If you turn on the 'Stars' in that, you'll see that its currently in the interesting direction of the sky - 'twards the center of the Milky Way where we've got other telescopes looking. I wonder if we would have detected it if it was 180° further in its orbit.
> Another factor that kept ED138 hidden is that it is on the same plane as our galaxy. “For the last 20 years, the object has kind of been looping its way across the Milky Way, from our point of view,” Holman explained. “People don’t typically search for things in the Milky Way, because there’s so many stars that it’s really hard to find moving objects.” [1]
This thing was actually found despite its location, not because of it. If it had been in the opposite part of the sky we almost certainly would have spotted it sooner, given that it's one of the brightest TNOs we've discovered.
We're finding it now because Vera Rubin (the instrument that found it) just came online and is exceptionally good at finding these kinds of objects specifically, even when they're hiding amidst a dense population of stationary background stars.
There was a lot of negative press and commentary about "demoting" Pluto from being a planet at the time. But its obviously true that it was the right move, and creating this "dwarf planet" class of objects was correct. It was hard to justify Pluto being a planet when Ceres exists, but Eris, Haumea, Makemake and all of these other possible Dwarf planets are the nail in the coffin.
> But its obviously true that it was the right move, and creating this "dwarf planet" class of objects was correct. It was hard to justify Pluto being a planet when Ceres exists, but Eris, Haumea, Makemake and all of these other possible Dwarf planets are the nail in the coffin.
I don't agree that "its obviously true that it was the right move". Instead of demoting Pluto, they could have promoted Ceres, and let Eris, Haumea, Makemake, etc, be planets too. Under that proposal, they could even have the category of "dwarf planets" encompassing them, it is just that "dwarf planet" would be a subcategory of planets rather than a disjoint category from them.
Classifications are useful. Even the 'proper' planets we split into terrestrials, Jovian giants, and ice giants. Each planet is individually interesting in itself. One might even argue all of the bodies in the Solar System exist on a continuum of mass from the tiniest mote of space dust, to rocks and asteroids, to moons and dwarf planets to terrestrial planets, ice giants, giants, and then the Sun, and then hypergiant stars, neutron stars, black holes, and then supermassive black holes, so let's call all of them 'celestial objects'—which we kind of do. But such a catch-all is pointless. Each object behaves very differently, with wildly different physical laws governing its interior and its neighbourhood.
Classifications impose structure on a collection of celestial objects, and we can derive relationships, commonalities, understand their development, the physics governing these objects, and build useful physical and mathematical models to widen our understanding of the universe.
Most reasonable scientists understand that some of these objects will blur the edges—like many moons being more massive and larger than Mercury. Or like the largest super-Earth exoplanets being nearly as massive as Neptune, or the largest gas giant exoplanets being 30-80 times as massive as Jupiter and hence having a surface temperature of ~2000 K. But that is part of science, and possibly even where the most interesting work is.
Classifications are useful! It is poor form to take a widely used and historic term and add on a whole different (dynamic!!) category of attribute as a filter on the definition. There is quite a bit of absurdity in the 2006 IAU's definition of a planet. If a massive object came into the solar system and perturbed orbits, caused chaos and collisions, then any "planet" with debris within its neighborhood would cease to be a planet. What do we gain from adding a whole different category of attribute (relationship to its neighborhood) as a criterion? Does it realistically outweigh decades or centuries of peoples intuition of the existing definition? Its not just a matter of science, but language and communication. If you want greater specificity, come up with a new term or use compounded terms.
It very much seems like the definition was just shoe-horned to stem any more planets needing to be added. It would be interesting to know if astronomers walk the walk when it comes the definition. If I research when planets were formed would I get the date when their neighborhoods were cleared? Do discussions around large events that send debris into the neighborhood of planets correctly reflect that they cease to be planets until it is cleared?
Alan Stern said it best
>"We're going to call it a cow, except when it's in a herd." A river is a river, independent of whether there are other rivers nearby. In science, we call things what they are based on their attributes, not what they're next to.
>And the IAU definition is even worse, because it produces different categorizations for identical objects, depending on where they are. Get this — Earth at the same distance from the sun [as Pluto] would not be a planet by the IAU’s measure, because Earth can’t clear that zone either. I would say any definition that produces a result where Earth is not a planet under any circumstance is immediately indicted as ridiculous, because one thing we all agree is a planet is planet Earth!
