Per the IAU (Resolution B5, adopted by the 2006 IAU General Assembly), there are five supposedly mutually exclusive categories of things in the solar system:
- The Sun (only one), which is distinguished by the ability to fuse hydrogen into helium in its core.
- The planets (eight, might possibly become nine if Michael Brown is correct), which are distinguished by having the ability to "clear their orbit" (a well-defined concept with numerical meaning) before the Sun turns into a red giant.
- The dwarf planets (unknown number, most likely well over 100), which are distinguished as being bodies that don't qualify as the sun or as planets, but are sufficiently massive to more or less have pulled themselves into a roundish shape.
- Small solar system bodies (unknown number, well into the millions) which are distinguished by not being distinguishable as any of the other categories.
- The satellites of planets, dwarf planets, and small solar system bodies (unknown number, the number of which change over time as satellites are captured or rejected) which are distinguished by being in orbit about a planet / dwarf planet / SSSB rather than the Sun.
Contrary to that resolution, these boundaries are fuzzy rather than sharp, particularly when one looks outside of our solar system. (Note well: The IAU definitions pertain to our solar system only. There are currently eight planets in the entire universe, with the possibility of growing to nine if the conjectures regarding a largish planet beyond Pluto turn out to be correct.)
Consider the first boundary, that which separates planets from stars. This becomes fuzzy with the smallest red dwarf and the largest super-Jupiter. There's no clear-cut boundary here. In fact, there's a good amount of debate over this very issue. This boundary is quite sharp in our solar system, but it almost certainly is anything but sharp in star systems beyond our own.
The second boundary, the one between planets and dwarf planets (which, despite the name, are not planets) also has the potential for being fuzzy. In our solar system, that boundary is once again rather sharp. Any time you can come up with a boundary that is separated by five orders of magnitude you have a nice sharp classification boundary. However, given the hugeness of the universe, there is almost certainly some star system out there, somewhere, where this boundary is as fuzzy as can be.
The third boundary, the one between dwarf planets and small solar system bodies, is fuzzy in our own solar system. Objects less than 100 meters in diameter are, as a rule, anything but "round". Objects over a kilometer in diameter are, as a rule, quite "round" (when the shape is resolvable). Astronomers have found lots and lots of objects between 200 and 600 meters in diameter. Squint hard enough at the smaller ones and you might be able to convince yourself that they are "roundish". Focus intensely on the larger ones and you might be able to convince yourself that they are not quite round. There's no sharp boundary here, even in our own solar system.
The fifth category, satellites vs non-satellites, opens yet another can of worms. Planets (particularly the giant planets) can temporarily capture objects as satellites, only to lose them as satellites after a short time (tens or even hundreds of thousands of years is a "short time" in this context).
Note well: I'm not saying Pluto should once again be promoted to the status of "planethood". That very sharp boundary (at least in our solar system) says that it most definitely is not a "planet".
But what about asteroids versus comets, which is what started this series of questions? That boundary is ridiculously fuzzy. Fuzzy logic (a real and serious concept) doesn't come close to addressing how "fuzzy" this is. Centaurs and formerly active comets are the boundary cases. There is no boundary. There instead is a soft melding from one to another.