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Why Isn't Pluto a Planet Anymore?

By O. Wallace
Updated Feb 27, 2024
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Nearly every schoolchild who has grown up in the latter part of the 20th century has been taught that Pluto was not only the furthest planet from the Sun, but also the smallest in our solar system. For now, science books and astronomical charts will have to be revised due to a decision made by the International Astronomical Union (IAU) during a meeting in 2006. The decision, which included eight days of zealous debate and a nearly split vote, resulted in Pluto being downgraded to a dwarf or minor planet status. It’s not so much that it was intentionally removed from the classification, but rather that the IAU outlined a new definition of what constitutes a classic planet, and Pluto no longer meets the standards. Now, the list of classic planets in our solar system includes Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The new standards dictate that, to be a classic planet, an object must be a celestial body orbiting the sun, with enough mass to allow its gravity to form into a round shape. In addition, must be gravitationally dominant enough to prevent anything of similar size, barring its own satellites, from floating around the planet. The term that the IAU used was “clearing the neighborhood of its orbit,” which was one of the most important aspects of the definition that was debated.

Although Pluto is now a dwarf planet, it also belongs to a third class of “lesser” objects that orbit the sun. The term used to describe it is “smaller solar system body,” which can also be used to refer to comets and asteroids. One would think that, because Pluto was downgraded from being the smallest of the planets, it would at least be the largest of the dwarf planets. This is not the case, as UB313, an icy object found further out in the solar system, is larger.

Regardless of the hot debate that has been raging since the 1990s, the National Aeronautics and Space Administration (NASA) felt that Pluto rated further study. In early 2006, NASA dispatched the New Horizons craft to further investigate the dwarf planet, and it is expected to arrive sometime in 2015. Some astronomers who were unhappy with the IAU’s decision believe that the decision to downgrade it will be overturned by then.

The conference, which included approximately 2,500 astronomers representing 75 countries, cast the vote amidst strong opposition. Some astronomers lament the fact that only 5% of the world’s astronomers took part in the decision to change the definition and believe that it will not stand as a result. Strong opposition came from the family of Clyde Tombaugh, the American who discovered Pluto in 1930 in Flagstaff, Arizona, as well as many other astronomers worldwide. For some, the decision was the correct one, as they believe that the original classification watered down the definition of a planet.

AllTheScience is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.

Discussion Comments

By anon937509 — On Mar 05, 2014

Technically, it is still a planet. A dwarf planet. For that kid who said Pluto was named for Mickey Mouse's dog, check your facts.

By anon319598 — On Feb 13, 2013

Folks, please read up about "Plutinos". The (Edgeworth-)Kuiper belt is differentiated into different regions as is the asteroid belt. Plutinos in the KB'a inner region have similar 2:3 orbital resonances with Neptune as does pluto (think non-collisions or slingshots-out-of-orbit). During early solar system formation Neptune is thought to have migrated outwards but didn't clear away some objects - these Plutinos. So what? Well, Pluto is the largest of these Plutinos, and notably so. (Brown's remarkable find, Eris, prompted much of the IAU concerns, contestably bigger than Pluto - some now calculate it is not; except Eris averages much further out, in the Scatter Disk, past the recognized Kuiper Belt region. It is nowhere near Pluto's neighbourhood. And Stern's probe New Horizons in the 2015 flyby may well see Pluto cratering indicating this largest Plutino was indeed busy clearing its orbit. BFenerty, Royal Astronomical Society of Canada

By anon307256 — On Dec 04, 2012

If Pluto does not fit the criteria of a planet, then it can't be a planet. It's that easy.

By anon269432 — On May 17, 2012

Everyone is missing the point; scientists did not demote Pluto because it is small. They removed Pluto's planetary status because it is one of many KBOs with similar mass and size.

Eris has been confirmed to be larger than Pluto, and objects such as Sedna and 2007OR10 are nearly 3/4 the size of Pluto. If scientists had classified every pluto-sized KBo as a planet, the list would grow far to long, and decrease the overall importance of every planet. It would be like having a lottery where half of the people win, albeit a very small sum.

Also, people are forgetting that the "clearing its orbit" part of the IAU planet definition refers to objects like each other. For example, Jupiter shares its orbit with hundreds of "Trojan asteroids," but is still classified as a planet because these asteroids are of entirely different composition and are thousands of times less massive.

