A little over a year ago, I mentioned the discovery of a new object in the solar system, a little larger than Pluto, that was being called the "tenth planet" by many observers. All that was needed to make this classification official for the International Astronomical Union to actually decide what the definition of a planet actually was, a surprising gap given the long-standing, if informal, use of the word "planet" in popular thought.
Yesterday, the International astronomical Union finally agreed upon and voted on a definition, and the results were something of a surprise. As recently as last week, we were being told that a "planet" would likely be defined as any object orbiting a star, with sufficient mass to assume a round shape under its own gravity. This would have meant that our solar system would have not 9 planets, or even 10, but twelve (including Ceres, the largest asteroid, and Charon, Pluto's moon, which would be reclassified with Pluto as a "double planet" since the center of gravity in their orbital system is a point outside of both bodies).
As it turns out, the IAU decided to define "planet" in such a way that Pluto no longer qualifies. So we really only have 8 planets in our solar system. The crucial distinction came by adding one line to the definition being discussed last week. A planet must not only be in orbit around a star, and round, it must also have "cleared the neighborhood around its orbit."
So teachers and textbook writers are scrambling to prepare lessons based on the new data, science fiction stories for the past several decades have just been declared invalid (I'm particularly curious to see how Dr. Who fans respond to the fact that the "Tenth Planet" of that franchise can no longer have that designation for several reasons), and a new mnemonic must be devised that leaves out Pluto.
But we'll live. After all, fans of Ceres (which at the start of the 19th century spent nearly 50 years classified as a planet before being demoted to being merely the largest asteroid) had to make do.
For Transformers fans, if it helps, here are my reasons why Unicron should still be considered a planet. :)