In light of the newly-found UB313, there's been a lot of controversy over what defines a planet. The main question is, what's the lower limit? I think we can agree it isn't low enough where non-spherical objects can be classified. Here are some popular theories I've seen recently:

1)Classify all spheroids as planets...which would bring the total up to ~20 and counting.

2)Retain the 4 terrestrial planets and 4 gas planets, but drop Pluto and UB313 due to being intertwined w/ a swarm or belt of other objects.

3)Set Pluto as the arbitrary limit...anything larger is a planet, anything smaller is not.

4)Regardless of limits, divide the different objects into categories by distance and structure...terrestrial planets, gas planets, and KBO planets.


And I may be forgetting other popular ones.

All of these have their pros and cons. Theory 1 is the most scientifically suitable, however having over 20 planets is a huge change. It reduces the meaning in a way too...not exactly the neat, orderly line of bodies to memorize. Theory 2 was the idea I was most comfortable w/ originally...however, this will be a huge problem if we find KBOs even larger than UB313 to the point where there are Mercury-sized ones. This theory would assume a Mercury-sized KBO isn't a planet because it's in a big asteroid. Not vey fair. Dividing objects by category seems reasonable enough...but then we're still left w/ the issue of where to draw the line between what's a KBO planet and what's just...a KBO.

Theory 3, having Pluto be the arbituary limit, sounds like it's giving into the cultural protests. With this idea in mind though...I decided to investigate the diameters of all the solid spheroids in the solar system. This was mostly to see if there are any evident groups or categories by size.

You can see it on this graph. Note, gas planets excluded for obvious reasons...and satellites are included as well. Also note some of the diameters are very rough estimates (esp w/ the new KBOs) and may be off by as much as a few 100km.



And the data...



I know what you're thinking. However, notice in the graph a few "gaps" in the bars? I do. What I see is that the spheroids CAN afterall be grouped into categories by size. This is how I see it based on the graph and data above. I have it grouped ascendingly, though it could also be the other way around.


Class I (Midgets):
Enceladus, Miranda, Mimas

Class II (Small):
Sedna, Orcus, Titania, Rhea, Oberon, EL61, FY9, Iapetus, Charon, Quaoar, Umbriel, Ariel, Dione, Ixion, Tethys, Ceres, Varuna, AW197

Class III (Middle-Sized...Planet Worthy?):
Io, Moon, Europa, UB313, Triton, Pluto

Class IV: (Large)
Ganymede, Titan, Mercury, Callisto

Class V: (Very Large)
Mars (could've been in the next class for simplicity, but it really is alone in close size)

Class VI: (Largest)
Earth, Venus


Maybe I'm out of line here. But that's why I'm posting it here...to see if you all see the groups that I outlined as I see it. What jumps out to me is the relatively noticeable gap between Pluto and the next-largest object.

So now that we have groups (IMO) clearly distiguished...it's a bit easier to draw the line. I think most here would agree it should either be between II/III or III/IV. But which? The former would make Mercury the smallest known planet...latter would include Pluto and UB313.

I'm favoring the latter for a few reasons.

1)Satellites of Class IV and higher are all generally referred to as the "large" satellites, whereas the remaining ones are midsized or chunky. Picking the former would split apart the Galilean Satellites in half for planet qualification...hypothetically assuming they orbit the sun.

2)As I was asking in another thread about atmospheres...generally, Pluto represents the smallest an object can be and being able to maintain an atmosphere.

3)Several objects in Class IV are differentiated and very much planet-like...certainly not any less impressive than the class above. Whereas most Class V objects tend to be more uniform in internal structure.

4)It keeps Pluto as a planet...suits the cultural aspect.


There IS a problem w/ this, however. What happens if objects intermeddiate in the classes are found? It would be hard to fit them anywhere, and they would fill in the gaps that separate the classes in the first place.

