Is Pluto a planet?

Pluto: Planet or not? Before we can answer this question, we need to know what the word "planet" is for, and that takes us back to the ancient Greeks who called Mercury, Venus, Mars, Jupiter, Saturn, the Moon, and Sun planets. Basically, if it moved across the sky and was bright, it was a planet. This is a terrible start for the word because, 1) it excludes Earth from the list, and 2) it groups wildly different things together.

But the Greeks couldn't know how different the Moon was from Saturn because the best technology they had to observe the Universe was sadly limited. It would take several thousand years until the industrious Dutch made the first telescopes, and astronomy became much more interesting. Astronomers could now confidently rearrange the solar system — an elegant scientific advance that no one could possibly object to — and reclassify its parts, dropping the Sun and Moon from the list of planets and adding Earth. Now, if it orbited the Sun, it was a planet.

As time went on and telescopes got better and better, each new century brought with it the discovery of a new planet. Which brings us to this familiar solar system: nine planets orbiting one star. And looking at this model makes people wonder, why do astronomers want to ditch Pluto? The problem is pictures like this in textbooks are lies. Well, not lies exactly, but unhelpful. They give the impression that the planets are similar in size and evenly spaced, but the reality couldn't be more different.

Here, dear Terrans, is our home planet Earth, and this is Jupiter next to it at the correct scale — rather bigger than you probably thought. If we take this diagram and adjust for the correct sizes of the planets, it looks like this. Unless you're watching the video in fullscreen HD mode, you might not even be able to see Pluto. So size differences are vast, and Pluto is the smallest by far. But it's not just small for a planet; it's also smaller than seven moons: Triton, Europa, our own Moon, Io, Callisto, Titan, and Ganymede.

Even if you show the correct relative sizes, the distances are still a problem. Think about it: if Jupiter was this close to Earth, it wouldn't look like a dot in the night sky but would be rather overwhelming — so it must be really far away, which makes drawing it to scale rather a challenge. If you want the length of a piece of paper to represent the distance from Mercury to Pluto, then giant Jupiter would be the size of a dust mite on that page, and Pluto, a bacterium.

But excluding Pluto from the planet club just for being tiny and far away isn't reason enough and quickly brings out the Pluto defenders. In order to understand what Pluto really is, we need to first discuss a planet you've never heard of: Ceres. Back in 1801, astronomers found a new planet in the huge gap between Mars and Jupiter — it was a small planet, but they loved it anyway and named it Ceres. The next year, astronomers found another small planet in the same area and named it Pallas.

A few years later, they found a third one, Juno, and then, funnily enough, a fourth one, Vesta. And for several decades, children learned the 11 planets of the solar system. But astronomers kept finding more and more of these objects and became increasingly uncomfortable calling them planets because they were much more like each other than planets on either side. So a new category was born: asteroids in the asteroid belt — and the tiny planets were relabeled, which is why you've never heard of them.

And it was a good decision too, as astronomers have now found hundreds of thousands of asteroids, which would be a lot for a kid to memorize if they were all still planets. Back to Pluto. It was discovered in 1930, making it the 9th planet. First estimates put Pluto about the size of Neptune, but with more observations, that was revised down, and down, and down. While Pluto shrank, astronomers started to find other, similar objects orbiting in the same zone.

Sound familiar? While school kids kept memorizing the nine planets, some astronomers grew uneasy about including Pluto because the size estimates continued to shrink. They learned that Pluto is made mostly of ice, and they continued to find lots and lots of icy objects at the edge of the solar system just like Pluto. This problem could be ignored as long as no one found an ice ball bigger than Pluto, which is exactly what happened in 2006 with the discovery of Eris.

Once again, astronomers recategorized the solar system and grouped these distant objects, including Pluto, into a new area called the Kuiper belt. And that's the story of Pluto — a miscategorized planet that finally found its home — just like Ceres. But this story is really less about Pluto than it is about realizing the word 'planet' isn't very helpful.

The first four planets are nothing at all like the next four, so it's even a little weird to group these eight together, which is why they often aren't and are separated into terrestrial planets and gas giants. And now that we have telescopes that can see planets around stars not our own, and we've found rogue planets drifting in empty space and brown dwarfs — objects that blur the very line between planet and star — the word "planet" becomes even less clear.

So as we increase our knowledge of the Universe, the category of 'planet' will probably continue to evolve, or possibly, fall out of favor entirely. But, for the time being, the best way to categorize the stuff in our solar system is into one star, eight planets, four terrestrial, four gas giants, the asteroid belt, and the distant Kuiper belt, home to Pluto.