How many planets are there?
The International Astronomical Union Exec Committee just finished their long-awaited two-year deliberations, and have submitted a proposal. There proposal to the IAU will be voted on next week, but they only submit resolutions likely to get a 2/3rds vote. After all the flack over some demoting Pluto to a mere sub-planetary body, they have now proposed a definition for a planet: anything large enough to collapse under it's own gravity into an oblate spheroid (necessitating approximately 800 km in diameter), or round; and anything orbiting a star that is not a star. So, for instance, the moon, Luna, larger than Pluto, is not a planet, as it orbits Earth, not a star.
By this definition, as expected, and as I have long argued to my classes, Pluto and Charon are now a double planet, the only one in the solar system, as they orbit a common center, and therefore both orbit the sun together, not a larger body. Xena was temporarily named such as they wanted to give a nod to Planet X and because the guys discovering it were all...guys. (It has a moon named, you guessed it, Gabrielle. Temporarily.) The true name at the moment is the rather unimaginative 2003 UB313. The amazing thing about this (now) planet is that it was discovered in 2005, is farther out than Pluto, and is larger than Pluto. So the IAU was being pressured to decide soon- if Pluto was a planet, then so was Xena. As indeed they now decided.
But the real shocker- Ceres! Who would have thought??? Now suddenly the Titius-Bode Law no longer has an exception- there actually is a planet between Jupiter and Mars! Lowly Ceres, only 950 km in diameter, is now a planet. Further, there are another 12 probabilities under consideration, that could shortly be planets, bringing the total number to 24. And some are saying there are at least 53 objects that could be planets under the new definition.
What does this mean? If you're teaching astronomy next year, good luck. You might have to have them memorize 53 different names. But it will be exciting, with all the new extra-textual developments to report. And this is just fun! Yeah, it's anthropomorphic, but our whole Solar System is changing! You can visit the IAU website to see a video of our new planets.