Pluto is NOT surrounded by larger KBOs, as there are no known KBOs larger than Pluto. It is surrounded by tiny, asteroid-sized KBOs, just as Ceres, which is spherical, is surrounded by much tinier objects, none of which are large enough to attain hydrostatic equilibrium. Pluto is both a KBO and a planet; the first indicates where it is, and the second indicates what it is.

Notably, Dr. Alan Stern coined the term “dwarf planet” in 1991 to refer to a third class of planets in addition to terrestrials and jovians, small objects large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never intended for dwarf planets to not be considered planets at all. Significantly, in astronomy, dwarf stars are a subclass of stars, and dwarf galaxies are a subclass of galaxies. So too, in spite of a decree by four percent of the IAU, dwarf planets are a subclass of planets.

This means that the only objects in the path of the planet’s orbit must be the planet’s moon(s), or any other smaller objects, such as asteroids about to crash into it. Take the earth. There are no objects that share our orbit around the sun, or block our orbit of the sun, save our moon, or an asteroid or meteoroid about to crash into us.

Pluto, however, is a KBO, surrounded by other larger KBOs, meaning that Pluto has yet to clear the neighbourhood of its orbit, and therefore is not a planet. This also goes for Ceres. It’s surrounded by asteroids who share its orbit and are not under its gravitational influence, therefore it is not a planet either.

… Pluto never stopped bieng badass to me.

hny-happy new year

– Kevin Heider

Any definition can be nitpicked. It would also be weird to call Pluto a planet based on geophysical characteristics and then exclude Triton, Titan, Europa, etc.

I still prefer to classify planets as either special (the 8 major dynamical bodies), or as a very inclusive group including spherical secondary planets. In the latter case we would already have about 70+ planets in the Solar System. We would have 8 dominant, ~18 secondaries, and 40-60 likely dwarfs.

– Kevin Heider

And the IAU’s stupid definition is not one of dominance, per se but rather explicitly one of orbital clearing. That’s why it’s silly.

We have found exoplanet systems with two giant planets orbiting in a 2:3 resonance with one another. By the IAU’s definition, neither would be considered planets. Does this make sense?

Why not just keep the term dwarf planet as a subclass of planets that are spherical and orbit a star but do not gravitationally dominate their orbits?

Pluto has an orbit that is controlled by Neptune (in a 2:3 resonance)

So from a dynamical stand-point, Jupiter and Neptune are dominant objects. Pluto does not dominate Neptune.

Yeah, Jupiter is a planet, and so is Pluto.

we will miss yuuuuuuu

A few comments about your comments…

-Thanks Yakumo Fuji for defending the cumulative effect building to badass status
-I didn’t expect such a heady debate, but it’s better than threadcrapping!
-Thanks to all you two initial folks for defending my assertion that Plutonium was named after the planet. Even if it’s not true (I believe it is, though) it still has a net positive effect on the badass quotient (BQ)
-L., Pluto is still a planet in my heart as well.

In 2011, Dawn will get to Vesta, and we will learn a lot about it, including information that will show it may be differentiated geologically and be more planet-like than formerly believed.

Classifying Pluto and Ceres as planets helps distinguish these objects from asteroids, which is important. As for the layout and formation of the solar system, even some of the largest planets are believed to have formed in places different from their current locations. Saturn, Uranus, and Neptune likely formed closer to the Sun and were pushed further out by the impact of Jupiter. An important issue to study is how and why objects like Pluto grew large enough to attain hydrostatic equilibrium and what stopped the intermediate objects, like Pallas and Vesta, from getting this far.

Also, Pluto is not just a ball of ice. It is actually estimated to be 70 percent rock. Earth is far more like Pluto than like Jupiter. And as said before, if you study the 400+ exoplanets found so far, in many systems, you will see several giant planets all orbiting in different planes. That does not make them not planets.

The other objects in a 2:3 orbital resonance with Neptune are much tinier, and none of them are spherical. Resonance orbits have been found in exoplanet systems where two giant planets orbit in 2:3 resonance with one another. These are gas giants larger than Jupiter. And resonance orbits don’t make something an asteroid. There are asteroids in very circular orbits between the Mars and Jupiter. Asteroids can have circular orbits just like planets can have elliptical orbits. This goes back to the fact that we cannot define an object solely by where it is. We also have to take into account what it is. Otherwise, we would end up with gas giants bigger than Jupiter in other star systems being called asteroids! The so-called “demotion” was wrong because it overlooks the issue of what an object is, classifying it only by where it is.

Classification schemes ought to help one understand the objects being classified. Calling Pluto, Eris, Ceres “planets” ignores the fact that they are in shared orbital spaces. Classifying them as KBO’s and asteroids because of their orbital natures leads us to understanding the layout and formation processes of the solar system.

Oh, and how is a reclassification a “demotion”? Is Pluto’s salary any less now…?

Classifying it as a KBO helps in all three. Thus, just as with Ceres, Pluto gets reclassified.

http://en.wikipedia.org/wiki/Kuiper_belt

On another point: how is being reclassified from “planet” to “KBO” a “demotion”?!