If we're only concerned from a planet or no-planet standpoint, this shouldn't be a big deal unless it's between Class III/IV. Since Pluto is the smallest object of its class, it is just barely qualifying for planetary status. The gap may be filled someday as more KBOs are being discovered...but generally, unless it's VERY close in size to Pluto (say 2200km or higher), the easiest is to just shrug at is a close-call but no-die for planet. The good thing is that it wouldn't be too close of a call really...because Pluto is just barely making the cut as it is.

SUMMING IT UP: I typed a lot, but it's not that complicated. Basically, I just see that the known spheroids can be organized into groups by size...and then picked a certain group (the one w/ Pluto and UB313) to be the lower limit for planet status. SO BASICALLY...I see 10 planets. And Pluto IS near the arbitrary limit...scientifically in this case.

Well I hope nobody passed out ready that. Thoughts and other issues I may have not caught would be great.

__________________
This space is for rent.

It looks good for some classification, but I don't know if everybody can accept this and there are other factors.

I'd say obiting the sun directly and having a 2000 km diameter is a good, arbitrary definition for a planet that keeps pluto in and the large moons out. I mean, the difference between a meteoroid and an asteroid is just as arbitrary. (Meteoroids are < 50m, asteroids are bigger) Let's call anything that assumes a spherical shape but is less than 2000 km in diameter a planetoid. Come on people, this isn't rocket science.

__________________
audentes fortuna iuvat

Perhaps anything following the basic rules of planetary composition (rocky planets close to star, huge gas planets further out) should be called a planet, and everything else by other classes - asteroids, KBOs, whatever. I get the feeling that a big reason Pluto is technically a planet right now is for old time's sake...recall the huge controversy when it was kept out of the planet lineup at the Hayden Center in New York.

There's no shame in a spheroid being called a KBO - hey, KBOs are cool - but if it formed due to completely different processes than, say, Neptune or even Earth, then that distinction needs to be made. Same goes for moons, which would keep the Galileans, Titan, etc. out of the fray. I'd say no more planets for our system, keep Pluto in the lineup with an asterisk by it, and recognize that there are just going to be huge chunks of rock out there that are going to keep getting discovered. After all, standards need to be in place when other solar systems are considered. And those standards should take into account a spheroid's composition and history, rather than size.

__________________
These aren't the droids you're looking for.

I wonder what people would have classified Sedna as if it had been in Mercury's orbit?

RBG

If it had been visible with the naked eye, then it would probably be a planet due to the ancient Greeks seeing it as another heavenly "wanderer."

You really should account for composition, temperature, orbital inclination, orbital eccentricity as well as size. Sure, we have solid rocky major planets and rocky minor planets (asteroids). We have fluid gas giant major planets, but much of the material of KBOs may sublimate if in closer orbits, so perhaps we should consider them frozen gas major planets and comets (sub-gravisphere) as frozen gas minor planets. Maybe the gaseous material that didn't form into a gas giant drifted out and froze into smaller spheres. (If a bunch of massive KBOs collided could they heat up enough and have the composition to form a gas giant?)

The idea of isolated objects as planets and group objects as members of a population may not hold up in the outer solar system. We think that the Kuiper belt is a population, but they seem to be more widely scattered than the Asteroid belt. The asteroids are not just at the same average distance but are co-orbital within a narrow band. The near earth asteroids and other asteroids that swing inside of that orbit are not considered part of the asteroid belt, IIRC (maybe I'm wrong). It may turn out that there are many large isolated masses beyond Neptune, but if they have wildly variant eccentricities and inclinations can we still consider them part of a population with similar characteristics? Instead of being a belt with the majority of masses being co-planar and co-orbital, it would extend so far in and out and north and south that it is practically a sphere. A definition so vague becomes useless.