When Ceres was discovered in 1801, it was called a planet. However, it was classified as an asteroid when people realized it was merely the largest of many asteroids in the solar system. The same thing has happened to Pluto. The only thing different about Pluto is its prolonged planetary status (1930-2006) and its lasting appeal.

By anon265739 — On May 02, 2012

I just held a meeting with my collected friends. We determined that the earth is no longer a planet. It is a gnoid. Gnoids have more than 10 artificial satellites. Now change all your textbooks

to reflect our views!

By anon262465 — On Apr 19, 2012

It is a good thing that we have scientists to figure out all our problems in the world. RIP Pluto. I will miss you!

By anon232337 — On Nov 30, 2011

I think it's so stupid to think Pluto isn't a planet. It's round, it orbits around the sun. So do all the other planets. Who cares if it's too small? Mercury is small and it's still a planet. In my opinion, Pluto is still a planet.

By anon212249 — On Sep 05, 2011

Well, if you stop and think logically, you will see the wisdom of it all.

Since planets have a consistent orbit and a consistent time frame for that orbit, then they qualify as planets, no matter what their size. Pluto therefore fits the bill.

By anon178283 — On May 20, 2011

RIP Pluto! I don't think that this is as much of a deal as some other things happening on Earth. Pluto isn't complaining if it's a planet or not, so why should we? Nobody seems to live on Pluto, so I don't think it is much of a deal. When life forms are found on Pluto, we'll see just what it is. This guy is a total jerk.

By anon131399 — On Dec 02, 2010

First off, who the hell are we to classify what is and what is not a planet? what makes us so all knowing? We have never even properly investigated most of our universe yet we claim to know more about it than our factual data would suggest.

Also, according to this article Earth is not a planet, because Earth has a moon! The universe is the one area of science we know the least about. I am not suggesting that I know if or if not Pluto is a planet, all I am saying is that I would recommend a subject be thoroughly investigated and any decision we make on a matter of this sort be based off of scientific evidence instead of speculation and irrational classifications.

By anon112637 — On Sep 21, 2010

Stop telling people to get over it, and don't put down someone's 10 year old daughter! Why are people getting so heated over this who don't even care? Let those of us who want things to stay the same complain without being put down. We are not stupid. We are just outer space fanatics who want our nine planets to stay the same! We can be emotionally attached. We are attached to this universe because we live in it.

By anon112622 — On Sep 21, 2010

If we can downgrade/debunk saints then Pluto can be downgraded also. Live and learn!

By anon111860 — On Sep 18, 2010

Pluto is an icy celestial body which nobody ever touched, let alone gotten close to. By definition it cannot be classified as a planet anymore. Get over it! Why are these people growing an attachment to something inanimate. You don't see Pluto complaining!

By anon89806 — On Jun 12, 2010

dude, pluto isn't gone, it's just not a planet. you guys sound like it died or moved to some new galaxy or something. it's still here, pluto just isn't a planet. let that sink in.

By anon89511 — On Jun 10, 2010

Look, Pluto's not a planet, so stop complaining. It wasn't named after mickey mouse's dog, it was named after the god of the underworld. Get over Pluto! I never thought it was cool. Stick to Uranus, Jupiter, Earth and Saturn to love. But Pluto? Get real.

By anon80086 — On Apr 26, 2010

i think it is still a planet!

By anon77787 — On Apr 15, 2010

Pluto deserves to be a planet. it was one for years. Come on astronomers, why do you have to rid of my favorite planet?

By anon71759 — On Mar 19, 2010

The unmitigated gall of so called astronomers that pull this. Just by inspection its obvious Pluto is a planet. Any third grader could tell you that.

We don't have to follow this lunacy, we can still call Pluto a planet!

By anon69924 — On Mar 10, 2010

Three years later and i find out that pluto is no longer a planet. i have been saddened beyond belief.

By anon67610 — On Feb 25, 2010

I don't here anyone complaining about the sun being called a Dwarf Star. Shouldn't our beloved Sun, a Dwarf Star, be allowed to have Dwarf Planets, like the most glorious Pluto?

I say we should celebrate all the Dwarf Planets, of which there are currently five: Pluto, Ceres, Makemake, Eris and Haumea.

