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Alpha Centauri in Context

If we’re finding planets in places 1500 light years away, as the TrES project just did, why don’t we know more about planets in the Alpha Centauri system? One problem is that Centauri A and B are relatively close to each other, with a semimajor axis of 23.4 AU. Leaving Proxima Centauri out of the picture (at 12,000 AU, its short-term effects can be disregarded), it’s still true that radial velocity studies have to take the complicated and varying spectra that binaries produce into account.

In other words, getting a read on binaries like these in terms of the slight wobbles that signal a planetary presence can consume lots of telescope time. Nonetheless, we do have some data thanks to observations with the Anglo-Australian Telescope. And we’ve learned this: No planet around either Centauri A or B induces a velocity variation as high as 2 meters per second. The implication is that any planet orbiting either star individually (in what is known as an S-type orbit) has to have a mass less than that of Saturn. Or if it is larger, and this is still possible, it must orbit in a plane that is substantially inclined to the line of sight to the system.

The Alpha Centauri system

That news comes from the latest paper by Elisa Quintana (SETI Institute) and Jack Lissauer (NASA Ames), whose recent work has examined planetary possibilities in binary star systems of various kinds (to review their previous work, run a search here under either name — you’ll find plenty of discussion on the question of what binaries may produce). While the new paper doesn’t limit itself to the Centauri stars, it does discuss simulations of the final stages of terrestrial planet formation using varying parameters to discover whether and where planets are likely there.

Image: Alpha Centauri A and B, with the position of the two stars, drowned by glare, shown in the imposed diagram. Proxima Centauri’s position is shown by the arrow. Credit: European Southern Observatory.

Quintana and Lissauer started their simulation assuming rocky planetesimals of Moon to Mars size that have already begun accreting from the protoplanetary disk. They then studied their evolution over a period of up to one billion years, with slight changes to the initial conditions to explore possible outcomes. For Centauri, the bulk of their simulations considered a disk centered on Centauri A, with Centauri B perturbing the system. The results here seem encouraging:

  • Using a protoplanetary disk centered on Centauri A and co-planar to the binary orbital plane, five terrestrial planets at least as massive as Mercury form around Centauri A within 100 million years, stable in orbits within 2 AU for the remainder of a 200 million year simulation.
  • With a change to the initial position of one planetesimal near 1 AU, the system still produces four terrestrial planets within 1.8 AU of Centauri A.
  • With a disk inclined initially by 15 percent relative to the binary orbital plane, three terrestrial planets are formed within 1.2 AU.

Indeed, the orbital inclination is clearly potent. From the paper:

We performed a total of 16 simulations of terrestrial planet growth around α Cen A in which the midplane of the disk was initially inclined by 30◦ or less relative to the binary orbital plane. In these simulations, when the bodies in the disk began in prograde orbits, from 3 – 5 terrestrial planets formed around α Cen A. Slightly more formed, from 4 – 5, when i = 180◦ relative to the binary plane. From 2 – 4 planets formed in a disk centered around α Cen B, with α Cen A perturbing the system in the same plane… The distribution of final terrestrial planet systems in the aforementioned cases is quite similar to that produced by calculations of terrestrial planet growth in the Sun-Jupiter-Saturn system.

The Centauri A and B stars are our focus today, but Quintana and Lissauer also look at terrestrial planet formation in wide binary systems and examine P-type orbits, in which planets circle both stars in a close binary system. Keep in mind that we have already found some thirty Jupiter-class exoplanets in orbits around one member of a binary system. While in most cases the two stars have a wide separation, three of these systems have a semimajor axis of only 20 AU. It becomes clear that binary systems may well support terrestrial planets as well, a significant finding given that the majority of Sun-like stars are found in such systems. In fact, say Quintana and Lissauer:

Approximately half of the known binary systems are wide enough (in this context, having sufficiently large values of periastron) so that Earth-like planets can remain stable over the entire 4.6 Gyr age of our Solar System.

And here is their suggestive conclusion:

As a result, ~ 40 – 50% of binaries are wide enough to allow both the formation and the long term stability of Earth-like planets in S-type orbits encircling one of the stars. Furthermore, approximately 10% of main sequence binaries are close enough to allow the formation and long-term stability of terrestrial planets in P-type circumbinary orbits. Given that the galaxy contains more than 100 billion star systems, and that roughly half remain viable for the formation and maintenance of Earth-like planets, a large number of systems remain habitable based on the dynamic considerations of this research.

So we have to put the Centauri question in context. These bright, tantalizing targets, so close to our own Solar System, will gradually yield their secrets. But work over the past decade has made it plain that planetary orbits around the two primary Centauri stars are stable within parameters that could put them in the habitable zones of each. And we’re learning that planetary formation is feasible under a wide range of initial scenarios. Alpha Centauri, it seems fair to say, may prove to be home to some of the most intriguing exoplanets we’ll find in the near future.

