Planet X

Back in grade school or middle school I remember reading about the hypothesis of an additional large undiscovered but massive object in our solar system with an orbit far out beyond Pluto.

This idea comes back to life in this story that argues that the orbit of one of the recently discovered large "dwarf planets" (Sedna) can't readily be explained given known solar system objects.

In particular note this graphic that is linked in the story showing the relative distance and difference in orbit from the other solar system objects.

Sedna's location doesn't make sense.

"Sedna shouldn't be there," said Brown. "There's no way to put Sedna where it is. It never comes close enough to be affected by the Sun, but it never goes far enough away from the Sun to be affected by other stars."

Perhaps a massive unseen object is responsible for Sedna's mystifying orbit, its gravitational influence keeping Sedna fixed in that far-distant portion of space.