Why is Pluto not considered a planet anymore?
There have recently been a number of discoveries of other things sufficiently like Pluto that, if Pluto is a planet, then they are too. So the International Astronomical Union had to make a decision. They could have decided: 1. All these other tings are plantes, so the solar system has at least 12 planets, (The usuals, plus Ceres and “Xena”), and expect a crowd more any day now as telescopes get better. 2. Pluto is a planet becasue it always was, for sentimental reasons, and these new ones aren’t becasue we say so. 3. Pluto is not a planet, but just another so-called “Kuiper Belt Object” – and then, of course, neither are all the other Johnny Come Latelies. The rules to be a planet are now: 1. In orbit around a sun. 2. Not a sun itself (i.e. not part of a double star) 3. Big enough to sweep all the debris out of its orbit Pluto fails test 3, and is demoted.
