The possibility of a brown dwarf companion has been raised and then “ruled out” a number of times, particularly since the Nemesis hypothesis emerged to try to make random asteroid strikes on Earth into something nice and uniform and predictable. So I give up. My own guess has been that there’s a largish planet but that its orbit is out of the ecliptic, and/or retrograde.
Given that it was only within the last 15 years when telescopes with “larger” mirror sizes than the Hale telescope on Mount Palomar started to find planets beyond Pluto at least Pluto’s size, we’re still only at the beginning to finding a potential brown dwarf companion to our Sun. With new, more advanced satellites going up within the next few years, we may finally get the technology to find it.