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Pluto's not a planet. We're sorry, but we think it's time you move on. If you've gone through all your breakup music and Meg Ryan movies, and you still can't get over it, then SciShow Space will get out the ice cream, cuddle up with you on the couch, and talk about how this could have happened.
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Guys, it’s done. It’s over. I know, I know, I was as upset as you were when I first heard the news. We were taught these songs and sayings to help us remember the planets, and now they’re all ruined. But Pluto hasn’t been a planet since, like, 2006, so it’s time to move on. But if you’ve gone through all your breakup music and Meg Ryan movies and you still can’t get over Pluto, then we here at SciShow Space will get out the ice-cream, cuddle up with you on the couch, and talk about how this could’ve happened.

Pluto was demoted at the 26th general assembly of the International Astronomical Union in Prague in 2006, and it was a controversial decision. There were astronomers who wanted to keep it, but the only argument they could make was tradition. There was, and is, no scientific justification for calling Pluto a planet. It’s a thing out there in the Kuiper belt which is like the asteroid belt, only it’s out past Neptune and it’s a lot bigger. There are about a trillion things in the Kuiper belt. A million million things! And nearly all of those things are chunks of ice and rock, like Pluto. Pluto is just the first one that we saw.

And Pluto is pretty big for one of those things, but it’s not the biggest thing in the Kuiper belt. That title goes to Eris which is about 25% bigger by mass and twice as far from the sun as Pluto. There are other big things too, like Haumea and Makemake and Ceres, which is in the asteroid belt between Mars and Jupiter, making it practically a local. 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 having over a hundred planets would have made those songs and sayings we learned in elementary school really, really long. And that’s really the crux of why we had to downgrade Pluto because if we call Pluto a planet we’d have to call so many other things planets too that the word "planet" would stop being useful.

So in 2006, astronomers decided that to be a planet an object had to: orbit the sun, have enough gravity to pull itself into a more or less spherical shape, and have cleared pretty much everything else out of its orbital path. The last one is where Pluto falls short. Pluto is only 0.07 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 this special place Pluto holds in our hearts, they decided to call all dwarf planets past Neptune plutoids which is… It’s pretty sweet. And the same year astronomers decided Pluto could no longer be considered a planet, NASA launched the New Horizons spacecraft with a mission of visiting Pluto. New Horizons will do a dramatic flyby of Pluto and its moons in July of 2015 and send back lots of photos and information that will tell us more than ever about our favorite little dwarf planet. And it will also carry my personal and deepest good wishes.

Thanks for watching this episode of SciShow Space, and thanks to all of our Subbable subscribers who make this channel possible. If you’d like to support us and score some cool gear, go to to learn more. And if you have questions or ideas for an episode you’d like to see, you can find us on Facebook and Twitter and in the comments below, and if you want to keep getting smarter with us, just go to and subscribe.