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IAU Proposes 3 New Planets
IZ Reloaded writes "Sources tell SPACE.com that the International Astronomical Union is preparing to include three new entries to the current list of planets in our solar system. From the article: The asteroid Ceres, which is round, would be recast as a dwarf planet in the new scheme. Pluto would remain a planet and its moon Charon would be reclassified as a planet. Both would be called "plutons," however, to distinguish them from the eight "classical" planets. A far-out Pluto-sized object known as 2003 UB313 would also be called a pluton."
The way I teach it in my classes is that there are 4 inner planets, 4 outer planets, and a (large) set of Kuiper Belt objects, of which Pluto is one of the largest and closest members. Why do we need a planetary definition? Historically, any serious attempt to classify natural objects eventually runs into problems anyway, especially when our first attempt includes objects that obviously belong to a number of sub-classes, each of which contains a continuum of members.
Planet: A celestial object orbiting a star that is massive enough for its own gravity to warp itself into a nearly round, spherical shape. A planet may not be massive enough to initiate thermonuclear fusion. In order for a pair of celestial objects to be considered a double planet, in addition to meeting the forementioned criteria, the barycenter of both objects must be located above their surfaces. Planetary systems orbit a barycenter, or their center of mass. Usually that center of mass is located at the center of the planet, but in the case of Pluto, the gravity of its "moons" pull the barycenter above the surface. As a result, Pluto is perpetually orbiting the center of mass of the planetary system, as illustrated in a chart located in the Wikipedia article. This is why Charon and Pluto are being considered double planets. I think that's the best set of criteria that can be offered. Why is the idea of over 50 planets so abhorent? Why must size and the number of planets be decided arbitrarily? We might as well use Isaac Asimov's mesoplanet suggestion, in which all objects with radii between Ceres and Mercury are mesoplanets, if this is how it is to be decided.