The abstract of a paper currently in-press in Icarus: The Reclassification of Asteroids from Planets to Non-Planets (Metzger et al. 2018, https://doi.org/10.1016/j.icarus.2018.08.026) says:

It is often claimed that asteroids‘ sharing of orbits is the reason they were re-classified from planets to non-planets. A critical review of the literature from the 19th Century to the present shows this is factually incorrect. The literature shows the term asteroid was broadly recognized as a subset of planet for 150 years. On-going discovery of asteroids resulted in a de facto stretching of the concept of planet to include the ever-smaller bodies. Scientists found utility in this taxonomic identification as it provided categories needed to argue for the leading hypothesis of planet formation, Laplace‘s nebular hypothesis. In the 1950s, developments in planet formation theory found it no longer useful to maintain taxonomic identification between asteroids and planets, Ceres being the primary exception. At approximately the same time, there was a flood of publications on the geophysical nature of asteroids showing them to be geophysically different than the large planets. This is when the terminology in asteroid publications calling them planets abruptly plunged from a high level of usage where it had hovered during the period 1801 – 1957 to a low level that held constant thereafter. This marks the point where the community effectively formed consensus that asteroids should be taxonomically distinct from planets. The evidence demonstrates this consensus formed on the basis of geophysical differences between asteroids and planets, not the sharing of orbits. We suggest attempts to build consensus around planetary taxonomy not rely on the non-scientific process of voting, but rather through precedent set in scientific literature and discourse, by which perspectives evolve with additional observations and information, just as they did in the case of asteroids. (emphasis added)

On the website there is also:


  • The literature on asteroid classification is reviewed from 1801 to the present.
  • Scientists considered asteroids to be planets until the 1950s.
  • Thousands of asteroids sharing orbits was not debarring from planet status.
  • They were reclassified as non-planets on the basis of geophysical characteristics.
  • Taxonomical terms like planet are determined by the scientific process, not voting. (emphasis added)

It sounds like they are saying that Pluto's status as a planet was removed by a vote, more than by scientific consensus, and that while the title suggests that the paper is about "Asteroids" it seems the scenario might actually be proxy for Pluto's demotion.

Question: Am I misunderstanding, or is that really the main point of the last few sentences of the abstract, if not the entire paper?

Here's an example of another situation of classification being in flux: Do astronomers generally agree that the distinction between comets and astroids is not so clear?

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    $\begingroup$ See this answer astronomy.stackexchange.com/a/592/14434 to a related question for a great explanation of why the IAU taxonomy makes sense. $\endgroup$
    – antlersoft
    Commented Sep 17, 2018 at 19:28
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    $\begingroup$ Oh, please. The primary alternative to voting by which science progresses is that the old fogies who inhibit progress eventually die. That science progresses as a result of someone publishing a paper or book that shakes the scientific establishment to its core is a pipe dream that happens but rarely. With regard to the reclassification of Pluto, a large number of papers were published prior to the vote. These papers went nowhere because too many old fogies (old does not necessarily mean "old", old fogies can be in their 20s) remained emotionally attached to Pluto being a planet. $\endgroup$ Commented Sep 18, 2018 at 0:45
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    $\begingroup$ I've pretty much lost all respect for all of the coauthors of that paper, Alan Stern in particular. The opening sentence of the paper is a flat-out lie: It is often claimed that asteroids‘ sharing of orbits is the reason they were reclassified from planets to non-planets. No, it's not. The primary reason a very few objects are designated as planets is that this has educational value and gets some children interested in astronomy. There is zero educational value if there are hundreds of objects in the solar system are classified as planets. $\endgroup$ Commented Sep 18, 2018 at 23:05
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    $\begingroup$ With hundreds of planets in the solar system, the list of planets becomes yet one more tedious list that teachers force students to memorize, ranking right up there in meaninglessness with the names the capitals of the states in the US, or the names of all the countries in the world. And like the list of countries, the list of minor planets is constantly changing. What is memorized as a ten year old has zero value ten years later. $\endgroup$ Commented Sep 18, 2018 at 23:08
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    $\begingroup$ Alan Stern knows this; he was the key author of one of the seminal papers that distinguishes the very small number of objects (and Pluto isn't one of them) as special. The only difference is that he wanted to call these special objects "uberplanets" as opposed to "planets". He designated as "unterplanets" the myriad number of roundish things that have not and never will clear their neighborhood. He wanted to collectively label the uber- and unterplanets as "planets", which defeats the purpose. $\endgroup$ Commented Sep 18, 2018 at 23:21

5 Answers 5


Part of the problem here is semantics. The meaning of "planet" was really poorly defined prior to 2006 (and some argue that it still is). In the early 2000's, this started to become an issue. You can blame Mike Brown.

