Mike Brown is convinced that beyond Neptune, at the far reaches of our solar system, there's an unseen giant planet. "This is the 5th largest planet in our solar system, lurking out there, waiting to ...

It's an evocative idea that has long bedeviled scientists: a huge and mysterious planet is lurking in the darkness at the edge of our solar system, evading all our efforts to spot it. Some astronomers ...

A newly discovered warp in the outer Solar System may have been created by a small, rocky world, much closer to the Sun than the hypothesized Planet Nine. According to a new measurement of the Kuiper ...

A possible new dwarf planet has been discovered at the edge of our solar system, so far-flung that it takes around 25,000 years to complete one orbit around the sun. Subscribe to read this story ...

There has been a lot of back and forth about the existence of a mysterious ninth planet, often referred to as "Planet Nine" or even "Planet X." Now, though, researchers say there is likely at least a ...

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 ...