Perhaps we should make a single differentiation between the classic 8 and the rest. From Neptune in the planets are orderly in the same plane and with low eccentricities, with two major subcategories based on composition. These 8 would be the Nuclear or Central Solar system. Everything with eccentricities that take it well beyond neptune and off the ecliptic are part of the Extended Solar System. The classic 8 are the Central (Primary Order) Planets and everything beyond are Extended (Secondary Order) Planets. So now we can keep Pluto as a planet, but make it the first of the new Order. The new discoveries, 2003 UB313, Quaoar, Orcus, would be Second Order Planets. Sedna might be part of a Third Order of Planets if it turns out that the Kuiper belt actually is a distinct population of comet-sized objects.

If you want another definition of planet, perhaps we could claim that a planet is of such a mass and orbit that it can generate a wobble in a G-type star (or m type) equal or greater to the planet's diameter (all things being equal with no other planet's influence). Or maybe earth would not even do that, I'm not sure. But maybe the idea of a planet being a dark object that can gravitationally influence it's primary star is a viable idea.

__________________
"What you think you thought you saw you did not see." Agent J, MiB - Manhatten Bureau

Clearly, planet classification has not been scientific. So, there are nine planets around the sun.

Another question would be how do you classify the idea of a Double-Planet system idea like Earth-Moon or Pluto-Charon

Our definition may allow us to call one a Moon and another a planet, but with future missions like Corot and TPF we will soon be finding very strange objects in Space. So it important to have solid definitions before we classify these objects.

But it's probably true that the reason that it hasn't been done before, is that it is not important.

I understand the role of classification in science. But in this case, I'm wondering if there are any real advantages.

One of the arguments for demoting Pluto was because apparently somebody had "found" Pluto and it wasn't in their minor planets database. Hey, make a new database, and call it "previously identified objects" database.

My stab:

Something that a) orbits a star in a stable orbit and b) is not a star itself, should be considered a planet, regardless of size and shape. Since comets and asteroids are susceptible to disturbances when interacting with larger bodies - which affects the stability of their orbits - they would not have rights to a planethood. Double - gravitationally bound - objects with a stable orbit around the star should be called "double planets" (I remark that, even in this case, both bodies still have an individual orbit around the star, each one performing a kind of cycloid; the shape of their orbits shouldn´t be an issue either).

We could have thousands of planets, considering future discoveries of small Kuiper belt objects. What´s the problem with that?

__________________
If you're careful enough, nothing bad or good will ever happen to you.

So, would you include Ceres, or not? Is Jupiter a quintuple planet?

hmmm, Ceres, yes. Planet Ceres.

Prior to considering the nature of the Jovian system, we have to check out the definition of satellite according to Argos: something that possesses less than 10% the mass of its companion. A satellite cannot be a planet. Thus, Jupiter is a single planet, surrounded by satellites.

__________________
If you're careful enough, nothing bad or good will ever happen to you.

So the Earth and Moon would not be a double planet? But I guess Pluto and Charon would?

Precisely. The Earth-Moon mass ratio is 0.0123, while Pluto-Charon ratio is 0.1496. So, according to Argos´ rules Pluto-Charon is a double planet, while the Moon is a satellite for Earth.

__________________
If you're careful enough, nothing bad or good will ever happen to you.

What about Vesta? Pallas? Hygiea?

If we're trying to base our classification on finding large gaps in size, there's an obvious one which hasn't been mentioned. And which leads to the conclusion that there are four planets, and some insignificant gravel.

Present me an object with a stable orbit (+1 G years) and I´ll call it a planet.

__________________
If you're careful enough, nothing bad or good will ever happen to you.

I sort of like the idea that anything with a 2000km or larger diameter is classified as a Planet, or those sharing elipital plane orbits with other planets, being classified as planets (or minor Planets if under 2000km size) with a few exceptions.

If it's orbiting another larger planent, then it's a moon.

If orbit brings it close engouh to sun where it behaves like a comet, in which case anything from 100km to 2000km size is classified as a Super Comet, and 2000km and up as a Rogue Planet.

__________________
"There is no problem that cannot be solved by a suitable application of high explosives" - US Army Demolitions School

http://worldsofothersuns.home.comcast.net/