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 M Jup 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.