By now you have probably heard that Pluto is no longer a planet. But why is that? What did it do to lose its status as the tiniest planet in our solar system?

American astronomer Clyde Tombaugh discovered Pluto in 1930. From 1930 until 2006, Pluto sat comfortably beyond Neptune as the ninth planet in our solar system.

In 2006, however, changes were made. The truth is, nothing about Pluto changed, but the definition of a planet did. Once these changes became official, Pluto no longer fit the definition of a planet.

According to new rules adopted by the International Astronomical Union, a celestial body must meet the following criteria in order to qualify as a planet:

  • A planet must be round.
  • A planet must orbit the sun.
  • A planet must have “cleared the neighborhood” of its orbit. This means that as a planet travels, its gravity sweeps and clears the space around it of other objects. Some of the objects may crash into the planet, others may become moons.

Pluto follows the first two rules: It is round, and it orbits the sun. It does not, however, follow the third rule. It has not yet cleared the neighborhood of its orbit in space. Because it does not follow this rule, Pluto is no longer considered a planet.

Don’t feel too sad for Pluto, though. It has a new title — “dwarf planet.” Dwarf planets are celestial bodies that only meet the first two criteria in the new definition of a planet.

Dwarf planets, like Pluto, have not yet cleared the neighborhoods of their orbits — and still have some cleaning to do if they will ever become “true” planets.

Pluto is in good company. There are currently five dwarf planets, but scientists expect more will be discovered over time.

Four of the dwarf planets — Pluto, Makemake, Haumea and Eris — are located beyond Neptune. The fifth dwarf planet, Ceres, lives in an asteroid belt between Mars and Jupiter.

Facts about Pluto:

  • Pluto is about the size of the moon.
  • It takes approximately 248 Earth years for Pluto to make one trip around the sun. Since astronomers discovered Pluto in 1930, it has not completed one trip around the sun. It still has more than 150 Earth years to go!
  • Pluto is so far away that it takes more than five hours for light from the sun to reach it. The sun’s light reaches Earth in only eight minutes!


107 Join the Discussion

1 Star2 Stars3 Stars4 Stars5 Stars  (139 votes, avg. 4.37 out of 5)
    • We think it’s GREAT that you and Callum would like to explore Pluto, Aidan! We think it would be a very interesting adventure, ourselves! Thank you for visiting this Wonder and for leaving us an AWESOME comment! :-)

    • Pluto is a cool planet and I always like to go to Pluto, but it’s hot for that you need food and water and turtles!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

      • Thanks for leaving us this SUPER comment, Anthony! You wouldn’t want to go to Pluto unless you packed some VERY warm clothes, because everything on Pluto is frozen solid. Thanks for hanging out in Wonderopolis today! :)

    • don’t forget that Pluto is smaller than what we classify as DWARF planets. Its like calling an asteroid or a moon a planet. Just because it was a planet. It doesn’t matter if it was a planet back then science changes every day.

      • Thanks for your comment, Matthew. Pluto is smaller than regular planets, but it is big enough to be a dwarf planet. In fact, dwarf planets are typically smaller than the planet Mercury. The main distinction between a dwarf planet and a planet is that planets have cleared the path around the sun while dwarf planets tend to orbit in zones of similar objects that can cross their path around the sun, such as the asteroid and Kuiper belts. Thanks for Wondering with us!

  1. Oh that’s great, Wonderopolis! I haven’t watched the video yet, but I promise I will! It’s great to see all kinds of wonders!
    I am welcome to tell you. Some other wonders, too!

    • Hi, Moa! Thanks for letting us know you visited this Wonder of the Day® about Pluto! There are LOTS of other Wonders in Wonderopolis just waiting to be explored! HAVE FUN learning new things! :-)

  2. I love Wonderopolis! I think it’s the BEST place to answer your wonders. I get on here every day to learn a new wonder. Thanks, wonderopolis!:)

    • It makes us super happy to learn that you love Wonderopolis so much and that you think it’s the best place to WONDER, Jourdan! Thanks for being such a GREAT Wonder Friend! :-)

  3. You know, this makes me think a lot about Pluto, like are there rules to be a dwarf Planet? This really helped because it was one of my wonders at school and at home… and know I have new wonders!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    • We think it’s AWESOME that you have Wonders, Leah! Make sure you let us know what they are by clicking on the “nominate” link at the top of every page in Wonderopolis and answering the questions! We can’t wait to hear all the WONDERful things you WONDER about! :-)

    • Hello, Ena! Thanks so much for visiting Wonderopolis with Leah! We think it’s GREAT to share learning with a friend or family member! :-)

    • It makes us really happy to read your comment, Leah! WONDERing is so much fun to do, and it helps us become smarter, too! When we WONDER, we want to learn more! Thanks for being a GREAT Wonder Friend! :-)

  4. Dear Wonderopolis,
    I wanted to know a wonder the wonder is…

    Why, on a computer, do people make it so it goes, “q w e r t y u i o p a s d f g h j k l z x c v b n m,” instead of the abc’s?????????