>That's crazy. No wonder teachers and students are frustrated, confused, unhappy. It reflects poorly on astronomy, to have something so illogical.
I find this interview very interesting, because around a decade prior, Stern himself proposed 'clear the neighbourhood'[1] as a finer-grained rule for planet classification involving the dynamics of the planetary body in question. That is, planets that could clear their neighbourhoods are überplanets; otherwise unterplanets. In fact, near the end of that paper he proposes not two, but five classifications of planets: subdwarf, dwarf, subgiant, giant, and supergiant.
Now, Stern's broader argument in that interview is that we should not take into account the dynamical behaviour of planets. I disagree, especially since dynamics determine the static future state of any given object. Take chthonian planets—former gas giants whose gaseous envelopes have been stripped away by the primary. Do we insist that they are still gas giants? No, we even came up with a name for the transitory period when these planets are losing their gas envelopes.
Personally, I think Stern was being a little more animated to drum up support for and shine a beacon on New Horizons, a then-ongoing mission that he was principal investigator of.
To directly answer your question:
> If a massive object came into the solar system and perturbed orbits, caused chaos and collisions, then any "planet" with debris within its neighborhood would cease to be a planet.
There actually are (multiple) mathematical formulations for 'clearing the neighbourhood'[2]; it's not purely empirical or based on vibes. Stern proposed one in the above-mentioned paper. Jean-Luc Margot suggests several more[3]. I suspect a more universal model will be probabilistic but grounded in celestial mechanics.
> Does it realistically outweigh decades or centuries of peoples intuition of the existing definition?
People's intuition for 'planet' in the centuries before telescopes were invented was 'something that moves against the fixed background of stars'—that is literally the Greek etymon for the English 'planet'. The Sun and Moon used to be a part of the 'classical planets'. We didn't even find the last two (or three) planets in our own Solar System until around two centuries ago. We didn't know about nuclear fusion until 1925, and we didn't know other galaxies existed until similarly recently. We didn't understand the inflationary model of the universe until the 1960s. The first exoplanet was discovered only in 1992.
We have repeatedly and retroactively renamed, retconned, reworked, revamped and rewound scientific definitions and models over and over again. I think planets will be OK.
I find it particularly amusing that the IAU definition, in a footnote, expressly declares the eight planets to be what they are. In other words, the true definition for being a planet is to be defined as a planet- so the criteria it gives are entirely superfluous, at least when it comes to our own solar system.
It's clear that they worked backwards from the eight planets to come up with a classification system that fit, but in realizing the problems with the approach, just gave up and created the carve-out.
Also, just as another example of the absurdity of the IAU definition, there's a pretty strong argument to be made that Earth's moon would be a planet under the criteria: it orbits the sun, it's round, and it's massive enough / positioned such that it's capable of clearing its neighborhood.
It orbits the Earth rather than the Sun by most conventional definitions of the word, although apparently the IAU doesn't have an official definition of orbit that I can find.
I wasn't arguing against classification, just that I remain unconvinced that the classification which IAU adopted is superior to the alternatives.
One problem with the IAU definition: it only works for our planetary system, not for exoplanets – with the end result that we have two different definitions of "planet", one for our system, one for everywhere else.
Now, of course, that in part reflects that we have vastly more information about our own system than any other – but, a few centuries or millennia from now, I expect we'll know a lot more about some of those other systems – we will probably have sent probes to explore the nearer ones by then – at which point people (or AIs?) will probably push to unify the two definitions. My own bet is they'll be more likely to unify them by dropping "clear the neighbourhood" from the our-system definition than extending it to apply to other systems. Splitting the common category of "planet" into two subcategories based on "does it clear its neighbourhood" works much better than insisting on two disjoint categories based on that criterion when you have very limited information on where that criterion is met.
It doesn't work for exoplanets because part of the definition is orbiting our Sun. But the exoplanet definition would define the same 8 for our star, so I don't really see where the issue is.
Thought experiment: Suppose we (somehow) discovered an exact replica of our solar system, n light years away; would its counterparts of Ceres and Pluto be classified as exoplanets?
Yes, regardless of the specific labels, there are clearly interesting and meaningful differences between these things, and it’s worth having vocabulary capable of distinguishing them. The whole universe is a continuum of stuff but it helps to have more than just the word “stuff”.
