Eris, the icy ball bigger than PlutoLast night was the conference banquet and after party. Many of us stayed up too late. Many of us looked at our conference program this morning in hopes that maybe something boring was going on that we wouldn’t mind sleeping through. All of us dragged ourselves out of bed and came to the 8:30am Sunday morning talk. Michael Brown, the premier finder of small icy-rocky things in the outer solar system, is giving a talk titled, “How I killed Pluto and why it had it coming.” How can anyone sleep through a talk like that?

He just showed us digitized versions of the original discovery plates for Pluto and it is now talking about how odd Pluto was known to be even at the moment it was found by Clyde Tombaugh. Everyone was looking for another Neptune or Uranus sized object. Instead, they found something smaller than the moon! This wasn’t expected, and astronomers at the time of its discovery didn’t have the right noun to attach to this new object. It wasn’t an asteroid – It reflects too much light and isn’t located in the parts of the solar system we expect to find asteroids. It wasn’t a comet – It doesn’t come into the inner part of the solar system grow a tail, and go back out (although we now know that if it came into the inner part of the solar system for some reason it would indeed grow a tail). When your only choices are comet, asteroid or planet, Pluto becomes a planet.

But today, we have more bins to throw objects in: Kuiper Belt objects, Oort Cloud objects, asteroids, planets (and various sub-divisions of each). And with these new bins it becomes important to consider the labels we give things.

But more on that later.

So Clyde found Pluto. If Pluto belongs to a non-planetary bin, where are the other objects. Well – Brown is finding them today. Next question: Why did it take so many decades to find other objects in the outer solar system? Well, the technology made it hard. Brown pointed out that Tombaugh’s images covered roughly the area on the sky of a hand (palm fingers and all) held out at arms. Modern images, however, cover an area the size of a small freckle on a hand held at arms length. When you are searching the sky a freckle at a time, it is hard to find things quickly. So, we are more sensitive to things we were never sensitive to before, but it is more time consuming to find them.

Currently there are 1100 other icy objects known to exist in the outer solar system. Some (or at least 1 named Eris) are bigger than Pluto. Most are smaller. They mostly have these crazy orbits that are tilted and stretched into ellipses. Kuiper Belt objects orbit in a swarm, buzzing about Neptune.

So, when you look at Pluto as just one of many objects darting around Neptune, and when you recognize Pluto’s not the largest of these objects, you need to work on defining bins. If Pluto is a planet, what other smaller objects are also planets? Or perhaps Pluto is not a planet? According to the International Astronomical Union, none of these tiny objects are planets. They are Kuiper Belt objects, plutions, dwarf planets… They are many things other than planets.

So what is a planet? Well, if you want the scientific reasons, go listen to our Astronomy Cast podcast on the topic. Let’s instead imagine, as Brown is telling us to imagine, that you are an alien flying into solar system. As you fly through you see this swarm of small icy things in the outer solar systems followed by these giant gassy blobs. Between the round gas blobs and the rocky worlds are potato shaped giant rocks. The rocky worlds then orbit the sun. An alien would dump things into 4 general bins, big round rocks orbiting the sun, small randomly shaped rocks forming a belt, gas giants, and icy swarming objects. Since the terrestrial planets and gas giants both orbit in orbits with nothing else (other than their moons), you can see these as two parts of the same (we call it planet) bin. At the same time, the asteroids and Kuiper Belt objects – the stuff that swarms – can be seen as two parts of the same (we call it minor planet) bin.

Pluto, left to an outside observer, just wouldn’t get called a planet. This, to me, is the best explanation of why it is silly to call Pluto a planet. One of these objects just isn’t like the other ones. That would be Pluto in a picture of the planets. Put it in a picture of Kuiper Belt objects and now it fits.

Pluto, get over yourself. You’re not a planet and you never were. Someone just misfiled you.

Interesting parting bit: Brown has been hinting at a new large (think Sedna) object may be getting announced in the not too distant future. Gotta love hints.

So, is anyone still wondering “Why is Pluto not a planet?