There have been questions about a mysterious ninth planet in our solar system for nearly a decade. Pluto was unseated as number nine in 2006. Now, a group of international researchers say they may ...

Scientists say they finally have "solid evidence" for Planet X, a true ninth planet on the fringes of our solar system.

Scientists reported Wednesday they finally have "good evidence" for Planet X, a true ninth planet on the fringes of our solar system.

Almost 20 years after Pluto was infamously downgraded from its status as a planet, scientists now believe they have discovered a new planet in our solar system.

A ninth planet the size of Neptune may be hidden in the far reaches of the solar system, but the evidence has left some experts skeptical.

CAPE CANAVERAL, Fla. -- The solar system may host a ninth planet that is about 10 times bigger than Earth and orbiting far beyond Neptune, according to research published on Wednesday.

New discoveries of objects in the Kuiper Belt have also presented challenges for the Planet Nine theory. The latest is known ...

A possible dwarf planet, like Pluto, has been observed in our solar system, according to a preprint published in arXiv. The area of space where this planet was found was previously thought empty.

CAPE CANAVERAL, Fla. — The solar system may have a ninth planet after all. This one is 5,000 times bigger than outcast Pluto and billions of miles farther away, say scientists who presented ...

Think you know Pluto? From icy plains to planetary debates, this quiz dives deep into the mysteries of our solar system’s most controversial—and captivating—dwarf planet.