CNN is running a “heartwarming” human interest piece about a fourth grader who came up with a mnemonic to help “those having trouble remembering the newly assigned 11 planets” as part of a National Geographic contest. Unfortunately, the contest, and thus the mnemonic, are wrong. It’s too bad they didn’t consult an astronomer before running with it. Here’s the mnemonic she came up with:
Her award-winning phrase is: My Very Exciting Magic Carpet Just Sailed Under Nine Palace Elephants.
The 11 recognized planets are Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto and Eris.
Ignoring the literary merits of her mnemonic (though personally I think “Pallid” would be a better replacement for “Palace”), it’s wrong because there are actually only eight planets. Ceres, Pluto, and Eris are dwarf planets. Where the contest got confused is they think that dwarf planet is a sub-classification of planet; it is not. According to the three-way classification system now in use, there are three entirely separate classes of solar system bodies: planets, dwarf planets, and small solar system bodies (SSSBs). At the time of the new definition in 2006, the International Astronomical Union took a bit of flak for putting the word “planet” in the name of the “dwarf planet” category because it has potentially confusing to laymen. It’s looking like those concerns were valid.
Now don’t think I’m just picking nits here. There’s another reason this mnemonic is untenable: it only includes the currently recognized dwarf planets. But there are dozens and dozens of likely dwarf planet candidates out there we simply haven’t officially classified yet (42 in the Kuiper Belt at last count alone). The definition of dwarf planet, an object that “has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape” (and is not a natural satellite), is pretty darn inclusive, and was intended to be. Its purpose is to fit in all of the large round objects that don’t make it into the exclusive planetary club. For example, here are a few of the objects in the solar system that are likely to be classified as dwarf planets in the near future: 2003 EL61, Sedna, 2005 FY9, Quaoar, Orcus, and Ixion. Some of these names will be familiar if you’ve been keeping up on your astronomy news. Note that all of them are larger than Ceres, which is already an official dwarf planet. So if we’re going to play the game of including dwarf planets in our planetary mnemonic, it’s quickly going to balloon to an unmanageable length. Better not even to try, and just stick with the eight.
So, here’s the takeaway. Dwarf planets are not a sub-classification of planets, but rather, are a completely separate category. There are only 8 planets and 3 current dwarf planets, but as astronomers get around to officially declaring more dwarf planets there will be dozens more. School children will continue learning all 8 planets but they will not learn all the dwarf planets because there are simply too many of them (and unlike studying the eight planets, each of which has many unique features making it worth learning about, most of the dwarf planets will be somewhat similar round iceballs too far out in the solar system for us to get much good information on). So this fourth grader’s mnemnoic, while cute, is destined for the rubbish bin of history. Not that I fault her at all (I blame National Geographic). I commend her for her astronomical precociousness, which puts her above 99.9% of kids her age. It’s just unfortunate that the facts aren’t correct.