The 4 smallest planets are ignored there because they’re too weak to have an effect on the outer solar system, but they still have a major effect clearing the region they’re in, thanks to masses orders of magnitude more than any of the little belt objects shown here. That’s a real and useful criterion too, one with probably much more to do with how they formed than just “roundness”.
But of course it’s always necessary on these topics to cast shade on the IAU picking a real, distinct dividing line that doesn’t include what America discovered and mistakenly thought was a much bigger object. The headline invites us to celebrate, but those of us who actually understood and agree with the decision always have to be the villains, even after almost two decades of most astronomers finding it actually works just fine for them (and a few having called for it much longer).
Sure, he’s putting a fun spin on it, but the actual truth is that Mike Brown has done great work discovering what the farthest reaches of our solar system are like…and instead of being honored for it, he got hate mail from children. So you’ll forgive me if this topic still being full of sneers is less amusing to me, someone who grew up reading about the possibility of the Kuiper belt and was once excited to see an actual understanding of Pluto’s place in the universe.
