It's an evocative idea that has long bedeviled scientists: a huge and mysterious planet is lurking in the darkness at the edge of our solar system, evading all our efforts to spot it. Some astronomers ...

For decades, our solar system was thought to have nine planets, with Pluto considered the smallest and farthest. But in 2006, Pluto was officially reclassified as a dwarf planet by the International ...

After studying thousands of computer simulations of the solar system, researchers at Rice University and the Planetary Science Institute think there’s a 40% chance an elusive “Planet Nine” or “Planet ...