Sometimes it's hard for a celestial body to get respect. In August 2006, the Rodney Dangerfield of celestial bodies, Pluto, was deemed by the International Astronomical Union (IAU) to no longer be a planet, but a "dwarf planet". Just as that definition was starting to settle in, the IAU decided in June 2008, to call Pluto a "plutoid". Perhaps in three more years they'll just call it a "Disney character" and have done with it.

This latest renaming has the danger of seriously undermining the credibility of the IAU with its own members...many of whom are still fuming about the 2005 vote.

But I'm getting ahead of myself. Let's start with a little history.

In the late 19th century, following the discovery of the planet Neptune in 1846, astronomers noted that the orbit of Uranus was perturbed by more than could be accounted for by Neptune alone. Taking up the challenge to find this mysterious body, which he called "Planet X", Percival Lowell began a search for this mysterious object; a search that would be terminated by Lowell's death in 1916.

Pluto was eventually discovered in 1930, after a year of renewed searching, by Clyde Tombaugh...at the same observatory that was founded by Lowell in 1894.

Through the years following its discovery, Pluto's place among the planet came to be increasingly tenuous as better measurements constantly down-sized the body in both volume and mass. After a fashion it was clear that Pluto's mass was insufficient to be the "Planet X" that was affecting Uranus.

Pluto's didn't much help maintain a family connection with the first eight planets. Mercury through Neptune have relatively circular orbits on the same relative plane. Pluto's orbit is rather elliptical, and is off of the plane of the other planets by more than 17-degrees.

In the 1990s, the discovery of the Kuiper belt, a sort of outer solar system asteroid belt that is much larger than the one between Mars and Jupiter. Unlike the more familiar rocky asteroids, the objects in the Kuiper belt were more like frozen hunks of methane, ammonia, water, and other volatile substances...a lot like Pluto and its largest natural satellite, Charon.

At this point, it was starting to look more an more that Pluto was just a big example of a Kuiper Belt object.

In the early 2000s, with the discovering of more Kuiper objects that were rivaling Pluto in size, some museum and planetarium directors started to unofficially demote Pluto from the list of planets.

This ultimately led to the IAU, in their 2006 assemblage, to reclassify Pluto as one of a member of "Dwarf Planets". Their definitions were as follows (followed by non-scientist-ese translations by me):

(1) A classical planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. [The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.]

A planet orbits the Sun, is round, big, and old.

(2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite. [An IAU process will be established to assign borderline objects into either dwarf planet and other categories.]

A round thing, smaller than a planet, orbiting the Sun.

(3) All other objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". [These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.]

Everything else orbiting the Sun.

To say that this was a controversial definition doesn't even begin to cover it. One of the biggest complaints of most of the members of the IAU that attended the conference was that the vote was taken at the end of the sessions, when most of the membership had already gone home. As a result, only 500 (or so) members of 10,000 astronomers voted on the proposed definitions.

The most recent action to define Pluto as a "plutoid" was largely the result of upper-level boards and committees, and wasn't put to a vote of the membership. Oh yeah, here's the definition:

Plutoids are celestial bodies in orbit around the sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit.

Small round objects, orbiting the Sun, outside of Neptune's orbit, with lots of things around to run into.

A meeting in August 2008 at the Applied Physics Laboratory is scheduled to discuss the definition of planets.

The thing that has saddened me the most in this whole mess isn't whether or not Pluto is or isn't a planet, but that the discussion has been centered so much on only our solar system. From the late 1980s thru June 2008, some 303 planets orbiting other stars (extrasolar- or exo-planets) have been discovered. Many have sizes and orbits that were even beyond the imagination of science fiction. When we consider planet definitions, it seems like we should be as all-encompassing as possible.

I came up with the following definitions:

(0) For the purpose of clarity, these definitions refer only to bodies in an established stellar system which has sufficient structure and temperature to be considered stable. This is not an unambiguous definition, so the example of the collision of a Mars-sized object with Earth would be an instance of a transition gray-area from the cataclysmic stage of formation to the stable stage of formation.

The Solar System needs to be "adult".

(1) A traditional planet is a celestial body that (a) is in orbit around its star or stars, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) is not so large that it can naturally initiate or sustain nuclear fusion, and (d) has cleared the neighborhood around its regular and/or local orbit of any non-stellar bodies capable of destroying it through impact or gravitational forces.

A planet orbits around one or more stars, is round, and big but not too big (i.e. smaller than a "brown dwarf").

