I'm writing this with sadness as I listen to a protest next to the lake below me.
The International Astronomical Union just came out with their new deliberations. Contrary to all expectations they reversed the recommendations of their board, a rare action indeed. The board had recommended that a planet be a body sufficiently large enough to be somewhat round by collapsing under its own gravity (somewhat, as Earth and most other planets are not actually round) and circling a star (so Charon would be a planet), resulting in three new planets: Charon, Xena, and Ceres. Instead the IAU voted to add one addition- the planet must have cleared all the objects out of its path. This refers to the development of a planet in the current Nebular Model. Planets start off as planetesimals, with every piece having some amount of gravity. The more gravity, the more pieces attract other pieces to them. The more they attract, the larger they get, the more gravity they get. Therefore in a matter of a few years you have a planet, clearing all other objects out of its path.
Theres a lot of controversy over this. The debates at the IAU were very contentious. Only 300 astronomers voted out of 2500 available at the IAU conference. The new guidelines arent completely precise. Pluto crosses Neptunes orbit, its true, and therefore has not yet cleared its path. Yet therefore neither has Neptune, though of course Pluto started orbiting long after Neptune did. All of the inner planets, including Earth, have major asteroids in their path, and therefore wouldnt be planets under the new definition. Even Jupiter has the Trojan Asteroids in its path. Therefore an object that isnt completely done with its work clearing paths can never fully be a planet, under the new definition.
But for now, rather than 12 planets, we are down to 8- Pluto is no longer a planet. At least until the IAU votes again and makes another decision. I must admit a certain sadness. Adding is always more enjoyable than subtracting. But there is a certain truth to it that I can see too. Round, yes. Circling a star, yes. And also, big enough that it controls the space around it- clearing its path so to speak.
But there is a great teaching opportunity here too. For we may not like the new guidelines, but that is what astronomers do. Sometimes students get the idea that science is merely competing beliefs or ideas, rather than a deliberative decision making process. They can begin to believe that a science teacher can choose what they teach, or change what they teach if they are convinced enough. But when we teach science, we don't get to decide what is true or what is not- we simply teach what the scientists do. That is the job of a science educator- to teach the scientific method of decision making. Whatever my pet beliefs or preferences, I must subsume them, if they run contrary to what scientists do, discover, and determine. I am not doing my job if I teach anything else.