With Pluto so much in the news lately, I’ve been thinking about the old mnemonic device for memorizing a Solar System made up of the nine planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto: “My very educated mother just served us nine pizzas.” When the International Astronomical Union demoted Pluto to non-planet status in 2006 (those bastards!,) you started hearing “My very educated mother just served us nachos.”
But of course the Solar System isn’t just the planets. It’s the Sun and everything that orbits the Sun, which can be subdivided into the six categories Sun, Rocky Inner Planets, Asteroids, Gas Giants, Kuiper Belt Objects, and Oort Cloud. So, to remember S R A G K O, I propose the mnemonic device “Some randy astronomers give kinky orgies.” It may not be quite as family-friendly as “My very educated mother just served us nine pizzas,” but the whole point of a mnemonic device is to be memorable, and it certainly is memorable.
Also, the topic of the outer Solar System reminds me of a grudge I have against one point of astronomical nomenclature. Not the decision nine years ago to reclassify Pluto as something other than a planet- that’s been discussed enough- but the name “Hills Cloud” for the inner part of the Oort Cloud. Since the real boundary of the Solar System is the Hill Sphere, the boundary beyond which objects in freefall do not tend to go into orbit around the Sun, I urge astronomers to find some other way to honor Professor Jack G. Hills than by naming a part of the outer Solar System after him.