Pluto as seen by New Horizons

Why is Pluto Not A Planet? – Witnessing The Demotion Of A Sphere

Pluto and Her Broken Heart

Why is Pluto not a planet?

This is a topic where Pluto’s broken heart crosses ours..

On this article we focus on IAU‘s rearrangement about classification and definition of planetary bodies in Solar System which consequently downgraded Pluto’s planetary status. For the opposing ideas and alternative planetary definition proposals, see our next article: Is Pluto A Planet?

Let this article begin with some questions:

Is Pluto still a planet? If not, what is Pluto and why is Pluto not a planet? What happened to Pluto? Where is Pluto now? Is Pluto feeling well?

Here we are to answer some heartbreaking questions.

Pluto as seen by New Horizons
A sad planet Pluto picture from New Horizons, captured on 13 July 2015 from 1.7 million kilometers away. Credit: NASA/JHUAPL/SwRI

You would feel hurt if you were downgraded right? Probably, so does Pluto!

Is there any way to fix Pluto’s broken heart? Let’s see.

Is Pluto A Planet? – Brief Explanation of What’s Known

Is Pluto a planet? A question of interest for more than a decade.

From the date of discovery (1930) till 2003, this beauty was an ordinary and content planet. After 73 years, this whole story began with Eris, the scapegoat.

On 21 October 2003, discovery of planet Eris took place. Similar to planet Pluto, Eris is also located in Kuiper Belt; and its orbit interferes with Pluto’s orbital path time to time.

Artists impression of Eris
Artists impression of planet Eris. Source: Hubblesite, Credit: NASA, ESA, and Adolph Schaller (for STScI)

Since the size and the orbit of Eris is similar to Pluto’s, astronomers started to think how to redefine such objects as there were a lot more candidates to be discovered from Kuiper Belt. In addition to that, characteristics of those were very similar to Pluto’s.
That limited the number of options with two: Either new discoveries should have been called ordinary planets or Pluto should have been re-classified among with others.

It took more than 2.5 years for IAU to consider this. And time helped to those and their hidden agenda. Discovery of 2 more planetary bodies, Haumea and Makemake took place in between helped to justify all. But not in our hearts!

IAU 26th General Assembly: Messenger of Dark Days

It was 24 August 2006 when IAU (International Astronomical Union) gathered for its 26th General Assembly. On this routine meeting, two main subjects of discussions out of six were as follows:

  • Definition of planets
  • Definition of Pluto-class objects.

IAU members had been voting and taking the decisions about those subjects, where only 5% of worldwide astronomers were participating.

On the announcement page of IAU, it’s stated that “the votes for Definition of Planet ‘was not counted’ but the statement passed with a great majority”. And next statement about the voting says: “Definition of Classical Planet” had 91 votes in favour, but many more were against. So actually there were no counts. Suspicious!

IAU members vote
IAU Members voting against planet Pluto. Source: IAU Website

Updated Definition of A Planet

New definition for an ordinary planet as announced by IAU on 24 August 2006 is as follows:

A planet is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Why Is Pluto Not A Planet Any More? – Being Kicked Out of Planetness

According to IAU, newly introduced definition of dwarf planets involved (a) and (b) above, but the opposite of (c). So this meant planet Pluto was not a planet any more, and was re-classified in a new concept: A dwarf planet.
Why is Pluto a dwarf planet? In other words, why does IAU call Pluto a dwarf? What is a dwarf planet? Let’s see in details.

Pluto as seen by New Horizons
Pluto is a planet unless you call her dwarf! Details about this fascinating Pluto image here. Source: NASA

The definition means a dwarf planet should directly be orbiting Sun, should have sufficient mass for a hydrostatic equilibrium and nearly round shape; but didn’t have cleared the neighbourhood around its orbit.

But wait, what does “clearing neighbourhood around the orbit” mean? Because it’s the only matter causing trouble to our beloved!

Clearing the Neighbourhood Around the Orbit

What scientists mean by clearing the neighbourhood is the gravitational power to remove other objects from the effective orbital path. And this includes being free from any kind of orbital resonance (like 2:3 resonance of Neptune and Pluto case).

