Forming Planets Around Binary Stars

Article written: 10 Dec , 2009
Updated: 24 Dec , 2015
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Fanciful science fiction and space art frequently depict the lovely visage of a twin sunset where a pair of binary stars dips below the horizon (think Star Wars). While it has been established that planets could exist in such a system by orbiting in resonances, that only holds true for fully formed planets. Can forming star systems even support an accretion disk from which to form planets? This is the question a new paper by M. G. Petr-Gotzens and S. Daemgen of the European Southern Observatory with S Correia from the Astronomiches Institut Potsdam attempts to answer.

Observations of single young stars with disks have revealed that the main force causing the dispersion of the disk is the star itself. The stellar wind and radiation pressure blow the disk away within 6 to 10 million years. Predictably, more massive and hotter stars will disperse their disks more quickly. However, “many stars are members of a binary or multiple system, and for nearby solar-like stars the binary fraction is even as high as ~60%.” Could gravitational perturbations or the added intensity from two stars strip disks before planets could form?

To explore this, the team observed 22 young and forming binary star systems in the Orion Nebula to look for signs of disks. They used two primary methods: The first was to look for excess emission in the near infrared. This would trace accretion disks as they radiate away absorbed energy as heat. The second was to look spectroscopically for specific bromine emission that is excited as the magnetic field of the young star pulls this (and other) elements from the disk onto the stars surface.

When the results were analyzed they found that as much as 80% of the binary systems had an active accretion disk. Many only contained a disk around the primary star although nearly as many contained disks around both stars. Only one system had evidence of an accretion disk around only the secondary (lower mass) star. They authors note, “[t]he under-representation of active accretion
disks among secondaries hints at disk dissipation working faster on (potentially) lower mass secondaries, leading us to speculate that secondaries are possibly less likely to form planets.”

However, the average age of the stars observed was only ~1 million years. This means that, even though disks may be able to form, the study was not comprehensive enough to determine whether or not they would last. Yet a survey of the currently known extra-solar planets reveal that they must. The authors comment, “[a]lmost 40 of all the extra-solar planets discovered to date reside in wide binary systems where the component separation is larger than 100AU (large enough that planet formation around one star should not strongly be inuenced [sic] by the companion star).”

Strangely, this seems to stand at odds with a 2007 paper by Trilling et al. which studied other binary systems for the same IR excess indicative of debris disks. In their study, they determined “[a] very large fraction (nearly 60%) of observed binary systems
with small (<3 AU) separations have excess thermal emission.” This suggests that such close systems may indeed be able to retain disks for some time. It is unclear on whether or not it can be retained long enough to form planets although it seems unlikely since no exoplanets are known around close binaries.

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8 Responses

  1. IVAN3MAN says

    For anyone who is interested, I stumbled upon this interesting paper on Binary Stars and Accretion Disks [PDF].

  2. SteveZodiac says

    This is good news for ET life given the tendency towards binaries.

  3. Lawrence B. Crowell says

    From what I know most binary stars do not appear to have planets. No evidence of planets around alpha centuri have been found from what I know.

    LC

  4. SuperKevin says

    I think they are called circumbinary planet’s, and I could swear I’ve read that a few had been discovered already.

  5. IVAN3MAN says

    @ Lawrence B. Crowell and SuperKevin,

    Gamma Cephei Ab is an extrasolar planet approximately 45 light-year away in the constellation of Cepheus (the King). The planet was confirmed to orbit Gamma Cephei in 2002, but was suspected to be a planet around 1988 (making this planet the first true extrasolar planet discovered).

    More…

  6. IVAN3MAN says

    D’OH! I forgot to mention, above, that Gamma Cephei is a binary star system. The star Gamma Cephei A has a companion star with a mass approximately 0.409 times that of our Sun. Gamma Cephei B is of stellar mass and is assumed to be of similar age to its primary. It is probably a red dwarf of class M4, 6.2 degrees of magnitude fainter than the K-type primary star at 3.22.

  7. Aodhhan says

    The problem isn’t whether or not planets exist in binary solar systems. It is whether or not they can form. If so, what is the envelope of all the variables?

    Quite a big difference, based on the fact there is more than one way for stars to come together. Just because the two stars are close together right now, doesn’t mean they always have been. Nor does it mean, they couldn’t capture satellites well into their life.

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