A Southwest Research Institute-led team has reported the first detection of gas on the distant dwarf planet Makemake, using ...

Though Pluto has formally been considered a dwarf planet for almost two decades, it still has many lessons left for planetary scientists — including hints about how the solar system formed.

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

Eighty years ago, the hunt for a mysterious Planet X culminated in the discovery of Pluto. Lying beyond Neptune in the Solar System, the orb was named after the Greek god of the underworld and ...

A mysterious 'Planet X' may be hiding behind Pluto By Kelly Dickerson and Matt Johnston Sep 10, 2015, 8:26 AM PT European Southern Observatory on Flickr ...

Astronomers believe a planetary body collided with Pluto early in its history to create a gigantic heart-shaped feature on the dwarf planet.

The discovery that Pluto is a wandering trans-Neptunian object and that many similar bodies exist argues against its status as a major planet.

Pluto, officially a dwarf planet, has surprising complexity on its surface. While certainly a world, not everyone agrees it is a planet.

The search for Planet X was over. Or was it? Pluto turned out to be much smaller than expected, and some astronomers, including Tombaugh, continued to search for a tenth planet, beyond Pluto.

That makes Pluto the furthest known x-ray source in our solar system. If confirmed, the finding could reshape our understanding of the dwarf planet’s atmosphere.

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

Its existence would satisfy the long-held hopes and hypothesis for a "Planet X" envisioned by scientists and sci-fi buffs alike.