Hmmm...I looked at my last post and mistakenly included a picture caption in the body of the message. I had to edit it. Sorry about that.<br /><br />So -<br /><br />I'm going to post the picture! It's in svg. format, I hope it's approved. The artist has graciously granted general permission (it's a wikipicture).<br /><br />I cropped it to fit, cutting out some black space. The scale on the right is inclination in degrees, on the top in red is semimajor axis in Astronomical Units, and the size depicted is a guess at the relative sizes, making (often questionable, IMHO) assumptions about the albedos of the objects.<br /><br />Notice the distinct population classes depicted here. There are a lot of objects in the same 2:3 resonance as Pluto, these are shown in red. Resonant objects are in red, there are other resonances, like 1:2.<br /><br />For those of you who don't know what a resonance is, that is a situation where (for example, Pluto) one object makes an integer number of orbits while another makes a certain ratio of an integer number of orbits. Pluto is in a 2:3 resonance with Neptune, in other words, when Pluto makes two orbits around the Sun, Pluto makes three. The other red objects clustered around Pluto have the same resonant orbit. The resonances are marked at the top of the image, superimposed on semimajor axis.<br /><br />There's something kind of strange about the nonresonant objects, which are depicted in blue. Most of them are clustered at low inclinations, orbiting near the plane of the ecliptic (or, the invariant plane, which is nearly the same only is based on all the planets, rather than just Earth). But there is another group of objects clustered around 25 or 30 degrees. Reality is probably more striking than that, because of the observational bias produced by the fact that the invariant plane is much more thoroughly explored than the high-inclination regions. Many of the high inclination objects were discovered as they crossed the invariant plan