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Why Pluto is no more planet?। What are the requirements to be a planet? Updated

Why Pluto is no more planet?। What are the requirements to be a planet? Updated 

Is Pluto a planet or not?, this is a common dilemma for us. If you were in elementary school before 2006, there’s a good chance you had to memorize something that the order of our solar system; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and finally Neptune and Pluto. Now if you are currently in elementary school you might be saying, there were nine planets?. 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. When even knew that was a thing that could happen?
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 Galelio 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 I massive air quotes orbiting the sun halfway between Mars and Jupiter. You may recognise this as where this as where the Asteroid belt is, and it isn’t because Ceres pulled an Alderaan and broke into a thousand pieces.
Astronomers noticed that Ceres was quite small, with only half the radius of Earth’s moon. The year after Ceres' discovery, the astronomy community was a buzz with discovery of another planet named Pallas. Then they found another a few years later and another and another. Four new planets are one thing. When it turns into thousands, it might be time to revaluate some definitions. Astronomers noticed that the rock had more in common with each other planets. They were tiny; they were barren; and the bust majority were not 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, Uranus. Neptune made in 1846 with Pluto joining the party 84 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. In 1978 astronomers discovered Pluto’s moon, Charon. At first this might seem to be strong evidence in Pluto’s favour.
If it’s big enough to have a satellite, it must be a planet, not exactly. Charon may be smaller than Pluto. But not that much smaller. One half diameter might seem like a big difference, but not compared to the difference 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 in planet like behaviour 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 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 revive their definition of what is or is not a planet.
From then on an object was only a planet if it’s feet the following three qualifications.

It must orbit the sun
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 n around it. Clearing it’s neighbourhood means that three are no nearby objects other than its own satellites. Pluto has failed to accomplish that feat, so the third box remain unchecked. Some scientists still doesn’t believe this theory. 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 while bunch of asteroids and other objects. All combined this new definition would take us from manageable eight planets to an unwieldy one hundred and fifteen.
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 surround the perspective planet instead of the worlds themselves. Even earth would fail to be a planet if it were out in the port cloud somewhere. On the other hand, what do yo 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 forms all the requirements, even it’s unquestionably a star. 


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