This hypothetical world, also known as Planet X, likely lurks somewhere within the Kuiper Belt — a giant disk of asteroids, ...

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 ...

which in the early 20th century became known as Planet X, a name popularized by astronomer Percival Lowell. He suspected that anomalies in the orbits of Neptune and Uranus were due to an undiscovered, ...

Is there a massive undiscovered planet on the outer reaches of the solar system? The idea has been around since before the discovery of Pluto in the 1930s. Labeled as planet X, prominent astronomers ...

A possible cousin of Pluto seems to be circling the far reaches of the solar system. The dwarf planet candidate 2017 OF201 travels in a superwide orbit, with the sun relatively near one end of its ...

When you buy through links on our articles, Future and its syndication partners may earn a commission. Planet Nine is a hypothesized massive planet, first proposed in 2014, that is speculated to orbit ...