Brown dwarfs form the same way stars do. But it is not known if there is a theoretical limit to how small an object can form in such a fashion. So it may be possible for cloud collapse to form even smaller objects, maybe even as small as Jupiter. This particular dwarf is in a binary with another brown dwarf, which means you could argue to call it a planet, (it orbits a star, and even on the high end of the mass estimate, there are known planets around that mass. However, the dynamics of its orbit suggest that it and its partner formed like binary stars, each from a cloud collapse, rather than as star-planet pairs do). But what if you found a lone dwarf of this mass range? Then what should we call it?

This is basically the other end (big rather than small) of the planet definition controversy that reclassified Pluto. Stars had previously been pretty definitely defined by hydrogen fusion (and it’s easy to determine if a star is fusing hydrogen, because that’s what makes it shine). Brown dwarfs sort of shred this definition, since they don’t fuse hydrogen. And they don’t necessarily fuse deuterium either, they only potentially fuse deuterium, and only in the early phases of their existence. And the mass cut-off is also theoretical, as there are other factors that affect the ability to fuse deuterium. The 13J limit is theoretical only, as in something bigger may be able to fuse deuterium, but might not, depending on circumstances. You can have two 13J brown dwarfs of identical mass, but one will be fusing deuterium, and one won’t be. And a brown dwarf that is sufficiently old (and this age, compared to the age of other stars, can be actually quite young) won’t be fusing deuterium because it’s used up all it’s fusable deuterium. (And with increasing age, less and less infrared radiation.)

Finally, large planets also can radiate infrared (that is not reflected starlight). Even Jupiter, cold as it is, radiates more infrared than it should from solar reflection alone.

Very well put! Ultimately, even though we like to have clear dividing lines for everything, large planets and small stars are both just blobs of matter held together by their own gravity. They’re all heated by gravitational collapse for an extended period of time. If they’re massive enough, they begin stable fusion at some point. But even fusion is dependent on temperature, which may drop over time as the energy of gravitational collapse is used up and equilibrium is reached.
And what if you drop an asteroid on a very very very large planet that’s on the cusp of deuterium fusion – does it instantly become a brown dwarf? What if it MIGHT have fused deuterium for a bit if the asteroid had hit it earlier? At some point the universe just plain defies our attempts to pigeonhole its inhabitants.

“It is the mark of an educated mind to rest satisfied with the degree of precision that the nature of the subject admits, and not to seek exactness where only an approximation is possible.” <– Badly Quoted Brilliant Greek Dude (name slips my mind)