When Is A Planet (Not) A Planet?...
When Nasa says so apparently. Calling it Sedna seems a bit odd though. Makes it look like a typo. "One group of astronomers believe that Pluto is not a true planet but merely one of the largest of a vast number of minor objects in the outer Solar System. The alternative standpoint is that Pluto is a planet and those who believe that will have to classify Sedna as the tenth planet." So there you go.