Dear doubletruncation;<br /><br />Thank you so much for this paper. I have found it so compelling that I have saved it in my computer for future reference.<br /><br />For the casual readers of this forum, the author proposes that a planet be defined as being greater that 100 times the mass of all the objects in a similar orbit. He devises elegant models that show this. Actually, most planets are about 10,000 times the mass of objects in a similar orbit, so the 100 figure is arbitrary to include some "pathological" cases, which always occur in science. Neptune is about 8500 times the mass of Pluto or similar-sized 2003 UB313 (note: the author did not have the latest size measurement for Xena when he wrote the paper, but that does not change it's validity, in fact it increases it).<br /><br />One of the factors in determining whether an object should be a planet is a, semimajor axis. This makes Mercury a planet, but an object the size of Earth at the distance of the Kuiper belt would fall short of being a planet.<br /><br />This paper is recommended reading.