Looks like there’s a lot more junk floating around in space than we knew about before.
For a good while I’ve complained about the “new” definition of planet that included the “clears local region” of debris. That’s the rule that got Pluto demoted to minor or dwarf planet. The big problem I’d seen was that this means planets can come and go as debris comes and goes. When our moon formed, we were not a planet, until the debris was “cleared” and then we were again. Then there’s the Trojans in the orbit of Jupiter. Asteroids that orbit in the same orbit as Jupiter, but before and after it at stable points. Not exactly “cleared”…
Well, now there’s a new problem. Seems Mercury and Venus both orbit inside a ring of debris. Dust and such. Likely originating from some bits of crap co-orbiting with them.
But doesn’t that mean they have not “cleared the nearby area” of debris? By the “new” definition, doesn’t that make Mercury and Venus no longer planets? Hmmm? (Hey, I didn’t make that daft rule. Heck, I even said it was daft as soon as I saw it…)
Things can be graviationally broken up at the Roche limit. Asteroids and comets fly all over the place and can cause a big chunk of “debris” to suddenly show up and than are you no longer a planet? We orbit through the debris field of Comet Encke (who’s arrival and breakup some 20,000 years or so ago likely resulted in the Younger Dryas impact event along with several other major Earth catastrophes) so are WE really clear of debris?
Hopefully this “issue” with Mercury & Venus will help get Pluto put back as a planet AND get some of the other KBO planet sized objects accepted as well.
Musings from the Chiefio 
Taxonomic debates always are fun. Lots of heat, little light. Does it really matter if the designation works for a useful purpose?
I propose a new definition of planet – if you can see it from your front lawn on a clear night, it orbits the sun, and it is made out of stuff, it is a planet.
(might have to put a visual magnitude limit on the definition but science is a process not a product)
But, but, Gary… we must must! have nine planets. Eight isn’t evenly divisible by three. Can’t have that now, can we?
With only eight planets, that would be the corners of a cube and their orbits around the Sun would be straight lines as they zip from corner to corner. The science is settled on that ;o)
Well I’d say we have 12 to 15 or more. Those are divisible by 3… Though I really like 18 better. Divisible by 2, 3, 6, 9 and of course 1 and 18.
https://chiefio.wordpress.com/2011/01/25/our-planets/
I just like “self rounding, maybe with a radius limit, orbits the sun concave all the way, never retrograde”. That’s all it takes. And don’t worry about junk near it. ALL the planets have junk in the area… Just some more than others.
The problem with “planet” is that it’s a cultural definition. People in ancient times saw these lights that behaved irregularly (going into “retrograde” phases due to normal orbital mechanics) and thought they were special.
You can see the current “definition” as an attempt to get back to the golden age of simplicity where we have a small, defined set of planets that fits into a simple number. But as you point out so well, the current definition of planet is arbitrary and isn’t really functional.
I prefer a life-based definition. A planet has to be at least large enough to hold an atmosphere that supports an active biosphere. Not necessarily at its distance from its star, but say at the distance of Earth or Mars from the Sun. “Dwarf” planets can be planets that are too small to hold an atmosphere for any appreciable time in ideal conditions.
We may still have only 8 planets with that definition (9 if you count Mike Brown’s hypotheical Planet X) , but it’s not a completely arbitrary one. And schoolchildren will still be able to count the planets on their fingers. Because “planet” is a cultural definition, having several hundred of them (if you count all the dwarf planets that will eventually be found) just isn’t going to fly.
If it travels 15° across the sky in one hour, it’s a star. Everything else is a planet, except for Luna.
She’s special.
In the (Star Trek) future, all such bodies will be “Planets” further identified by “Class”. The most interesting will of course be “Class M” — rocky spheres orbiting in a “Goldilocks zone” with surface gravitational acceleration near 10.0 meters per second ( + / – 20% ) and an atmosphere with oxygen in the mix. Note these could easily be satellites of other classes of “planets” — so Ganymede orbiting Jupiter might be the first instance of, say, a “Class G” *planet* while Jupiter itself might define a “Class J” planet rather than a failed or sub-standard or proto-“star”.