Why do we care if Pluto is classified as a planet? Classification doesn't change anything fundamental about Pluto, like the orbital parameters or composition. We can't hurt an inanimate object's feelings, correct?

In the case of our Solar System, do we expect the taxonomy to change as our understanding changes due to new discoveries? I wouldn't think the taxonomy would at all drive or affect the science of Astronomy, but rather be driven by it. I don't see how astronomers' execution of the scientific method changes at all depending on Pluto's classification.

Is it that Pluto scientists worried about reduced funding or status?

Note: This question is not meant to provoke an argument of whether Pluto should be classified as a planet. Instead, I am genuinely curious as to why it matters for the field of Astronomy.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – called2voyage Mod
    Commented Jan 22, 2021 at 14:32

7 Answers 7


Is it that Pluto scientists worried about reduced funding or status?

I did some searching to see if and how the classification as a planet would matter.

According to a piece by the Atlantic:

A planetary designation may be largely cultural, but that doesn't mean it isn’t meaningful. Funding for research relies on popular understanding. Surely it helps when an object of interest—be it a brontosaurus or a celestial body—is widely known.

I think this makes sense, it's the human factor. While Pluto does not care whether we call it a planet or a large-*** asteroid, the people who approve funding for space exploration do. Imagine you are a scientist pleading for funding before some government organization. The people who would approve your project may or may not be scientifically inclined. If the latter case, it makes a huge difference whether you wish to explore "the last, least explored planet on the solar system" or "some very popular rock on the Kuiper belt".

Is it that Pluto scientists worried about reduced funding or status?

While I have never heard of astronomers being so specialized as to be called plutologists, I do think this is the case. Our generation grew up learning in schools that Pluto is a planet, so for now research on Pluto may be safe. But consider Haumea and Makemake. Those guys could probably hold the keys to many secrets about the origins of planets and the early days of our solar system, even more so than Pluto. But the vast majority of people have never even heard of them... So trying to convince a panel that it is a good idea to send a probe in a Makemake flyby trajectory is a much harder battle than sending another one to Pluto.

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    $\begingroup$ IMHO, Pluto's reclassification was actually a promotion. Rather than being the ninth (ho hum) planet, is the first object which was discovered to be in a solar orbit that has a resonance relationship with Neptune (or, I believe, any planet). While there are today many known objects that have such orbital relationships, Pluto will now and forever have the distinction of being the first one discovered, which set the way for the discovery of the rest, much as Ceres has the distinction of being the first asteroid to be discovered. $\endgroup$
    – supercat
    Commented Jan 21, 2021 at 17:22
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    $\begingroup$ @supercat It can be a floor wax and a dessert topping. $\endgroup$
    – Barmar
    Commented Jan 21, 2021 at 18:27
  • $\begingroup$ @supercat And as M. A. Golding mentions in his answer, Ceres was also originally classified as a planet. astronomy.meta.stackexchange.com/a/677/34975 $\endgroup$ Commented Jan 22, 2021 at 9:35

While it's possible that there could be legal or financial ramifications, I think the true reason is simply that scientists prefer their terminology to be precise. For instance, when they write articles that describe features of planets in general, they want their conclusions to be true of all the bodies that are in that category. And it would be bothersome if they frequently had to write "all planets except Pluto" to handle its differences from the other planets. I expect that they'd noticed that they often had to treat Pluto as a special case like this, so it made sense to recategorize it.

You can find similarities in other areas of science. For instance, anthropologists obsess over which genus early hominid fossils belong to.

Also, this isn't really just about terminology. Much of science is about modeling, and categorization is important in creating useful models. Of course, they could accomplish this with their own scientific jargon, and leave things as they are in casual language used in public media. But that also makes things more confusing.


It could matter one day on a legal point of view.

The exploitation of space resources is already subject to international treaties. For example the Outer Space Treaty, with an article that goes:

Outer space, including the moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means.

Note the use of the very broad "the moon and other celestial bodies", which encompass basically everything outside the Earth.

Some people are not so happy with these treaties, and have actually started to walk away from them (for example with the SPACE Act in the US). Because of this will to exploit space resources (whether we think it is legitimate or not), some day there might be a need for a new, more flexible treaty. The classification made by scientists could be used by policymakers to define what is exploitable and what is not. I can imagine that they could preserve the international status of planets to keep some people happy, but open asteroids to private appropriation and mining. Depending on its classification, Pluto could be open to exploitation by this new treaty.

