Wikipedia:Today's featured article/April 9, 2019
Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the unlikely clustering of orbits for a group of extreme trans-Neptunian objects, bodies with average distances from the Sun that are more than 250 times that of Earth. These objects tend to make their closest approaches to the Sun in one sector, and their orbits are similarly tilted. Their improbable alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known Solar System objects. Planet Nine would have a predicted mass five to ten times that of Earth, and an elongated orbit extending 400 to 800 times as far from the Sun as the Earth's. It may have been ejected from its original orbit by Jupiter during the genesis of the Solar System, wrested from another star, captured as a rogue planet, or pulled into an eccentric orbit by a passing star. (Full article...)