Arguably there are just two categories: stuff and not-stuff (aka vacuum). And then you go from intergalactic space (almost entirely not-stuff) via interstellar space (marginally more stuff) through planets and stars up to neutron stars (lots of stuff) to black holes (well, we don't actually know what is inside a black hole exactly, our models break down).
Is this classification useful for everyday life? Not at all, but looking at things from different perspectives for a change can be quite amusing.
The backlash was all just populist ignorance on display. There were multiple concrete measures of planetness constructed and Pluto is an outlier in all of them. One of these things is not like the others.
I think the backlash was fed by insistence dwarf planets are not just a subtype of planets, it's such a "well ackchyually" type thing to choose that name for it regardless of how valid it was to break out the category that I'm not sure it'll ever stop being talked about.
Even something as simple as "planets" contains "major planets" and "minor planets" would have avoided 20 years of people arguing about the names instead of caring to learn what's different about them. But you never hear people explaining why the IAU insists "dwarf planets" aren't "planets" even when "dwarf planets" has several subcategories which don't share the "planet" moniker, you just hear people argue about missing Pluto or call other people ignorant of the technical details of the categories.
I'm saying there were two reasonable options: one where they were different subcategories of planets and one where they were their own category but not called dwarf "planets" to clearly reflect that separation from "planets".
I'm not saying anything about what we call dwarf planets and planets being the same as each other. The technical details are fine, it's purely the oxymoronic naming which screwed public acceptance.
Yeah. The gas giants are clearly way different from rocky planets and we still keep them all under one taxonomical grouping. Pluto and other bodies should have been an extension of that: make them dwarf planets or whatever other classification you want. But they're still planets. Theres no "demotion" just a further categorization.
Right. Somewhat similar to other dual-use words like “work”, “energy”, “power”, “momentum”.
With such “well, ackshually”s as “you didn’t really do any work at all when you were pushing on that tree trunk” and “you specified watts, but you really wanted to use lumens”.
Yes. I wouldn't call a dwarf person any less of a person, nor would i refer to a dwarf planet as being any less a planet. Between the terrestrial planets, the gas giants, the ice giants, and arguably a couple of rounder moons, we have far more than 50 planets in our solar system.
By some extreme definitions, Earth might not be a planet. Some say the moon is too big to qualify the earth "cleared out its orbit" criteria, making Earth-moon some sort of double planet... like Pluto-Charon.
Setting aside the whole "demote Pluto" and "dwarf planet" aspects, I find it amusing that we use the same term for small rocky Mercury (2,440 km radius, 5% of Earth's mass) and immense gaseous Jupiter (69,000 km radius, over 300 times Earth's mass). Only we don't, of course. Jupiter is a gas giant, while Mercury is a rocky planet, or iron planet; there's a lot of 'planet' terminology.
Ultimately these sorts of quibbles are fundamentally semantic in nature thus (in general) entirely pointless. It's only the subjects themselves - the myriad celestial bodies - that are interesting.
It's semantics as well as a collision with our inability to work with gradients. We can look at two colours and can name the discrete ones - but when does red stop being red? When does a species emerge?
Yeah, I don't like when a modifier is mandatory. If they chose to label them "dwarf planets" then it should be expected that people will leave off the modifier "dwarf" when being concise.
If they really didn't want people to continue to refer to them as planets, then they should have chosen a completely different noun.
Why is Jupiter in the same category as planets then?
Jupiter does not orbit the sun (the first requirement per IAU definition of planet), the barycenter for the system is not inside the surface of Sol at 1.07 solar radius.
It is not planet then yes ? There is no push to promote it to a sub-brown dwarf or a new category .
Classifications are always arbitrary and only fits some bias one way or other.
a) orbits a star, and is not a star nor a brown dwarf itself;
i) a star is an object that is massive enough and hot enough to conduct protium fusion in its core
ii) a brown dwarf is an object that is massive enough to conduct deuterium fusion, but insufficiently massive (and hence insufficiently hot) for protium fusion
b) has attained hydrostatic equilibrium, that is, it is spheroidal;
c) it has cleared its local neighbourhood of 'rival planets' since its birth;
Ergo, Jupiter is a planet. It is a certain type of planet—a gas giant—but a planet nonetheless.
