Talk:Dwarf planet

The first sentence says that a "dwarf planet" is one that is in "direct orbit around the Sun", where "Sun" is the one star at the center of our Solar System.

Is it true that dwarf planets by definition never orbit around other stars? I though the term was so carefully defined so that we could use it in our investigation of other planetary systems.)

(I've done some hunting for the definition, and many/most seem to reference orbit around the Sun.) -- Dan Griscom (talk) 22:47, 3 March 2025 (UTC)

The 2006 IAU definition only applies to the Solar System. It will be many centuries, most likely, before we ever have to worry about making that distinction in other systems. There are other definitions that apply to other systems as well but they are not officially in use. Serendi^podous 12:57, 4 March 2025 (UTC)

'Many centuries'? You sound like the man who said that we'll never know what the stars are composed of ... It seems to me very likely that we'll have plenty of information within at most 50 years to have to start discussing this matter in detail. — Preceding unsigned comment added by ~2025-36588-21 (talk) 11:38, 4 January 2026 (UTC)

I agree that trying to predict the rate of scientific progress isn't a good idea, especially on Wikipedia (where it borders on WP:CRYSTAL). However, Serendipodous is right that dwarf planets around other stars are unlikely to be of much interest in the near future; and when they do become of interest, adjusting our definition is simple. Renerpho (talk) 15:50, 4 January 2026 (UTC)

For what it's worth it seems like the IAU is/was working on a formal definition of an exoplanet. I don't believe it's been fully adopted, but it can be seen here:

Notably, it states that The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System. This implies that extrasolar objects like Pluto and Ceres would not be considered exoplanets, though it is unclear what they would be categorized as. ArkHyena (it/its) 22:45, 4 March 2025 (UTC)

Pluto and Eris would be considered planets, because they are large enough to become spherical under thier own gravity. The reason they are not planets is they are part of larger structure, the Kuiper belt. Serendi^podous 23:06, 4 March 2025 (UTC)

So apparently Alan Stern coined the term "dwarf planet" in a 1990 Icarus paper according to this 2008 interview, but I can't seem to find it on either Google Scholar or the Astrophysics Data Service. Even when broadening the year range to 1988-1992, I don't see any Icarus papers by Alan Stern that explicitly mention the term "dwarf planet". Instead, I found one paper by Gonzalo Tancredi and Julio Fernandez, submitted to Icarus in 1989 and published in 1991, which actually does use the term "dwarf planet", but only in passing:

"Due to this characteristic and its small mass compared to the rest of the planets, Pluto resembles more a "gigantic" minor body of the Solar System than a "dwarf" planet."

If that wasn't enough of a headache, I also seem to can't find a source backing up the claim that Alan Stern coined the term dwarf planet with the specific intention of being "an analogy to the term 'dwarf star'" and "as part of a three-way classification ... classical planets, dwarf planets, and satellite planets." Or to rephrase, I can't seem to find a source (preferably pre-2010s) that explicitly attributes this three-way classification to Stern. I don't have much time to investigate this more deeply, so I would really like some help getting this sorted out. Nrco0e (talk • contribs) 06:58, 15 April 2025 (UTC)

Interesting.

There are only a handful of papers in Icarus during 1989-1991 for which Stern was a (co-)author, none of which appears to be the correct one. This seems to be the only paper by Stern published in Icarus during those years that includes the word "dwarf", and that's in the context of dwarf stars, not planets (the paper is not concerned with classification). When Stern said in 2008 that he "coined the term dwarf planet in 1990 in the academic journal Icarus," I think he was mistaken about at least one thing: the year, the journal, or that it was him. I would guess he's wrong about the year at least, since the term "dwarf planet" already existed in a paper written in 1989 by someone else. If he is right about the journal then it must have been after 1984, which is when he published his first Icarus paper. Renerpho (talk) 07:27, 15 April 2025 (UTC)

The terms involved in that particular three-way classification (classical planet, dwarf, satellite) don't appear together anywhere in the literature prior to the IAU's redefinition of planets. In 2000, Stern proposed a definition that distinguishes between überplanet and unterplanet, with subtypes subdwarf, dwarf, subgiant, giant, and supergiant, but no three-way classification of any kind, and nothing that distinguishes between satellites and "others". Renerpho (talk) 08:28, 15 April 2025 (UTC)

