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 the 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 one, it excludes Earth from the list, and

two, 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 the 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. 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-ish in size and evenly-ish-spaced, but the'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 full-screen HD mode, you might not be even 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 nine 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 small 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. Series. 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 series.

The next year astronomers found another small planet in the same area and named it Palace.

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