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Why Pluto Is Not a Planet

January 23, 2018


I know, I know. It’s tragic. It’s confusing. We were taught these songs and sayings to help us remember the planets, and now they’re all ruined. Pluto hasn’t been a planet since 2006, so maybe it’s time to move on.


But if blasting breakup music and binging Nicholas Sparks movies still can’t help you get over Pluto, then right now is the perfect moment to bring out the ice-cream, cuddle up on the sofa, and talk about how this could’ve happened.


When Pluto was demoted at the 26th general assembly of the International Astronomical Union in Prague, 2006, it was a controversial decision.


There were astronomers who wanted to keep it, but the only argument they could make was based on tradition. There was, and is, no scientific justification for calling Pluto a planet. Yes, it sits out there in the Kuiper Belt - but there are about a trillion things in the Kuiper belt, and nearly all of those things are chunks of ice and rock, just like Pluto. The only distinguishing quality about Pluto is that it just so happened to be the first one we noticed.


And okay, Pluto is pretty big for one of those things, but it isn’t even the biggest thing in the Kuiper Belt. That title goes to Eris, a planetary mass which is about 25% bigger by mass, and twice as far from the sun as Pluto.


Scientists predict that there may be up to a hundred other “big” things out there in the Kuiper Belt, just waiting to be discovered. But if we have over a hundred planets in our Solar System, it would make those songs and sayings we learn in elementary school really, really long, and is really the reason why we had to downgrade Pluto. If we call Pluto a planet, then we’d have to call so many other things planets, too, and that would pretty much cause the word “planet” to lose its meaning.


So after the big debate in 2006, astronomers decided that in order to be a planet, an object had to: orbit the sun, have enough gravity to pull itself into a spherical shape, and outsize pretty much everything else in its orbital path. The last one is where Pluto falls short.



Pluto is only .07 times the mass of everything else in its orbit. Earth - just to give you some sense of perspective - is 1.7 million times the mass of everything else in its orbit.


But the International Astronomical Union wasn’t completely heartless; It came up with a new category of celestial objects that satisfies only the first two criteria. We call them dwarf planets - and in honor of Pluto, they decided to call all dwarf planets beyond Neptune “plutoids.” Which is… pretty sweet.


That same year, NASA launched the New Horizons spacecraft with a mission of visiting Pluto. It completed a dramatic flyby of Pluto and its moons in July of 2015, and sent back lots of photos and information that taught us even more about our favorite little dwarf planet. (Want to read about the flyby? Leave a comment!)


In the end, Pluto IS NOT a planet, but just a small, ordinary clump of icy rock. Unless you think it’s a yellow dog. Then it’s totally a yellow dog.


Thank you for choosing SkyFeed! Be sure to follow my social media (Instagram: @astrolia) and subscribe to get the best of space exploration delivered to your inbox every week.


Sources: This story was originally published on SciShow Space. I am republishing a lightly edited version on SkyFeed in light of interest in the subject. Riemers, Ried. “Why is Pluto Not a Planet?” SciShow Space, YouTube. 15 April 2014. Web video.

Citation: Rovira, Lia N. "Why Pluto is Not a Planet." SkyFeed. 22 Jan, 2018. Web article.


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