In January 2005, Mike Brown's team at CalTech in Pasadena, California, discovered an object we now know as Eris (they nicknamed it Xena early on). The early observations of Eris suggested it was larger than Pluto (which was later conclusively proven incorrect with the New Horizons flyby of Pluto in 2015). But larger or not, it was close in size to Pluto. Did this make it the tenth planet? If so, then what about the other bodies like Sedna? Haumea? MakeMake? For that mater, what about Ceres, or Chiron? By some counts, you could say we had upwards of 23 planets. Or do we only have 9? Or 8?

For that matter, what makes a planet a planet?

The word planet comes to us from the ancient Greek "planetes asteres" which meant "wandering stars." To the ancients, even well before the Greeks, a few of the lights in the night sky were definitely different from the others. The stars remained in fixed patterns. Sure, they rose and set, and they rose and set at different times through the year, but their relative positions were always the same - as far as the ancients could tell. But five of them were different. Their positions changed. Two of them would rise before the sun in the morning for a while, then disappear and set after the sun in the evening for a while. These are Mercury and Venus. The other three, Mars, Jupiter, and Saturn, all seemed to move in the same direction, interestingly through the constellations they knew as the zodiac, but now and then they'd back-track a little bit, then move forward again. Very odd.

The ancients had no idea that they were, in fact, other worlds, completely different in nature from the burning stars. Of course, for that matter, they had no idea what the stars really were either. They just knew the planets, though they looked pretty much the same as stars, moved differently. They were wandering stars. It wasn't until Galileo turned his scope on the heavens that we learned that these aren't just bright points of light like the other stars - but whole different worlds. The ancients barely could see Uranus, and gave it little heed, and couldn't see Neptune at all. Other, smaller bodies like Pluto, Ceres, and others were far to small to see without a telescope.

When Herschel discovered Uranus in 1781, he first thought it was a comet. In 1783, observations and calculations had lead to the discovery that Uranus' orbit was nearly circular, and it behaved like the other planets, and so it was accepted as one (and Herschel wanted to name it George). When the predicted movements of Uranus didn't match the observed motions, astronomers and mathematicians suggested the reason was due to another planet, further out, pulling on Uranus and affecting its orbit. In 1846 it was finally discovered, largely based on mathematical predictions. At the time, no telescope could show either Uranus or Neptune as more than a very small disk. But they were in nearly circular orbits, and that seemed to be good enough.

Ceres was discovered in 1801, and for several years was considered a planet. German Astronomer Johann Elerte Bode had previously observed what he thought was a mathematical relationship between the planetary orbital radii. According to his calculations, there should be a planet between Jupiter and Mars, right where Ceres was discovered by Guiseppe Piazzi. Bode's observation was actually erroneous - there was no such mathematical realationship, but it was convenient that an object was found there.

As it turned out, Neptune's orbit didn't quite fit the mathematical predictions either. This lead to a belief that there might be a tenth planet causing the orbital discrepancies. In the late 1800's and into the 1900's, the hunt was on. Wealthy eccentric and astronomer Percival Lowell founded the Lowell Observatory in Fagstaff, Arizona, and set out to find "Planet X." In 1929, the director of the Lowell Observatory hired a 23 year old farmer from Kansas who'd been constructing his own telescopes, mostly out of salvaged farm equipment, and doing his own observations. Clyde Tombaugh was brought in to work on the hunt for Planet X, a rather arduous task involving taking pictures of the same patch of sky a few days apart and then comparing them using a device called a "blink comparator" and looking or something different between the two images. On February 18, 1930, Tombaugh noticed a tiny dot moving between images taken a few weeks before. Looking back at a previous image a few days before, he added to the evidence. After capturing a few more images still and comparing, he finally reported the discovery on March 13, 1930. He had discovered Pluto - the ninth planet!

Well, at least, that's what the hype said. It didn't take too long to notice that Pluto was a bit different. While no two planets orbit in the same precise plane, the previously-discovered eight are very close to co-planar. They also have orbits that are very close to circular. Pluto's orbit is tilted dramatically with respect to the other planets, and its orbit is significantly more elliptical. In fact - Pluto's orbit crosses inside that of Neptune (though they'll never hit each other).

Pluto was also tiny. Our own moon is larger, as are several other moons of other planets. In 1978, Pluto's moon Charon was discovered. With further research, it was found that Charon was so large, that the point of orbit between them, what is known as the barycenter of the orbit, is actually well outside Pluto's radius. Many astronomers were starting to talk of Pluto-Charon as a "co-planet."