There is some controversy about when giant planets stop being giant planets and start being brown dwarfs—typically around 30 Jupiter masses—but this is an active area of research, to understand the fusion ignition in these objects. As I mentioned in another comment, classifications are useful. Of course they are arbitrary and they are biased—they were decided by humans. But the entire point of science is to push the edges of our understanding and reduce these biases and make our models more general and less arbitrary.
Everything orbits a barycentre. The position of that barycentre within or without the primary is not really relevant within several orders of approximation—and when it does become relevant, then that system becomes interesting in itself. Chances are the planet in question is so massive relative to its primary that we need to ask a more fundamental question: are we even looking at a star-planet relationship, or is it closer to a binary star system? Then we can investigate how that system formed and developed, and generalise and improve our model of stellar/planetary system development.
There are plenty of binary star systems where this orders of magnitude standard is not met .
There is no academic literature or accepted consensus barycenter has to be orders of magnitude away - the accepted consensus is that a body orbiting another means internal barycenter, if they have external barycenter then they orbit each other period.
The fact that Charon and Pluto do not have internal barycenter is why Pluto-Charon (also Eris-Dysonmia) are considered dwarf planet systems ; and that was used partly in support (while not used in definition) of arguing for dwarf status of Pluto .
Yes — their point is that as-written, it orbits a barycenter external to the star: hence doesn’t orbit the star, but they’re in a mutual orbit.
Thus the suggestion for a new dwarf category — precisely because we do label times the barycenter is outside the star differently, as you yourself note.
You were so busy trying to “correct” me, you failed to address the topic.
'In orbit around' is frequently used as short-hand to refer to a secondary with mass << primary, to a first order approximation. In the Solar System, even for satellite transfers, the effect of Jupiter on the Sun can and is ignored.
The topic is pointless hair-splitting and nostalgia. It's like saying we can't ever use F = ma to describe free-body diagrams; we need a full relativistic per-particle QFT simulation to accurately and precisely model every single interation. That is impractical, at least for now. Likewise, we come up with classifications that are useful for the present time. If 'dwarf planet' is a catch all for all the small roundish bodies in our planetary system, then so be it. I suspect the IAU will make things more fine grained in the relatively near future. We even classify the asteroids as more than just rocky lumps.
I agree with this picture as the barycenter argument seals it for me. The only leap left is for me to believe Ol'Jove as a failed star (it still emits heat in excess of what it derives from Sol) and we find ourselves cozily herded about by a binary star system.
Nah, what we need to do is to just extend our heads to be 2 categories and not think of one as lesser or demotion.
Like, in high fantasy, right, we have humans and elves and dwarves and orcs and all that. Are dwarves 'less' than elves? Are orcs 'less' than humans? No, they exist on a spectrum or line that is not less or more. Maybe less or more evil, maybe less or more aged or fancy. But in high fantasy you can't demote a elf to be a dwarf or a human into an orc. |
So, my proposal: Include all the dwarf planets in all the science exhibits, the kindergarten posters, the rhymes and meters. Just like with the upper and lower cases of the Latin alphabet. Sure you can forget that the dwarf ones exist, but if it's on everything, then that'll be hard.
Here's a quick one that I put like no thought into:
>Mercury, Venus, here comes Earth!
>Mars is next, let's shout with mirth!
>Jupiter's big, Saturn has rings,
>Uranus and Neptune make eight amazing things!
>Ceres, Pluto, Haumea too!
>Makemake and Eris join the crew!
>Sedna, Quaoar, Orcus, Gonggong,
>Dwarf planet dancing all day long!
So, again, sure, dwarves, fine. But just don't make being a dwarf planet a 'thing', celebrate them too!
I initially assumed from your comment that all of those objects are larger than Pluto, but none of them are, so what exactly is "obvious" about the decision? They haven't resolved any inconsistency by demoting it.
You merely need to define the term "NinePizza", which is a collection of nine pizzas—and then have your very educated mother serve that. This doesn't present any problems philosophically, since we already have terms like "Solar System" for collections of disconnected objects.
Does Saturn have enough density to count, though? I mean, from a certain point of view, it's more of a spheroid of hydrogen with a bit of helium. Maybe we need a new term for it, like "Planetary nebula"? ;)
Ceres by itself doesn't really challenge Pluto's old classification. Pluto has 14 times the mass of Ceres, and five moons. Ceres is the only "dwarf planet" classified so far that has zero moons. There are still good arguments for a definition that doesn't consider Ceres a dwarf planet.