We write that Several years before the IAU definition, he used orbital characteristics to separate "überplanets" (the dominant eight) from "unterplanets" (the dwarf planets), considering both types "planets". -- This makes it sound as if Stern had just used different names for the same thing, which is not the case. His definitions specifically included planetary satellites, for example, and also free-floating planets (not gravitationally bound to the Sun). Renerpho (talk) 08:32, 15 April 2025 (UTC)

As for claims to have coined the term, that honor may belong to V. V. Kesarev, who, while discussing the types of planets that might exist in the Solar System, came up with the concept of a "dwarf planet" in 1967 -- envisioned as something between a comet and a planet. From the abstract (p. iii):

This book sets forth an original concept of the author concerning the nature of the Earth and the planets. [...] It is shown that the proposed chemical model of the Earth can be extended to other inner and outer planets.

And later (p.114-115):

If the chemical processes penetrate from the surface of the dwarf planet to the interior of the body, they may acquire a progressive nature as a result of the fact that the heat of reaction remains in the reaction zone and will not be exchanged with the space surrounding the body. [...] In connection with the above characteristics of a dwarf planet, there arises the question of whether such planets exist in the solar system and what are the signs of their existence. We must give a positive answer to this question. Such planets have been observed ...

Renerpho (talk) 09:56, 15 April 2025 (UTC)

In his book "Chasing New Horizons" (Chapter 10, sorry audio don't have page number) Alan Stern claims he coined the term dwarf planet in his 1991 paper arguing that Pluto-sized objects may be plentiful in the trans-Neptunian region. It's probably this one, but I can't access it. Serendi^podous 09:59, 15 April 2025 (UTC)

@Serendipodous: The word "dwarf" appears once in that paper ("Similarities between the apparently common IRAS disks around main sequence dwarfs (Backman and Gillett 1987) and our Kuiper disk therefore strengthen the rationale for determining the small-planet content of our outer Solar System.") No word about dwarf planets. Renerpho (talk) 10:41, 15 April 2025 (UTC)

@Serendipodous: To expand on what you wrote, here's the relevant quote from the book: Many planetary scientists had long been referring to the rich harvest of newly discovered small planets in the Kuiper Belt as “dwarf planets,” a term Alan coined in a 1991 research paper mathematically calculating that the solar system might contain as many as one thousand of them. He chose the term “dwarf planet” in analogy to the well accepted astronomical term “dwarf stars,” like the Sun, that are the most common type of stars in the universe. (p.136/301 in the ebook version, chapter 3, sub-chapter "The rise of the 3rd zone").

This is definitely about the paper you mentioned, as they discuss that paper already in an earlier chapter (p.56, chapter 10, sub-chapter "The astronomers eject Pluto"): In 1991, Alan took this idea further, publishing a research paper called “On the Number of Planets in the Outer Solar System: Evidence of a Substantial Population of 1000-km Bodies,” arguing mathematically, from several kinds of forensic evidence in the outer solar system, that there should be hundreds or perhaps thousands of small planets that had been created early on, constituting a whole new “3rd zone” of the solar system, beyond Neptune.

That paper, as mentioned, does not include the term... Renerpho (talk) 11:04, 15 April 2025 (UTC)

Well, I think once again we have fallen foul of Alan Stern's well-documented habit of bare-faced lying. Serendi^podous 11:08, 15 April 2025 (UTC)

@Serendipodous: I'm not sure I'd call it an outright "lie", but I agree he may not have been entirely truthful. The 1991 paper includes this:

This hypothesis naturally begs the question of the definition of a planetary body. I employ a simple, three-criterion test. To be considered a planet, an object must (a) directly orbit the Sun (or some other star), (b) be massive enough that gravity exceeds its material strength (so that the bulk object is in approximate hydrostatic equilibrium), but (c) not be so massive that it generates energy through nuclear fusion. For typical planetary equations of state, criterion (b) implies a demarcation size near 150-200 km between strength-dominated "rocks" and gravitation-dominated planetary bodies. By these criteria, Pluto as well as Ceres and several other large asteroids are classified as planetary bodies; conversely, however, meteorites, comets, and most asteroids are not. In what follows we refer to bodies in the outer Solar System orbiting the Sun which grew no larger than 10^22-27 g as planetary "embryos."

