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-ash in size and evenly-ish 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's 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 plant 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 the 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 a 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 the 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 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 Kuiper belt. And that's the story of Pluto -- a miscategorized

planted 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