If Pluto is considered a planet and size matters, then it may be time to up the number of planets in the solar system to 10:
In another Nature paper about the Kuiper Belt published this week, Frank Bertoldi (University of Bonn, Germany) and four colleagues report thermal observations taken with the Institute for Millimeter Radio Astronomy's 30-meter telescope in Spain that confirm that the distant object 2003 UB313 is indeed larger than Pluto. By comparing UB313's brightness and reflectivity, Bertoldi and his colleagues measure a diameter of about 3,000 km, which is 700 km larger than Pluto's. These results are consistent with previous Spitzer Space Telescope observations of this so-called "tenth planet" that were announced four months ago. "Since UB313 is decidedly larger than Pluto," says Bertoldi, "it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status."
I've recently been introducing Powerkid, The Eldest to the heavens. We've spoken some about the planets and he thinks there are 9 or 10. That's one good thing about kids... they aren't always set in their beliefs :-).