At this point, there has to be a clear definition about the limits of this orbital suppression because most of the planetary bodies (depending on the distance between each other) are able to influence each other.
Take Earth and Moon for example.
First of all, Moon is always around Earth’s orbital neighbourhood similar to other satellites and planets.
In addition, it’s not only Earth dictating its gravitational power to the Moon, Moon effects Earth’s orbital path and motion as well. And that surely applies to Neptune-Pluto interaction!

Orbit of Pluto with others
Orbits of Pluto, Neptune, Uranus, Saturn and locations of the distant unmanned exploration spacecrafts. Source here, Credit: NASA

Although IAU did not directly refer any of them on its statements, there are some definitions and calculated discriminants offered by different scientists which are related to this subject.

Formulation of Neighbourhood Clearance

In that aspect, there are three seperate constants calculated via dedicated formulas: Stern-Levison‘s Λ (lambda), Steven Soter‘s µ(mu) and Jean-Luc Margot‘s Π (pi) calculated in different manners. Among all these formulas, there are calculated limits for the orbital clearing. And whichever formula is taken into account, all 8 planets successfully exceed the limits where Pluto, Ceres and other Kuiper Belt objects fail by far. (For details of those calculations, follow this link)

Although above formulas scientifically make sense and can draw a line, IAU’s famous article (c) still does not do that. If a planetary object could not clear the other object from its orbit, than the other object (let’s say with a bigger mass) faces the same problem as well. In that manner, when we go one step further and check all 8 “planet“s orbits, none of them will remain as planets, including Earth! For example if Pluto is not a planet, that makes Neptune “not a planet” too.

Why Is Pluto Not A Planet – Main Motive Behind The Decision

Despite above contradiction, we find it logical in its extent when and if discriminants taken into account and clearly stated.
Since statement (c) and hence the planetary definition of IAU is not 100% clear, it did not fully satisfy us and many scientists who contributed for other definitions. This leads with other conclusions about what has happened.

Pluto NASA Images
High resolution picture of Pluto atmosphere, taken by New Horizons. Source: NASA

One important thing to mention at this point is: Above stated “IAU’s definition of a planet” does not mention anything about a planet’s (or a dwarf planets’) distance from Sun.

However when you ask the question: “Why is Pluto not a planet any more?” to IAU, one of the answers you’ll get will be: “because Pluto is a trans-Neptunian object”. We as Plutopic believe that this is one of the basic motivations and excuses behind Pluto getting definitively kicked out, although it’s not a part of official definition.

It’s very likely that there won’t be any more planetary discoveries from Sun till Neptune orbit from now on*.

However, there are quite a lot of dwarf planet candidates, which are beyond Neptune and some are similar in size with planet Pluto. In case Pluto somehow becomes a planet again, most of others should be planets as well.

To sum up, we believe this 3-sectioned planet definition of IAU is here to mostly avoid further and frequent changes in textbooks. That’s it!
But if that is the case, what is the scientific or educational benefit of that? It’s hard to say.

Pluto Is Not a Planet Anymore! – What Has Changed About Planet Pluto?

So as a conclusion;

Is Pluto still a planet? Pluto is called a dwarf planet by IAU for some time. And the worldwide press and textbooks mostly agreed with this. However, there are scientists who do not stick with this definition. We will get there.

Pluto Mosaic
A Pluto Mosaic, where discoverer Clyde Tombaugh’s photos embedded inside. Source: NASA

As mentioned above, the unfortunate statement of IAU took place in 2006. From that date till now, the majority of our knowledge about many Pluto facts is supported by New Horizons Mission. And the mission showed the world that Pluto is an active planet with her geological and atmospherical diversity.

One important and indirect consequence of New Horizons Mission was discovery of new Pluto Moons in Pluto System. And the next one is about the rings of Pluto. Because although the scientists believed the opposite before 2015, New Horizons Flyby proved that Pluto has no rings.

However, whether Pluto possessed some moons, rings or not; that does not change anything about IAU’s definition. Because the definition is only about the “orbital neighbourhood“, not the planetary distance. That’s worth to be noted here as it causes confusion for many.

