A new study has proposed the existence of Planet Y, an alternative Planet Nine candidate that is smaller and closer to Earth ...

The planet Neptune wobbled in its orbit around the Sun. That could only mean one thing, astronomers said: There was a ninth planet out there, somewhere, lurking in the fringes of the solar system.

Siraj’s Planet Y search started about a year ago when he was trying to find out whether the shape of the Kuiper Belt is flat. “The planets of the solar system have slight tilts up and down, but ...

Editor’s note: A version of this story originally appeared in Astronomy’s 2007 special issue 50 Greatest Mysteries of the Universe. For a more recent discussion of Pluto’s planethood, see “An ...

For such a small member of the solar system, about which relatively little is known, Pluto has an impressive following. When the news that the ninth planet had been stripped of its planethood got out, ...

Pluto may have lost its status as a full-fledged planet in 2006, but that doesn't mean it's a joke of a world this April Fools' Day and the folks behind Pluto TV want to make that clear. On April 1, ...

In late August 2006, new discoveries upended a traditional, comfortable way of viewing our solar system: Scientists decided Pluto wasn’t a planet after all. Some space nerds like to mourn Pluto’s loss ...

You know what a planet is, right? A big round thing that orbits a star. Uh, not so fast. You know what a planet is, right? A big round thing that orbits a star. Uh, not so fast. The surprisingly ...

For New Mexico, the fight for Pluto is not over. Seven months after a conclave of scientists downgraded the distant heavenly body to a "dwarf planet," a state representative in New Mexico aims to give ...

The solar system’s retinue of known, faraway worlds has gained another member: a small, icy body that takes 40,000 years to plod once around the sun, traveling farther away from our home star than all ...

Scientists discovered an extremely distant object billions of miles beyond Pluto. The object has an orbit that hints at an even-farther-out “Super-Earth” or larger “Planet X” that could be lurking out ...

After passing in the state senate, a new bill that would officially designate Pluto as Arizona's state planet is now at the will of Gov. Katie Hobbs. Since its discovery in Flagstaff, Pluto has become ...