But you can't just define words however you like. We have a current, working definition for the word "planet" based on what we know now. Pluto no longer fits in that definition. It's silly to ignore the facts of today just because we might learn more later.
In point of fact, there are a lot of valid criticisms of that current, working definition. It's a flawed definition largely
because irrelevancies like nostalgia and politics ended up influencing it, so there are a lot of valid questions to be raised about the definition from a scientific standpoint. For instance, the paradoxical assertion that dwarf planets aren't planets, which seems to arise mainly from a resistance to change (since if they are planets, then the number of planets goes up hugely, while if they aren't, it only changes by one). And the various lingering questions about some of the arbitrary lines drawn by the "official" definition.
In truth -- and this is one reason why treating this like a huge controversy is so silly -- the IAU's definitions aren't really binding on anybody. They're more suggestions than anything else. Astronomers are free to use or ignore them as they see fit. But of course, a large percentage of astronomers were saying that Pluto shouldn't be considered a planet long before the 2006 ruling. As I said, the astronomical community has been uncomfortable about that designation for decades.
But you know what? That's okay. Science isn't about making up pat, easy answers, or about sticking labels on things and pretending they represent some final truth. Science is about asking questions, about continuously adding to our understanding of things. There's always more to learn.
I like to tell people who ask that the solar system right now consists of four giant planets, four rocky planets, upwards of two hundred dwarf planets, several hundred moons and several thousand asteroids and comets.
Well, to date, there are only five officially recognized dwarf planets, and three of those are only tentatively accepted. Again, our knowledge is always expanding. Eris, Haumea, and Makemake are currently accepted as dwarf planets because it's a reasonable conclusion from their observed mass that they must be spherical, but we aren't absolutely certain. And there are dozens, maybe hundreds of other candidates for dwarf planet status, but we don't have good enough observations of them yet to determine their sphericity. It's
probable that the Solar System contains dozens of dwarf planets and possible that it contains hundreds, but it's far too early to pretend that's a proven fact.
Sedna is not officially regarded as a dwarf planet, and Wikipedia does not claim that it is. It only says that it probably fits the parameters. It's a dwarf planet
candidate, like so many others.
Like I said, science is not about rushing to conclusions and pretending you have some final truth. It's about asking questions and gathering progressively more information to help you improve your answers to them. There's nothing wrong with saying "We don't know yet."
And the number of asteroids and comets is in the millions, not thousands, and that's just in the observed parts of the Solar System. Take the whole Kuiper Belt and Oort Cloud into account and the number of small Solar System bodies is probably in the billions, even trillions.
Basically, we can break the Solar System down into several major zones with transitional areas between them:
Inner system: Contains the four Terrestrial planets, Luna, and assorted asteroids. Lacking in volatiles (such as ice, hydrogen, nitrogen, etc.) except on the larger bodies.
Main/Inner Belt: Transitional zone. Mainly stony or metallic asteroids, with increasing proportion of volatiles as distance from Sun increases. Contains one dwarf planet, Ceres.
Middle system: Contains the four giant planets (two Jovian/gas giant and two Neptunian/ice giant), their associated satellite and ring systems, their associated Trojan asteroids, and the
centaur asteroid/comets. Hydrogen, ice, and other volatiles are abundant.
Kuiper Belt: Transitional zone. Consists of icy bodies of many sizes including multiple dwarf planets. Far larger in mass and volume than the "Main" Belt.
Outer system: Contains icy bodies of various sizes and orbits, mostly still undiscovered. Probably contains many dwarf planets, possibly contains undiscovered icy planets. Includes scattered disc and inner Oort cloud. Contains magnetopause, the boundary with the interstellar medium.
Oort Cloud: Conjectural cloud containing billions of comets and possibly larger bodies, extending out to nearly a light-year.
Of course, you could treat the magnetopause as the outer boundary of the system, but there are still plenty of objects beyond it that are still gravitationally bound to the Sun.
And I should make it clear that the "Inner/Middle/Outer system" designations are strictly my own proposed terms for them, not something official.