What Constitutes a Planet?

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 :-).

Here and here are a couple of other previous posts on the new planet/kuiper belt object/giant ice globe.

Comments

[cpurick]

I think there were a lot of astronomers who felt Pluto should not have been called a planet from the beginning. They weren't looking for a planet as much as they were trying to explain Neptune's irregular orbit.

[Phi]

Yep, and Pluto's orbit is also very irregular and tilted quite a bit with respect to the other planets. Some astronomers believe it is only a matter of time until someone finds other Kuiper Belt objects larger than Pluto and, thus, of planetary size. There's also a sort of scarcity problem at work... if too many bodies around the sun are called "planets," then planets, in a sense, lose their significance.

[Douglas]

I hereby predict that in a few years, school children will be reading about how Pluto was once considered a planet, but is now known to be the first of a number of KBO's (Kuiper Belt Objects) to be discovered.