This helps illustrate how arbitrary these classifications are. They depend entirely on what properties are included in the definition. The updated IAU definition is essentially reverse engineered to fit how they wanted to classify the planets.
So, a factor of 14.4 is enough to put the line between Ceres and Pluto, but a factor of 23.0 is not enough to put it between Pluto and Mercury?
Yes, those classifications are arbitrary, but then any classification is. If you want to consider Pluto as a planet, you have to include quite a lot of other bodies, which are all qualitatively and quantitatively different from the 8 (which themselves are in 2 qualitatively and quantitatively different groups. Or 3, depending on who you ask).
In the grand scheme of things, it does not matter and astronomers won’t care about the general public’s feelings. It is utterly pointless to get worked up about these things.
> So, a factor of 14.4 is enough to put the line between Ceres and Pluto, but a factor of 23.0 is not enough to put it between Pluto and Mercury?
Why not? It's an arbitrary line, and there has to be a lower limit somewhere.
In the IAU definition, hydrostatic equilibrium loosely defines a lower limit for all planets including dwarf planets, but at the limits that's a fuzzy property that gets debated for small bodies. In fact Ceres is less massive than what some studies have found to be required for hydrostatic equilibrium, e.g.: https://iopscience.iop.org/article/10.3847/PSJ/ad55f3 . The abstract of that paper starts out with, "The current IAU definition of 'planet' is problematic because it is vague and excludes exoplanets."
If we had more small planetary candidates in the solar system, they would have had to come up with some additional constraint to separate planets from non-planets.
The problem is not experts having precise terminology in technical settings; the problem is the army of “experts” who will descend upon you if you use the incorrect arbitrary term in colloquial settings. The same sort that will descend on you if you dare refer to a planet orbiting a different star as a planet, not an exo-planet.
> The updated IAU definition is essentially reverse engineered to fit how they wanted to classify the planets.
I wholeheartedly agree with this take. I have something of a personal hatred of poorly executed retconning thus I present the following proposal.
For the sake of simplicity we should use even powers of 10 with earth as the primary reference point. Note that the earth is 10^24, mars is 10^23, and the earth's moon is 10^22 (pluto is also 10^22 and less massive than the earth's moon). It seems "obvious" then that 23-25 are planets, 22 or less are relegated to dwarfs, and 26 and over are giants (jupiter is a 27 and the sun a 30 for reference).
By that logic, if we want to consider pluto a planet then the earth's moon also has to qualify. I don't really care one way or the other but that just doesn't seem sensible to me.
Why did we add this requirement? To fit how you want to classify planets? That retcon was not executed well.
It shows that the line of reasoning makes no sense, because any classification is constructed to fit some of our preconceived notions. Choosing a classification that builds off our social history is perfectly fine, as long as the rules for classification are applied consistently.
It's easy to exclude the moon, like the IAU does, by requiring that planets orbit the Sun. (Although you might want to avoid the IAU's parochialism, which excludes exoplanets, and say "a star" instead.)
> By that logic, if we want to consider pluto a planet then the earth's moon also has to qualify. I don't really care one way or the other but that just doesn't seem sensible to me.
Ganymede is larger than Mercury, so a definition that doesn't specify orbiting a star would have to include some moons, or exclude Mercury.
This exposes another problem with this kind of definition - it seems to make more sense to define a body in terms of its intrinsic properties rather than whatever location it happens to be in.
For example, a rogue planet is not a planet by the IAU definition. You'd have to call it something like a free-floating planetary-mass object. ("Free-floating" is not the right term either, but I don't have the research funds to come up with a better one.)
> I have something of a personal hatred of poorly executed retconning
Totally agree. Wanna join me outside George Lucas' house to protest the Kessel Run?
This is one of the reasons I think it's too early to talk about the Fermi paradox. We still know so little about what's around us in the universe
We've barely confirmed the existence of 10_000 exoplanets in our galaxy, and the methods we use to detect them are heavily biased toward certain ranges of planetary mass and orbital distance
As far as we know, everyone could very well be right here
> The new dwarf planet candidate, called 2026 ED138, orbits on a plane tiled nearly 25 degrees
I've noticed similar glaring typos in 2 recent articles. In both cases very early in the article. Is it possible that they are adding typos intentionally to mask the AI authoring?
If anyone is interested, here is its orbit, my old day job was to compute orbits for asteroids and comets.