Then, in the late 1990's and into the 2000's, a bunch of other objects started turning up out past Pluto. Some of them were spherical, like a planet. So, were they planets?

If you take a look back at Pluto's discovery, you can make an argument that the only reason Pluto was called a planet from the start was because Tombaugh was trying to find a planet. He found something, there's no doubt there. But would it have been called a planet if they weren't specifically looking for a planet? As it turns out, Pluto's mass (including the whole Pluto/Charon system which includes four other "moons") isn't nearly enough to explain the discrepancies between the observed and calculated positions of Neptune (which, as it happens, were later explained away by calculations based on Einstein's discovery of special relativity).

When Mike Brown discovered something that was potentially larger than Pluto, but much further out, and just as weird, people started questioning what it meant. If Eris was a planet, then what made it a planet and not Sedna, or Orcus, Or MakeMake, or the others. And then, if those were planets, why not Ceres, which had been downgraded to an asteroid. What does "planet" mean?

So here come the semantic police. The problem is, who decides what a word means. We run into this in language all the time. But for science, we need precise definitions so we can make and categorize observations. If I simply define a star as a bright light in the sky, that includes planets, the moon, and even airplanes. For science to work, we have to have agreed upon definitions - operational definitions.

Of course, in many cases, there's no single body to define things. There is no "official" body to make these decisions. The purpose of the International Astronomical Union is to provide an agreed-upon group to set standards like this. When they met in 2006, they had a working group to formulate a definition of what should define a planet and differentiate it from another body. To some greater or lesser extent, it IS arbitrary - as is all language.

When I say fruit, you might think "banana" or "apple." But not everyone would think of "tomato" as a fruit, even though it is, by definition. How about a cucumber? Both a tomato and a cucumber are the fruiting bodies of their plants. They are, by definition, fruits. Yet how many people do you know that talk of a cucumber as a fruit? Most people consider fruits to be sweet while vegetables are savory. So, to an extent, it will depend on your point of view.

But for science, we need a standardized set of definitions. And this is what the IAU did in 2006. Their working group formulated a set of criteria to define a planet, and put it to a vote of the general assembly, which passed it.

The authors of the cited article state that "taxonomical terms like planet are determined by the scientific process, not voting." However, when you are arguing semantics, then you are talking about consensus. Looking at another taxonomic system, scientists agree that a "cat" is a cat because it has certain characteristics. More recently, those characteristics include genetics and assumed evolution, but historically taxonomy was more based on observed characteristics. In defining a planet, we have the same process at work - only the defining characteristics were put to a vote.

Regardless of what we call Pluto, no one argues that it is not a definite body of some substantial size orbiting the star Sol. The argument does not deny Pluto's existence, only its classification. Pluto, along with the eight other fully-accepted planets, along with the thousands of asteroids, along with the myriad other trans-Neptunian objects, Plutinos, Kuiper Belt objects, and everything else orbiting the sun, are all planet-like in some ways, and not in others. At some point, a definition must be agreed upon, and this is exactly what the IAU did in 2006. Not everyone agreed, but that wasn't necessary. We weren't stating a scientific fact, we were assigning a label. It's not a FACT that anything is a planet, it is simply a label of convenience. In fact, if you wanted to be nit-picky, you could argue that Jupiter and Earth are not at all the same type of object, and that they shouldn't both be called planets but should have different classifications.

By reclassifying Pluto, the IAU basically played a game of "one of these things is not like the others" and drew a line. And whether or not you agree with that line, the majority of the assembled members of the convention did, and adopted the definition. Sure, it could be changed, but why bother? It really makes no difference other than in the minds of people who don't understand otherwise. It's PR, and nothing more.

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    $\begingroup$ ...majority of the remaining astronomers on the last day of the conference news.bbc.co.uk/2/hi/5283956.stm . It's great to include some perspective in your answer but could you indicate which part of your answer directly addresses my question? The "semantics police" will likely have some thought on what "question" and "answer" mean as well. Thanks! $\endgroup$
    – uhoh
    Commented Sep 26, 2018 at 2:01
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    $\begingroup$ I guess your criticism of my answer is warranted. I focused more on the apparent implied questions than the stated question. And yes, I think that is the overt argument of the paper (though it's hard to say from only an abstract), but it appears obvious that the authors were implying that Pluto's "demotion" was not handled scientifically, where I would suggest that it was, in as much as the determination of terminology is a matter of a group agreeing upon a term to use - I do not see how THAT can be scientific. $\endgroup$ Commented Sep 26, 2018 at 16:02
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    $\begingroup$ yep, I appreciate the work you've put in to this substantial, thoughtful, and well-reasoned post, and I'd up voted your post along with the comment. $\endgroup$
    – uhoh
    Commented Sep 27, 2018 at 1:20

To respond to the question as is: yes the paper is trying to point out that the concensus was not scientific.

