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Message: <blockquote data-quote="occam" data-source="post: 3022877" data-attributes="member: 39815">What's the point of a classification other than to draw essential distinctions? The distinction of roundedness isn't scientifically interesting, in and of itself.No, they don't. Planets are not asteroids, nor comets, nor meteors, nor stars, nor moons. These are all orthogonal categories, that relate to details of formation, composition, etc. That the category of "planets" may also be subcategorized into gas giants and rocky planets doesn't change that.The proposed definition of "planet" only means "big, orbiting a star, but not a star itself". How is that useful? You still need a way to refer to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune as different from everything else in the Solar System, including Ceres and any large KBOs (including Pluto and Charon). What do you call them, then? "Planets that aren't also asteroids or KBOs or large examples of some other things?" Or do you just call them planets, with the unspoken caveat that you really don't mean to include Ceres?The large Jovian moons, and Titan, and Triton and other large moons (including Earth's) formed differently than the planets around which they orbit. It's an important distinction.Limiting the discussion to objects in the Solar System*: Planets orbit the Sun. They all orbit within the ecliptic (not counting Pluto), and in the same direction. They all have nearly circular orbits (not counting Pluto). They all have substantial atmospheres (not counting Pluto, or Mercury, which has extenuating circumstances). They all probably formed in a similar manner (possibly not counting Pluto). Taken together, those characteristics help define a useful category of objects (which can be further subcategorized).* No doubt, future discoveries in other star systems will muddy the waters, but we're not quite there yet.No, it doesn't, other than that it happens to be big (or more specifically, massive). By itself, it doesn't provide any real insight into the processes of formation, so limiting the definition of planethood to this one thing (OK, two things, with the star-orbiting criterion) doesn't further any discussion. You still need to make further distinctions, so what's the point, other than to allow Pluto to retain its legacy status?Pluto is the edge case! Heck, it's not really even an edge case; it's been an oddball inclusion from the start. Drop it, and there's no further confusion (in this star system, at least).</blockquote>

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