Detecting planets around brown dwarfs is tricky business, but it’s worth pursuing not only for its own sake but because planetary systems around brown dwarfs can tell us much about planet formation in general. A new paper from Andrzej Udalski (Warsaw University Observatory) and colleagues makes this point while noting four brown dwarf planets we’ve thus far found, all of them much larger than Jupiter. An extremely large planet well separated from a brown dwarf suggests a scaled-down binary star system rather than one growing out of an accretion disk.

Fortunately, we can use gravitational microlensing to go after much smaller worlds around brown dwarfs, a method that is not compromised by the faintness of both planet and dwarf. In microlensing we don’t ‘see’ the planet but can infer its presence by observing how light from a more distant star is affected as a brown dwarf system passes in front of it. Udalski and team have used microlensing to discover OGLE-2013-BLG-0723LB/Bb, which appears to be a Venus-mass planet orbiting a brown dwarf.

Image: A brown dwarf in relation to the Sun, a smaller star and planets. Credit: Jon Lomberg.

Andrew Tribick, a Centauri Dreams regular who passed along the link to this paper, notes that the discovery blurs the line between conventional star/planet and planet/satellite configurations. What that suggests is that similar processes are at work within the accretion disks that form around stars and those found around brown dwarfs and even planets. Here’s how the paper reports this:

OGLE-2013-BLG-0723LBb is a missing link between planets and moons. That is, its host OGLE-2013-BLG-0723LB, being a brown dwarf in orbit around a self-luminous star, is intermediate between stars and planets, in both size and hierarchical position. Moreover, the scaled mass and host companion separation of OGLE-2013-BLG-0723LB/Bb are very similar to both planets and moons in the solar system…

As the snip from the paper shows, the brown dwarf in question is itself accompanied by a low-mass M-dwarf star separated some 1.7 AU from the brown dwarf. The planet and brown dwarf, meanwhile, are separated by 0.34 AU. The system is about 1600 light years away in the direction of galactic center. The paper continues by noting similarities to planets and moons in the Solar System:

Both Uranus and Callisto are believed to have formed in the cold outer regions of their respective accretion disks, and are mostly composed of the raw materials of such regions: ice with some rock. In the case of Uranus, it is believed to have been formed closer to the current location of Saturn (10 AU) and to have migrated outward. In the table [showing the physical parameters of the brown dwarf system], the companion-host separations are scaled to the host mass. This is appropriate because the “snow line”, the inner radius at which icy solids can form (2.7 AU in the solar system) increases with host mass, probably roughly linearly. A plausible inference… is that these processes scale all the way from solar-type stars hosting planets, to brown dwarfs hosting “moon/planets”, to giant planets hosting moons.

Several issues remain to be resolved, however. While the researchers believe that the Venus-class planet is orbiting the brown dwarf, this does not guarantee that it was born in an accretion disk around it — planets in close binaries can become perturbed and move from one star to another. If this is the case here, the planet would have similarities to Triton, which is evidently a captured satellite — the authors add that the Neptune-Triton system is scaled to roughly the same parameters as OGLE-2013-BLG-0723LB/Bb.

The other issue is that there is evidence, in the form of excess light in the detection aperture, for a fourth member of this system, one that is likely more massive and luminous than the other three, and separated from them by roughly 100 AU. The presence of this fourth object would obviously affect the system dynamics at work here, complicating the issue of the Venus-class planet’s origin. What we’re left with as this question is followed up is a roughly terrestrial-mass planet orbiting a brown dwarf, a configuration that may be common, with the implication of a formation process that scales down to large planets and their own family of satellites.

The paper is Udalski et al., “A Venus-Mass Planet Orbiting a Brown Dwarf: Missing Link between Planets and Moons” (preprint).