Pluto -by popular demand

I've gotten some requests to clarify the Pluto situation for my non-geology friend so here it is:



over the last 15 or so years if you asked any astronomer if Pluto was a planet, they would say no. I called it a "psuedo planet" in my astronomy class. I actually even had a favorite exam question asking students to explain why or why not Pluto was a planet. Its alwaybeen the one that doesn't belong. Recall that Sesame Street song "one of these things is not like the others..."? That is Pluto. Not terrestrial (rocky) like Earth, not Jovian (gassy) like Jupiter. Just an icy ball with an irregular orbit and a really big moon.



With the discovery of other large, nearly round objects in the outer Solar System, the IAU (international astronomical union) decided it was time to have a set definition for what a planet was. At there recent big meeting they presented a draft, had some heated discussions ( I guess some people are very attached to the idea of Pluto as a planet) and finally decided on a definition. The original draft included Pluto, Ceres (an asteroid in the asteroid belt between Mars and Jupiter) and an object called 2003 UA313 (hasn't been named yet) as planets. If this definition held we would have gone from the nine traditional planets to over 53!

This is what the IAU came up with:

The IAU therefore resolves that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:



(1) A "planet"1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.



(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.



(3) All other objects3 except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".



This means that there are eight planets. Pluto, Ceres and 2003 UA313 are dwarf planets.

I kind of like this definition.

Additionally the IAU decided:



Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.1



So Pluto is actually the flagship for this new class of bodies and is more significant than it was before.



So for all you kids who had mnemonics to remember the planets:

My Very Eager Mother Just Served Us Nachos

instead of Nine Pizzas