Solar System Planets and Moons

Is Pluto A Planet? – Pluto’s Planetary Status, Alternative Definition and Reviews

Is Pluto A Planet?

Beginning with August 2006 and IAU’s controversial decision, Pluto’s planetary status and the concept of the planetary definition has been on debate.

On our related article, we mentioned about Pluto’s demotion, the motives behind IAU’s decision, its worldwide outcomes and our comments based on the IAU announcement.

On this article we would like to refer to the objections against IAU‘s definition, alternative planetary definitions that popped up in last years and neutrally consider suggested definitions and comments in scientific context.

IAU’s Planetary Definition

Before starting with the alternatives, let’s remember IAU’s definition once more:

Following several internal debates and voting, on 24 August 2006, IAU introduced a new definition for planets. According to IAU, a proper planet should be orbiting Sun, should have reached to nearly round shape settling a hydrostatic equilibrium and should have cleared the neighbourhood around its orbit. Those who can not clear the neighbourhood are redefined as dwarf planets. While this changed Pluto‘s planetary status from an ordinary planet to a dwarf planet; Eris, Ceres, Haumea and Makemake were also categorized in same title.

The concept of “demotion of a planet” was surely an interesting subject for everyone including media. Consequently, public together with several scientists were interested in and some were opposed to this definition.

Pluto Protests in September 2007
Al and Cherylee Tombaugh, Clyde Tombaugh’s son and his wife join demonstrations on 1 September 2006 in New Mexico State University. Clyde Tombaugh is the discoverer of Pluto. Image source here, Credit: NMSU/Darren Phillips

IAU’s Planetary Definition – Pros and Cons

What we mostly find problematic about IAU’s definition is, as criticised by the opposers, its unclear language which lacks straightforward definitions and words.

While it’s also criticised as being a definition “only for Solar System“, we do not see much problems with that. At least for now! Because although thousands of them discovered, we do not have enough knowledge about other planets (or star systems so to speak) to classify them yet.

But on the other hand, those critics does have a point since this is classifying a sample system of 1 (one) and this sounds little bit odd. What’s more, at some point we will need to classify them with a universal concept.

Another universality problem with the definition pops up for the planets not orbiting any stars. Again that can easily be neglected as “out of concept” for now, but not for future.

But why to paper it over now and why should science ignore exoplanets in such classification? That’s a fair question to ask.

A Planet with no stars
Artists impression of a starless planet in the universe. Source here, Credit: NASA/JPL Caltech/ R. Hurt

In addition to above, the word phrases: “nearly round shape” and “clearing the orbit around its neighbourhood” are little bit “fuzzy“. The phrase “being rounded” is not causing much problem as hydrostatic equilibrium can “mostly” distinguish the planetary bodies from each other.

The Issues About “Clearing The Orbit Around The Neighbourhood”

But than it comes to clearing the orbit, which is the key part of the definition discriminating a planet and a dwarf planet. The opposing scientists go against this section of the classification because the definition itself does not set a clear minimum limit. And it sure will remain controversial until “clearing the neighbourhood” is properly settled to a mathematical concept. See this link for the related calculations present.

Even if above mentioned mathematical concept is clarified, there is another problem with the definition considering other star systems. While the Solar System has its own gravitational dynamics, other star systems have complete different ones. And the ability to clear the orbit for a planetary body varies by many parameters including a planet’s distance to its star, its distance to other planets / belts together with sizes of its star and neighbouring planets.

Asteroid Belt
An illustration of the Asteroid Belt. Source here, Image credit: NASA/JPL-Caltech

This will have funny consequences on other star systems. The reason is, if IAU’s current definition is taken into account for another star system, Earth can be a dwarf planet. And in another scenario, moon Charon can easily become a regular planet. It’s just a matter of playing with the sizes of the stars and orbits of the planets. And you can easily do that considering the existence of trillions of star systems in the universe.

Possible Deficiencies of IAU Definition

What IAU definition lacks is the primary definition of a planet: Not being a star. Considering only the Solar System, IAU definition saves the day since there are no other stars in solar system.

