When I remarked that I was amused by the fact that some astronomers don't consider Sedna a planet, our teenage daughter Kate joined in- agreeing that Sedna shouldn't be classified a planet. Surprised, I asked why. "Dad, if Sedna is a planet, then Ceres is too, and there are probably lots and lots more things this big that we haven't discovered. You all should leave it to just the normal nine we learned about in school. We can't have so many planets that you can't name them all!"
Flawed, as it was, Kate's logic about exactly what should and should not qualify as a planet is about as good what I have heard lately from some professional astronomers. I explained to Kate that no one knows the names of all the stars, or all the galaxies, but that doesn't mean we limit the number of stars and galaxies to just the first few handfuls that were named. For that matter, I remarked, if your brain was so completely full of names of people that it just couldn't take any more, would anyone new who you met after that, therefore not be a person? Of course not! We decide whether a person is a person based on their genetics, just as we do when classifying any given living thing into its species. Likewise, astronomers decide whether a star is a star or not, and whether a galaxy is a galaxy or not, based on its physical properties. It might be a dwarf star or a giant star, a dwarf galaxy or a giant galaxy, but the basic qualification is based on some physical characteristic of the object. Stars, for example, are objects that generate the bulk of their energy as a result of sustained nuclear fusion in their interiors. If an object is too small to generate the bulk of its energy as a result of sustained nuclear fusion in its interior, then it isn't termed a star - period. Astronomers do not exclude tiny stars - called dwarf stars - as stars because they are too small; if they have the salient characteristic of a star, i.e., energy generation by fusion, they are termed a star. Despite that, however, some of my brethren think that dwarf planetary bodies like Sedna shouldn't be termed planets. The idea that I do like is very simple. It identifies a physical characteristic for setting a lower boundary to planet classification, akin to the "fusion energy generation" criterion for stars. Any kid knows that when you draw a picture of a planet, you have to draw something round. So the idea I like is this: If an object is large enough for gravity to round its shape, then it is no longer just a structure ruled by mechanical strength, like a rock, a building, or a mountain - instead, it is a wholly different kind of structure that we call a planet. I like to call this criterion, "Gravity Rules." One can calculate the minimum size body that will become rounded by its own gravity starting from very basic principles of physics. Doing so, you find the boundary is a diameter of a few hundred kilometers. JDG: I presume that he would also add that it be a rounded body that is orbiting a star, rather than another planet. http://www.spacedaily.com/news/outerplanets-04b.html JDG _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
