If our sun does have a companion star it has to be pretty far out there. First off a brown dwarf will be 10 to 75 times the mass of Jupiter. Jupiter is a very bright object. So for an object 10 to 75 times more massive not to be seen and not to have a obvious impact on the sun and planets gravitationally, it has to be atleast a lightyear or more away. Still it is plausible.


[link to news.discovery.com]