Getting Planets Right

The IAU did what I originally thought they would do and demoted Pluto from a “planet” to a “dwarf planet”. I agree with the decision. The reasoning is pretty simple. If we want the word “planet” to be reserved for a small, unique group of large objects orbiting a star, then there is no easy way to have Pluto be a “planet” the way Jupiter and the Earth are planets. Pluto is one of the largest (but not the largest) members of a vast swarm of objects orbiting the Sun in what is called the Kuiper Belt. The new definition excludes objects that are not the dominant objects in their orbital zones. This has some physical basis for it, as planets originally grow by gravitationally gobbling up (or kicking away) everything in their gravitational zone of influence. If you can’t manage to unite your neighborhood, then the best you can be is a dwarf planet. But the resolution is pretty vaguely worded about just how “clear” the “neighborhood of the orbit” has to be. Jupiter, the king of the planets, shares its orbit with thousands of Trojan asteroids that are in a special 1:1 resonance with Jupiter. So what rules out Pluto: the dozen or more “Plutino” Kuiper Belt Objects that share its 3:2 resonance with Neptune; the fact that its orbit crosses that of Neptune; other KBOs whose orbits cross that of Pluto?

Now if only we can find a better term than “small solar system bodies” for all the stuff smaller than dwarf planets.

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