How Do You Define a Planet? How Do You Define a Planet? By Joyce Wang Grade 7 Dear Editor, What would be your definition of a planet, and would Pluto match it? In 2005, there was no official definition for a planet, so Pluto was still considered the ninth planet, but in 2006 the International Astronomical Union made an official definition for a planet. This definition sparked a huge controversy about Pluto and its classification. Pluto should be reclassified as a dwarf planet for three main reasons, which are that Pluto does not match the official definition of a planet, Pluto does not match up to the other planets’ qualities, and that if Pluto is a planet, the other objects of the Kuiper Belt should be planets as well. First, Pluto does not match the official definition of a planet given by the International Astronomical Union. According to the IAU, a planet must match these three criteria: a planet must orbit the sun, be large enough to have its own gravitational pull to make it a sphere, and be able to clear any other objects from its neighborhood. Pluto would have been a planet if not for the last rule. Page 15 in ‘When is a Planet Not a Planet?’ says, “Pluto fails to meet the third criterion: It shares its neighborhood with far too many chunks of rock and ice. So, Pluto isn’t a planet—it’s a dwarf planet.” Pluto couldn’t clear its neighborhood of rock and ice, so they did not identify it as a planet. Second, Pluto does not match the traits of other planets. Pluto’s orbit was also strange. All the other planets orbit the sun in an almost perfect radius on each side, but Pluto’s orbit intersects other planets. Page 14 in ‘When is a Planet Not a Planet?’ says, “It also had a strange orbit, as the diagrams on the next page show. But it was round and it moved around the sun, so it stayed a planet.” This shows that Pluto has an irregular orbit that’s different from the other eight planets. Also, Pluto is made mostly of ice and rock, while the other planets are made of rock and gas. Page 14 in ‘When is a Planet Not a Planet?’ says, “Second, they found other objects in the Kuiper Belt very much like Pluto—other balls of rock and ice.” Other planets are made of either gas or rock, but Pluto is made of rock and ice. This quote shows that there are many objects in the Kuiper Belt are similar to Pluto, meaning that if Pluto was a planet, then all those objects would be as well, which would make the number of planets much, much bigger and almost impossible to memorize. Lastly, Pluto is a circular object that orbits around the Sun, and we counted it as a planet. Does that mean similar objects in the Kuiper Belt should be classified as planets as well? Since astronomers considered Pluto to be the limit of size between what was a planet and what wasn’t, if there was a circular object in the Kuiper Belt that was larger than Pluto, then technically that would be a planet. Astronomers have actually discovered an object like this in the Kuiper Belt, called Eris. Page 11 in ‘What is a Planet?’ says, “It was the recent discovery of an object larger than Pluto within the Kuiper Belt that changed everything. Is this object, now named Eris, our 10th planet since it is larger than Pluto?” This shows how Pluto is the set point for what is and what is not a planet. If Eris exceeded Pluto’s size, does that mean we have another planet? Page 11 in ‘What is a Planet?’ says, “Then our solar system could grow to dozens of planets as more and more Kuiper Belt Objects are discovered.” The Kuiper Belt probably has many more objects that are larger than Pluto, and since Pluto does not match the third criterion for being a planet, that means all the objects in the Kuiper Belt that are larger than Pluto are technically planets too. Making Pluto a planet will most likely spark a controversy about whether all those objects in the Kuiper Belt are planets or not. If Pluto were kept as a planet, then that would mean all the circular objects larger than Pluto in the Kuiper Belt that orbited around the Sun would be planets as well. Then, the number of planets in our solar system would jump to over 50. Some may say it would be better if we kept it a planet because it would be too inconvenient to reclassify Pluto as a dwarf planet because then, all the books and museums would have to change their information completely. Page 17 in ‘When is a Planet Not a Planet?’ says, “People had grown used to the idea. Textbooks and museums would have to change what they said.” This quote explains how inefficient it would be to reclassify Pluto as a dwarf planet, but this new, massive, and important discovery about Pluto would be worth updating every article, book, and museum because learning based on false information is not beneficial. Others may say that we should keep Pluto a planet because it had been like that for many years, but we shouldn’t let history get in the way of science. If some people say that doing this would be wasting material, I still think it would be worth it to do it for the sake of learning. In conclusion, I believe that Pluto should not be reclassified as a planet, but should remain a dwarf planet because it does not clear its orbit of neighboring asteroids or meteors, its properties are not the same as the eight planets, and that since it was a planet, the other objects in the Kuiper Belt should be too. What do you think about this controversy? From, Joyce
