One question being asked right now is whether our own Solar System has a very faint and small brown dwarf star orbiting our Sun at a distance beyond the Oort Cloud. We won’t be able to find out until the James Webb Space Telescope is launched in 2020. If confirmed, that could explain the occasional orbital changes with Neptune, Pluto and the recently discovered “mini planets” even further beyond.
Some years ago, if not still?, some thought the Sun might have a distant companion ("Nemesis") which was responsible for periodic mass extinctions on Earth, the idea being the companion would periodically stir up comets in the Oort Cloud, sending some toward the inner solar system and Earth.