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  • Now listen up. My very eager mother just served us nine pizzas. Ooh, yum! Hey, if you were

  • in elementary school before 2006, there's a good chance you had to memorize something

  • similar to that sentence. This mnemonic device was used to teach children the order of the

  • planets in our solar system; Mercury, Venus, Earth, Mars, Jupiter, Saturn, URinusalso

  • pronounced UrANUS (yeah, you can’t hide from it either way), and finally Neptune,

  • and Pluto. Now, if you're currently in elementary school,

  • you might be saying, "Wait, there were nine planets?” before going back to playing Fortnite

  • Go or whatever kids are into these days. So, what happened to Pluto? It’s not like

  • it’s gone anywhere. It’s still out there on the edge of the solar system, as cold and

  • far away as ever, so what changed? Pluto hasn't, but our understanding of it

  • has. We know way more about space than we did one hundred years ago. Pluto itself was

  • only discovered in 1930, so it's not like the planet lineup hasn't been modified before.

  • Still, that doesn’t change the fact that there’s something inherently strange about

  • a planet being demoted. Who even knew that was a thing that could happen?

  • What do you think? Should Pluto be a planet or is it right where it belongs. Give me your

  • opinion in the comments. Things might get a little clearer once we

  • figure what the word planet means. The exact definition has changed a lot over time, but

  • from the age of Galileo to the nineteenth century, it referred to any object orbiting

  • the Sun. This might seem a little vague, but worked perfectly well until the year 1801.

  • That was the year astronomers discovered Ceres, a planet, in massive air quotes, orbiting

  • the Sun halfway between Mars and Jupiter. You may recognize this as where the Asteroid

  • belt is, and it isn’t because Ceres pulled an Alderaan and broke into a thousand pieces.

  • If it had, I’m pretty sure kids would pay a lot more attention in science class.

  • Anyway, astronomers noticed right away that Ceres was quite small, with only half the

  • radius of Earth's Moon. The year after Ceresdiscovery, the astronomy

  • community was abuzz with the discovery of another planet named Pallas. Then they found

  • another a few years later. And another. And another. And I think you see where this was

  • going. Four new planets are one thing...

  • Well, four things, but you get the idea. When it turns into thousands, it might be

  • time to reevaluate some definitions. Astronomers noticed that the rocks had more in common

  • with each other than any of the other planets. They were tiny; they were barren; and the

  • vast majority weren't even spherical. These small objects became known as the asteroids

  • in the asteroid belt, and the world went back to learning the seven planets, Mercury, Venus,

  • Earth, Mars, Jupiter, Saturn, URinus (UrANUS, whatever…). Neptune made eight in 1846,

  • with Pluto joining the party eighty-four years later.

  • With the benefit of hindsight, it might be easy to guess that Pluto would go down the

  • same path as it’s long lost cousin Ceres. But we all know what they say about hindsight,

  • and there are things we know about Pluto now that weren't obvious in the early twentieth

  • century. For example, while we know Pluto is nothing more than a tiny ball of ice and

  • rock, initial measurements gave it a similar size to Uranus or Neptune.

  • Pluto’s size was revised down to that of the Earth the year after its discovery. In

  • 1948, it shrank again. It would keep shrinking until Astronomers were finally able to get

  • an accurate measurement in 2006. We know that Pluto is only one 459th the size of planet

  • Earth, making it smaller than the moon and only about twice the size of the former planet

  • Ceres. Pluto’s planet status was in trouble long

  • before that, however. In 1978 astronomers discovered Pluto's moon, Charon. At first,

  • this might seem to be strong evidence in Pluto's favor. If it's big enough to have a satellite,

  • it must be a planet, right? Not exactly. Charon may be smaller than Pluto,

  • but not that much smaller. One half the diameter might seem like a big difference, but not

  • compared to the differences in size between the other planets and their moons. In fact,

  • they're similar enough in mass for Charon to noticeably affect Pluto's orbit around

  • the Sun, causing it to wobble to and fro as it travels through space.

  • That’s some very un-planet like behavior, and it led more than a few astronomers to

  • feel uncomfortable about using that word to describe Pluto. And they got even less comfortable

  • every time a new, Pluto-like object was discovered beyond Neptune's orbit.

  • Still, Pluto had been on the list for decades by this point, so not everyone was ready and

  • willing to give it the boot. All of that changed with the discovery of Eris in 2005. While

  • Eris is slightly smaller than Pluto, initial measurements placed it as somewhat more massive.

  • This added one more strike against Pluto’s status as a planet, and in 2006 the International

  • Astronomical Union decided it was once again time to revise their definition of what is

  • or is not a planet. From then on, an object was only a planet

  • if it fits the following three qualifications. First, it must orbit the Sun. Number two is

  • that the object must be a sphere, or at least nearly so. Pluto checks the first two boxes

  • but runs into trouble with number three, which says a planet must have "cleared the neighborhood"

  • around it. Clearing its neighborhood means that there

  • are no nearby objects other than its own satellites. Pluto has failed to accomplish that feat,

  • so the third box remains unchecked. One strike might be okay in baseball, but

  • it’s a deal-breaker if you're trying to stay on the exclusive list for Club Planet.

  • Now, not everyone was thrilled to find out Pluto got kicked out. It's hard not to feel

  • bad for the little guy, and even today, there are a few scientists who disagree with the

  • IAU’s ruling and want to call Pluto a planet once more.

  • They propose that the first and third qualifier be removed. Under this definition, any object

  • with enough mass to maintain a spherical, or nearly spherical, shape would qualify as

  • a planet. While this would let Pluto back in, it would also let in our Moon, as well

  • as several of the moons of Jupiter, Saturn, and Neptune. That's not to mention Ceres,

  • Charon, and a whole bunch of asteroids and other objects. All combined, this new definition

  • would take us from a manageable eight planets to an unwieldy one hundred and fifteen.

  • Just imagine the pneumonic device you would need for that.

  • Now, the current definition is far from perfect. As some astronomers have pointed out, it excludes

  • rogue planets not orbiting any star. Some feel that it also puts too much emphasis on

  • what surrounds the perspective planet instead of the worlds themselves.

  • To quote Ethan Siegel from Medium.com, “Mercury, at the distance of Jupiter, would never clear

  • its orbit and wouldn’t obtain planetary status. A world much smaller than Mercury

  • could be a planet around a red dwarf star, while even Earth would fail to be a planet

  • if it were out in the Oort cloud somewhere.” On the other hand, what do you do about stars

  • such as Proxima Centauri, a red dwarf star that orbits the larger and brighter Alpha

  • Centauri A and B. Is this a planet? It fulfills all the requirements, even though it’s unquestionably

  • a star. Can you be a star and a planet at the same time? Conventional wisdom says no,

  • but this is the problem you run into when trying to define words like planet.

  • All of this might indicate that it may be about time to take another look at how we

  • define planets. That said, increasing the number by a factor of fourteen doesn't sound

  • like a great solution. Whatever definition science eventually settles on, sadly Pluto

  • probably won’t be on it. Ooh, and we didn’t even talk about how Pluto

  • was named after that Disney Dog! Oh wait, he wasn’t? shucks. Never mind.

  • Hey, if you learned something new today, then give the video a like and share it with a

  • friend! And here are some other cool videos I think you'll enjoy. Just click to the left

  • or right, and stay on the Bright Side of life!

Now listen up. My very eager mother just served us nine pizzas. Ooh, yum! Hey, if you were

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冥王星が惑星ではない理由 (That's Why Pluto Is Not a Planet Anymore)

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    林宜悉 に公開 2021 年 01 月 14 日
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