The issue is, if we start calling Ceres, Charon, and 2003UB_213 planets, aswell as maintaining that Pluto is a planet, that pretty muchs opens the door for for dozens, and quite possibly eventually hundreds of planets within just our solar system if the Kuiper belt is as large as predicted.

The real issue is, at what point is something just a floating ball of rock and ice.

The composition of the Asteroid belt objects and Kuiper belt objects is for all intensive purposes uniform, where as the inner most, classical 8 have more unique compostions, aswell as sufficient mass to maintain a minimal atmosphere.

Also, the classical 8 have more consistent orbits with low inclination with regards to the Sun's equator, where as Kuiper belt objects have extremely eccentric orbits, at times Pluto is closer to the Sun than Neptune.

All that said, whether it matters or not depends on personal opinion. I think planetary status should be reserved for unique, more massive planets with more consistent orbits.

Also, I doubt most kids would enjoy learning about the 83 planets of the solar system as opposed to the 8 or 9.

In reply to Pharcyde, #4:


Well to be perfectly honest, were Pluto discovered today, it would never even face consideration for planetary status. It's just one of many, many Kuiper belt objects.

Another proposed solution is giving Pluto the status of "classical planet," allowing it to retain it's planetary status for historical purposes while simultaneously removing the issue of having to consider every other Kuiper belt object's planetary status. Personally, though I don't consider Pluto to be a planet, this solution seems to be the best compromise for all sides of the issue.

1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet.”

(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.

(4) All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

I've never really considered "There'd be a lot of them" a valid objection to any general definition. That's like saying we don't have a good definition of "mammal" because it includes too many species.

If the asteroids are essentially uniform, then the same argument goes for the terrestrial planets. They're as similar to each other as the asteroids. They're just larger. Jupiter and Saturn, too, are almost identical in composition, and Uranus and Neptune are twins in their own right. You're trying to justify the "dominant body" definition with phantom qualifiers.

This was a very good argument until we started finding superjovian exoplanets in bizzare orbits. Now, it seems that the classical planets are the unusual ones. More importantly, this limits the definition of "planet" to OUR solar system, making it almost as worthless as it is undefined. If we can't apply it to any other system, there's no reason to apply it here.

I think planet should convey more than "this object we like better for entirely subjective reasons". You say it should be limited to "more massive objects", but more massive than what? The orbits should be consistent with what? What if there's a solar system with only one planet in a very eccentric orbit? Is it not a planet? What if it's 50 jovian masses? 30? 10? 1? 0.1? What if it's the mass of Earth? Mars? Mercury? By your definition, when does it stop being a planet?

The definition has to stand, if you'll excuse the pun, in a vacuum.

Science isn't a democracy. It doesn't matter if kids will enjoy learning the names of all the planets. Hell, it shouldn't matter if they can name all the planets; they should be able to explain what a planet is instead. We don't expect them to know what all the mammals are, after all.

Your definition boils down to "big" (with no scale given) and orbiting in the same plane as all the other planets the solar system (which limits it to multi-body planetary systems), while being compositionally unique (which limits it to single-body planetary systems). Your definition falls.

Really though, you are right, the issue is finding a definition that is universally accessible, that can be used with regards to all solar systems. To me, the only real logical solution would be then would be to divide a class of rocky bodies up into many sub groups. I.E., comets are rocky bodies of insufficient mass to maintain permanent atmosphere with orbital inclinations usually around blank to blank.