Planet X, a hidden giant seven times Earth’s mass, challenges astronomers with its faint light and distant orbit, potentially redefining our understanding of the solar system.

Forbes contributors publish independent expert analyses and insights. An award-winning reporter writing about stargazing and the night sky. Is there an unseen planet — a "planet X" or "Planet Nine" — ...

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 ...

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 ...

For decades, astronomers have searched for the mysterious Planet X. This documentary explores the evidence behind the theory, the strange orbits that sparked the search, and whether our Solar System ...