Is Pluto being a planet just goofey?

Moonflake asks:

In serious news, an asteroid with multiple moons has been discovered. Adding to this the recent discovery of the as-yet-unnamed Planet X, a supposed 'planet' orbiting within the Kuiper Belt (a second asteroid belt beyond the orbit of Neptune, whose most famous member is Pluto), this begs the question: when is an asteroid just an asteroid, and when is it a planet? Is there some arbitrary line in the sand drawn by astronomers? The existence of a moon or moons can't be it, as the above article shows. Nor can size, composition or proximity to an established asteroid belt be a consistent qualifier as this would rate Planet X and Pluto firmly in the asteroid category. In addition, while Pluto is technically labelled a planet by the IAU, most astronomers would disagree with this classification. O SchpatDope, please help us on this one.

Schpat answers:

Mmm, toughie. Actually Moonie it doesn't beg the question, "begging the question" refers to flawed logic requiring circular reasoning, this discovery instead raises the question. I don't mean to be pedantic about this but when you make as many leaps of logic as I do you'd better know what they are called! Don't feel bad, this usage has been commonly accepted since about 1983 so I'll let it slide.

By the way have you seen the size of Pluto? It's almost microscopic in relation to the five gas giants. It's smaller than our moon. If you were driving to Icon and back you would have travelled far enough to pass directly though its centre by the time you got back to Beufort West.

Back to the question at hand, what is the difference between an asteroid and a planet? Well astronomers have not in fact drawn an arbitrary line in the sand, none of them are brave enough... yet. While the matter was under discussion for some years over at the IAU no decision was ever made. The discovery of 2003 UB313 (or Planet X) has forced the IAU to review the matter as people are asking them to finally come up with and answer.

In order to define the rules for planetary classification let's first investigate the meaning of the word planet. Planet is derived from greek and not-so-literally means "wanderer". The Greeks thought that planets were a specific subset of stars that wandered across the sky for some reason known only to themselves and the gods after which they were named. So by greek standards a planet is anything that wanders across the sky, this isn't really going to help us is it?

While looking into this I came across a site describing how star trek weaponry was way cooler than Star Wars weaponry because they could vaporise a large asteroid with a single photon torpedo but it took the entire death star to destroy a planet. I didn't really look into that too much, but it did give me the idea to see if maybe Starfleet had some kind of defining rule that we could use. Their definition: "Planets are large objects orbiting a star [...] size-based distinctions will prove somewhat arbitrary". Gene, I can't believe you whimped-out, Starfleet is just as indecisive as the IAU.

The best idea I've seen was from Mike Brown of Caltech. He proposes that a planet be defined as any body that has a mass greater than the sum of the masses of all other bodies in a similar orbit. This seems like a good idea to me. I'd make a few minor tweaks, like saying 'approximately' the same or greater mass, and allowing for double planet systems like the ones described in Anne McCaffrey's Pern and another very cool sci-fi book I lent to zenstar and can't remember the name of.

If the IAU decide to go with this definition then Pluto may have to be relegated from the planetary league. There is precedent for this. The first few asteroids discovered were considered planets for quite a while, almost 40 years. Later, once more and more of these bodies were discovered, they were reclassified as asteroids. Pluto has been considered a planet for about 75 years and now that more and more of these plutonoids are being discovered demotion would seem to be a logical decision. However logic is not always the prevailing factor when it comes to decision making.

When Pluto was originally discovered by Clyde Tombaugh in 1930, totally by accident I might add, there was debate by the IAU as to whether or not it should be awarded planetary status. Active lobbying ensured that the only "planet" discovered by an American got the status it truly deserved. Demoting the planet now may be considered a serious affront by the American public. Already letter writing campaigns have been started at primary school level to stop this from happening.

As to whether or not Planet X, or the latest incarnation thereof anyway, should be designated as a planet the following may play a part in the decision making process. Currently interest in space is at almost an all time low. If it wasn't for a few failed shuttle missions the general public would have all but forgotten about actual space exploration. The discovery of a new planet would without a doubt result in a renewed interest and probably in increase in funding for NASA. That is possibly why NASA has already tacitly accepted 2003 UB313 as a planet and is putting pressure on the IAU to declare it as one.

Mitsy says that all this sciency stuff is making her head hurt and has found an alternate criteria on her teen forums. One poster says: "The 'planet versus asteroid' debate isn’t too tricky for me [...] I can personally attest to the strength of Pluto’s influence [...] therefore, Pluto is a planet [...] this new one is bigger than Pluto so it's a planet too". In case you didn't realise this commentator is talking about proving that 2003 UB313 is a planet based on astrology. Well, at least it's decisive.

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