The paper is Quintana and Lissauer, “Terrestrial Planet Formation in Binary Star Systems,” slated to appear in the book Planets in Binary Star Systems, ed. Nader Haghighipour (Springer, 2007) and available online. Clearly, the upcoming title is one we’ll be examining with great interest.

Comments on this entry are closed.

  • Christopher L. Bennett June 4, 2007, 13:05

    What do “S-type” and “P-type” stand for?

  • Administrator June 4, 2007, 13:47

    Christopher, P-type stands for ‘planetary type’ orbit — this is an orbit around both binary stars (obviously, in a close binary situation). The S-type is a ‘satellite type orbit,’ one that goes around one or other of the two stars in question.

  • Christopher L. Bennett June 4, 2007, 14:12

    Thanks.

    Another question: Why does the paper assume the planetesimal disk cuts off at 2 AU? Is this an arbitrary value used in the simulations? I know our system’s planetesimal disk extended much farther, or we wouldn’t have the giant planets and asteroids and Centaurs and such. And a number of extrasolar systems have pretty wide dust disks. So why the 2 AU cutoff here?

  • Darnell Clayton June 4, 2007, 14:20

    Great article!

    A quick question though. Has SETI ever looked into whether or not radio signals may be coming from Centauri, or is the interference too high to pick up anything useful?

    Also, I think (off topic) that we may have to redefine our perspective on “galactic habitable zone” as we are increasingly finding massive planets around small, red dwarf stars. IMHO, as long as stars are not near the galactic center, they may have the chance of hosting habitable worlds.

  • Administrator June 4, 2007, 14:27

    Re Christopher’s question, I think the 2 AU limit is simply the result of the mass distribution model used in this study — I notice Quintana and Lissauer start with ‘rocky embryos’ and planetesimals in a zone between 0.36 AU and 2.05 AU, and their paper gives references on their choice of disk distribution model. But I don’t have anything more definitive than that — I’ll be glad to see if I can get a comment from Elisa Quintana on that question. If I’m not mistaken, Wiegert and Holman’s much earlier work on Centauri planetary orbits suggests stable orbits further out than 2.0, but once you get around 4.0 and beyond things get dicey given the disrupting influence of the other star.

    Re SETI and Centauri, I’m aware of no search that specifically targets Centauri, but maybe someone else can give a better answer to that. I’m not sure on the red dwarf question, though. I think the sample is still so small that we may yet find that massive planets (I assume you’re talking about gas giants, Darnell) may still be relatively rare around M dwarfs.

  • Luis June 6, 2007, 4:54

    Hi,

    First let me congratulate you on the quality of your site. I love reading it !

    In this feature article you mention a study of radial velocities of the Alpha Centauri system by the AAO that pose some restrictions on possible planets in the system:

    “Nonetheless, we do have some data thanks to observations with the Anglo-Australian Telescope. And we’ve learned this: No planet around either Centauri A or B induces a velocity variation as high as 2 meters per second.”

    Could you provide me with a reference or better still, a link to a preprint or paper where I can read about this work ?

    Thanks !

    Luis

  • Administrator June 6, 2007, 8:29

    Luis, sure, the paper with the Centauri velocity reference is the one referred to in the article:

    http://arxiv.org/abs/0705.3444

    The authors mention in the paper that Geoff Marcy communicated the information about the Anglo-Australian Telescope to them. To my knowledge, more details about these observations have yet to be published.

  • ljk June 7, 2007, 17:10

    arXiv:0706.0732 [ps, pdf, other] :

    Title: Hot Jupiters in binary star systems

    Authors: Yanqin Wu, Norman W. Murray, J. Michael Ramsahi

    Comments: 4 pages, submitted to ApJ Letters

    Radial velocity surveys find Jupiter mass planets with semi-major axes (a) less than 0.1 AU around 1.2% of solar-type stars; counting planets with a as large as 5 AU, the fraction of stars having planets reaches 7%. An examination of the distribution of semi-major axes shows that there is a clear excess of planets with orbital periods around 3 or 4 days, corresponding to a ~ 0.03 AU, with a sharp cutoff at shorter periods. It is believed that Jupiter mass planets form at large distances from their parent stars; some fraction then migrate in to produce the short period objects. We argue that a significant fraction of the `hot Jupiters’ (a

  • Shaun June 29, 2007, 5:07

    I share the owners of this site’s passion even fixation with the Alpha Centauri system. Like yourselves and a lot of the site’s readers, I sincerely hope that we’ll have some confirmations of planets around if not all the 3 stellar components, then at least for one of them.

