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

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

A US-based trio hunting this elusive world has stumbled on what appears to be a new dwarf planet in the solar system's outer ...

Elon Musk, under the username "Kekius Maximus," has joined William Shatner in advocating for Pluto's reclassification as a ...

Some astronomers say the strange, clustered orbits of icy rocks beyond Neptune indicate that something big is out there, ...

A new dwarf planet discovery in the outer solar system is challenging the long-standing theory of a hidden giant planet ...

"The debate became even more animated ... For decades, students learned about the nine planets of the Solar System, with Pluto holding a special place as the farthest member.

That makes it three times smaller than Pluto. But that is still big ... up to 10 times larger than Earth, naming it Planet Nine and kicking off a debate that has rumbled since.