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  • 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-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

Pluto: Planet or not? Before we can answer this question we need

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B1 pluto planet solar system jupiter solar belt

Is Pluto a planet?

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    VoiceTube posted on 2013/03/22
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