They didn't end up counting Sedna as a planet because it was smaller than Pluto and they have started to regret even calling Pluto a planet because it's become clear that there is a lot of junk out there of roughly that size. (Also, when they first classified Pluto as a planet, they had overestimated its size because they didn't know it had a moon.) Some people have tried to get Pluto demoted, but there was outcry from traditionalists and it didn't happen.

The new thing they found is larger than Pluto, and many people including at least one of the discoverers have argued that if Pluto is a planet, anything larger than Pluto should also count as a planet, and that should be the cutoff. However the International Astronomical Union has not yet made any declaration as to whether this new thing counts as a planet. "Planet" is a fairly arbitrary category to begin with; when people are arguing about what is and isn't a planet, that isn't an argument about scientific facts but rather about how different people want to use the word.

Thanks for the update Daniel. I didn't find anything about the IAU changing their minds on Sedna. Yes there is an ongoing debate about Pluto, but it seems unlikely they will take it off the planetary list.

I do think that it is interesting that the "artist's" depiction of both planets seems eerily similar. Maybe their "artist" is just not very creative, or maybe they will use the same picture (modified slightly) for all new planetary discoveries. After all, it's not like most of us will ever get to see them in person.

From the article (end): "The discovery of 2003 UB313 comes just after the announcement of the finding of 2003 EL61, which appears to be a little smaller than Pluto."

So there's the numbers for reference. It's interesting, the whole "what makes a planet" discussion.

Did the IAU ever say Sedna was a planet? I thought the press just jumped the gun... what I can find them saying is at http://www.iau.org/IAU/FAQ/sedna.html".
(Another wrinkle is that although Sedna is not called a "planet", it IS called a "minor planet"--there are more than a quarter million of those.)

I agree about the "artist's depiction"... I sort of wish they would wait until we have actual images and then show us those. I don't assume people would actually mind that much

So is that why they dropped Carla who they were so excited about finding a couple of decades ago -- she didn't cut the mustard, size-wise?

We can all hope that NASA will spend many, many billions of dollars that aren't needed elsewhere on figuring out just who gets to say what is and isn't a planet and why, and maybe even test a few nukes out there while they're at it. Inquiring minds and aerospace investors want to know.