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A Discouraging Outlook for Centauri A Planets

The news about planetary prospects around the Centauri stars has been positive enough lately that a paper suggesting otherwise introduces a rather jarring note (to me, at least). After all, we’ve detected more than forty extrasolar planets in multiple systems, a significant percentage of all detected exoplanets, and while most of these are in systems where the stars are widely spaced, there are planets around stars like Gliese 86 or Gamma Cephei where the separations are in the range of a Centauri-like 20 AU. Moreover, key studies have shown that planetary orbits in the habitable zone of the Centauri stars are viable.

But what Philippen Thébault (Stockholm Observatory), Francesco Marzari (University of Padova) and Hans Scholl (Observatoire de la Côte d’Azur) bring to the table is a different question. Never mind that planetary orbits may be stable — how likely are planets to form in these settings in the first place? It turns out that the last stage of planet formation has been studied, with no deal-breaking restraints on at least some planets in close binary systems if the stars are separated by more than 5 AU. The new work, however, focuses on an earlier stage, the accretion of small planetesimals that leads to so-called planetary ’embryos.’

Planetesimals around a Sun-like star

And here the news is much less positive for those of us hoping to find terrestrial worlds around such stars. The companion star can cause such perturbations that the kilometer-sized planetesimals involved collide at high speeds, moving too rapidly to allow them to gradually merge into a larger object. Earlier work by these authors has shown that the efficiency of runaway growth is quite sensitive to encounter velocities in budding planetary systems. The current paper now applies these results to the situation around Centauri A.

The results: Beyond a close 0.5 AU, Centauri A “…is hostile to kilometre-sized planetesimal accretion.” In fact, say the authors, using any realistic distribution of planetesimals, these high-impact events outnumber low-velocity encounters by far. What’s worse, “…our approach can be regarded as conservative regarding the extent of the impact velocity increase which is reached, the ‘real’ system being probably even more accretion-hostile than in the presented results.” That makes Centauri A an unlikely place for habitable worlds like our own.

Image: Artist’s conception of the debris disk surrounding the Sun-like star, HD 12039. Planetesimals like these are thought to be the building blocks of planets. But will they form planets in the binary environment of Centauri A? Credit: NASA/JPL-Caltech; T. Pyl, SSC.

Contrasting this outcome with earlier studies that found planet formaton possible in the region inside 2.5 AU of Centauri A, the authors explain why their work comes to such a radically different conclusion:

This is because these studies focused on the later embryo-to-planet phase, thus implicitly assuming that the preceding planetesimal-to-embryo phase was successful. The present study shows that this is probably not the case. This confirms that the planetesimals-to-embryos phase is more affected by the binary environment than the last stages of planet formation.

How to extrapolate these results to other stars? The authors are reluctant to do so, not even to nearby Centauri B, the perturbing star in the Centauri A scenario. The reason is that the mathematical modeling here is based both upon the perturbations caused by the nearby star as well as the relationship between those perturbations and the effects of gas drag in the early system, which shape the movements of planetesimals. The byplay between these forces is complex and obviously varies from star to star, which is one reason we have so far to go in understanding how planets form in binary systems.

Can we imagine planetary migration from inner orbits into the habitable zone? Perhaps, but this would assume interactions between multiple planets and a sizeable disk of remaining planetesimals. The authors run through other scenarios that would produce Centauri A planets at 1 AU or greater, including the possibility that the Centauri stars were originally in a different configuration than today, thus setting up a new set of initial conditions. The latter can’t be ruled out, but the authors’ conclusion remains:

…we think that our results on planet formation, with the present binary configuration, are relatively robust. Our main result is that it is very difficult for s < 30 km planetesimals to have accreting encounters beyond 0.5-0.75 AU from the primary, which makes planet formation very unlikely in these regions.

Observationally, of course, we lack the data to know just what we might find around 1 AU at Centauri A. Radial velocity studies have shown that we can say only one definitive thing about planets around this interesting star: If they exist, they are smaller than 2.5 Jupiter masses. The new work, which takes what the authors believe is a more realistic set of assumptions for differences in sizes among planetesimals, suggests that if we find smaller worlds around this star, they will be relatively close to the star, too close for habitable conditions to exist there. A look at Centauri B using these parameters would be welcome.

The paper is Thébault et al., “Planet formation in Alpha Centauri A revisited: not so accretion-friendly after all,” accepted for publication in Monthly Notices of the Royal Astronomical Society (abstract). Centauri Dreams has covered a good deal of recent work by Elisa Quintana, Jack Lissauer, Greg Laughlin and others — you can use the search function on the top page to find these entries, or to get a representative sample, look here and here and here, where citations are available.

