Sun’s mass, it’s much dimmer, cooler, and redder than the Sun or Alpha Centauri AB.  The “e” means it’s an actively flaring star.  More on that later.

Moving In

Sometimes it’s only mentioned in passing, but fictionally speaking, Alpha Cen is a common waypoint or colonization target.  It gets mentioned as such, and occasionally featured, in such things as Star Trek, Lost in Space, Buck Rogers, Doctor Who… the list goes on.  A couple of books that mention or feature Alpha Centauri are The Songs of Distant Earth by Arthur C. Clark and Foundation and Earth by Isaac Asimov.  In the movie Avatar, the moon Pandora orbits a gas giant which in turn orbits Alpha Cen A.  Sending a colony ship there is a method of winning the game in Civilization, and it’s the name of the game Alpha Centauri.  Typically, these works imply or require the presence of at least one habitable planet orbiting either A or B.

The recently discovered planet is Alpha Centauri Bb.  It has a mass somewhat greater than Earth’s, and orbits Bb at a radius of 0.04 AU.  Despite the fact that B is dimmer than the Sun, that means this little planet is baked to a surface temperature of at least 1500 K (depending on its albedo and atmosphere).  That’s hot enough to melt silicate rocks, and is at least twice as hot as Venus (which averages 735 K).  Odds of anything living there are pretty slim.

Artist’s conception of Alpha Cen Bb, which really needs a better name, in orbit around Alpha Cen B. The color scale is logarithmic, with brightness ranging over a factor of many trillions. Alpha Cen B is ~100,000 times brighter than Alpha Cen A as seen from Bb, the Sun is not quite a billion times fainter than Alpha Cen A, and the galactic plane is fainter still.

Because Alpha Centauri is so close, there’s enough data to give good limits on what other planets could be in the system.  Anything habitable must be orbiting either A or B relatively closely in order to be warm enough and to avoid having its orbit perturbed too much by the other star — but not too closely.  The combination of these two requirements makes it difficult for a planet to stay in Alpha Cen A’s habitable zone — it’s likely to get scattered out by B.  This may be a problem for works that put the habitable planet around A, such as Foundation and Earth.  B, on the other hand, may have less trouble with this, since its habitable zone is closer in.

Planets far enough out to orbit outside the AB pair would be too cold.  C is an unlikely candidate — it’s so small that any planet close enough to be warm enough for life would be close enough to be seriously zapped by radiation from the flares, which would (probably) erode its atmosphere.

Back to Avatar.  The moon Pandora orbits a gas giant, which orbits A.  Even assuming it gets away with orbiting A, there’s another problem.  Despite some false alarms, current limits indicate that there are no gas giants or brown dwarfs anywhere close to A, B or C.  Pandora doesn’t exist.  (And who names their moon Pandora, anyway?  But we’ll cover Avatar another day.)

On the other hand, that leaves plenty of room for speculation — smaller, rocky, habitable planets are possible, and more plausible around B.  The stars in the system are also older than the Sun, which means there’s been plenty of time for life to develop…

ALIENS!!!

If the aliens are the technologically primitive (no radio) Na’vi, it’s pretty obvious why we haven’t heard from them yet.  Or they could all be a hundred years dead.   On the other hand, if we’re talking about aliens like the fithp in Footfall (by Larry Niven and Jerry Pournelle), then we’ve got a problem.

Contact with aliens from Alpha Cen is another common theme, with technologically advanced aliens an option.  In Footfall, they’re in the form of an unusually plausible group of alien invaders.

There’s just one problem.  We’ve been putting out radio signals for over a century, and relatively strong ones for television broadcasts for over seventy years now.  Those signals, while usually not decipherable, are still detectable and clearly artificial for a sphere sixty or seventy light-years in radius and expanding.  If we ourselves were sitting at Alpha Centuari, with our current technology, we could unambiguously detect those radio signals and notice both Earth and the fact that it’s inhabited.

By the same token, if Earth with all its radio chatter were orbiting Alpha Cen B in the habitable zone… we’d have heard them by now.  At least one of the books listed in Wikipedia (Factory Humanity) seems to get this right — contact with aliens at Alpha Cen is established first via radio.  Why we didn’t hear the fithp before they decided to take a detour through our solar system isn’t clearly explained.  Maybe they’ve replaced all their radio tech with subspace transponders.

Either that, or they’ve seen us, and are deliberately hiding.  Just like the Martians.  Insert your conspiracy theory here.