The Speculist

Articles like the one you linked are just symptoms of people whining about Pluto losing its planet status. And the main reason they are whining is because they grew up with Pluto as a planet, and have fond memories of it as thus. However, it's just a tiny useless ice rock, and doesn't deserve planet status.

Good riddance, Pluto.

I think the rules should be something simple--just pick a minimum diameter. 1000km? 1500km?

The rules would be simple: a planet is any body more than Xkm in diameter that is not orbiting another planet.

Then--a moon is any body more than Ykm in diameter that is orbiting a planet.

I agree with the idea that the criteria should be simple. The irony of the current debate is that this is supposedly guidelines for labeling something a planet in the Solar System. Ie, the rules aren't intended to apply yet to other star systems. As just about everyone knows, the IAU failed to define a critical term "clearing the neighborhood", meaning this definition is far from complete.

However, reading up on the debate, I think the crucial paper is this paper by Steven Soter (submitted August 16 BTW). Apparently, the idea of "clearing the neighborhood" is worked on in some detail here.

Here's the proposed definition of planet from the paper. Take you candidate object that is in orbit around the Sun. Find its closest and most distant points from the Sun. You then consider the shell of the space that is in between these two distances from the Sun as the "orbital zone" of the object. If the collective mass of everything else (the author excludes objects with vastly different orbital periods, but this is insignificant for the Solar System) that is in orbit around the Sun and passes through the orbital zone is less than 1% of the mass of the candidate object (and the object is too small to be a star, what the paper calls a "substar"), then it is considered a planet in this paper.

Part of makes this definition work for the Solar System (with the exception of Pluto) is that the other eight planets easily exceed this limit (Earth with it's Moon is the only one that doesn't though the combined mass of the two objects does, and probably would be considered a binary planet in the paper). OTOH, the nonplanets don't come close. Ceres is probably the closest and it masses about a third of the rest of the stuff that passes through its orbital zone.

So you have a gap from about 10^-2 for Earth to 3 for Ceres. That's two orders of magnitude. If you combine the Earth and Moon as one object, then the gap is far greater.

Michael, I think the IAU didn't do a good job. They haven't defined a critical term. The whole situation is absurd since we still don't have a proper definition of "planet" except to say that Pluto isn't one any more.

Having said that, the orbital dynamics of Pluto really do preclude it as a traditional planet. It's locked in a 3:2 orbital resonance with Neptune. Other objects also have this resonance with Neptune.

What was the trigger is the recent discovery of larger objects than Pluto in the Kuiper belt. The thinking here is that there could be a lot of Pluto-sized objects out there. There could be dozens or hundreds of objects that meet the mass threshhold of being a planet.

D. Vision, there is a definition of a planetary mass object, called appropriately "planemo". No dependence at all on orbital dynamics.

I also note that this isn't the first time that the IAU has come up with ill-defined terms.

For example, parsec and astronomical unit (or "AU") are both poorly defined. The astronomical unit is defined to be the radius of a circular orbit from the Sun that has a precise orbital period which is almost a year.

The problem is that the gravitational constant is poorly measured compared to all other basic physical constants. So if you wish to use standard units to describe celestial mechanics, you quickly find that the mass of the Sun is poorly known merely because the gravitational constant is poorly known. So the IAU decided to use astronomical units and solar masses to describe the mechanics of the Solar System. The key problems with this approach is that first, the AU is a complex definition and second it changes over time as the Sun loses mass (according to the above link it changes as much in 3000 years as the current uncertainty in the unit's length in meters).

The parsec in turn is defined to be a certain multiple of the astronomical unit (original such that the parallax of a star as it appears from the Earth is 1 arcsecond).