Comments on this entry are closed.

  • dad2059 July 14, 2008, 12:43

    It would be a shame if planets didn’t form around Centauri A, but I think the idea an accretion disk would form just asteroids is an old theory if I’m not mistaken Paul, check me if I’m wrong on this.

  • Bynaus July 14, 2008, 14:43

    There is still hope… Check out the work by A. Johansen and coworkers from MPIA Heidelberg (e.g. this conference presentation: http://www-theorie.physik.unizh.ch/~ryuji/ascona/Ascona08_Johansen.pdf), who studies planetesimal formation by gravitational instabilities: typical planetesimals formed by this process range from 100 to 1000 km in diameter (depending on the turbulence model used). If there is any truth to it, the “harmful” < 30 km regime could be overcome. In the end, only observations will tell.

  • Administrator July 14, 2008, 16:32

    Bynaus, thanks, and you’re right, the Thébault model does mention the gravitational instability model, although the authors point out that how gravitational instability would proceed in a close binary is still up for question.

    dad2059, the theory itself is still evolving, as per the above, and in this case the way the accretion disk is affected by particular circumstances of perturbation is going to be the subject of new work around other stars. So I guess we could say it’s an older theory that’s getting tuned up as we go…

  • Dave Moore July 14, 2008, 18:03

    We may be able to answer these questions by looking for a zodiacal dust cloud around the star. It’s size and density should tell us something about the the extent of any asteriod belt. The plane of the system is almost edge on to us so we will be looking through nearly the full depth of the cloud.
    The question is how far are we from being able to image the cloud if it exists.

    Dave.

  • spaceman July 16, 2008, 17:54

    Don’t Worry My Friends

    “Planet formation in Alpha Centauri A revisited: not so accretion-friendly after all,” is a rediculous paper. It is an example of theory at odds with observation. I often wonder sometimes if writers of papers like such as this one are motivated by the need to be contrary more than anything else.

    It is highly likely that the prospects of finding planets around Alpha Centauri A and B are good. Let us keep in mind that we already have evidence that planets can form and remain stable in close binary systems. As merely one of several examples look at the Gamma Cephei gas giant; this was actually the first planet found around another star (it was thought to be a false positive but better data confirmed that the original 1988 planet interpretation is actually on the mark)! I think astronomers even know of a triple star system with a planet. So, papers which attempt to show that for whatever reason planets can’t form in these types stellar environments are just plain at odds with the facts– they are flat out contradicted by several counter-examples and probably more to come!

  • spaceman July 16, 2008, 18:03

    I think there was another paper from several years back in which the authors assert that planets cannot form in binary systems in which the stars are 50 A.U. apart or less. It was also thoroughly refuted by observation. The Gamma Cephei gas giant is over 2 AU from its primary so this essentially means that enough dust and gas coagulated at this distance for a planet larger than Jupiter to form. All the observational data is converging on the notion that is easier to make terrestrial worlds than it is to make gas giants so it is rediculous to expect when gas giants have been found around close binaries that terrestrial planets won’t be formed around them. QED.

  • andy July 17, 2008, 6:04

    spaceman: I think you are assuming the paper is saying something it isn’t – the paper is NOT saying that terrestrial planet formation in close binary systems is impossible. It is saying that for the specific case of Alpha Centauri A, there are significant obstacles to planet formation under the accretion paradigm. The paper itself states that the conclusion cannot be generalised to other binary systems due to strong sensitivity to the properties of the binary system in question (in fact, it says the results cannot even be transferred to the properties of planet formation around Alpha Centauri B). In fact, the paper is not at odds with observations at all, since we haven’t found any planets around Alpha Centauri A yet.

    Systems like Gamma Cephei or Gliese 86 have different mass ratios, separations, eccentricities to Alpha Centauri, so they do not provide good counterexamples to the paper.

  • philippe July 17, 2008, 8:33

    Dear Spaceman,
    As one of the authors of the “ridiculous” paper, let me simply add that Gamma-Cephei is a completey different matter (please have a look at our own 2004 paper on this object), mainly because the mass ratio between the 2 binaries is much lower then for alpha centauri. As for the sentence “It is highly likely that the prospects of finding planets around Alpha Centauri A and B are good”…well, this is a rather tentative claim not backed (to my knowledge) by any reasonable argument. Let me finish by saying that we do not rule out the possibility for finding a planet in the terrestrial region around alpha Cen, all that we say is that *with the current binary configuration*, it is difficult to *form* a planet there *following the “standard” scenario*….simply because the standard scenario requires an initial dynamically cold phase (for the “runaway growth”) which is very difficult to obtain at 1 AU from a very eccentric equal-mass close binary.
    I would of course be the first one to be delighted by the detection of a planet around alpha Cen (maybe that’s what is hidden behind G.Laughlin’s mistery anagram), but the formation of such a planet would have to be explained taking into account the problems pointed out in our paper.