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    $\begingroup$ Sorry if this sounds more like science fiction than astronomy! I am currently reading the Mars trilogy by Kim Stanley Robinson, so I got carried away a bit. :) $\endgroup$ Commented Jan 21, 2021 at 12:52

Essentially you will need to ask the individual people who want to see it classified as a planet - and the answer can not be objectively but will always reflect a personal view

However, obviously the majority of astronomers (or at least attends at the IAU assembly) did NOT care (enough) to see it as a planet. Scientifically it's just a category name tag we put on it, and it doesn't change its physical significance or anything whatsoever about Pluto itself.

So what you see and here here and other places around the categorization and definition of what constitutes a planet is a minority, yet a loud one.

So essentially it all boils down to human factors and animosities:

  1. change-adversity: it was always this way, I don't want change
  2. money: I want to make science in the Kuiper belt. I can sell a planet better than one of possibly many dwarf planets.
  3. attention: I do research in the Kuiper belt. Planets give me more exposure and traction in the public
  4. prestige: Doing planetary research may sound more relevant than investigations on one-of-many remote ice- and dirt-balls.
  5. history: I (or my institution / country / ...) want also be recognized as a discoverer of a planet

You can probably come up with more sentiments why one might want Pluto remain or become again a planet. And some of these sentiments and motivations may even be around inconsiously.

That said, I believe that even the notion about money or attention are not necessarily very well founded. The perceived controversy in the public about Pluto being a planet or not, or once having been named planet, gave the issues with the Kuiper belt objects MUCH more attention in the broader public than it would have gotten otherwise. So... maybe even this 2nd order phenomenon might play a role in this issue: just keeping up awareness and the topic in the public without actually being interested in a change back itself.

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    $\begingroup$ You omitted religion. We have had more than one user who ultimately admitted that their objection to the relegation of Pluto's planetary status was based on religion, specifically, Young Earth Creationism. I've seen this elsewhere as well. There is no place at this site for those who conflate big bang theory and evolution as one and the same. There is no place at this site for those who object to science in general. This is a scientific site. There are plenty of nonscientific sites within the SE network. They can go there, or to plenty of other nonscientific sites on the internet, $\endgroup$ Commented Jan 21, 2021 at 12:08
  • $\begingroup$ @David Hammen How would our classification of an object that was unknown before the early 1900's be of any concern to Creationists (or any other religion)? $\endgroup$
    – D. Halsey
    Commented Jan 22, 2021 at 23:44
  • $\begingroup$ @D.Halsey The key characteristic in the IAU definition of a planet that makes Pluto not a planet, the "clearing the neighborhood" clause, implicitly assumes the solar system is about 4.6 billion years old (which it is). Young Earth Creationists believe the Earth, the solar system, the entire universe, is less than 10000 years old. For them to acknowledge that Pluto is not a planet would mean their beliefs are wrong. So they insist that Pluto is a planet. $\endgroup$ Commented Jan 23, 2021 at 1:33

Why do we care if Pluto is classified as a planet?

The IAU doesn't care whether Pluto is a planet or not. They didn't change the classification of Pluto: They defined the word "planet."

There is no practical reason to aribitrarily assign specific objects to different categorgies. On the other hand, it facilitates communication if categories are defined in clear, objective ways so that everyone can immediately see what to call any newly discovered object.

The IAU was aware that there would be sentimental backlash if their definition of "planet" excluded Pluto, but they really had only three choices;

(a) Define "planet" in a simple, natural way that excluded Pluto, and tell the world that the solar system has eight of them,

(b) Define it in a slightly more aribitrary way that includes Pluto, and tell the world that we don't know for sure how many planets are in the solar system because there could be others out there waiting to be discovered, or

(c) Define it in a completely arbitrary way just so that we could continue to say that we have exactly nine planets.

Option (c) never was on the table. It would have been no use to the IAU at all. Option (b) was maybe a possibility, but it would be a compromise between science and sentiment. In the end, they decided not to compromise.

You can call Pluto a planet, and probably nobody will demand that you stop unless either they are a hopeless geek, or you are trying to publish an academic paper.


I note that Pluto is a planetary mass object (PMO), or planemo.

