Brown dwarf stars are the stage between planets and stars. They can fuse deuterium and create sufficient heat for convective mixing to alter their chemistry, but they cannot sustain the kind of hydrogen burning that powers young stars.
There has been debate over how these things actually form – do they come from collapsing gas clouds like stars, or do they form due to instabilities in the disc of gas and dust around stars – like planets. There has been evidence both ways and modelling has shown that instabilities in the discs of lone stars could form these things, but a team has now shown the same to be the case in discs surrounding stars in groups – a more common scenario as stars tend to form in clusters and then either drift apart like our own Sun’s cluster did, or solidify into a loose association or denser globular cluster. In this case, the instabilities arise due to passing stars exerting gravitational tugs on the discs. Not only do these lead to brown dwarf formation, but the high inclinations of the resultant brown dwarfs match those seen in planetary systems.
Of course, can doesn’t mean does, and it will be up to observationalists as well as theorists to determine the ratios of rogue brown dwarfs to brown dwarfs stuck in ‘planetary’ or binary systems.