Over the past 250 years, the number of "planets" in our solar system has ranged from six to nine — and, briefly, even 11 — depending on what astronomers knew at the time and how they defined a planet.

But what exactly is a planet? It seems like an easy question. After all, you’re standing on one right now! But the scientific definition was fairly loose until recently. The discovery of other ...

In 2006, updated research led to Pluto being controversially demoted to dwarf planet status by the International Astronomical Union. The reasoning was that Pluto's location in the far-flung ...