(2) A satellite planet is a celestial body that (a) orbits around another tradition planet where its barycenter rests within the physical mass of the traditional planet, or shares an orbit with another body that could be called a traditional planet except that they orbit around a barycenter outside of the physical mass of either body; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; (c) is not so large that it can naturally initiate or sustain nuclear fusion, and (d) has cleared the neighborhood around its regular and/or local orbit of any non-stellar bodies capable of destroying it through impact or gravitational forces excepting its parent traditional planet or of its satellite planet sibling(s).

A planet that doesn't orbit a star, but a planet.

(3) A satellite (or moon) is a celestial body which does not meet the requirements of a satellite planet and which (a) orbits around a larger object with a barycenter located within the larger object, or (b) is orbitally bound to the barycenter of a local multi-body system of a traditional planet, with or without one or more satellite planets, or of a satellite planet system that lacks a traditional planet.

A round or non-round object that orbits a bigger object that isn't a star.

(4) A lilliput is a celestial body which does not achieve the requirements for a traditional planet or a satellite planet but which (a) is in orbit around its star or stars; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) is not so large that it can naturally initiate or sustain nuclear fusion.

A small round object that orbits one or more stars, whose orbit is not ordinarily clear of non-trivial collision-causing objects.

(5) All other objects orbiting within the star or stellar system shall be referred to collectively as "Small System Bodies".

Everything else.

So, what would my definition result in? Well, I don't have tons of data around me, but this is my guess (TP=traditional planet; SP=satellite planet; SM=satellite/moon; L=lilliput; SSB=small system body):

• Mercury (TP)
• Venus (TP)
• Earth (TP), Luna (aka "The Moon") (SP)
• Mars (TP), Deimos/Phobos (SM)
• Ceres (TP)
• Vesta (L/SSB)
• Pallas (L/SSB)
• Jupiter (TP), Io/Europa/Ganymede/Calllisto/etc (SP), other non-round Jovian (SM)
• Saturn (TP), Titan/Rhea/Iapetus/Dione/etc (SP), other non-round Saturnian (SM)
• Uranus (TP), Oberon/Titania/Umbriel/Ariel/etc (SP), other non-round Uranian (SM)
• Neptune (TP), Triton (SP), other Neptunian (SM)
• Pluto/Charon (SP)
• Eris (TP/L), Dysnomia (SM)
• Sedna (TP/L)
• Quaoar (TP/L)
• 2003 EL61 (SSB)

With this, Pluto still doesn't make traditional planet status, but Ceres does (barely). The large round moons get promoted to satellite planets (also where Pluto is now positioned), while the definition of Eris, Sedna, and other Kuiper-belt objects are dependent on how much in danger they are from other bodies in their locale (doesn't have to be a totally clear orbit, exactly, but definitely safe enough for them to be called a planet).

It should be noted that during the craziness of the 2006 IAU meeting, that a lot of astronomers said that this was all a bunch of hot-air, as they never worry about what, exactly, a planet is. Personally, I find this somewhat disingenuous. Classifications matter. True, much of the mathematical work worries only about mass and stuff, but it isn't just about that. The fact that it matters to the public means that it does matter. Seriously, a lot of debate has gone on in astronomical circles about what is and isn't a "brown dwarf", it seems that no less attention should be given to their smaller brethren.

I will confess that I do have a certain nostalgia for Pluto being a planet, but honestly, no matter how you cut it, it really doesn't measure up. Sorry, Pluto fans.

Some have protested that we could conceivably end up with hundreds of planets in our own solar system as detection methods improve. I say to that, great. When I was a kid, Jupiter had 12 moons, and Saturn 10. Now we count 63 moons for Jupiter, and 60 (or more) for Saturn. Amazingly, even with this explosion from 22 to 123+, my life hasn't become one of celestial anxiety. I know the main ones. If it turns out to be similar for planets, we'll all probably know just the main ones for that, too.

So, what does all of this matter? If you aren't into astronomy, probably very little. If you are into astronomy, it also probably matters very little. The fact of the matter is that we humans like to categorize things. It makes the universe neat and manageable for us. It makes us feel like we have some control and have more knowledge than we do. We need to know what a planet is because it makes us feel good...which is probably reason enough.

One last thing..."plutoid" is a pretty dumb and unimaginative name. If the IAU is going to muck things up, they should at least give us good names.

1. Eurocommute, Creative Commons