Just as Ceres began to make sense only when it was recognized as one of a vast population of asteroids, Pluto fell into place only when astronomers found it was one of a vast population of Kuiper belt objects (KBOs) [see "The Kuiper Belt," by Jane X. Luu and David C. Jewitt; Scientific American, May 1996, and "Migrating Planets," by Renu Malhotra; Scientific American, September 1999]. Astronomers began to reconsider whether it should still be called a planet. Historically, revoking the planetary status of Pluto would not be unprecedented; the ranks of ex-planets include the sun, moon and asteroids. Nevertheless, many people have argued for continuing to call Pluto a planet, because almost everyone has grown quite accustomed to thinking of it as one.

The discovery in 2005 of Eris (formerly known as 2003 UB313 or Xena), a KBO even larger than Pluto, brought the issue to a head. If Pluto is a planet, then Eris must also be one, together with scores of other large KBOs; conversely, if Pluto is not a planet, neither are the other KBOs. On what objective grounds could astronomers decide?

1 Comments

1. 1. laurele 11:51 PM 10/2/08

   You are clearly misrepresenting the work of Stern and Levison. Yes, they distinguished between dynamically dominant "uber-planets" and non-dominant "unter-planets," but they never say that the smaller, "unter-planets" are not planets at all. That is what the IAU definition, which was adopted by only four percent of its members, most of whom are not planetary scientists, dictates--a statement that makes no linguistic sense. Lumping dwarf planets in with the asteroids and non-round KBOs blurs the very important distinction between round and non-round objects. The round ones are in hydrostatic equilibrium, meaning they have enough self-gravity to have pulled themselves into a round shape. When this happens, differentiation and geological processes akin to those on the larger planets occur. These processes do not occur on shapeless, inert asteroids. Yes, in some cases of very small objects, it may be difficult to tell if they have achieved hydrostatic equilibrium, at least with the observation technology we have now. There will always be objects on the fuzzy edges of any definition. However, Pluto, Ceres, Eris, Makemake, and Haumea are not at these edges. Their spherical shape and resulting geological processes are well known. It is not a useful scientific classification scheme to ignore this and lump these as just more objects in belts. That defines the objects solely by where they are while ignoring what they are. If we use only dynamics, Earth would not be a planet if it were placed in Pluto's orbit. The best way to fix this is to amend the IAU resolution and include dwarf planets as a subclass of the broader umbrella term "planet." The number that results in does not matter, as memorization is far less important than is understanding the different types of planets and their characteristics. As for the round moons of planets, these were in the past referred to as "secondary planets," reflecting their composition being that of a planet but their orbital path being around another planet rather than around a star. We can still colloquially refer to them as moons or satellites.

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