Planet? I usually make it up as I go along

Wed 23 Aug 2006 09:07 AM

The International Astronomical Union (IAU) votes tomorrow in Prague on a proposed definition of the word 'planet.' has a nice discussion of the proposal here and here.

There are nine canonical planets. The problem begins because one of them, Pluto, is really not up to the measure of the others. It is just an icy rock ball on the periphery of the solar system, but common usage and popular opinion holds that it is a planet. Worse still, there are comparably sized rocks out there; for example, 2003 UB313 aka Xena. Whatever the scientific merits of the case, the guys down at the Dairy Queen won't stand for Pluto being demoted from planet to mere rock. As Janet explains, "Pluto is too a planet! It was a planet when I learned it in school, and it hasn't gotten any smaller since then!" This is a nice example of the way that scientific concerns and common usage together shape how we decide to use words.

The proposed definition is this:

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.

This is not oddly disjunctive; neither does it merely stipulate a list of members. So it surely picks out a natural kind.[1]

This definition includes all nine canonical planets. The trick is that it would include many objects not currently considered planets. Xena, for example. Also Charon, which is typically considered a moon of Pluto.

Ceres, the largest asteroid, would also be a planet. This militates against common usage-- the guy at the Dairy Queen knows that asteroids ain't planets-- but Ceres was classified as a planet when it was first discovered in 1801. Asteroids were, in the early days, called 'planetoids' and 'minor planets.' Moreover, Ceres is not just some Oldsmobile-sized chunk of rock. It is 930 km across and comprises about a quarter of the asteroid belt's total mass.

So: The definition is principled, preserves Pluto's planethood, and satisfies the desideratum that every planet we learned about in school should count as a planet. All the same, I think the definition is tantamount to kicking Pluto out of the planet club.

As a rough approximation, people will remember lists of seven items, plus or minus two. Nine planets, fine planets-- yes. But twelve planets, three too many. Once lists get too long, people cluster the items. Instead of having more items, there is one list of kinds of items and seperate lists for each kind.

This is already happening. The eight planets from Mercury to Uranus are classical planets. Ceres is a dwarf planet. Pluto, Charon, and Xena are plutonic planets; dropping the 'planet' moniker entirely, these are plutons. The club of eight retains its aristocratic standards.

I predict that, if the proposal is accepted, then in two decades 'classical' will be dropped in ordinary conversation. Dwarf planets and plutons will have to be specified as such.

[1] As readers will recall, I am inclined to be ecumenical about natural kinds. I might count a stipulated list of the canonical nine planets as a natural kind. That does not mean that we should use the word 'planet' to pick out just those nine things, however.