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Excluding Alpha Centauri Planets

You would think Alpha Centauri would be a prime hunting ground for extrasolar planets simply because of its proximity. But the problem for direct imaging is the sheer brightness of Centauri A and B, creating a halo of diffuse light around the pair. Getting through the glare isn’t easy, but a search based on twin techniques — adaptive optics and CCD imaging — covering a wide-field around the Centauri system has just been completed. Results on the CCD work, using European Southern Observatory equipment, have now been made available and they’ve come up short on planetary detections.

As reported by Pierre Kervella (Observatoire de Paris-Meudon) and Frederic Thévenin (Observatoire de la Côte d’Azur), the team found no co-moving companion objects between 100 and 300 AU. And that’s useful information, because it puts some constraints on possible planets around these stars. From the paper:

Within the explored area, this negative result sets an upper mass limit of 15-30 M J to the possible companions orbiting α Cen B or the pair, for separations of 50-300 AU. When combined with existing radial velocity searches…and our adaptive optics results…this mostly excludes the presence of a 20-30 M J companion within 300 AU.

First of all, note what this is not telling us. We can draw no conclusions about possible terrestrial-sized worlds orbiting within 3-4 AU of either Centauri A or B, for the equipment is not sensitive enough to detect planets that small. Thus the scenario that continues to fire the imagination of many of us — habitable planets around one or both Centauri stars — is still viable. We’ve simply learned that we can rule out massive super-Jupiters in wide orbits.

And that gives us further insight into the Alpha Centauri system itself, for some recent work has indicated that the mass of Centauri B could be higher than what earlier models have suggested. Specifically, radial velocity studies have come up with mass estimates that differ by 28 Jupiter masses (plus or minus 9) from the results of long-baseline interferometry. If the missing mass is in the form of an unseen companion, we can now exclude at least one planetary configuration that might have accounted for it.

The paper is Kervella and Thévenin, “Deep imaging survey of the environment of α Centauri,” accepted as a research note by Astronomy & Astrophysics and available as a preprint online. The team’s earlier work using adaptive optics (which feeds directly into the present paper) is Kervella et al., “Deep imaging survey of the environment of α Centauri: I. Adaptive optics imaging of α Cen B with VLT-NACO,” available here. Centauri Dreamsearlier story on the latter is also available.

Comments on this entry are closed.

  • Stephen December 11, 2006, 15:04
  • Darnell Clayton December 11, 2006, 19:28

    If Alpha Centauri does not contain nearby worlds, what is the closest star that does? (even if they are Jupiter sized, it may hint at a possibility towards other planets)

  • Administrator December 11, 2006, 20:04

    Darnell, Alpha Centauri may well have planets around either Centauri A or B or both. This study just says that it doesn’t have super-Jupiter class planets in wide orbits. Actually, the chances of terrestrial worlds in the habitable zone around B look pretty interesting. As to the nearest confirmed planets, I think that would be Epsilon Eridani’s massive gas giant, at about 10.5 light years. But we may well find planets around Proxima Centauri and Barnard’s Star.

  • Zen Blade December 12, 2006, 17:15

    Wasn’t there a simulation (or something) about planet formation around one of the Centauri stars… and the simulation commonly came up with earth-like planets… or at least with a planetary system somewhat like ours.

    also, with regards to this report what technological improvements are needed to look for earth-sized planets?

    -Zen Blade

  • Administrator December 12, 2006, 18:25

    Here’s a story on terrestrial worlds around Alpha Centauri that may fill in some of the background (but yes, the outlook for these worlds is improving markedly):

    https://www.centauri-dreams.org/?p=304

    And here’s something about Greg Laughlin’s recent work that implies the real possibility of these being planets with water:

    https://www.centauri-dreams.org/?p=774

    Laughlin (UC-SC) has also pointed out that with state of the art equipment today we could probably detect planets in the Centauri habitable zone if we were willing to devote enough observing time to the project. The second of the stories linked to above goes into this and I hope it’s helpful.

  • Zen Blade December 12, 2006, 18:53

    yes yes, that is what I was thinking of.
    Many thanks!

    -Zen Blade

  • ljk December 14, 2006, 9:44

    Astrophysics, abstract
    astro-ph/0612242

    From: Amaya Moro-Martin [view email]

    Date: Sat, 9 Dec 2006 18:49:22 GMT (187kb)

    Are Debris Disks and Massive Planets Correlated?