A planetary-mass object (PMO), planemo,[112] or planetary body is a celestial object with a mass that falls within the range of the definition of a planet: massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain core fusion like a star.[113][114] By definition, all planets are planetary-mass objects, but the purpose of this term is to refer to objects that do not conform to typical expectations for a planet. These include dwarf planets, which are rounded by their own gravity but not massive enough to clear their own orbit, planetary-mass moons, and free-floating planemos, which may have been ejected from a system (rogue planets) or formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs).



At the pressent time the planetary mass oobjects in the solar system includes eight objects classified as planets (including four terrestrial planets, two gas giants, and two ice giants), five classified as dwarf planets (including one asteroid and four trans Neptunian objects or TNOs), and nineteeen natural satellites or moons. And there are some candidates for classification as dwarf planets among TNOs.

And I think that most professional astronomers, amateur astronomers, and science fiction fans can list most of those planetary mass objects from memory.

So if I was an astronomy student studying for an astronomy career around 2006 I might be a little annoyed by the redefiniton of a planet. I might take the objection to increasing the number of recognzied planets in the Solar System and the redefinition of planet to avoid that as insulting to my ability to remember the names and data of a large number of objects.

Since the old definition of a planet left out the nineteen planetary mass natural satellites, continuing to use it would result in astronomy students and school children having to remember the names of only fourteen planets so far, with another five presently considered strong contenders to be classified as dwarf planets or planetary mass object, and the hypothetical and as yet unconfirmed Planet Nine. So by 2021 there would be at most 20 objects listed as planets.

And possibly the largest trans Neptunian objects, the ones of planetary mass, could have been classified as a category of trans Neptunian planets, in a separate category from terrestrial planets, gas giants, and ice giants.

I also note that the dwarf planet and asteroid 1 Ceres, discovered in 1801, and the other three first asteroids to be discovered, were originally listed as planets. So after Neptune was discovered in 1846, there were usually at least twelve planets listed in astronomy books.

Actually the fifth asteroid to be discovered, 5 Astrea, was discovered in 1845, and at least one asteroid was discovered each year afterwards, and those newer asteroids were aleo often listed as planets. All the asteroids dicovered up to 15 Eunomia, discovered in 1851, were sometiems considered planets, until they were reclassifed as minor planets or asteroids in the 1850s.

So for a short period after 15 Eunomia was discovered, there were 23 recognized planets in the Solar System.

I also note that many science ficiton fans are familiar with many fictional planets, and that many astronomers who search for exoplanets around other stars may be familiar with many of the over four thousand exoplents which have been discoverd so far.

  • $\begingroup$ I think Pluto would be better classified as not just being a trans-Neptunian planet-mass object, but as the first discovered object in a class of object with a solar orbit that is resonant with that of a planet, just as Ceres went from being the eighth (ho hum) discovered planet to the first (hey, cool) discovered solar orbiting object that shares its orbital zone with other such objects. $\endgroup$
    – supercat
    Commented Jan 22, 2021 at 16:33

My problem with the whole topic is that it is a big bikesched. Everybody has an opinion, we get a lot of chat, visitors and upvotes from it, but honestly it is not what I could consider an interesting Astronomy read.

The other side of the story is that it attracts visitors. It is not only useful for the site, but also the popularization of science is something what this site can do to make the whole world a little bit better.

Summing these together, I suggest to not have any specific rule about the topic.

If an essentially similar question was already asked, close it as a dupe. If not, allow it - it is still on-topic. Simply there are not too much essentially different questions about the planet-ness of the Pluto what was not already discussed.

  • $\begingroup$ Was this answer accidentally posted to the wrong question? Should this answer have been posted to: astronomy.meta.stackexchange.com/questions/667/… ? $\endgroup$
    – Connor Garcia Mod
    Commented Jan 22, 2021 at 19:31
  • $\begingroup$ @ConnorGarcia I tried to answer this, from the main site imported question, as a meta question. Although my answer would be the same also there. $\endgroup$
    – peterh
    Commented Jan 30, 2021 at 22:48
  • $\begingroup$ Unfortunately, they closed it on the main site, migrated it here, and then closed here. I think it was a legitimate question, but who knows? $\endgroup$
    – Connor Garcia Mod
    Commented Jan 30, 2021 at 23:59

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