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Sunday, February 27, 2011

Why Pluto Was Removed As a Planet

Ever wondered why Pluto was declassified as a planet in 2006?

The largest known trans-neptunian objects.
Source: Wikimedia Commons

Discovered in 1930 by Clyde Tombaugh, Pluto was classified as the ninth planet in our Solar System. This would be taught in classrooms for nearly a century, until science began having a better understanding of what shape our Solar System really has and how it works.

In 2006, Pluto was declassified as a Planet because of new definitions set forth by the IAU (International Astronomical Union), which listed three mandatory rules in order for a solar object to be classified as a "planet" in our Solar System. In order to be classified as a planet, the object should:

1) Be in orbit around the Sun,
2) Has sufficient mass to assume hydrostatic equilibrium (a round shape), and
3) Have "cleared the neighbourhood" around its orbit.



Pluto failed to meet the standards of rule three, which essentially means that the solar object must have sufficiently much larger mass than its neighboring bodies. For example, Earth's mass is much larger than the moon and other neighboring bodies that are around a radius close enough to Earth.

The reason why Pluto failed to meet the third rule is essentially because of the Kuiper Belt. The Kuiper Belt is a large "ring" of asteroids and other foreign objects that orbit our Sun, but are outside of the general Solar System's vicinity. The green marks below identify objects that are in the Kuiper Belt:

The Solar System and the Kuiper Belt.
Source:
Wikimedia Commons


Contrary to what astronomers originally thought, Pluto was not orbiting the Sun on its own. It was among thousands of other objects, some near the mass of Pluto, all orbiting the Sun in the Kuiper Belt. By studying the trajectory of these objects in the Kuiper Belt, astronomers were able to understand that Pluto was simply a larger mass that stood out among the other Kuiper Belt objects.

A large reason why the search for a 9th planet was started was due to the fact that Neptune's orbit is not currently completely explained. This is because Neptune's orbit is more skewed than it should be due to Uranus--meaning that something with a large mass--further out of Neptune's orbit is acting on Neptune, changing its trajectory. This search for this "Planet X" continues to this day. Scientists have identified a few new "plutoids" such as Eris and Sedna. However, they do not have the mass necessary to cause the skew in Neptune's orbit.

Pluto was simply a misidentification by scientists in the search for Planet X which is responsible for the skew in Neptune's orbit.

68 comments:

  1. I've always argued that pluto did revolve around the sun. but then again it did go into the path of other planets so i don't know.

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  2. Yeah, it's not a planet. Just an oversized dust/ice ball, probably related to a comet?

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  3. pluto will remain as a planet for me. always :D

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  4. I was wondering about this the other day, interesting.

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  5. Poor Pluto. I guess it'd be hard to call all of the objects in the K-Belt by name, though, so it would make sense to cut down rather than include more.

    Enjoying the interesting content, sir- keep it up.

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  6. i heard about it not metting the requisites but didnt exactly know what they where

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  7. Replies
    1. I bet you still think the earth is flat too, kiddo.

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  8. oh the dwarf planet fiasco. People were so upset about that, as if it made any real difference. It's still tiny and cold.

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  9. I love your blog, so interesting and informative

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  10. poor pluto always wanted to be a planet

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  11. I've grown up with pluto being a planet and so it will stay that way for me.

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  12. Ahh thats cool, I didn't even know it wasn't classed as not being a planet!

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  13. I was like, "how can the astronomers do that???" But then when I read "3) Have "cleared the neighbourhood" around its orbit." Well, that makes sense. It's funny, though, because since I graduated, there is one more ocean, and one fewer planet, and now indigo isn't even a color!!!

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  14. To be fair, I don't agree with the third conditional for planets. In fact, in some solar systems, we've observed pairs "planets" the size of jupiter orbiting each other AND the sun. By our definition, these super-sized spherical masses would not be considered planets. Just a thought.

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  15. @MTensor: There is actually a lot of controversy between scientists on the three conditions to this day. So yes, it is nowhere near a perfect standard for determining what is a planet and what's not.

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  16. As an astronomer, there is really no consensus on these rules. They were pretty much made up by a bunch of guys trying to make an excuse for why Pluto isn't a planet. It isn't a planet, but its more of a "I know it when I see it" then any sort of definition. If Pluto were to move into the inner solar system, it would look like a huge comet. The other thing, the reason why Neptune's orbit was predicted wrong turns out not to be because of some outside influence. Earlier astronomers had mistaken its size, it's actually smaller then originally thought.

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  17. but why oh why did they have to make rules in the first place!? it's so exclusive!

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  18. The trend is solid planets go in the inner solar system and gas giants go in the outer. But I am a Pluto-is-a-planet fan.

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  19. i had wondered why but never researched it. i know so little about the solar system.

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  20. my childhood shattered when pluto was removed

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  21. I didn't know this actually, thanks for the info.

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  22. I love your blog, I'm totally following!

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  23. I love Astrophysics too...
    gonna follow ;)

    PS: And thanks for sharing :D

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  24. This comment has been removed by the author.

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  25. very well written post, very accurate info. I would only add stress on the current understanding of there being a whole shit load of other trans-neptunian objects.

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  26. Wow, interestin! Followed
    alphabetalife.blogspot.com

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  27. I always feel stupid when I read your blog posts. :(

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  28. Lol I just can't think that we only have 8 planets in our solar system. it's weird.

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  29. Still disappointing. I dislike the notion of being told my childhood is a lie.

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  30. I don't get all the people who think the declassification of Pluto has any effect on them personally. The truth is it doesn't even have an effect on Pluto...It's not like any of these high drama people who've decided to personalize its change in class grew up spending summers at their lake house on Pluto. And I'm pretty sure any actual inhabitants of the dwarf planet (if there are any) couldn't care less about what the Earthlings think about their home. Newsflash: your entire childhood was already a lie...Santa Claus, The Tooth Fairy, the Easter Bunny, 'work hard and get ahead', 'follow your dreams', and all other such foolishness were the lies that defined the lives of every child who grew up in the U.S. Get over yourself already.

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