
On this date (Aug 24), in 2006, sentimental geeks the world over were heartbroken to discover that their favorite little ball of ice & rock was demoted.
For more than 76 years, Pluto enjoyed its status as the 9th planet in the Solar System. It was one of the big dogs – granted, it was by far the smallest, but it was in the cool kids’ club. There can only be one sun in our system, so the best chance for a hunk o’ rock to be in an exclusive clique was to be a planet!
After all, despite its small size, Pluto is pretty cool. It has 5 known moons: Charon, discovered in 1978; Nix and Hydra, discovered in 2005; S/2011 P 1, discovered in 2011; and S/2012 (134340) 1, just discovered last month. Five moons! Yeah, sure, Pluto and all its moons together aren’t as big as our moon, but still… that’s pretty cool!
Of course, there’s still some controversy there for geeks like me… ya see, the barycenter of the orbits of Pluto and Charon doesn’t actually lie within either of them – it’s somewhere in between – so technically they would be a binary system. But the International Astronomical Union (IAU) doesn’t have a formalized definition for binary dwarf planets – so for the moment, Charon is officially still a moon.
Anyway, back to the important stuff… Pluto’s demotion!
In the late 70s, astronomers began to discover other little bits o’ ice & rock in a region that became known as the Kuiper belt – an area that ranges from 30 AU to around 50 AU from the sun. For those who weren’t nearly named after actors from the original Star Trek series, AU is astronomical unit – just under 93 million miles, the approximate mean distance between Earth and the sun. So the Kuiper belt is a long way away.
As more hunks o’ rock were found in the Kuiper belt, more and more scientists began to question Pluto’s place in the cool kids’ club. Then, in 2005, they found Eris, a hunk that’s 27% larger than Pluto. That was the beginning of the end for our little underdog.
Serious debates ensued – some argued for Pluto to be demoted, and some argued that Eris and all the other hunks out there should just be added to the list of planets.
But really, if you keep adding kids to the cool kids’ club, before long, it just isn’t cool anymore. So, on August 24, 2006, the IAU (remember them from a few paragraphs ago? The International Astronomical Union?) redefined what it means to be a “planet” within the Solar System. This new definition excluded our poor little Pluto, and put it in a new category called “dwarf planet” along with Eris, Ceres, and a few others. Members of the minor planet category have numerical designations, so now Pluto’s official name is 134340 Pluto, and thanks to that bully Eris, Pluto is only the second-most-massive dwarf planet in the club.
So today is the sixth anniversary of Pluto’s demotion. It’s a sad day. I was thinking of flying my astronomical telescope array at half mast… but two things stopped me. First, I don’t actually have an astronomical telescope array. And secondly, well, at least Pluto got to be in the cool kids’ club for 76 years… that’s not something everyone can say!
But for me, and for many sentimental geeks out there, Pluto… you will always be our 9th planet!




