If you're in the "Pluto is a planet" crowd, then you might consider selling all your 9^th planet merchandise on eBay while you still can: astronomers have found that Eris is more massive than Pluto. Eris was discovered a few years back, and observations indicated it might be bigger (that is, have a larger diameter) than Pluto. This is pretty hard to do, because it didn't look like much more than a dot in telescopes; the diameter had to be inferred by its known distance and its brightness. If it's made of something dark (like organic chemicals, common on distant objects), it must be big to look as bright as it does; if it's made of something reflective (like snow or ice) then it's smaller. Subsequent Hubble observations indicated it was indeed bigger than Pluto, and the former ninth planet took one more body blow. Now it's known that not only is Eris bigger, it's more massive. About 30% more massive, in fact.
Artist drawing of Eris and Dysnomia. Remember, the camera adds ten pounds. Copyright Robert Hurt, IPAC
The mass is derived by observing the orbit of its moon Dysnomia (remember when they were called Xena and Gabrielle? Man, I'm glad they changed the names). By seeing how long it takes the moon to orbit Eris, the mass of Eris can be found. Mike Brown from Caltech (that evul librul who discovered Eris), and his grad student Emily Schaller, determined Eris to have a mass of 1.66 x 10^22 kilograms. Pluto's mass is 1.27 x 10^22 kilograms. Eris wins. (Not that it's all that big in the first place: Earth, for comparison, is 6x10^24 kg, about 200 times more massive than both Pluto and Eris combined.) First off, let me comment on how cool it is that we can determine the mass of an object that is currently 14 billion kilometers away. That's amazing. Second, of course, this news will probably mildly rekindle the "Is Pluto a planet?" debate. As I have said many times, there is only one answer to this: it doesn't matter. Why? Because the word "planet" is ill-defined; the group of astronomers who tried to define it last year did an OK job, but the scientific definition left the public rather cold. People want to define the word viscerally, emotionally... I might even use the word unreasonably. In the case of the public, most people want Pluto to be a planet, and won't be open to all the reasons why it shouldn't be. And in the end, we're arguing over semantics. Pluto doesn't care whether it's a planet or not. Worse, lumping it into a category where it might not fit -- and one with arbitrary boundaries -- may make it easier to miss important facts about it. It's a sort of mental illusion. If you think of it as a planet, you might miss a bit of data about it that fits better with Pluto being a big Kuiper Belt Object, or comet nucleus, or some other weird bit of flotsam. Words have impact. They shape our thoughts. Call it a planet if you want. It doesn't matter that much to me -- and less to Pluto itself -- but you'll be doing the object, and yourself, a disservice. And Eris is bigger anyway. Nyah nyah.
