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 ...

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 ...

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.

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.