    It would be great if you give me the answer!!

    Thanks, Leah

    • We’re not sure why computer keyboards are set up that way, Leah, but you know what? Your question would make a GREAT future Wonder of the Day®! Thank you so much for suggesting it…KEEP WONDERING! :-)

  5. I was so shocked about how long does Pluto orbits around the sun…248 EARTH YEARS?? TOO LONG! And pluto was not finished yet!

    • Hey there, Wonder Scientist Kim! We’re glad you’re back today! :)

      It’s awesome that you shared your favorite part of our Pluto Wonder– keep up the great WONDERing! :)

  6. Wonder:
    Jaylen- Why is Pluto so small?
    Kaia- Is is night time everyday on Pluto?
    Nathan- Why is Pluto so far away?
    Katelyn- Is there any form of life on Pluto?
    Jake- Has anybody ever walked on Pluto?

    Ramon – It reminded me of a circle
    Jaylen – It reminded me of a sphere
    Jazmyn – It reminded me of a bouncy ball
    Ashlee – It reminds me of Cat and the Hat when they go into space
    Noah – It reminds me of the super ball show we watched because it is round

    • Happy Friday, Mrs. Farr’s class! We are SO excited to see so many of you WONDERing and making such smart connections! Special WONDER shout out to Jaylen, Kaia, Nathan, Katelyn, Jake, Ramon, Jaylen, Jazmyn, Ashlee and Noah! You guys ROCK! Wonder On! :)

    • Hey there, Taylor and Dellon! Thank you so much for sharing your comments with us today! We’re glad you learned all about the celestial body, or dwarf planet, that describes Pluto! :)

    • So do we, Taylor! Thanks for sharing your comment with us today! We are glad Pluto is at least considered a “dwarf planet”! :)

    • Thanks for letting us know, Lauryn! We are so glad you’re WONDERing about Pluto and it’s “dwarf planet” status. Thanks for sharing your comment and telling us about your project, Wonder Friend! :)

    • Hmm…

      If another planet crashed into Pluto, that would definitely be Neptune, since their orbits intersect with each other. But they probably won’t.

      Pluto will come within 150,000 kilometers from Neptune on three separate years: 2161, 2259, and 2417.

      P.S: This is coming from a fifth grader who skipped fourth grade. You can trust me that my ideas are correct.

    • Hi Gage! Thanks for WONDERing with us! It is actually so cold in space that plants cannot grow there. Keep WONDERing with us! :)

  7. ‘Neighborhood’ is too vague a term for my likings. This ‘neighborhood’ should be defined in terms of the radius of the plante in question: how many times should it sweep clean its own radius? If too many times, then Mars and Jupiter would have to be demoted for not sweeping the asteroid belt clean!!
    Who is the moon and who is the planet? The planet would be the nearest object to the barycenter of the system, disrespect of whether the barycenter is inside or outside one of the members of the system.

    • Thanks for letting us know you thought this Wonder was COOL, Danielle! What kinds of things are you still WONDERing about Pluto? We hope you join us to WONDER again soon! :)

    • We like your thinking, Wonder Friend Jordan. Thanks for visiting Wonderopolis today and leaving us a comment! :)

  8. I like Pluto because it has a lot of colors and Pluto is the best. I like it but I didn’t know that Pluto was a planet?

    • Hi, Ermis. We’re happy to hear that you like Pluto and think it is the best. Remember from the Wonder Pluto is considered a dwarf planet. We’re glad you spent time WONDERing with us today! :)

    • Can Pluto ever get bigger? Can it change its status to a different one?
      Will we ever live on Pluto? I have so many questions!!!!!!

      • Those are some SUPER questions, Wonder Friend! We’re so glad are Wonder on Pluto encouraged you to WONDER some more. We encourage you to explore the answers to your questions on your own. Thanks for visiting Wonderopolis! :)

    • How COOL is that, Wonder Friend Tyler! We bet your science project was WONDERful! Thanks for WONDERing with us today! :)

    • Hello, Jamiesha! We encourage you to go back and explore this Wonder again. You might be able to find the answer to your questions. Happy WONDERing! :)

    • Thank you for your kind words, Wonder Friend Chris! We’re glad you hear you find our website interesting. We’d like to know what is your favorite Wonder? :)

  9. I already knew that Pluto wasn’t a planet anymore.
    Great thoughts, Wonderopolis! Thanks for WONDERing with us today!