    Authors: Amaya Moro-Martín, John M. Carpenter, Michael R. Meyer, Lynne A. Hillenbrand, Renu Malhotra, David Hollenbach, Joan Najita, Thomas Henning, Jinyoung S. Kim, Jeroen Bouwman, Murray D. Silverstone, Dean C. Hines, Sebastian Wolf, Illaria Pascucci, Eric E. Mamajek, Jonathan Lunine

    Comments: 24 pages, 3 figures. Accepted to Astrophysical Journal

    Using data from the Spitzer Space Telescope Legacy Science Program “Formation and Evolution of Planetary Systems” (FEPS), we have searched for debris disks around 9 FGK stars (2-10 Gyr), known from radial velocity (RV) studies to have one or more massive planets. Only one of the sources, HD 38529, has excess emission above the stellar photosphere; at 70 micron the signal-to-noise ratio in the excess is 4.7 while at wavelengths

  • ljk December 14, 2006, 9:44

    Link to the previous article abstract:

    http://arxiv.org/abs/astro-ph/0612242

  • Matt January 15, 2007, 16:11

    Wouldn’t a trinary system like Alpha Centuri have a lot more debris left over from its formative years? So even if there are terrestrial worlds orbiting one of them there would be a higher frequency of catastrophic impacts.

    I would also imagine that the radiation levels experienced by a hypothetical world would be greatly increased. Nothing life couldn’t adapt to, but something to be wary of should we get our act together and go.

  • ljk June 25, 2007, 0:40

    The dust, planetesimals and planets of HD 38529

    Authors: Amaya Moro-Martin, Renu Malhotra, John M. Carpenter, Lynne A. Hillenbrand, Sebastian Wolf, Michael R. Meyer, David Hollenbach, Joan Najita, Thomas Henning

    (Submitted on 22 Jun 2007)

    Abstract: HD 38529 is a post-main sequence G8III/IV star (3.5 Gyr old) with a planetary system consisting of at least two planets having Msin(i) of 0.8 MJup and 12.2 MJup, semimajor axes of 0.13 AU and 3.74 AU, and eccentricities of 0.25 and 0.35, respectively. Spitzer observations show that HD 38529 has an excess emission above the stellar photosphere, with a signal-to-noise ratio (S/N) at 70 micron of 4.7, a small excess at 33 micron (S/N=2.6) and no excess less than 30 micron. We discuss the distribution of the potential dust-producing planetesimals from the study of the dynamical perturbations of the two known planets, considering in particular the effect of secular resonances.

    We identify three dynamically stable niches at 0.4-0.8 AU, 20-50 AU and beyond 60 AU. We model the spectral energy distribution of HD 38529 to find out which of these niches show signs of harboring dust-producing plantesimals. The secular analysis, together with the SED modeling resuls, suggest that the planetesimals responsible for most of the dust emission are likely located within 20-50 AU, a configuration that resembles that of the Jovian planets + Kuiper Belt in our Solar System. Finally, we place upper limits (8E-6 lunar masses of 10 micron particles) to the amount of dust that could be located in the dynamically stable region that exists between the two planets (0.25–0.75 AU).

    Comments: 23 pages, including 1 table and 5 figures. Accepted for publication in ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0706.3368v1 [astro-ph]

    Submission history

    From: Amaya Moro-Martin [view email]

    [v1] Fri, 22 Jun 2007 16:12:30 GMT (172kb)

    http://arxiv.org/abs/0706.3368

  • Paul Morris February 26, 2009, 18:52

    I understand that in the star field around Alpha Centauri (AC), as viewed from Earth, there are many more distant backgound stars. If ACi A or B has planets, wouldn’t they occasionally eclipse the more distant background stars? For example, an Earth-sized planet around AC could entirely block the light from a background star at about 400 ly distance. Why not look for such eclipses? Has this been done?

  • Administrator February 26, 2009, 20:20

    That’s an interesting question, Paul, and I have no idea whether this particularly kind of ‘transit’ hunt has been suggested before. However, a major problem seems apparent: The glare from Centauri A and B would almost certainly mask the kind of effect you’re talking about. And once we have the kind of high-end coronagraph or sunshade technology in place to eliminate the worst of the glare, a direct detection of a planet would surely be easier. At least that’s how I see it; maybe others will have a comment.