Poor Pluto. Only discovered in the last century, the on-again/off-again furthest planet from the sun in our solar system, not even recognized as having a moon until 1978 (at which point it was realized to be even smaller than we thought). And now, it doesn’t even get the distinction of being called a planet anymore.
The IAU voted this week on a formal definition for what constitutes a planet, and it turns out Pluto doesn’t make the cut anymore. At best, it’s considered a dwarf double-planet (its moon, Charon, is half the diameter of Pluto), and is now classed along with Ceres (the largest asteroid in the inner belt) as a lesser rock of the solar system. Also making the list of “dwarf planets” is the yet-to-be-officially named ‘Xena,’ which, like Pluto, is a Kuiper Belt object.
We’ll see if it stands. As an astronomer points out in this article, the vote about Pluto’s classification was taken from among the 400-odd astronomers present at the IAU conference, out of thousands of astronomers in the world. Some of them are going to appeal that decision (somehow; I’m not real sure what kind of court takes those cases) and more than a few are pissed off about Pluto’s demotion. So stay tuned.