(This uses wasm, meant for a desktop, and will download the asteroid catalog of about 65mb, you have been warned)
https://dahlend.github.io/ketev/#t=2009-07-01.717&sel=2026%2...
This is a big boy.
If you turn on the 'Stars' in that, you'll see that its currently in the interesting direction of the sky - 'twards the center of the Milky Way where we've got other telescopes looking. I wonder if we would have detected it if it was 180° further in its orbit.
> Another factor that kept ED138 hidden is that it is on the same plane as our galaxy. “For the last 20 years, the object has kind of been looping its way across the Milky Way, from our point of view,” Holman explained. “People don’t typically search for things in the Milky Way, because there’s so many stars that it’s really hard to find moving objects.” [1]
This thing was actually found despite its location, not because of it. If it had been in the opposite part of the sky we almost certainly would have spotted it sooner, given that it's one of the brightest TNOs we've discovered.
We're finding it now because Vera Rubin (the instrument that found it) just came online and is exceptionally good at finding these kinds of objects specifically, even when they're hiding amidst a dense population of stationary background stars.
[1] https://edition.cnn.com/2026/10/28/science/ed138-dwarf-plane...
That is a great little app. Thanks for sharing!
Thanks!
You can also turn on the known asteroid population:
https://dahlend.github.io/ketev/#t=2026-11-12.9717&sel=2026%...
Everything runs in the browser, served as a static github pages so I don't have to think about hosting or anything.
It would help on mobile if you could you make left panel dismissable / reappearable so that the main rendered animation is visible by default
Sadly this site is not usable on mobile
it worked for me, you can drag the sidebar to make it smaller.
but it's not great for mobile
There was a lot of negative press and commentary about "demoting" Pluto from being a planet at the time. But its obviously true that it was the right move, and creating this "dwarf planet" class of objects was correct. It was hard to justify Pluto being a planet when Ceres exists, but Eris, Haumea, Makemake and all of these other possible Dwarf planets are the nail in the coffin.
> But its obviously true that it was the right move, and creating this "dwarf planet" class of objects was correct. It was hard to justify Pluto being a planet when Ceres exists, but Eris, Haumea, Makemake and all of these other possible Dwarf planets are the nail in the coffin.
I don't agree that "its obviously true that it was the right move". Instead of demoting Pluto, they could have promoted Ceres, and let Eris, Haumea, Makemake, etc, be planets too. Under that proposal, they could even have the category of "dwarf planets" encompassing them, it is just that "dwarf planet" would be a subcategory of planets rather than a disjoint category from them.
Classifications are useful. Even the 'proper' planets we split into terrestrials, Jovian giants, and ice giants. Each planet is individually interesting in itself. One might even argue all of the bodies in the Solar System exist on a continuum of mass from the tiniest mote of space dust, to rocks and asteroids, to moons and dwarf planets to terrestrial planets, ice giants, giants, and then the Sun, and then hypergiant stars, neutron stars, black holes, and then supermassive black holes, so let's call all of them 'celestial objects'—which we kind of do. But such a catch-all is pointless. Each object behaves very differently, with wildly different physical laws governing its interior and its neighbourhood.
Classifications impose structure on a collection of celestial objects, and we can derive relationships, commonalities, understand their development, the physics governing these objects, and build useful physical and mathematical models to widen our understanding of the universe.
Most reasonable scientists understand that some of these objects will blur the edges—like many moons being more massive and larger than Mercury. Or like the largest super-Earth exoplanets being nearly as massive as Neptune, or the largest gas giant exoplanets being 30-80 times as massive as Jupiter and hence having a surface temperature of ~2000 K. But that is part of science, and possibly even where the most interesting work is.
Classifications are useful! It is poor form to take a widely used and historic term and add on a whole different (dynamic!!) category of attribute as a filter on the definition. There is quite a bit of absurdity in the 2006 IAU's definition of a planet. If a massive object came into the solar system and perturbed orbits, caused chaos and collisions, then any "planet" with debris within its neighborhood would cease to be a planet. What do we gain from adding a whole different category of attribute (relationship to its neighborhood) as a criterion? Does it realistically outweigh decades or centuries of peoples intuition of the existing definition? Its not just a matter of science, but language and communication. If you want greater specificity, come up with a new term or use compounded terms.
