All of you readers out there (all three of you!) may remember that Pluto has been demoted from the lofty position of "planet". Indeed, it was quite the ruckus when it happened, and now everybody is struggling to figure out how to end the sentence: "My Very Excellent Mother Just Served Us Nine ..." (it used to be "Pizzas!") This isn't the point of this post.
I just figured out what Pluto really is now. I was confused by the terminology, and kept telling people that Pluto is now a "minor planet." Not so! Pluto is officially classified as a "dwarf planet". According to Wikipedia:
A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but which has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass for its self-gravity to overcome rigid body forces in order to assume a hydrostatic equilibrium and acquire a near-spherical shape.
Got that? So, it's gotta orbit the sun, clear most of it's neighbors away, and not be orbiting some other larger, non-sun object. According to this same article, this means there are now officially four "dwarf planets", namely:
-- Ceres, a big honkin' rock in the asteroid belt. It was discovered in 1801 (45 years before Neptune!) and was considered a "planet" (my, how we miss those simple days) before being reclassified as an asteroid. It's just under 1000 kilometers in diameter.
-- Pluto, our previously demoted planet. It was discovered in 1930 and was considered a "planet" until two years ago. It's just over 2300 km in diameter.
-- Eris. It was discovered in 2003 and was the reason this whole "planet" thing got kicked off in the first place. People just couldn't find it in their hearts to make room for a tenth planet. It's about 2400 km in diameter (um, yeah, that's bigger than Pluto).
-- Makemake. It was discovered in 2005 and was just classified (and officially named) two weeks ago (July 11, 2008). It's about 1500 km in diameter.
There's a whole pile of other objects out there contending to go into this category, including Pluto's very own "moon" called Charon. Add to this all the confusion surrounding "minor" planets (a term which used to be more widely used but has fallen out of favor in lieu of the far-less-simple-to-say "small solar system bodies" or, acronymically, "SSSBs!", pronounced, um, "sssbs!") which can also be variously considered "asteroids," "centaurs," or "trans-Neptunian objects", and you've got a whole pile of names for the little rocks and ice balls zipping around our solar system.
As for me, there's plenty of room in my heart for as many planets as we can find, dwarf, minor, or otherwise. At least now, though, I know that Pluto is indeed a dwarf planet! (And not that ... other ... thing.)