Some researchers are suggesting that the sun is not the only star in our area, but may in fact be the dominant half of a binary system. I don't quite see how we could miss another star if it was that close to us, even if it is a brown dwarf. I would think that there would be a more dramatic effect on the orbits of the gas giants, but I guess I don't know for sure. The article says that the Kuiper Belt Objects Sedna and Xena have erratic orbits that can't be explained with current models for a single star system. Personally, I suspect the models might need some work.