Mathematicians calculated where the planet might be based on its effects on Uranus, and astronomers searched for less than an ...

Planet Nine was previously suggested as a possible explanation for distant orbital clustering. It was thought to be ten times Earth’s mass, orbiting far beyond Neptune.

A Planet Parade (or planetary alignment, if we’re being specific) is when a bunch of planets line up just right so they look like they’re forming a straight line across the night sky. IRL, they’re ...