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

The small world was found during a search for the hypothetical Planet Nine, and astronomers say the next time it will reach ...

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 ...

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

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 ...

Astronomers in Japan have published their theory of an “Earth-like planet,” dubbed Planet Nine, that’s hiding in plain sight just a few billion miles behind Neptune. Published last month in ...

Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news on fascinating discoveries, scientific advancements and more. Our solar system ...

When I was a kid, there were nine planets. Now we know of thousands! But that includes exoplanets, alien worlds that orbit alien suns. Only eight planets call our solar system home. Or there might ...