Things started out really well for Pluto in the eyes of those of us Earthlings. It was formed 4.6 billion years ago with the rest of our solar system, and since then had been hanging out in a distant orbit around the sun. Clyde W Tombaugh discovered Pluto in 1930, and it was named after the Greek god of the underworld, apparently based on the advice of an 11-year old girl. At this point, it became the 9th planet in our solar system, and it remained a planet in the eyes of humanity for many years, the Pizza that our mothers served us in the old school mnemonic:

My Very Excellent Mother Just Served Us Nine Pizzas.

But things started to change for Pluto in 1978. It was then that Pluto’s moon Charon was discovered. The presence of the moon allowed astronomers to calculate Pluto’s size accurately based on Charon’s orbit. It turned out that Pluto was really small. Originally thought to be larger than Mercury, it turns out Pluto was way smaller – only 1/25th the size of Earth and smaller than 7 moons in our solar system: Jupiter’s Ganymede, Callisto, Io, and Europa, Saturn’s Titan, the Earth’s moon, and Neptune’s Triton. That didn’t seem very planet-like…

In 1992, other similarly sized objects near Pluto where discovered. These were considered to be part of the Kuiper belt, a large disc of objects orbiting the sun on the far side of Neptune, like an asteroid field, but huge. It seemed like Pluto was a part of the Kuiper belt, and as more and more objects in this belt were discovered, Pluto started to look less special and less “planet-like”. There were two other objects (Makemake and Haumea) in the Kuiper belt that were 50-60% as large as Pluto.

The turning point came in 2005, when an object called Eris was discovered in the far reaches of the solar system beyond the Kuiper belt. Eris is slightly smaller than Pluto, but it’s denser, so it actually weighs about 25% more. Astronomers were faced with a difficult decision. If they wanted to keep Pluto as a planet, they would have to add Eris, and maybe some others too. The bigger question was, exactly what makes a planet a planet?

In truth, there really wasn’t a strict definition of “planet.” Most people considered a planet be a “large” object that orbited the sun, but now that we were discovering more and more of these objects, we needed to define what “large” meant.

The task fell to the International Astronomical Union (IAU). After much debate, they issued the following definitions to clear things up:

1. A 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) shape, and (c) has cleared the neighborhood 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) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite (e.g. a moon).
3. All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies.”

Pluto, Eris, Makemake, and Haumea don’t meet this new definition of a planet, because they haven’t “cleared” their neighborhoods, meaning that there are a bunch of other objects floating around them instead of a clean “planet-like” orbit.

So in the end, the IAU didn’t actually define “large,” they just changed the requirements for planethood. It may seem arbitrary, but it makes sense  in some ways. Without the “clearing the neighborhood” requirement, Pluto, Eris, and Ceres (long considered an asteroid) would be considered planets, and it seemed like Pluto’s moon Charon might have been considered a planet as well as part of a “double planet” system with Pluto. This is because the two objects actually orbit each other to some degree. To avoid introducing 3 new planets that almost no one had ever heard of (all smaller than our moon), the IAU decided instead just to kick Pluto out of the club. No more pizza for us, only Nachos:

My Very Excellent Mother Just Served Us Nachos

So where does that leave Pluto now? There is some controversy around the “clearing their neighborhood” clause, in particular because several real planets (including the Earth) do have nearby asteroids, but these complaints haven’t changed the IAU’s mind. It seems like there is no coming back now. Changing the definition would probably make things unnecessarily complicated and add a bunch of additional planets for kids to remember for no apparent reason.

Certainly, Pluto could go ahead and clear it’s neighborhood. That would show us! However, it’s had 4.6 billion years to clean things up, and it hasn’t done so – things aren’t likely to change in the Kuiper belt over the next few billion years either. So Pluto appears to be resigned to dwarf planet status for good, but at least it had a good run as a planet for a while, right?