Neptune's mass and location in the solar system has long intrigued scientists.  They are puzzled by the planet's anomalous heat radiation, as well as its large mass, and how it could have formed so distant from the sun where material would have been scarce during it's creation.  A new theory proposes a series of events that includes the collision of Neptune with a larger-than-Earth-sized planet and its moon.

Image: Jet Propulsion Laboratory, NASA