It very much seems like the definition was just shoe-horned to stem any more planets needing to be added. It would be interesting to know if astronomers walk the walk when it comes the definition. If I research when planets were formed would I get the date when their neighborhoods were cleared? Do discussions around large events that send debris into the neighborhood of planets correctly reflect that they cease to be planets until it is cleared?
Alan Stern said it best
>"We're going to call it a cow, except when it's in a herd." A river is a river, independent of whether there are other rivers nearby. In science, we call things what they are based on their attributes, not what they're next to.
>And the IAU definition is even worse, because it produces different categorizations for identical objects, depending on where they are. Get this — Earth at the same distance from the sun [as Pluto] would not be a planet by the IAU’s measure, because Earth can’t clear that zone either. I would say any definition that produces a result where Earth is not a planet under any circumstance is immediately indicted as ridiculous, because one thing we all agree is a planet is planet Earth!
>That's crazy. No wonder teachers and students are frustrated, confused, unhappy. It reflects poorly on astronomy, to have something so illogical.
https://www.space.com/12710-pluto-defender-alan-stern-dwarf-...
I find this interview very interesting, because around a decade prior, Stern himself proposed 'clear the neighbourhood'[1] as a finer-grained rule for planet classification involving the dynamics of the planetary body in question. That is, planets that could clear their neighbourhoods are überplanets; otherwise unterplanets. In fact, near the end of that paper he proposes not two, but five classifications of planets: subdwarf, dwarf, subgiant, giant, and supergiant.
Now, Stern's broader argument in that interview is that we should not take into account the dynamical behaviour of planets. I disagree, especially since dynamics determine the static future state of any given object. Take chthonian planets—former gas giants whose gaseous envelopes have been stripped away by the primary. Do we insist that they are still gas giants? No, we even came up with a name for the transitory period when these planets are losing their gas envelopes.
Personally, I think Stern was being a little more animated to drum up support for and shine a beacon on New Horizons, a then-ongoing mission that he was principal investigator of.
To directly answer your question:
> If a massive object came into the solar system and perturbed orbits, caused chaos and collisions, then any "planet" with debris within its neighborhood would cease to be a planet.
There actually are (multiple) mathematical formulations for 'clearing the neighbourhood'[2]; it's not purely empirical or based on vibes. Stern proposed one in the above-mentioned paper. Jean-Luc Margot suggests several more[3]. I suspect a more universal model will be probabilistic but grounded in celestial mechanics.
> Does it realistically outweigh decades or centuries of peoples intuition of the existing definition?
People's intuition for 'planet' in the centuries before telescopes were invented was 'something that moves against the fixed background of stars'—that is literally the Greek etymon for the English 'planet'. The Sun and Moon used to be a part of the 'classical planets'. We didn't even find the last two (or three) planets in our own Solar System until around two centuries ago. We didn't know about nuclear fusion until 1925, and we didn't know other galaxies existed until similarly recently. We didn't understand the inflationary model of the universe until the 1960s. The first exoplanet was discovered only in 1992.
We have repeatedly and retroactively renamed, retconned, reworked, revamped and rewound scientific definitions and models over and over again. I think planets will be OK.
[1]: https://doi.org/10.1017/S1539299600013289
[2]: https://en.wikipedia.org/wiki/Clearing_the_neighbourhood
[3]: https://doi.org/10.1088/0004-6256/150/6/185
I find it particularly amusing that the IAU definition, in a footnote, expressly declares the eight planets to be what they are. In other words, the true definition for being a planet is to be defined as a planet- so the criteria it gives are entirely superfluous, at least when it comes to our own solar system.
It's clear that they worked backwards from the eight planets to come up with a classification system that fit, but in realizing the problems with the approach, just gave up and created the carve-out.
Also, just as another example of the absurdity of the IAU definition, there's a pretty strong argument to be made that Earth's moon would be a planet under the criteria: it orbits the sun, it's round, and it's massive enough / positioned such that it's capable of clearing its neighborhood.
It orbits the Earth rather than the Sun by most conventional definitions of the word, although apparently the IAU doesn't have an official definition of orbit that I can find.
Well yes, but otoh https://physics.stackexchange.com/questions/266426/what-does...
> Classifications are useful.
I wasn't arguing against classification, just that I remain unconvinced that the classification which IAU adopted is superior to the alternatives.
One problem with the IAU definition: it only works for our planetary system, not for exoplanets – with the end result that we have two different definitions of "planet", one for our system, one for everywhere else.
