Does a massive, extremely distant planet orbit the Sun? A new analysis of distant solar-system orbits argues that it should exist.
Astronomers are debating the existence of a big planet beyond the orbits of Pluto and Neptune. Dr. Michelle Thaller talks with planetary scientists Scott Shepard and Mike Brown about when we'll discover it.
The discovery of trans-Neptunian object 2011 KT19 in a strange orbit deepens a key mystery in the outer solar system.