When astronomers found a large world farther out than Pluto, it became one of the final nails in the coffin of our ninth ...

The number of planets that orbit the sun depends on what you mean by “planet,” and that’s not so easy to define ...

For decades, Pluto was celebrated as the ninth planet of our solar system. However, in 2006, the International Astronomical ...

Pluto isn't alone—it's part of the Kuiper Belt with possibly hundreds of other planet-sized objects. Because Pluto hasn't ...

is a disc-shaped region beyond the orbit of Neptune’s in the outer solar system—50 Earth-sun distances—that’s home to dwarf planets like Pluto, Makemake and Eris. It’s also thought to be ...

In 2005, Eris was discovered. It was considered the largest known dwarf planet in the solar system until a year later when Pluto was downgraded from being a planet. In 2008, tribal violence following ...

For one thing, Eris shares the region and isn't stuck in Pluto's orbit. The end result? Pluto is bumped from "planet" to "dwarf planet." Now, Stern argues that a dwarf planet is a kind of planet.