New Definition of Planet

Astronomers are debating a new definition of the word "planet."

Under the proposed definition, an object is a planet if it is at least 500 miles in diameter, orbits the sun, and has a mass at least about one-12,000th that of Earth.

Pluto would keep its planethood while three other bodies would be added, including Pluto's moon Charon, the asteroid Ceres and Brown's object 2003 UB313, which he nicknamed Xena.

What I don't get is why Charon would be considered a planet under this definition. It orbits Pluto, not the Sun. Or, if you prefer, Pluto orbits Charon. (Charon is the most massive moon relative to its planet in the solar system so it's hard to say which is the planet and which is the moon.) If orbitting a planet that orbits the Sun qualifies as also orbitting the Sun, then our own Moon would have to be considered a planet. The moon has a diameter of more than 2000 miles and a mass about 1% of the Earth.

Personally, I agree with George Musser at Scientific American:

The trouble with this line of argument is that categorizing objects has played an important role in the development of science. Just because categories involve subjectivity doesn't mean they can't capture objective truths about nature. To make sense of stars, for example, astronomers divvied them up according to color and brightness, discerned patterns, and explained these patterns theoretically. Categories overlap to a degree -- and that's fine. In fact, a researcher trying to understand a phenomenon does well to look at the marginal examples.

Definitions in astronomy are indeed often imprecise and putting objects into multiple, seemingly mutually exclusive categories is not uncommon. In my own domain of astronomy, quasars and BL Lac objects have mutually exclusive definitions (one has emission lines and one doesn't). But then you find active galactic nuclei that have emission lines that right at the boundary, so which are they?

Update USA Today may have the answer about why Charon would qualify as a planet under the proposed definition:

The definition would make a planet of the asteroid Ceres and also reclassify Pluto's moon Charon as a planet, on the logic that the center of gravity around which Charon and Pluto orbit is not inside Pluto but rather in the space between them. (Earth's moon orbits our planet around a center of gravity that is inside Earth.)