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Circumbinary Planets: A New Class of Systems?

Last week’s meeting of the American Astronomical Society is still much in the news, and I want to cover several more stories from the Austin conclave this week, starting with yet another circumbinary planetary system, in which a planet orbits two stars. Not long ago we looked at Kepler-16b, a circumbinary planet orbiting two stars in this mode — as opposed to a binary system where planets orbit one or the other of the two stars. Kepler-16b was interesting but perhaps unusual given the perceived difficulties in finding stable orbits around close binaries.

But things are happening quickly on the exoplanet front. Needing more information about the prevalence of this kind of planet and the range of orbital and physical properties involved in such systems, we now get news of not one but two more, Kepler-34b and Kepler-35b. Note the nomenclature: We could as easily call these Kepler-34(AB)b and Kepler-35(AB)b. We confront the real possibility that ‘two sun’ systems are not necessarily rarities. At least, that’s the view of William Welsh (San Diego State University), who presented the findings at the AAS meeting:

“It was once believed that the environment around a pair of stars would be too chaotic for a circumbinary planet to form, but now that we have confirmed three such planets, we know that it is possible, if not probable, that there are at least millions in the Galaxy.”

Here’s what we know about these worlds. Kepler-34b orbits two Sun-like stars every 289 days, while the two stars in question orbit and eclipse each other every 28 days. At 4,900 light years from Earth, the planet is in the constellation Cygnus, as is Kepler-35b, although the latter is more distant at 5400 light years. Both are thought to be Saturn-sized gas giants. Kepler-35b orbits its two stars (80 and 89 percent of the Sun’s mass) every 131 days, with its central stars orbiting and eclipsing each other every 21 days. Between these worlds and Kepler-16b, we are building our knowledge of a new class of planets, one Kepler may supplement with still more examples.

Image: Twin suns would yield not only spectacular visual effects but climate changes that could be equally breathtaking. Credit: Lynette R. Cook.

Laurance Doyle (SETI Institute), a co-author of the paper on this work, speaks of “…the new field of comparative circumbinary planetology,” which he believes is now established by these findings. Here is yet more fodder for science fiction writers looking for unique settings, for planets in such orbits would receive continually changing amounts of sunlight. The effects on local weather patterns alone would be enough to spin an absorbing tale, a year’s worth of seasonal change packed into short and dramatically changing time frames. “The effects of these climate swings on the atmospheric dynamics, and ultimately on the evolution of life on habitable circumbinary planets,” says Welsh, “is a fascinating topic that we are just beginning to explore.”

The paper is Welsh et al., “Transiting circumbinary planets Kepler-34 b and Kepler-35 b,” published online by Nature on 11 January, 2012 (abstract).


Comments on this entry are closed.

  • Tarmen January 16, 2012, 13:47

    That kind of variation in plantary climate and weather would drive the evolution of some very tough, adaptable and smart ‘animals’ . Such creatures would make excellent colonists if they evolved in that direction. The longer that Earth avoids contact with ET, the better. ET may be tougher and smarter than many locally evolved beings such as us. Hawking is right to warn us of this possibility.

  • andy January 16, 2012, 16:04

    I see the Kepler team are still at it with minimising the discoveries of other people. The first circumbinary planet discovered was found in 1993: PSR B1620-26 consists of a millisecond pulsar and a white dwarf, with an orbiting Jovian-type circumbinary planet. Apparently Steinn Sigurðsson (who was involved in the PSR B1620-26 discovery) called them out on this at the Extreme Solar Systems II conference. Furthermore there are several examples of circumbinary planets around post-common envelope systems (HW Virginis, DP Leonis, NN Serpentis and HU Aquarii) that were discovered before Kepler-16.

  • Rob Henry January 16, 2012, 20:12

    Andy, I suspect that the reason for that shocking behaviour is due to the sword of Damocles hovering over funding extensions. This is not the only known case of a research group dancing like bears given the heated metal plate treatment.

  • Aleksandar January 17, 2012, 7:41

    The inhabitants of that world would have discovered gravitational waves pretty soon :)

  • A. A. Jackson January 17, 2012, 11:14

    Not that many years ago , a little over 30, Celestial Mechanics was a kind of backwater of esoteric study.
    (Tho I must say that the numerical dynamics of aggregations of stars like galaxies and globular clusters did have a life of its own.)
    Then came the discovery of the chaos associated with the 3 to 2 resonance of asteroids with Jupiter. A mechanism than can send main belt asteroids across the Earth’s orbit. Plus the rediscovery of symplectic numerical integrators that could explore the orbital dynamics of solar system for gigayears.
    Now there is an explosion of orbital dynamics studies of the the multitude of strange extra solar planetary systems.
    For multi-star planetary systems each discovery has to be checked for stability.
    There could billions of ‘free range’ planets slung from their parent systems roaming the Galaxy.
    I am not sure if a wide binary has been discovered with planets around each star.
    PSR B1620-26 and PSR B1257+12 have planets, there must be more, orbital dynamics could get weird in those systems.
    (Just think of Black Hole binaries!)
    Gliese 644 appears to be a quintuple system , if there are planets there a tip of the hat to Asimov!

  • Joy January 17, 2012, 14:52

    Than You Andy. Yes, the Kepler team discovered the 6th, 7th, and 8th circumbinary systems. Very nice, but hardly enough to claim credit for discovering the class.

  • andy January 17, 2012, 18:39

    I am not sure if a wide binary has been discovered with planets around each star.

    There’s only one example currently among the planets listed in the Extrasolar Planets Encyclopaedia: WDS J03201-2851, which comprises the stars HD 20781 and HD 20782. The planetary systems around each star are quite different: HD 20781 has a pair of Neptune-mass planets in mildly eccentric orbits, while HD 20782 has a single Jovian on an extremely elliptical orbit (e=0.97). The two stars have a projected separation in the plane of the sky of over 9000 AU.

  • Ronald January 18, 2012, 9:15

    Just as there seems to be a minimum (AU) separation for binary stars to have a (stable) planetary system around at least one of both, or both (sometimes put at around 10-15 AU), likewise, there is probably a maximum separation for binaries to be able to have a circumbinary planetary system.
    Then there may also be a transitional separation range in which neither can exist: to wide for circumbinary, to narrow for individual systems.

  • FrankH January 18, 2012, 15:43

    Su-Shu Huan wrote about binaries and habitable zones in 1960. Still relevant:

    “Life-Supporting Regions in the Vicinity of Binary Systems”
    Publications of the Astronomical Society of the Pacific, Vol. 72, No. 425, p.106