But consider the hypothetical Planet X in our Solar System. Planet X’s gravitational influence –bigger than Earth’s– is still known and researchers still look for it. And one of the recent studies regarding this gravitational influence speculates that the source of this mass might also be a primordial black hole. If that’s the case, we might have to call this black hole a planet, which obviously will not be appropriate. Although that’s a minimal chance, this is technically possible and is a clear reflection of a classification deficiency.

A black hole illustration
A black hole illustration. Source here, Credit: NASA/ESA and G. Bacon (STScI)

Geophysical Definition of A Planet – Is Pluto A Planet Than?

Above matters have been discussed by scientists and public from day one, and the opposing ideas matured in time. Even as an instant reaction, more than 300 planetary scientists opposed the definition with a statement immediately after the IAU announcement. And the conflict went on and on for over a decade.

Finally planetary scientists came up with an alternative definition for planetness, especially focusing on the absence of fundamental properties of a planet in IAU classification.

The geophysical planet definition introduced by Kirby D. Runyon, a planetary geomorphologist is as follows:

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

Geophysical Planetary Definition – Pros and Cons

Above definition classifies every rounded body (that including the moons) as a regular planet.

One of the many matters criticised -especially- by planetary scientists in the past was that IAU formed a definition without contribution of many of them. And well, their argument obviously grounds on the fact that it is their profession! Consequently, this developed a “reactional language” which refers to the “clearing the orbit” issue.

Pluto's orbit
Pluto’s orbit intersects with Neptune’s. Among with the orbits, major events are marked on this image. Credit: NASA/JHUAPL/SWRI

The definition, as its name refers, is focused on geophysical properties of a planet. The words “sub-stellar” and “nuclear fusion” clearly draws a line between a planetary body and a stellar body.

Runyon‘s definition disregards a planetary body’s orbit, which in geophysical sense does not distinguish moons and planets from each other. Although planetary scientists refer this as a “usual” denomination within the community and planetary researches, this is something new for the rest of the world.

One advantage of this classification is that it will definitely be universal with current global astronomical knowledge. It is applicable for any planets in any star systems. Even it covers planets not orbiting any stars.

One minor issue on this definition is, similar to the “nearly round shape” of rival definition, its clarity with a universal approach regarding the shape of a planet. Although it looks like it draws a strict line, it might not be doing that considering possible real cases when an “almost triaxial ellipsoid shaped” body is detected.

Planet Haumea
An illustration of Haumea with its moons. Credit: NASA, ESA, A. Feild (STScI)

In any case, overall clarity of the definition sounds sufficient enough comparing to IAU’s. On the other hand, Runyon also declares on his published paper that the definition leaves the classification of “brown dwarfs” to the future which might partially drop charges regarding IAU’s definition.

The Purpose of Scientific and Planetary Definitions

At this point, it is worth to remember why scientific definitions and classifications exist and to get to the bottom of this matter.
We believe they exist in order to place the objects of interest to the scientific context, to better understand and handle further researches and to serve the proper way of information for educational purposes in terms of historical perspective of nomenclature.

It is clear that humanity’s knowledge revolves fastly. And consequently the way we understand things varies in dramatic ways. In the end of the day, it is of utmost importance to modify our knowledge and classifications by means of above stated purposes.

That brings us to following questions: What do those definitions offer? How useful are they in terms of scientific and historical concept? What are their scientific benefits and which parts are most useful in terms of science and education? The answers in common sense should end this debate, but the subjectiveness of the concept makes it harder to conclude.

Solar System Planets
Sun and the Eight Planets. Credit: Wikipedia

Historical Context of Planetary Nomenclature

Historical background of humanity’s astronomical knowledge is one important fact here. Although the history of astronomy began with the first person looking up to the skies and thinking about the lights on heavens, our basic written knowledge goes back to few milleniums back. And for centuries we have seperated stars from planets and moons from planets. Should we just throw that classification into trash?