  • Administrator July 17, 2008, 13:33

    A pleasure to have you with us, Dr. Thébault, and congratulations on a strong paper indeed. It will be fascinating to see these methods extended to other binary stars, each of which, as you point out, presents a unique configuration.

  • philw1776 July 18, 2008, 11:45

    I for one appreciate the creative insight and analysis in the “ridiculous paper”. The caveats are well defined, perhaps planet formation and possible migration is subtlly different than we currently understand (quite likely IMO) and the specific study is not facily generalized.

  • spaceman July 18, 2008, 13:17

    Maybe I was a bit too extreme in my assertion that the paper is rediculous. For that I apologize, but I still think astronomers need to be careful about the types of environments in which they say planets cannot be found. The Kepler mission website puts it quite well:

    “The formation of stars and planets is complex, making it almost impossible to predict the diversity of planetary systems from first principles (Boss, 1995, Lissauer 1995).”

    And yet, studies like the one mentioned in this paper do precisely that—they are based on first principles and they try to predict the limits of planetary system diversity. If you asked an astronomer 30 years ago if he or she thought that gas gaints would be found in torch orbits, or if a planet could exist in the Gamma-Cephei system, or if planets could form around pulsars or brown dwarfs they would probbaly laughed at you with incredulity for even asking such a question. If there is one thing that we have learned in this golden age of exoplanet science it is this: the diversity of planetary systems is greater than anyone would have predicted using studies like yours. So, although take back my original assertion that your paper is rediculous (in fact, it is good in many ways) I don’t think it will be the final word on the issue of terrestrial planets being able to form at ~1 A.U. around either Alpha Centauri A or B. And I guess I should have said that counter-examples exist not for your study specifically, but for other previous pessimistic assessments of the prospects of planet formation around close binary systems.

  • spaceman July 18, 2008, 13:20

    What is this mystery anagram that people are talking about?

  • Administrator July 18, 2008, 15:05

    spaceman, the reference is to this jeu d’esprit on Greg Laughlin’s systemic site:

    http://oklo.org/?p=279

  • ljk October 28, 2008, 23:03

    Excitation and damping of p-mode oscillations of alpha Cen B

    Authors: W. J. Chaplin, G. Houdek, Y. Elsworth, R. New, T. R. Bedding, H. Kjeldsen

    (Submitted on 28 Oct 2008)

    Abstract: This paper presents an analysis of observational data on the p-mode spectrum of the star alpha Cen B and a comparison with theoretical computations of the stochastic excitation and damping of the modes.

    We find that at frequencies > 4500 micro-Hz, the model damping rates appear to be too weak to explain the observed shape of the power spectral density of alpha Cen B. The conclusion rests on the assumption that most of the disagreement is due to problems modelling the damping rates, not the excitation rates, of the modes. This assumption is supported by a parallel analysis of BiSON Sun-as-a-star data, for which it is possible to use analysis of very long timeseries to place tight constraints on the assumption.

    The BiSON analysis shows that there is a similar high-frequency disagreement between theory and observation in the Sun.

    We demonstrate that by using suitable comparisons of theory and observation it is possible to make inference on the dependence of the p-mode linewidths on frequency, without directly measuring those linewidths, even though the alpha Cen B dataset is only a few nights long. Use of independent measures from a previous study of the alpha Cen B linewidths in two parts of its spectrum also allows us to calibrate our linewidth estimates for the star.

    The resulting calibrated linewidth curve looks similar to a frequency-scaled version of its solar cousin, with the scaling factor equal to the ratio of the respective acoustic cut-off frequencies of the two stars. The ratio of the frequencies at which the onset of high-frequency problems is seen in both stars is also given approximately by the same scaling factor.

    Comments: Accepted for publication in ApJ; 19 pages, 6 figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0810.5022v1 [astro-ph]

    Submission history

    From: William Chaplin [view email]

    [v1] Tue, 28 Oct 2008 13:50:17 GMT (204kb)

    http://arxiv.org/abs/0810.5022

  • andy November 6, 2008, 7:44

    …and now a discouraging outlook for Alpha Centauri B planets.

    arXiv: Planet formation in the habitable zone of alpha Centauri B