That's without ever calling them "dwarf planets". Stern calls them "planetary embryos" instead, which I suppose he could have misremembered (although why not check when you're writing a book about it?). He coins a possible definition here which resembles the later IAU definition, in particular the criterion of "approximate hydrostatic equilibrium", but without the "clearing its orbit" stuff. That, at least, is consistent with what we knew about him. Renerpho (talk) 11:15, 15 April 2025 (UTC)

I think we have established that Stern did not coin the term, and I don't believe that he originally came up with a three-way categorization of any kind. The sources might be difficult to reconcile.

The article is undergoing a Featured article review since 3 June 2025, in part due to the dubious history of the term. Renerpho (talk) 02:47, 19 June 2025 (UTC)

this is at least partly my fault. i've removed the claim from the lead. even if it were accurate, imo it doesn't belong in the lead per notadictionary. the history of the term is rather incidental to the topic. — kwami (talk) 00:26, 20 June 2025 (UTC)

@Kwamikagami: Thanks! I think that's alright. This just leaves the other mention of it (also tagged as dubious+citation needed) in the Dwarf planet#Name section. I do think the history of the term is important. I don't know if we need all the coverage we currently have (it seems excessive), but we do need some. Renerpho (talk) 01:25, 20 June 2025 (UTC)

agreed — kwami (talk) 01:44, 20 June 2025 (UTC)

@Renerpho: I am not a fan of Alan Stern, nor any witness to his varacity, but he definitely did conceive of a tripartite classification system for planets. That he later decried it when it demoted Pluto makes him a hypocrite in this case, but not a liar. Serendi^podous 20:38, 14 July 2025 (UTC)

Thanks! Here is the final version of that paper: Renerpho (talk) 23:53, 14 July 2025 (UTC)

A bit of searching turned up this curiosity: "The perihelion of Pluto's orbit is interior to the orbit of Neptune. The difference between the perihelion distance of Pluto and the semimajor axis of Neptune's orbit is only 0.2 AU. Due to this characteristic and its small mass compared to the rest of the planets, Pluto resembles more a "gigantic" minor body of the Solar System than a "dwarf" planet." Tancredi and Fernandez (1991). Here's a use of "dwarf planet" in 1980 to refer to Pallas: . And I also found a 1981 book that apparently treats "dwarf" as synonymous with "terrestrial" . Stepwise Continuous Dysfunction (talk) 01:31, 19 July 2025 (UTC)

The comparative masses of several dwarf planets are given below the figure "Comparative masses of the likeliest dwarf planets, with Charon for comparison." How can Charon have a comparative mass of 1.59 in comparison to Charon? Numbers for the other dwarf planets to be checked as well ? 47.62.20.3 (talk) 13:26, 27 October 2025 (UTC)

The unit of mass is 10^21 kg, per the description of the figure. Charon's mass is (1.5897±0.0045)×10^21 kg so that's accurate. Charon is not a dwarf planet, but is added for comparison, as the best studied trans-Neptunian object in the relevant size range. Renerpho (talk) 17:17, 27 October 2025 (UTC)

I misinterpreted the pie chart.

I though it was like describing star weights, for example the Sun has a solar mass of 1 by definition.

I would maybe suggest to slightly reword the legend below the pie chart to avoid any confusion with a comparison index :

"Masses of the likeliest dwarf planets" or something similar.

By the way, thanks for your response and your work on Wikipedia in general. 47.62.20.3 (talk) 08:45, 28 October 2025 (UTC)

I've removed the word "comparative", thanks. Renerpho (talk) 10:14, 28 October 2025 (UTC)

• Reassess the article to C-class.

The article is in the following categories:

• Articles with unsourced statements from July 2025,
• Articles with unsourced statements from December 2025.

There is entirely too much unsourced material with several instances. The B-class criteria #1 states, The article is suitably referenced, with inline citations. It has reliable sources, and any important or controversial material which is likely to be challenged is cited. -- Otr500 (talk) 14:52, 20 December 2025 (UTC)