not every star system is a binanary system
its rare but some are just a single star,its not like our solar system is the only one with only one star

if there was a rogue brown dwarf about it would play havoc with the ort cloud(a band of rocks that surronds our solarsysyem)first
if said browndwarf was on a regual orbit it would also play havoc with the rest of the planets well before you could see it

so since the ort cloud aint doin much and the planets are where they usually ar id say no brown dwarf anywhere near
to say its having an effect on earth but not the other parts of our solar system is plain daft imo