While ruling out any kind of orbital property, the geophysical planet definition brings a complete new way of comprehension and a revolutionary point of view of planetary bodies; both for the textbooks and for public. And it positions itself as a superior classification by means of better understanding about what planets (and moons in hydrostatic equilibrium state) are, how they are formed and evolved.

Although it sounds logical, the critical point is to take the perception of public and students into account in combination with above. If it is evaluated that it will create a scientific and social virtue in long term, than there is no point to stick with the older denomination which might have relatively lost its value.

With above discussion, another (double) question pops up;
Who are the authorized people to answer the question: “Who can choose the authorized people for such nomenclature?” ? Those are tricky questions to answer and are couple of side reasons of this polemic.

Nine Planets
Source here, Credit: NASA

IAU’s or Planetary Scientists’ Definition?

Elaborative version of this question is, should we classify planets in terms of their size and motion or their physical attributes? Or should we combine these two? A common, global agreement will sort this issue out. Regardless of how the planetary definition is revised, we believe it must include the maximum amount of benefit for everyone, independent of astronomy’s sub-divisions.

IAU’s definition might have problems, discrepancies and deficiencies. But despite it is problematic and unclear, when you make sure what it means, it still works. Or alternatively we can say, when it is clarified and considered only as a “Planetary Definition for Solar System Objects“, we find it scientifically sufficient and still useful.

The geophysical definition on the other hand pretends to include a more functional classification and to offer a better perspective especially for students and public. They claim if people learn planets in their way, they will be capable of grasping the physical features of planets and non-planets. And they believe it will lead to a better understanding about planets’ formation.

Meanwhile, IAU definition supporters object to such a classification with the problem of “number of ordinary planets“. The number will probably be more than a hundred and will eventually increase. That is also a point, but the benefit of “having less number of planets in books” is truely a controversial issue.

Another approach on this matter may also be combining the definitions. For example the moons can still be named as moons (or planet moons), and planets can be classified like Inner Solar System, Gas Giants, Kuiper Belt Planets and so on under the big roof of Planetness. That might sort out the majority of discussions.

Solar System Moons
Credit: Wikipedia

The Matters Out of Scientific Context

Besides all these, there are matters which jeopardize the convenient discussion platform for the concept.

First thing is the way some people align when it comes to Pluto. Explaining above matters “specific to Pluto” looks like the concept is slightly shifted from its scientific frame. In addition, the demonstrations and campaigns prepared for Pluto looked like a hollow circus to many people. But as you can see from above, the opposing scientists clearly have a point!
Another criticism to the “Pluto Forces” is that they are a US community and they defend Pluto because Pluto is the first planet discovered by a US astronomer.

Above matters cause the “Pluto opposers” to underestimate and dismiss the opposing claims, which leads to an impossibility for a consensus. What’s more, they mostly blame the Pluto supporters with pragmatism. They believe the IAU’s definition is clear and the opposers are just tweezing the words in IAU’s definition for saving Pluto. Keeping above facts in mind, that might be a partially fair judgment from that point of view.

All we can say about this chapter is: Come on guys! This is only a planetary definition!

Pluto Protests
Ok, that was fun.. 😀

Conclusion (Or Not!)

As we frequently face different reactions in terms of Pluto’s planetary status, we would like to note once for all:
Plutopic considers all scientific facts including such classifications in a scientific point of view. Similarly on our articles about Pluto and her neighbourhood, our only intention is to serve the scientific knowledge with minimum subjective comments.

On the other hand, nomenclature is another expertise which also involves historical and cultural knowledge. And we do not feel that competent on this subject.
That’s why this article does not conclude and “choose the side” of the “most effective or useful planetary definition” as we do not think we are qualified to lead readers in that manner. We just present the arguments, add our humble comments and leave the decision to the experts.

But we are open to any comments! Please use below comment box and let us know what you think.

And final words: We hope to see a global consensus very soon!


  • 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. Laurel Kornfeld December 10, 2019
    • Plutopic December 17, 2019

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