Planets in order largest to smallest diameter counting just some of the ice dwarfs and asteroidal / rock dwarfs candidates :

Jupiter, Saturn, Ouranos, Neptune, Erath, Venus, Mars, Mercury,

*Pluto* (# 9 now upgraded from #10)

Eris, Makemake, Haumea, Sedna, Quaoar, Varuna, Orcus, “Buffy” or 2004 XR190, Ceres, Huya, Ixion, 2002 TC302, “Snow White” or 2007 OR10, 2002 AW197, Vesta and maybe Pallas.

So that’s 15 or 16 worlds *smaller* than Pluto versus only eight that are larger. Not so small relatively at all but right in the middle of the planetary size range! ;-)

PS. No, I wouldn’t make the schoolchildren learn all the names of the likely 30 + planets – just the key one’s incl. Pluto and breaking them into major types : the gas giants, the rocky terrestrial types and the ice dwarfs. Or the gassy, the rocky and the icy! ;-)

@ Laurel : Seconded – great comment there. I think @plutosavior is a great username although I’m not on twitter just email, facebook and here. (I was posting formerly as ‘StevoR’ btw.) Wishing you all the best & please let me know if I can help – you have my full support on this issue. :-)

Pluto and its three moons deserve more respect than that from someone who claims to be a professional astronomer. This is the best story of 2010. What beautiful poetic justice!

Please consider coming to Honolulu in August 2015 to fight for the replanetization of Pluto during the IAU’s trienniel General Assembly. I hope to be there.

Also, check out my space art and other art on my page. I have made art from New Horizons images, too, during its Jupiter encounter.

Nice post, Laurel! Would love to see high res photos of Eris and its moon, even though it is a pipsqueak planet! LOL!

Pluto and Eris are both planets and Kuiper Belt Objects. One does not preclude the other. They are planets because they are large enough to be rounded by their own gravity. They are Kuiper Belt Objects because they are located in the Kuiper Belt. Ceres too is a small planet because it is large enough for its gravity to pull it into a spherical shape. The IAU misappropriated the term “dwarf planet,” which was first coined by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto, to indicate a third class of planets which are 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 be classed as not planets at all. The IAU did not “have” to do anything other than allow Eris’s discoverer to name it while holding off on any additional classification until more information is discovered about remote planets in this solar system and all planets in other solar systems.

Significantly, there are quite a few exoplanet systems in which multiple planets orbit the host star in various different planes. Some have orbits far more eccentric than Pluto’s, yet they are giant planets the size of Jupiter or larger. According to the IAU definition, none of these objects are planets!
Saying there are more differences between Pluto and the eight closer planets to the Sun depends on what aspects one considers. Earth actually has far more in common with Pluto than with Jupiter. Both have surfaces on which we can place rovers and landers. Both have a large moon formed by giant impact; both are geologically differentiated into core, mantle, and crust, and both have nitrogen in their atmospheres. Other than orbiting the Sun, what do Earth and Jupiter have in common?

It is premature to pronounce declarations that these faraway objects are definitively not like the other planets or that one is larger than the other. We just do not have enough data at this point to do more than make educated estimates. What we really need to do is send robotic missions like New Horizons to Eris as well as Haumea and Makemake. Yes, that will take time and money, but it is a far better investment than the black holes the endless wars in Iraq and Afghanistan have become.

Also, memorization is not important. It is much more important to teach the characteristics of each category of planet than to ask kids to memorize a bunch of names. We don’t ask them to memorize the names of rivers or mountains on Earth, so why do so with planets, and why allow a need for convenient memorization to determine how we classify them?

Note: I tweet as @plutosavior

A dwarf star is still a star so why isn’t a dwarf planet counted as a planet?

Moreover, NO planet could clear an area the size of Pluto’s orbit – not even Jupiter.

Plus Pluto *does* gravitationally dominate its local region – hence its three moons one of them relatively huge! ;-)

I’d define planet as :

1) Rounded by self-gravity thus too big to be an asteroid or comet,

2) But too small to ever shine by fuse elements at its core thus not a star or brown dwarf,

&

3) Not orbiting another planet but the Star thus not a moon.

It then make sense, I think, to divide this broad category of planets into sub-classes – the gassy, the rocky and the icy, Hot Jupiters, SuperEarths, gas dwrafs, ice dwarfs, water worlds and so forth. Instead of teaching all the planets name we then teach kids the sub-classes and a few examples of each eg. Jupiter, Earth, Pluto, 51 Pegasi b (aka “Bellopheron”), Gliese 581 g, etc .. as examples.

The whole “orbital clearnance”criterion strikes me as raising and creating more problems and confusion than it solves. It also seems superflous & really pretty needless thus violating Occams razor.

I think the IAU got it badly wrong and the sooner they correct their error on the Pluto / Planet question the better.

Besides, if Pluto ever feels bad about its status, it always has Charon’s gravitationally locked shoulder to cry on.

A Pluto by any other categorization would still be as big…