<p>Clyde William Tombaugh discovered Pluto in 1930. He saw that it is pretty big, so he gave him the classification of planet. But then the scientists discovered more objects, sometimes even bigger than Pluto. If they would classify all of them for the planets, then we would have around 20 or 30 of them. But that wouldn't be so great. So they just decided, that Pluto is too small. In fact, Ganymede, Titan, Callisto, Io, Moon, Europa, Triton are all larger than Pluto (they are moons). Eris is just some kilometers smaller than Pluto.</p>
<p>But the main thing here regards the IAU definition of the planet. Planet must clear its neighbourhood, but Pluto clearly hasn't done it.</p>

CC BY-SA 4.0
2021-01-31T17:32:20.940Z
Oh,  well i always thought it was because of what Joseph Casey said, As it just got too far away from the system but yes that would work too.
TheBigJoe1489
CC BY-SA 4.0
2021-01-31T18:54:13.927Z
@ProfRob Size **is** the issue.
David Hammen
CC BY-SA 4.0
2021-01-31T21:36:36.833Z
@DavidHammen Maybe you could explain your comment to help others who have read the IAU definition and can't see how size (or even mass, since Pluto is spherical) comes into it.
ProfRob
CC BY-SA 4.0
2021-02-01T08:28:22.220Z
@ProfRob See [this answer](https://astronomy.stackexchange.com/a/592/2618) by Emilio Pisanty.
David Hammen
CC BY-SA 4.0
2021-02-01T08:53:50.337Z
That answer says "Size, for example, is not a good discriminant, because solar system bodies come in a continuum of sizes from Jupiter down to meter-long asteroids. ". It then goes on to talk about a dimensionless mass *ratio* which is just a proxy for rule #3 in the IAU definition. @DavidHammen . Size is not relevant
ProfRob
CC BY-SA 4.0
2021-01-31T17:30:28.590Z
2021-01-31T22:30:08.513Z
2021-01-31T22:30:08.513Z
User123
<p>I have been wondering this and the teachers at my high school never really gave me an answer. Please help, why was Pluto kicked out of the solar system?</p>

CC BY-SA 4.0
2021-01-31T18:13:46.303Z
Normally, you should wait 24 hours before accepting an answer so you can receive perhaps better answers and get more feedback on your current answers.
WarpPrime
CC BY-SA 4.0
2021-01-31T18:50:36.550Z
https://en.wikipedia.org/wiki/IAU_definition_of_planet ?
Craig Constantine
CC BY-SA 4.0
2021-01-31T18:53:01.567Z
Downvoted for early acceptance of a bad answer.
David Hammen
CC BY-SA 4.0
2021-01-31T18:56:35.317Z
And why are we still debating this? Voting to close as off topic.
David Hammen
CC BY-SA 4.0
2021-01-31T18:57:34.707Z
I’m voting to close this question because questions about the planet status of Pluto are old, and are now explicitly off-topic.
David Hammen
CC BY-SA 4.0
2021-01-31T19:25:28.120Z
Please see [this meta post](https://astronomy.meta.stackexchange.com/questions/667/can-we-make-arguments-that-pluto-should-be-considered-a-planet-off-topic) concerning this topic of discussion, as @DavidHammen said.
WarpPrime
<planet><pluto>
Why was Pluto declassified as a planet?
TheBigJoe1489
# Why was Pluto declassified as a planet?

(Asked by: username_0 on 2021-01-31)

I have been wondering this and the teachers at my high school never really gave me an answer. Please help, why was Pluto kicked out of the solar system?

**Tags:** planet, pluto
> **Comments:**
> **username_1**: Normally, you should wait 24 hours before accepting an answer so you can receive perhaps better answers and get more feedback on your current answers.  
> **username_2**: https://en.wikipedia.org/wiki/IAU_definition_of_planet ?  
> **username_3**: Downvoted for early acceptance of a bad answer.  
> **username_3**: And why are we still debating this? Voting to close as off topic.  
> **username_3**: I’m voting to close this question because questions about the planet status of Pluto are old, and are now explicitly off-topic.  
> **username_1**: Please see [this meta post](https://astronomy.meta.stackexchange.com/questions/667/can-we-make-arguments-that-pluto-should-be-considered-a-planet-off-topic) concerning this topic of discussion, as @DavidHammen said.  

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## 1 answers

### Answer by username_4 on 2021-01-31. Score: 3

Clyde William Tombaugh discovered Pluto in 1930. He saw that it is pretty big, so he gave him the classification of planet. But then the scientists discovered more objects, sometimes even bigger than Pluto. If they would classify all of them for the planets, then we would have around 20 or 30 of them. But that wouldn't be so great. So they just decided, that Pluto is too small. In fact, Ganymede, Titan, Callisto, Io, Moon, Europa, Triton are all larger than Pluto (they are moons). Eris is just some kilometers smaller than Pluto.

But the main thing here regards the IAU definition of the planet. Planet must clear its neighbourhood, but Pluto clearly hasn't done it.

> **Comments:**
> **username_0**: Oh,  well i always thought it was because of what Joseph Casey said, As it just got too far away from the system but yes that would work too.  
> **username_3**: @ProfRob Size **is** the issue.  
> **username_5**: @DavidHammen Maybe you could explain your comment to help others who have read the IAU definition and can't see how size (or even mass, since Pluto is spherical) comes into it.  
> **username_3**: @ProfRob See [this answer](https://astronomy.stackexchange.com/a/592/2618) by Emilio Pisanty.  
> **username_5**: That answer says "Size, for example, is not a good discriminant, because solar system bodies come in a continuum of sizes from Jupiter down to meter-long asteroids. ". It then goes on to talk about a dimensionless mass *ratio* which is just a proxy for rule #3 in the IAU definition. @DavidHammen . Size is not relevant  

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