In reply to no_more_scotch_eggs:
> what's it doing out there then...? taken a bit of a shunt in an evolving solar system?
Most likely yes, some multi-body near collision with something heavier that sling-shotted it out of its solar system. (Or, much less likely, it formed in situ in a region without enough stuff to form anything heavier.)
> and is that quite big for a planet? i thought jupiter was pretty big already. how big can a planet
> get before it becomes a brown dwarf star...?
They're claiming that one as 4 to 7 Jupiter masses, which is fairly common; planet masses peter out at about 20 Jupiter masses. Brown dwarfs are then heavier than about 30 Jupiter masses. You need about 75 M_Jup to burn hydrogen and so be a star. (You can burn lithium at > 13 M_Jup, but nowadays that isn't considered sufficient qualification to be a brown dwarf.)