Scientists are speculating that the outer Oort Cloud may contain more than comets. In fact, it might be home to a bruiser of a world, one up to four times more massive than Jupiter.
Scientists have noted there seems to be a pattern in the timing of mass extinctions on Earth, and they have tied one of those events-- the one that ended the Age of the Dinosaurs-- to a huge comet impact. The idea is that the pattern in mass extinctions might be explained by the Earth being struck periodically. That would require a trigger-- a big body out at the edge of the Solar System that alters the orbits of comets on something like a regular basis, sending them into the inner System. Astronomers studying the orbits of some comets think they have evidence of such a body. Similarly, astronomers mathematically inferred the existence and position of Neptune, leading to its optical discovery, by studying quirks in the orbit of Uranus.
The reason such a world has never been seen or photographed, they say, involves its incredible distance from the Sun. It would be too cold to be imaged in infrared, and receive too little sunlight to be seen in visual light. How a body so huge could form that far out remains a puzzle. Most stars like the Sun do have stellar companions, however, so this object, if it exists, could be a companion that didn't quite reach stardom.