Rather than above, nothing changed about our Pluto knowledge since 2006.

Evaluation of IAU’s Definition

After 1990’s, number of Kuiper Belt Objects discovered has been increasing exponentially. And that was going to cause confusions in terms of classification of Solar System rocks. With that pressure, IAU organization considered a new definition and it was truely painful even just to debate and vote.

A Kuiper Belt Illustration
A Kuiper Belt illustration including close-by planets’ orbits. Source: NASA

It literally took years to conclude the definition for the union. This was a real burden and that’s why we appreciate the efforts of IAU members who are precious astronomers.

IAU is a union consisting of astronomers. And those people are some of the most authorized people on their field. The union makes decisions via leading influences of members and side influences from other astronomers in the world. Considering above, it’s not really wise to expect such committees to take public demands into accounts and make their decisions accordingly.

However, there are lots of planetary scientists, astronomers and people who do not agree with IAU about new planet definition. Even there are scientists who offered seperate planet definitions and they have many supporters. See related article related to geophysical planetary definition.

On above mentioned article we also discuss scientific benefits of this and other possible definitions. Because that’s the only important evaluation of any kind of scientific classification in our point of view.

For now we would like to briefly mention that we believe with proper scientific referrals and a worldwide consensus, IAU’s definition would be a sufficient classification and would be consistent in its own scientific context. But how helpful is it to science, researchers, students and public? That’s another question to answer.

Pluto Is Not A Planet – What To Do Now?

Downgrading of Pluto from ninth planet to a dwarf planet is probably one of the unique scientific classifications running into this much public reaction. While that’s the case, most of the public interest about this topic consists of funny campaigns and jokes. But this makes scientists happy as well, since astronomy and far worlds of Solar System gets more attention one way or another.

Make Pluto a Planet Again!
A placard from a one-person protest: Source Link: ( )

Besides, you are reading this article.. The campaigns worked for Plutopic too!

As stated above, we believe we have to leave scientific classifications to qualified scientists and nomenclature experts. Besides that, it could sound harsh but we do not believe that this is public’s or individual’s duty or perfection to correct a scientific classification.

However, the question “Why Is Pluto Not a Planet?” did not only become a global phrase, but also a question asked by several scientists.

Pluto Is Not A Planet – Is There Something You Can Do About Planet Pluto?

In that perspective, we believe there is not much public can do to bring Pluto back to her ninth planet rank. Ok, public pressure generally works and is wise to raise when it comes to governmental decisions and public benefit (which might effect astronomy as well). But this time it is a context of pure science where none of the people have any direct interest.

That’s why it’s hard to say what kind of a scientific contribution “fixing a definition” might provide and what kind of a benefit you could obtain out of this, except a naive happiness. Well, happiness is good in any case. We like happiness 🙂 Anyway, at least this is how we approach this matter.

On the other hand, there are people who made quite a lot of serious efforts including building up campaigns for raising the interest in this topic. And this might have triggered planetary scientists as well. Wait, did we say you can not do anything earlier?

Despite above, if you are still willing to join the Pluto Forces, another thing we can recommend you is to persuade majority of IAU members to revise IAU’s definition of planets as IAU seems to be the only organization handling such astronomical classification job.

In case you accept such challenge, feel free to start with quoting below:

Is it worth to offend a planet for the sake of a better definition? That’s something to think about.

Most rational comment by far, comes from Jerry: “Morty, nobody change the planets!”

* Our hypothesis excludes dwarf planets (as if it is a thing!) where there is still a slight possibility.




  • We aim to build a proper Pluto encyclopedia and your contributions are highly appreciated!
  • Any deficiencies on this article? Please type to below comment box or contact us from this link. We are ready to check and clarify.


  1. Raj Pillai December 6, 2019
    • Plutopic December 17, 2019
  2. Saney August 21, 2019
  3. Laurel E. Kornfeld July 11, 2019
    • kevin July 12, 2019
    • Plutopic July 12, 2019

Leave a Reply