However, this claim is not true at all. In order to understand how classifications within the solar system are made, one can look at the history of the planets.

The short answer is, the demotion of a planet is a result of much scientific consensus than the classification of a body as a planet.

For thousands of years, we had 7 planets: Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. These were the seven wanderers in the night sky. However, as we understood more about our system, we dropped the moon and the sun and added Earth. That process happened through a lot of bloodshed, since back then, voting upon an idea seemed not very logical(!).

Then the time came that we discovered a planet for the very first time: Uranus. Without doubt it was classified as a planet because its place perfectly fit the Titus-Bode law.

Uranus's discovery did not stop the astronomers because, according to Titus-Bode law, there was a missing planet between Mars and Jupiter. And so the day came and in the year 1801, Ceres was discovered and announced as the new planet (again because it fit the law). However, as time passed by and we understood more and more about the Asteroid belt, 46 years later, Ceres (and others that were considered a planet) was demoted from the planet status.

The same story (more or less) happened with Pluto, and after much consideration (and a lot of published papers about Pluto, Kuiper Belt objects & planetary taxonomy) 76 years later Pluto was demoted as well.

Here, the difference between 1847 and 2006 is IAU, which is the organisation to decide astronomical classifications and facts by looking at scientific evidence and reaching a consensus by vote! Through all these information, it is a blatant ignorance to tell that planetary taxonomy relies on non-scientific process of voting.

  • $\begingroup$ "to decide astronomical classifications and facts by looking at scientific evidence and reaching a consensus by vote" No, it was much messier than that. $\endgroup$
    – Dave
    Commented Sep 21, 2018 at 20:52
  • $\begingroup$ Thanks for the answer in the beginning where you address the question as asked. $\endgroup$
    – uhoh
    Commented Sep 21, 2018 at 22:40
  • $\begingroup$ @Dave your linked BBC article is well-worth the read, thanks!! it was a strange, polarizing moment and hasn't sat well with some people, and some find the current definition less than ideal. Your linked 2006 article highlights those well, and that this paper was accepted indicates, a little bit like the CMB, that sentiment is still echoing today. $\endgroup$
    – uhoh
    Commented Sep 21, 2018 at 22:44

If they wanted to make this paper about Pluto, why would they not just discuss Pluto?

You may be right that the comment about voting could be linked to the Pluto debate, but I don't see any reason why they would need to conceal their intentions. I think it is more likely that they are actually researching and discussing asteroids, and they have linked their work briefly to the issues around Pluto because that gets more attention than asteroids.

I have seen many other examples of this: authors with throw in a paragraph at the end of a paper to link their work to a current hot topic (such as exoplanets or gravitational waves) because it boosts their readership, but it is not the main point of the paper of the focus of their research.

  • $\begingroup$ What is Icarus exactly? My field is nothing to do with asteroids/planets/the solar system so that is jargon to me. $\endgroup$
    – FJC
    Commented Sep 18, 2018 at 12:51
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    $\begingroup$ Icarus is a scientific journal that specializes in papers on the solar system. Given this narrow focus, it is a rather highly regarded and highly impactful journal. It's not amongst the very top astrophysics journals, but it is close to to it. The narrow focus inevitably keeps it from being one of topmost journals in astrophysics. $\endgroup$ Commented Sep 18, 2018 at 22:31
  • $\begingroup$ David— Icarus isn't a top astrophysics journal for the simple fact that it is not an astrophysics journal. It's a planetary science journal, owned by the Division of Planetary Science of the American Astronomical Society (though published by Elsevier). It is one of the two primary long-form-article planetary science journals in existence today (the other being the American Geophysical Union's Journal of Geophysical Research— Planets). $\endgroup$ Commented Apr 29, 2019 at 20:01
  • $\begingroup$ @StuartRobbins - Planetary science is a subcategory of astrophysics. $\endgroup$ Commented May 11, 2019 at 15:06
  • $\begingroup$ @DavidHammen - As a working planetary scientist, no. If you're talking about solar system dynamics, okay it's a subset of astrophysics. But the bulk of planetary science is closer to geology and geophysics than astrophysics. That's what happened when we got close to Pluto: It turned from an astrophysical object to a geophysical object once it was resolved. Please don't tell me what my field is and is not. $\endgroup$ Commented May 12, 2019 at 17:38

We suggest attempts to build consensus around planetary taxonomy not rely on the non-scientific process of voting, but rather through precedent set in scientific literature and discourse ...

