The International Astronomical Union (IAU) got together on August 24 and voted on a definition of “planet” — a task that has been hanging fire, apparently, since the 1930’s when Pluto was accorded planet status essentially by acclamation. Now, though, the IAU recognizes eight “classical planets”. Pluto and other remote denizens of the Kuiper belt need not apply, including the 2003 UB_313 object that is 10% larger than Pluto.
There’s a lot of emotional baggage that accompanies this redefinition. Basically, astronomers have discovered the “heap paradox” and made it their own. The verdict is that pre-1930 heaps are fine, and post-1930 heaps are not. There were arguments over a definition that would have made planets out of any body orbiting the sun whose gravity formed it into a mostly spherical shape. That was too inclusive for the IAU, who adopted a resolution that included that plus added a requirement that a planet have “gravitational dominance” over other things in its orbit.
I think that I have to agree with those responding that children shouldn’t have to re-learn what planets there are that children aren’t going to be bothered by this much, by and large. Our educational system needs to improve a fair bit before the effect of changing from nine planets to eight “classical planets” and three “plutons”, “Kuiper belt objects”, “hip-hop planets”, or whatever the IAU eventually decides to call them makes any sort of difference. Let’s see students actually coming out of school knowing what a year is, then we can argue over smaller stuff.