They went from absurdly capricious to far too broad. My objection is that it shouldn’t include moons, nor planets which are round only because they are ice balls. Here’s what everyone intutively defines planets as: Really big and in orbit around a star. So we need to find a definition of how big is big enough. “Large enough to clear their orbit” was a nonsensical defintion chosen to include Mercury and exclude Pluto, once they realized that their confident declarations that there was no Planet X were proven flatly false.
“Large enough to be in hydrostatic equilibrium” is good, but disliked by many because it’s difficult to tell from a distance why a planet is round.
A universally functional characteristic needs to be intrinsic. But any intrinsic property would be hard to observe from great distances (like hydrostatic equilibrium) or based in turn on something ELSE which is potentially arbitrary to that given planet.
I like “Large enough to retain an atmosphere,” as useful for space exploration, but Jovian moons are smaller than Mercury, but Mercury can’t retain an atmosphere because it’s so close to the Sun. Many of the easiest planets to find will be VERY close to their stars. But here’s the point: a planet the size of a Gallilean moon (one of the four Jovian moons discovered by Gallileo) will be VERY much worth studying; Mercury... not so much. So let’s stick to hydrostatic equilibrium.
(Besides, I’m guessing that’s why they called the Death Star “a moon” at first.)
“once they realized that their confident declarations that there was no Planet X were proven flatly false.”
There is no planet X. That was an erroneous prediction based upon some incorrectly recorded observational data. That’s it. I enjoy listening to coast to coast AM at night sometimes, but there is no planet X.