Ah, Pluto, for such a small and distant world you are able to stir controversy.
In 2006 the International Astronomical Union (IAU) formally defined a planet as an object which 1) orbits the Sun, 2) is massive enough to be in hydrostatic equilibrium (basically that means it’s round), and 3) it has cleared the neighborhood around its orbit. Since Pluto didn’t satisfy the third criteria, it was tossed out of the official list of planets, much to the outcry of the general public. Pluto is still considered a dwarf planet, along with Ceres, Eris, Haumea, and Makemake, but that was small consolation. Then when New Horizons flew past Pluto in 2015, it found that Pluto was a rich geological world, with mountains and thin blue skies. The images captured by the probe showed that Pluto was very planet-like, and there were new calls to redefine Pluto as a planet.
Even some astronomers have issues with the current definition of a planet. To begin with, a planet must orbit the Sun, so the thousands of exoplanets orbiting other stars are not planets. That’s fine if you want to keep planets and exoplanets separate, but most people would figure that an Earth-like body orbiting any star would be a planet. Then there is the third criteria, the whole “cleared it’s neighborhood” business. If it weren’t for that, Pluto would still be a planet. The problem with this criteria is that the more distant a planet is, the more difficult it is to clear an orbit. Earth is a planet under the current definition, but if Earth were beyond Pluto in the Kuiper belt, it wouldn’t be a planet. That seems rather arbitrary.
So Alan Stern (principle investigator for the New Horizons mission) and others have proposed a new definition: A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters. In other words, if a body is basically round but too small to be a star, then it’s a planet.
This would broaden the definition of planet significantly. If this definition were adopted by the IAU Pluto would officially be a planet once once again. So would all the exoplanets we’ve discovered so far, and so would rogue planets that wander cold deep space all alone. Anything with a diameter larger than about 500 km, all the way up to bodies 15 times more massive than Jupiter would be considered planets.
But such a definition might too broad. Not only would Pluto be a planet, but so would its largest moon Charon. So would our Moon, making Earth a double planet. So would the largest moons of Jupiter and Saturn. The definition would shift our solar system from 8 planets to more than a hundred. Sure, Pluto would be a planet, but who cares at that point?
Personally, I think the proposed definition is too general. You could argue that Pluto should be a planet (I disagree), but Saturn’s small moon Enceladus shouldn’t be a planet. Even the common definition of a moon is that it orbits a larger body. In science fiction we have no problem with moons being large and Earth-like, even habitable. They are still moons. I think there is also something to be said for some kind of orbit-clearing condition. It is quite likely that there are thousands of objects larger than Enceladus in the outer edge of our solar system, and they aren’t the same as closer objects like Mars or Mercury.
All of this is worth discussing. As we learn more about both our own solar system and others our definition of what a planet actually is will have to adapt. Whether we end up with hundreds of planets or only eight will depend on what we want the word “planet” to mean.