A year ago, Pluto was declassified as Planet and shunted into the category of Dwarf Planet by the International Astronomical Union. This was accomplished through a series of three qualities that a ‘planet’ must possess, according to the IAU:
- A planet must orbit its star, not another body
- A planet must have achieved hydrostatic equilibrium, i.e. be a sphere
- A planet must have “cleared its neighborhood” of other bodies or debris
In case you’ve forgotten the ruckus from last year, Pluto fails the third item. In addition to its moon, Charon, there are several other small bodies that are orbiting Charon, and in the same basic orbit as Pluto. Therefore, it hasn’t cleared its neighborhood.
There are several discussions concerning this topic over at Bad Astronomy, but I’d like to reiterate my opinion that this whole thing was, and still is, very silly. Whether or not Pluto is considered a Planet, or a Dwarf Planet, or a Plutino, or a Kuiper Belt Object (KBO), is entirely immaterial to Pluto. It will continue to orbit the sun, most of the time at a distance greater than Neptune, and be the 9th Planet in the minds of a whole lot of people. The definitions of planet are arbitrary and difficult to quantify. For example, how spherical does a body really need to be to be in hydrostatic equilibrium? Saturn has a large ellipsoidal eccentricity caused by its fast rotation (although I’m not arguing that it’s not in hydrostatic equilbirum). If we find a star with one lone pile of rocks orbiting it, with a shape close to spherical, is that a planet?
I continue to paraphrase the opinion that “I don’t know what a planet is, but I know one when I see one.” I think that Pluto should have been grand-planeted into the family of planets, and all other KBO’s arbitrarily excluded. It has come to light since last year that Eris, formerly known as 2003 UB313 and/or Xena, is actually larger than Pluto, which would have made it a better candidate for Planethood. Again, my feeling is, “too bad, Eris. You’re too late,” and allowed Pluto to maintain its title.
So long, poor Pluto. It’s too bad you were demoted, but we won’t forget you.