Remember “Eris?” It was in all the papers a couple years ago - a snowball larger than Pluto out in the Kuiper Belt. The discovery of Eris by Mike Brown (And his team) was the thing that re-ignited the whole “Is Pluto a planet or not?” debate. In a more general, and more interesting effect, it generated the larger debate of what the heck a planet is, anyway. Turns out there was no definition, just “I know it when I see it.”
So the International Astronomical Union debated and debated, and ultimately chose a completely disingenuous definition of planets that operates by size, rather than something more reasonable. Why is size unreasonable? Because size is entirely capricious. There moon Ganymede is larger than the planet Mercury. Using size as the only significant definition is like saying “Midgets aren’t people because they’re too short.”
Anyway, the net result of all this is that object 2003 UB313 was named “Eris” after the Greek goddess of discord, for all the fuss the discovery had caused. And, of course, Pluto was demoted to “Not a planet” status.
Well, it turns out the story isn’t done yet, as Eris is quite a bit smaller than thought. Likely smaller than Pluto itself!
And a more technical piece here