Good luck with that.

There are (at least) two distinct camps in the planet versus not a planet debate. There is a third camp as well: What should those things that orbit stars other than the Sun be called? Per the current definition, there are exactly eight planets in the entire universe. I'll ignore that issue for now.

One side of the planet versus not-a-planet debate looks at things from the perspective of roundness. That Titan and Pluto are not called planets, despite having active geologies, apparently much more active geologies than Mercury or Mars, is a bit befuddling to this group. The other side of the debate looks at things from the perspective of dynamics. That Titan and Pluto are not called planets while Mercury and Mars are is crystal clear. There's a three to six order of magnitude difference (depending on the metric) between Mars and Pluto.

To me, while the planetologist viewpoint does have some credence, it has a boundary problem. There is no clearcut boundary between planet and not-a-planet from this perspective. Any definition would be highly arbitrary, and there will be boundary cases on both sides that go against the grain of the definition. On the other hand, the three to six order of magnitude difference between the dynamicists' distinction of planet versus not-a-planet means that boundary is crystal clear, at least in our solar system.

That is not a problem given that there are only eight planets in the entire universe. The thousands of objects discovered in the last few decades to be orbiting other stars are not planets. They're exoplanets. Eventually the exoplanet problem will have to be solved. (Given the amount of rancor that occurred, and continues to occur, over the demotion of Pluto to not-a-planet, that eventuality is probably well into the future.)

Someday in the not too distant future, a group of astronomers will have discovered a 20 Jupiter mass object that they will have deduced to have formed from planetary rather than stellar formation processes, making it an exoplanet, with a Neptune sized object orbiting this exoplanet. At the same time, another group of astronomers will have discovered a 10 Jupiter mass object that they will have deduced to have formed from stellar rather than planetary formation processes, making it a brown dwarf, with a Mars-sized object orbiting this brown dwarf. The Neptune-sized object will not be deemed to be a planet (better: an exoplanet) while the Mars-sized object will.

  • $\begingroup$ can I ask you to read through my comments here and consider a re-open vote based on them? Thanks! $\endgroup$
    – uhoh
    Commented May 9, 2019 at 23:56

Indeed, the IAU's definition of a planet has been determined by vote (of actually a very small part of the IAU as the majority didn't come to the notorious meeting in 2006) and bares any scientific footing. If Pluto (and Eris) were no planets due to not having "cleared their neighbourhood" then many other planets including Earth and Jupiter wouldn't be planets either but so-called "dwarf planets". So all in all the IAU decision has no logic and shouldn't be taken seriously by anybody.

A scientific definition of "planet" has been proposed by Kirby Runyon, PhD student from the Department of Earth and Planetary Sciences at Johns Hopkins University, which is known as the geophysical definition of a planet:

"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."

According to Runyon's definition any spherical celestial body which isn't a star is a planet, including moons. One doesn't have to agree with it (I don't stand up for that definition) but it is a logical scientific and consequent possible definition of a planet and it is better to stand up for this one than to hold to a definition that was voted in by astronomers who obviously have not much clue of science. Sorry to say it so harshly but the IAU definition really lacks any science or logic, but it would be too much to describe it all here. But one more point: planets don't have to orbit stars so that part of their definition is wrong too.

  • $\begingroup$ I think the expression is "tell us what you really think" ;-) Great first answer, welcome to Stack Exchange! $\endgroup$
    – uhoh
    Commented Apr 28, 2019 at 12:24
  • $\begingroup$ Thank you. It's actually my 2nd answer, my first was there: astronomy.stackexchange.com/questions/18995/… $\endgroup$
    – Guest55
    Commented Apr 28, 2019 at 12:26
  • $\begingroup$ +1. Your post is more a useful comment than an answer to the actual question, but to some extent that's true of each of the other answers too! One query though: when you say "planets don't have to orbit stars", are you thinking of "rogue planets", or of planets orbiting stellar remnants such as a white dwarf or neutron star, or of some other circumstance? $\endgroup$ Commented Apr 28, 2019 at 12:41
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    $\begingroup$ I'm thinking of rogue planets. I consider white dwarves, neutron stars and brown dwarves being stars. $\endgroup$
    – Guest55
    Commented Apr 28, 2019 at 13:12
  • $\begingroup$ That's not what "cleared their neighborhood" means. Nothing would clear absolutely everything out. $\endgroup$ Commented Apr 29, 2019 at 1:06

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