U.S. astronomers hunting for "Planet Nine" have instead stumbled on what appears to be a new dwarf planet in the solar system ...

It's an evocative idea that has long bedeviled scientists: a huge and mysterious planet is lurking in the darkness at the ...

If you were born in the last century you might have memorized that there were nine planets in ... the summer of 2006, Pluto was demoted. This wasn't the first time such a debate happened.

Discovery of a possible dwarf planet suggests our cosmic backyard holds many more secrets to uncover It is an evocative idea ...

Astronomers find evidence of a possible ninth planet beyond Neptune, reigniting the debate about the formation of the Solar ...

Discovery of 2017OF201, a large trans-Neptune Object, challenges previous beliefs about the Kuiper Belt and hints at the existence of a theoretical Planet Nine.

To everyone who was happy about Pluto's planetary demotion, consider this: If Earth rode the same orbital highway as Pluto, it too would be a dwarf planet. In fact, we live on a geological body ...

For adults who grew up learning about the nine planets in school ... “Instead of concentrating on Pluto and the Pluto debate,” he says, he wants them to realize “that our knowledge of ...

A new dwarf planet, discovered beyond Neptune and described as Pluto's 'cousin,' could void the hypothesis of a Planet X in ...

(Correction: An earlier version of this story misstated how far Pluto is from the sun. The correct number is 3.7 billion miles.) What was discovered in Flagstaff, Arizona, and killed off in Prague?