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's outer reaches.

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

In the cold, dark outskirts of planetary systems far beyond the reach of the known planets, mysterious gas giants and ...

If the object really is Planet Nine, it would be larger than Neptune and exist 700 times further from the sun than Earth.

Science textbooks may be in for another revision. Our solar system shrank from nine planets to eight after the International Astronomical Union demoted Pluto in 2006. But there may yet be another ...

The chance of a planet forming in the outer reaches of the solar system - a hypothetical Planet Nine - could be as high as 40 ...

The presence of another planet in the solar system could explain the odd movements of some of the bodies in the Kuiper Belt.

That will allow the observatory to detect anything that changes — like, say, Planet Nine moving across the sky. Sponsor Message "If it's there, we should be able to find it, pretty easily," says ...