    Off the top of my head, I recall coming across a paper that was archived on arxiv where the authors insinuates/suspects that Alpha Centauri B may possess a

  • Adam June 29, 2007, 19:40

    Hi Shaun

    Could you finish that sentence?

  • Shaun June 29, 2007, 23:39

    Ok Adam I’ll try. The 1st time around somehow it got arbitrarily truncated.

  • Shaun June 29, 2007, 23:41

    Somehow I have greater hope in us finding at least 2 or more Mars sized planets if not some Sub Neptunes or super Earths around the A primary component. However I guess that what many of us would really want is the discovery of a planetary system around what is the closest known star to Sol i.e. Proxima Centauri. Offhand, I can’t recall where I’ve read it (it could be from this site, I can’t be sure or on Steinn’s blog) but there seems some indications of a sub Neptune in orbit around this flare star. Whatever the case, I earnestly wish that an announcement either confirming or refuting the presence of planets around this VLM SpT M 5.5 dwarf 3rd component in the Alpha Centauri system can come about very soon.

  • Shaun June 29, 2007, 23:43

    Just in case the 1st of the 2 replies didn’t get through. Reposting it again here. Off the top of my head, I recall coming across a paper that was archived on arxiv where the authors insinuates/suspects that Alpha Centauri B may possess a

  • Administrator June 30, 2007, 9:24

    Shaun, I don’t know what’s happening to truncate that particular message, but send me the above in its entirety by e-mail and I’ll post it from here. Sorry about the inconvenience!

    Paul

  • ljk June 30, 2007, 11:59

    I have noticed that text that has the greater and less than
    signs (obviously I cannot show them here) cuts off messages
    I have tried to post here with those symbols in them, if this
    is any help.

  • Administrator June 30, 2007, 13:08

    I’ve noticed something of the same sort. WordPress evidently interprets those symbols as something other than intended.

  • ljk May 12, 2008, 13:53

    An Algorithm For Photometric Identification Of Transiting Circumbinary Planets
    Authors: Aviv Ofir

    (Submitted on 8 May 2008)

    Abstract: Transiting planets manifest themselves by a periodic dimming of their host star by a fixed amount. On the other hand, light curves of transiting circumbinary (CB) planets are expected to be neither periodic nor to have a single depth while in transit. These propertied make the popular transit finding algorithm BLS almost ineffective so a modified version of BLS for the identification of CB planets was developed – CB-BLS.

    We show that using this algorithm it is possible to find CB planets in the residuals of light curves of eclipsing binaries that have noise levels of 1% and more – quality that is routinely achieved by current ground-based transit surveys. Previous searches for CB planets using variation of eclipse times minima of CM Dra and elsewhere are more closely related to radial velocity than to transit searches and so are quite distinct from CB-BLS.

    Detecting CB planets is expected to have significant impact on our understanding of exoplanets in general, and exoplanet formation in particular. Using CB-BLS will allow to easily harness the massive ground- and space- based photometric surveys in operation to look for these hard-to-find objects.

    Comments: MNRAS accepted. 8 pages, 8 figures (fig. 1 may appear cut – less important, and will be replaced later with a correct version)

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.1116v1 [astro-ph]

    Submission history

    From: Aviv Ofir [view email]

    [v1] Thu, 8 May 2008 17:01:10 GMT (168kb)

    http://arxiv.org/abs/0805.1116

  • ljk July 3, 2008, 23:40

    Identifying Transiting Circumbinary Planets

    Authors: A. Ofir

    (Submitted on 3 Jul 2008)

    Abstract: Transiting planets manifest themselves by a periodic dimming of their host star by a fixed amount. On the other hand, light curves of transiting circumbinary (CB) planets are expected to be neither periodic nor to have a single depth while in transit, making BLS [Kovacs et al. 2002] almost ineffective. Therefore, a modified version for the identification of CB planets was developed – CB-BLS.

    We show that using CB-BLS it is possible to find CB planets in the residuals of light curves of eclipsing binaries (EBs) that have noise levels of 1% or more. Using CB-BLS will allow to easily harness the massive ground- and space- based photometric surveys to look for these objects. Detecting transiting CB planets is expected to have a wide range of implications, for e.g.: The frequency of CB planets depend on the planetary formation mechanism – and planets in close pairs of stars provides a most restrictive constraint on planet formation models.

    Furthermore, understanding very high precision light curves is limited by stellar parameters – and since for EBs the stellar parameters are much better determined, the resultant planetary structure models will have significantly smaller error bars, maybe even small enough to challenge theory.

    Comments: To appear on the IAU Symposium 253 proceedings. 4 pages, 4 figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0807.0527v1 [astro-ph]

    Submission history

    From: Aviv Ofir [view email]

    [v1] Thu, 3 Jul 2008 10:32:14 GMT (71kb)

    http://arxiv.org/abs/0807.0527