By anon65607 — On Feb 14, 2010

Goodness people are dumb. Who cares if your 10 year old daughter misses Pluto or not; keep calling it a planet if you want, you'd just be wrong. It simply doesn't qualify as a planet and this article explains why. Science isn't like religion; it changes with time. Beliefs aren't based on faith; it's based on what we continue on learning.

By anon65306 — On Feb 12, 2010

Pluto rocks! I will never accept it as a 'lesser' planet, because it has just as much a right to be a planet as any of the others.

By anon63355 — On Feb 01, 2010

OK, we understand the rationale behind the decision but in our house, we will let Pluto stay if it wants. My 10 year old son agrees with those who want Pluto to remain a planet.

By anon59958 — On Jan 11, 2010

I will miss you Pluto, so much.

I think it should stay a planet. Now people have to get rid of all thew science books stating that Pluto is a planet. Doesn't that confuse everyone.

By anon56774 — On Dec 17, 2009

I think it's hilarious that someone who says "Science is science" can think Pluto is "light years away."

Pluto is an average of 39.5 AU away from us, a light year is 63,000 AU.

By anon56422 — On Dec 14, 2009

Yeah we grew up thinking Pluto is a planet, but don't argue with science! Some of you sound like you're in the sixth grade. If it's not large enough to clear its own atmosphere it can't be classified as a classic planet. We should be appreciative of this new information. Read all the information and learn something.

By anon55649 — On Dec 08, 2009

this is so stupid. just like people nobody can change us. we are what we are and that's it.

By anon51293 — On Nov 04, 2009

I know that many of you will not believe this, but Pluto, is my home planet. We first came to earth in the early 70s but found the conditions too volatile. We have just recently returned to attempted to get to know your species better but were deeply hurt when your race no longer recognized out status. Our planet may be small compared to yours, but it is still a proud and noble planet. I hope that our influential leaders can negotiate with your scientists and convince them of their insulting error.

By anon51022 — On Nov 02, 2009

figures they would get rid of the one named after mickey mouse's dog =(

By anon50889 — On Nov 01, 2009

I think that is so dumb! Pluto should remain a planet. It's not the same without Pluto. Pluto is my favorite planet. Who cares if it is too small? A planet is a planet no matter how small. Just like a person is a person no matter how small. It's unfair they took Pluto out!

By anon50722 — On Oct 31, 2009

Whether Pluto will regain its status as a planet remains to be seen. The vote to declassify Pluto as a classic planet was close, and hotly debated. Moreover, there is strong opposition to Pluto's declassification and the decision by the IAU may be overturned. I'll await further developments before I mourn the loss of our cool little buddy in the outer reaches of the solar system.

By anon47880 — On Oct 07, 2009

We hung the nine planets in my son's room when he was three years old. He stares at them at bedtime and memorized their order long ago. He is five now and I just broke the news to him about Pluto (I didn't have the heart to before, besides I figured his 3-4 year old brain would not get it). He was totally bummed out! It is cute how many people, myself included, are attached to this dwarf planet. It has a fun name and is little and cold like santa's home. Kids relate to it and love it. Pluto will be missed!

By anon21040 — On Nov 09, 2008

I honestly don't understand why people are so bummed. Science is Science and Pluto is scientifically not a planet, that's just the way it is. I mean how could you grow an emotional attachment to an object that is light years away :) move on. just remember this: MY VERY EAGER MOTHER JUST SERVED US NOTHING

By k050173 — On Oct 27, 2008

I understand why people are so upset about this since I feel the same way. I feel like something has been lost. I grew up with the nine planets, and when I list the names, I can't stop at Neptune. I still think of Pluto as a planet, dwarf or not. Karen E. W.

By anon19491 — On Oct 13, 2008

i think Pluto should still be a planet. i really don't get how Pluto isn't supposed to be a planet. i think it's stupid. I MISS PLUTO!!!

By anon6751 — On Jan 08, 2008

I still don't understand why it was such a big deal i believe pluto should still be a planet so what if it's the smallest i think that pluto should still be a planet it represents people so what if i was a dwarf and i was super tiny and i wanted to be president but i couldn't because i was too short these planets represent people so what if it's small...

RIP PLUTO!!! WE WILL MISS YOU!!! much love, me

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