    • You ROCK, Calub! The best thing about Wonderopolis is that we have LOTS of Wonders. If you already know something about one Wonder, you can check out a different Wonder. Thanks for WONDERing with us again today! ;) We can’t wait for you to stop back by tomorrow! :)

    • We’re thrilled this Wonder has you doing more WONDERing, Cesar! Scientist don’t believe there is life on Pluto because it gets so cold and everything is frozen solid. Thanks for your WONDERful question today! :)

    • Hello, Mrsick! Thank you for your nice comment about today’s video. We’re glad you liked it! What did you learn from visiting this Wonder of the Day? :)

    • We like how you turned this Wonder into a story, Wonder Friend pink Bryce. Sounds like it would make a neat picture book! Thanks for WONDERing with us today! :)

    • Those are some SUPER questions, Lawrence! According to scientist nothing could survive on Pluto because it is so far away from the sun and extremely cold there. We hope to see you back here in Wonderopolis soon! :)

    • Hi, Wonder Friend Jacob! We think you could answer your question if you go back and re-explore today’s Wonder of the Day. :)

    • We’re proud of your thinking and WONDERing today, Roman. We’ll have to think about that question ourselves. What do you think? Happy WONDERing! :)

    • We don’t think so, Wonder Friend Lawrence. According to scientist Pluto is very cold because of its distance from the sun. We hope you KEEP WONDERing! :)

  10. It has a unique size,pluto had what it take to be a planet like neptune, it orbited the sun, but it had a little problem it had a unusual size,it needed change,so a astronomer clyde tombaugh waited 76 years it had no change so it was considered a dwarf planet.

    • We like the way you summarized your learning today, Sian Pi! Thank you so much for being a friend of Wonderopolis and for posting today! :)

    • BRRRRRR, Wonder Friend Diavionne! Luckily we are in Wonderopolis not Pluto, because it’s SO cold there! We enjoyed having you WONDER with us today! :)

    • That makes us smile :) here in Wonderopolis that you enjoyed WONDERing about Pluto, Khyreese! What was the most interesting fact you learned? We hope to see you back tomorrow! :)

    • Great question, Wonder Friend Brandon. There are five dwarf planets. They are Pluto, Makemake, Haumea, Eris, and Ceres. We’re SUPER glad you joined us today! :)

    • We’re so glad this Wonder of the day made you WONDER some more, Daniel! We aren’t sure if there is a planet like earth. Maybe you could explore the answer to that question on your own? Let us know what you find out! Happy WONDERing! :)

  11. This is really cool! It must take a really long time for Pluto to go around the sun but I’m sure it’ll get there eventually! :) I think this wonder was the best so far (in my opinion) I think you guys should do another wonder about our solar system or outer space!

    ~Sydney :)

    • Wow, Sydney! What a great comment you left us today! We appreciate your positive words! You can check out more solar system related Wonders HERE Happy reading and WONDERing! :)

    • Thanks for letting us know that you thought the Wonder about Pluto is a GREAT one, Even! We appreciate your enthusiasm! Join us to Wonder again soon! :)

  12. I dont understand how planets make the neighborhood clean!! And why does it have to be those 3 rules in order to become a planet? Because here surely has to be some other rules that the other planets don’t have in common!! Because, think about pluo. He’s with all the other dwarf planets when instead he could be with our planets!! So why does it have to be those 3 rules in order to become a planet and how long do you guys think it’ll take for Pluto to become planet??

  13. Is there anyway of choosing the right definition for Pluto in this article. It says it is a dog from Disney, but if students are reading this it would be nice for them to know the definition that relates to the article.

    • Hi M! Thanks for pointing out that we were using the definition for a different Pluto than what we discuss in today’s Wonder. We’ve updated the definition to avoid further confusion. Did you know that Pluto was named after the Roman god of the underworld, and that the name was suggested by Venetia Burney (1918–2009), an eleven-year-old schoolgirl in Oxford, England? What do you think about words that have more than one meaning? Have you seen Wonder #576: What is a homonym? Thanks for WONDERing with us!

    • Hi, Rebecca S.! Thanks for sharing this WONDERful video with our WONDER friends! We love your enthusiasm for WONDERing and learning! :)

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Have you ever wondered…

  • Why is Pluto no longer a planet?
  • What is the definition of a planet?
  • What is a dwarf planet?

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Wonder #73- Pluto Static ImageVimeo Video

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Most children (and most adults, too!) cannot fully grasp the immensity of the solar system we live in. If you have a little time, a few materials and a desire to get a little exercise, learn just how big the solar system really is. It’s a neat exercise you won’t soon forget!

Once you understand the immensity of our universe, bring the solar system back down to size by creating a scaled-down mobile of the planets — from largest to smallest. Hang it in your bedroom to inspire out-of-this-world dreams!

A little overwhelmed by the project? Try this simpler, more modest solar system mobile.


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