字幕表 動画を再生する 英語字幕をプリント 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, URinus—also 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 Ceres’ discovery, 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!