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A Binary ‘Rogue’ Planet?

‘Planetary mass binary’ is an unusual term, but one that seems to fit new observations of what was thought to be a brown dwarf or free-floating large Jupiter analog, and now turns out to be two objects, each of about 3.7 Jupiter masses. That puts them into planet-range when it comes to mass, as the International Astronomical Union normally considers objects below the minimum mass to fuse deuterium (13 Jupiter masses) to be planets. This is the lowest mass binary yet discovered.

A team led by William Best (Institute for Astronomy, University of Hawaii) went to work on the L7 dwarf 2MASS J11193254–1137466 with the idea of determining what they assumed to be the single object’s mass and age. It was through observations with the Keck II telescope in Hawaii that they discovered the binary nature of their target. The separation between the two objects is about 3.9 AU, based upon the assumption that the binary is around 160 light years away, the distance of the grouping of stars called the TW Hydrae Association.

Let’s pause on this for a moment. The TW Hydrae Association has come up in these pages in the past, as a so-called ‘moving group’ that contains stars that share a common origin, and thus are similar in age and travel through space together. Moving groups are obviously useful — if astronomers can determine that a star is in one, then its age and distance can be inferred from the other stars in the group. Best and colleagues determined from key factors like sky position, proper motion, and radial velocity that there was about an 80 percent chance that 2MASS J11193254–1137466AB is a member of the TW Hydrae Association.

Image: Keck images of 2MASS J11193254–1137466 reveal that this object is actually a binary system. A similar image of another dwarf, WISEA J1147-2040, is shown at bottom left for contrast: this one does not show signs of being a binary at this resolution. Credit: Best et al. 2017.

Determining a brown dwarf’s age is tricky business because these objects cool continuously as they age, which means that brown dwarfs of different masses and ages can wind up with the same luminosity. The authors point out that this mass-age-luminosity degeneracy makes it hard to figure out their characteristics without knowing at least two of the three parameters. Membership in a moving group like the TW Hydrae Association gives us an age of about 10 million years but also provides mass estimates from evolutionary models.

And a binary system hits the jackpot, for now we can study the orbits of the two objects to work out model-independent masses, which is how Best drilled down to the 3.7 Jupiter mass result for each binary member here. The authors consider the binary a benchmark for tests of evolutionary and atmospheric models of young planets, and go on to speculate about its possible origins:

The isolation of 2MASS J1119−1137AB strongly suggests that it is a product of normal star formation processes, which therefore must be capable of making binaries with ≲ 5 MJup components. 2MASS J1119−1137AB could be a fragment of a higher-order system that was ejected via dynamical interactions (Reipurth & Mikkola 2015), although the lack of any confirmed member of TWA within 10° (projected separation ≈ 5 pc) of 2MASS J1119−1137 makes this scenario unlikely. Formation of very low mass binaries in extended massive disks around Sun-like stars followed by ejection into the field has been proposed by, e.g., Stamatellos & Whitworth (2009), but disks of this type have not been observed.

Image: The positions of 2MASS J11193254–1137466A and B on a color-magnitude diagram for ultracool dwarfs. The binary components lie among the faintest and reddest planetary-mass L dwarfs. Credit: Best et al. 2017.

So there is much to learn here. An object’s composition, temperature and formation history all come into play when determining whether it is a brown dwarf or a planet, and some definitions of brown dwarf take us below the 13 Jupiter mass criteria. But at 3.7 Jupiter masses, these objects clearly warrant the authors’ careful tag of ‘planetary mass binary.’

The paper is Best et al., “The Young L Dwarf 2MASS J11193254−1137466 Is a Planetary-mass Binary,” Astrophysical Journal Letters Vol. 843, No. 1 (23 June 2017). Available online.


{ 8 comments… add one }
  • Harry R Ray July 12, 2017, 9:28

    What are the odds of binary stars BOTH having EXACTLY THE SAME MASS? I know of NONE with the exact same masses(the CLOSEST THAT I KNOW OF are Capella and CM Draconis). This effects the nature of their orbits about each other(ALWAYS at OPPOSITE SIDES of a SINGLE circle or elipse as opposed to an inner circle and outer circle or weird elipse configurations). It MAY be just a CO-INCIDENCE that the FIRST such binary discovered has two objects of equal mass, but I doubt it. My sense is that the MAJORITY of such binaries will have components of equal mass, and this will be determined to be a property of planet-planet scattering and re-capture than stellar birth. We’ll just have to wait and see!

    • Alex Tolley July 12, 2017, 11:42

      My sense is that the MAJORITY of such binaries will have components of equal mass, and this will be determined to be a property of planet-planet scattering and re-capture than stellar birth.

      Can you expand on this a little?

      • Harry R Ray July 13, 2017, 9:39

        This may be wrong, but a MUTUAL CAPTURE after a chaotic planet-planet scattering event has a much higher chance of occurring than if one planet exerts a lot more gravitational influence on another than the other planet does on it. In that scenario, the “other planet” is MOST LIKELY going to be EJECTED out of the system while the larger planet STILL REMAINS IN IT. Keep in mind, even a MUTUAL CAPTURE that ejects BOUT PLANETS is obviously still an EXTREMELY RARE EVENT. That’s why it has taken us THIS LONG to discover JUST ONE! Also: Circumbinary planets orbiting two stars are common. How common are circumbinary moons orbiting two planets?

  • Michael July 13, 2017, 2:39

    I wonder if these globules of matter ejected during star formation are responsible for some of these free floating planets.


  • Jeff Wright July 14, 2017, 14:18

    Sounds like the “Gypsy moons” of Posita and Negato from ROCKY JONES: SPACE RANGER

  • Geoffrey Hillend July 15, 2017, 17:36

    I doubt Jupiter sized planets have to be the exactly the same sized in a binary to be ejected when you consider the gravity is much stronger with gas giants larger than Jupiter. A Jupiter sized planet and larger planet have a barycenter or center of mass. Due to the inverse square law of universal gravitation, it seems more likely to me that the whole barycenter would be ejected along with both planets which don’t have to be equal in size. If the planets were smaller like the Earth Moon system or asteroid sized bodies then one might get left behind. It might depend on how far apart the binary system is. If it’s close together then it might be more likely that they both get ejected.

  • Harry R Ray July 27, 2017, 10:10

    A SECOND binary planet candidate has emerged, and this one orbits a solar-mass subgiant star on the verge of entering the giant phase, and for that reason, has a radius yet to be determined. The star is designated Kepler 1625. Kepler 1625b is roughly Jupiter sized and the as yet undesignated planet candidate is roughly Neptune sized. The authors(Kipping et al)are calling the candidate an exomoon candidate instead of a binary planet candidate in their paper, but, if it is CONFIRMED(observations with the HST are underway), I very seriously doubt that the IAU would accept an exomoon with the size of Neptune.

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