Home > Asimov, Avatar (2009 film), Clark, Clement's Game, Larry Niven > SciFi Round Seven: Alpha Centauri

SciFi Round Seven: Alpha Centauri

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…


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.

  1. michaelbusch
    2012/10/25 at 4:22 am

    The Dumusque et al. paper describing the discovery of AC Bb is available for free on ESO’s website: http://www.eso.org/public/archives/releases/sciencepapers/eso1241/eso1241a.pdf

  2. 2012/10/29 at 11:34 pm

    Two questions regarding binary systems:

    A) Wikipedia suggests that about 1/3rd of stars in our galaxy are binary, so it seems like we should have lots of data on them and there are presumably a bunch of simulations as to how binary systems develop as a result of star formation. Is it possible for binary systems to be only temporarily stable (although still on the order of millions of years), especially through the interaction of a smaller 3rd body, like a gas giant? Have we observed binary systems that are in the process of collapsing? Can this form a new star, and could we know if an existing star was a recent merge from a binary collapse?

    B) More on the scifi theme, is there anything special that can happen at binary Lagrange points? It would seem like this is a good place to put “interesting” things. Can a Lagrange point be consistently in the habitable zone? Is it common/possible that a planet could form or get stuck there during the star formation process? I would say this is where I would hide life on AC, but I’d wager that none of them are even close to habitable.

  3. 2012/10/30 at 3:36 am

    As far as binary stability goes — not really. So long as the binary is by itself, it’s stable. A smaller 3rd body like a gas giant generally won’t have enough mass to perturb the much-larger stars. On the other hand, in a trinary (or bigger) system, or a binary in a a globular cluster with many other stars, it may be possible to perturb the system, generally ejecting one of the stars. The odds of a collision between stars are really, really, really low.

    Binaries don’t ever really “collapse,” per se. Very close binaries, where the two stars are essentially touching, or where one is drawing hot gas off of the other, have been observed. They may be luminous x-ray sources, catastrophic variables, or produce a nova or supernova, depending on the details. This doesn’t form a new, young star with habitable planets — though it can produce a neutron star or black hole.

    As for the binary Lagrange points… well, in a case where the ratio in mass between the two stars is less than about 25, none of the Lagrange points are actually stable. (L1, L2, L3 are always unstable; L4 and L5 are stable if that ratio is more than ~25. Those are where, for instance, the Trojan asteroids that co-orbit with Jupiter are.) That means, although there may be quasistable orbits around those points, an object orbiting near the Lagrange points will be easily perturbed away from that position if there are any other objects in the system.

    Maybe you could set up a binary where that could work, and be habitable, but I don’t think this is going to work on Alpha Centauri, for two reasons. First, the two stars are fairly close in mass — the difference is less than a factor of two. Given the distance between them, any object far out enough from both stars to be in a Lagrange point would be too cold for life. And then there’s the issue with the mass ratio that makes the L4 or L5 points stable. You’d need one star to be ~25 times more massive than the other… which is unusual and poses other problems in terms of the stellar types (one star would be much brighter and shorter-lived than the other).

    So, I’d expect that while there may be debris hanging around the Lagrange points, it’s not possible to put a habitable planet there in the Alpha Cen system. (Michael, correct me if I’m off on this.)

  4. michaelbusch
    2012/10/30 at 4:03 am

    Rachel is quite correct. The only stable orbits in Alpha Cen are distant orbits around both stars or very close orbits around each star.

    It is possible to have planets around two closely-spaced stars, and then have another star or pair of stars at much greater distance. Meg Schwamb and the Planet Hunters team just found one like that: http://blog.planethunters.org/2012/10/15/ph1-a-planet-in-a-four-star-system/ .

    It is also possible to have two planets sharing the same orbit, effectively sitting in each others L4 and L5 points (although the equilibrium orbits are bit more complicated in that case). Kepler has found such a system, Kepler-36 (http://en.wikipedia.org/wiki/Kepler-36). Some of Saturn’s moons show similar behavior.

    There are binary star systems that are unstable on very long timescales, but only if they are close together, as Rachel described. Adding more bodies of comparable mass to the N-body problem can produce shorter-term instabilities. Usually, the bodies concerned are of planetary mass. This happened here a few hundred million years after the solar system formed, when Jupiter and Saturn entered a 1:2 mean-motion resonance, at least according to the Nice model (http://en.wikipedia.org/wiki/Nice_model). The resulting gravitational scattering of the planets made the Kuiper Belt the way it is today.

  5. michaelbusch
    2012/12/19 at 10:31 pm

    And now we learn that Tau Ceti, another nearby Sun-like star, may have a generous allotment of planets as well: http://arxiv.org/abs/1212.4277

    Unlike Alpha Cen, Tau does not have the complexities of a binary system, so something like the proposed New Worlds Observer could take direct images of the planets in it.

    Another crowd of science-fiction stories, including one of my own, just became impossible.

  6. michaelbusch
    2014/03/31 at 3:58 pm

    Update from 2014 March:

    Over the last 12-18 months, both of the claimed discoveries of planets around Alpha Cen B and Tau Ceti have been challenged on the grounds that other observers have as yet been unable to replicate them. In the particular case of Alpha Cen Bb, there is also controversy about the methods used to subtract off variability due to activity on the star itself. We all await more data to figure out what is actually going on.

    Rachel’s criticisms of fictional representations of the place remain correct, though.

  7. michaelbusch
    2015/10/19 at 6:23 pm

    Update from 2015 October:

    Claimed planet around Alpha Cen B demonstrated to be an artifact of the data processing procedure used to analyze the data. Alas, there is apparently no lava world at Alpha Cen: http://arxiv.org/abs/1510.05598 .

  1. 2012/12/24 at 8:57 pm
  2. 2013/02/09 at 6:15 pm
  3. 2015/08/08 at 4:35 pm

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