Quote from: Star One on 06/25/2015 03:43 pmI personally think the debate will recede if further planets in between the Keiper belt and Oort Cloud are discovered as it kind of becomes irrelevant then.Er, define "planets" to be discovered there.Mike Brown has said--I think somewhat tongue-in-cheek--that he would not be surprised if a Mars-sized object is found way out in the Kuiper Belt. I think it is certainly possible to find something as big as, if not bigger than Pluto. But if something significantly larger than Pluto is found, it's going to really force a reevaluation of the definition of "planet."But wouldn't that be f'ing great?!
I personally think the debate will recede if further planets in between the Keiper belt and Oort Cloud are discovered as it kind of becomes irrelevant then.
Actually, IIRC, one of the lesser-known definitions of a planet in the current IAU codes is that a planet orbits the sun within the plane of the ecliptic. (They must have given a plus-or-minus in there somehow, since I know the "eight planets" vary from that plane by, what, up to five degrees?)Pluto, of course, has an orbit that's well... different. Which leads me to the best reason I ever read as to why, under the new IAU rules, Pluto is not a planet:"Because it is not so inclined."
What Mr. Russell has done is bypass the politics and get back to first principles, and I think his work has given superior results.
Also, as to Nilof's point, I know that when material is spread out into an orbit around a body, be it a planet or a star, the material fairly quickly organizes itself into an equatorial ring. The orbital characteristics cause the "sports" in higher-inclination orbits to be pulled into the equatorial plane. It breaks down a certain distance from the primary, but as far as Sol is concerned, the material that organized itself into orbit around our star back nearly five billion years ago organized itself into a pretty thin ring or disk of material that seems to have extended out at least as far as Neptune, and all of the eight currently recognized planets evolved into final orbits no more than a few degrees outside of this plane in inclination.
Quote from: the_other_Doug on 06/26/2015 06:20 pmAlso, as to Nilof's point, I know that when material is spread out into an orbit around a body, be it a planet or a star, the material fairly quickly organizes itself into an equatorial ring. The orbital characteristics cause the "sports" in higher-inclination orbits to be pulled into the equatorial plane. It breaks down a certain distance from the primary, but as far as Sol is concerned, the material that organized itself into orbit around our star back nearly five billion years ago organized itself into a pretty thin ring or disk of material that seems to have extended out at least as far as Neptune, and all of the eight currently recognized planets evolved into final orbits no more than a few degrees outside of this plane in inclination.The fact that planets migrate as the disk becomes thinner tends to break the nice pattern of small relative inclinations as they can end up scattering each other during the migration. The major planets in our solar system happen to have survived with a low inclination, but there is nothing that says they had to survive, and even main belt asteroids are seriously inclined compared to the planets which is not compatible with the naive picture that everything is in the plane of the old disk.In a large portion of other solar systems, the planets themselves do have strongly inclined orbits to each other. This is almost always the case for systems that have a hot Jupiter. Any definition of a planet should be applicable to other solar systems than our own.
Never before in history has there been so much debate about terminology.
So it is important to debate this.Back in Christopher Columbus's time, it seems that there was not enough debate about whether the Earth was flat or spherical.
Back in antiquity, there was probably a large majority that actually believed the Earth was spherical. Then there was the minority that thought it was flat. Obviously both were wrong and the Earth is more ellipsoidal.
So having a good debate about whether Pluto is a planet or a dwarf planet is important to help guide the future state of how the glossary section is written.
If history is an indicator, Pluto will not be termed a planet or dwarf planet in the future. I'm sure that the scientists of the future will look back into our days thinking of us as odd and say "why did scientists actually believe Pluto was a planet, or even a dwarf planet?". I think the future will define the object type from details on how the object was formed. Asteroids seem to imply that a planet was destroyed and is simply debris from that process. Planets seem to imply formation thru some form of hot magma condensing plus a large amount of bombardment. Then there seems to be objects that have appeared from the Oort cloud. If a rogue planet was captured from interstellar space, obviously that should be termed differently than something like a simple native planet formed in our solar system.Size doesn't matter.
Yet we draw the line when it comes to grains of sand. Nobody would reasonably go running around a beach in the attempt to identify each grain, let alone classify the differences.
Quote from: Mr. Scott on 06/27/2015 04:41 amYet we draw the line when it comes to grains of sand. Nobody would reasonably go running around a beach in the attempt to identify each grain, let alone classify the differences. I have a geologist friend you should meet Believe me, they've done it.The utility of terminology is mainly in its ability to be as specific or vague as is needed in the context of whatever conversation you're having. In general conversation, "moon", "planet" & "star" are just fine, but if the terminology used by the experts is based on arbitrary factors, it doesn't remain in use for long. New Horizon's exploration of the Pluto system and Dawn's exploration of Ceres will no doubt teach us a lot more about "dwarf planets" and (I suspect) add further material to the argument that this is a poor label. Give it a year or two and I reckon the IAU will be compelled to supply another definition.As Mr Russell discusses in his definition paper linked above, you can have bodies in the size range of giant planets/brown dwarfs which form by different processes (gas collapse vs accretion in a planetary disk), so by using a different label, that immediately communicates certain things about the body you're talking about.Likewise, I suspect they'll eventually have a different category for moons that are captured objects (as is suspected about Phobos/Deimos) versus moons that resulted from an impact and accretion around a planet (like our moon). It tells you much about the expected composition of the body in question, but they are still moons (or "planetary-mass" moons as I suggested).
I still like the term "moon" or "satellite" for a body in orbit around a planet (i.e., in orbit around a body that is itself in orbit around a star). I'd go for "dual planet" or "double planet system" if the barycenter of the orbit is outside of the sphere of the primary, as I believe it is between Charon and Pluto, but is not between Luna and Earth.