Now, of course, that in part reflects that we have vastly more information about our own system than any other – but, a few centuries or millennia from now, I expect we'll know a lot more about some of those other systems – we will probably have sent probes to explore the nearer ones by then – at which point people (or AIs?) will probably push to unify the two definitions. My own bet is they'll be more likely to unify them by dropping "clear the neighbourhood" from the our-system definition than extending it to apply to other systems. Splitting the common category of "planet" into two subcategories based on "does it clear its neighbourhood" works much better than insisting on two disjoint categories based on that criterion when you have very limited information on where that criterion is met.
It doesn't work for exoplanets because part of the definition is orbiting our Sun. But the exoplanet definition would define the same 8 for our star, so I don't really see where the issue is.
Thought experiment: Suppose we (somehow) discovered an exact replica of our solar system, n light years away; would its counterparts of Ceres and Pluto be classified as exoplanets?
Yes, regardless of the specific labels, there are clearly interesting and meaningful differences between these things, and it’s worth having vocabulary capable of distinguishing them. The whole universe is a continuum of stuff but it helps to have more than just the word “stuff”.
Arguably there are just two categories: stuff and not-stuff (aka vacuum). And then you go from intergalactic space (almost entirely not-stuff) via interstellar space (marginally more stuff) through planets and stars up to neutron stars (lots of stuff) to black holes (well, we don't actually know what is inside a black hole exactly, our models break down).
Is this classification useful for everyday life? Not at all, but looking at things from different perspectives for a change can be quite amusing.
The backlash was all just populist ignorance on display. There were multiple concrete measures of planetness constructed and Pluto is an outlier in all of them. One of these things is not like the others.
I think the backlash was fed by insistence dwarf planets are not just a subtype of planets, it's such a "well ackchyually" type thing to choose that name for it regardless of how valid it was to break out the category that I'm not sure it'll ever stop being talked about.
Even something as simple as "planets" contains "major planets" and "minor planets" would have avoided 20 years of people arguing about the names instead of caring to learn what's different about them. But you never hear people explaining why the IAU insists "dwarf planets" aren't "planets" even when "dwarf planets" has several subcategories which don't share the "planet" moniker, you just hear people argue about missing Pluto or call other people ignorant of the technical details of the categories.
They are a separate category and it can clearly be demonstrated.
I'm saying there were two reasonable options: one where they were different subcategories of planets and one where they were their own category but not called dwarf "planets" to clearly reflect that separation from "planets".
I'm not saying anything about what we call dwarf planets and planets being the same as each other. The technical details are fine, it's purely the oxymoronic naming which screwed public acceptance.
Yeah. The gas giants are clearly way different from rocky planets and we still keep them all under one taxonomical grouping. Pluto and other bodies should have been an extension of that: make them dwarf planets or whatever other classification you want. But they're still planets. Theres no "demotion" just a further categorization.
To me this subject is just a combo of bikeshedding[0] lumper and splitter[1] stuff by people who aren't domain subject experts.
I find people do this a lot with space stuff because they're interested in it but don't really know much about it.
It's ultimately irrelevant to the subject at hand and detracts from the conversation.
https://en.wikipedia.org/wiki/Law_of_triviality
https://en.wikipedia.org/wiki/Lumpers_and_splitters
And a spattering of domain subject experts being offended that they don't have veto power over colloquial terminology
You can just keep calling it a planet in everyday conversation if that's your jam. There's no astronomy police that will come to get you if you do.
Just expect to be corrected and "well, achually"-ed by people who follow the current naming system, especially in science circles.
I'm amused that you described a group policing terminology right after saying there's no group policing terminology.
Right. Somewhat similar to other dual-use words like “work”, “energy”, “power”, “momentum”.
With such “well, ackshually”s as “you didn’t really do any work at all when you were pushing on that tree trunk” and “you specified watts, but you really wanted to use lumens”.
Ok so we have 50 planets?
I mean, why not?
It would make the mnemonic for memorizing the order of the planets very long and difficult for the children to remember.
My Very Elegant Mother Just Served Us Nachos
Yes. I wouldn't call a dwarf person any less of a person, nor would i refer to a dwarf planet as being any less a planet. Between the terrestrial planets, the gas giants, the ice giants, and arguably a couple of rounder moons, we have far more than 50 planets in our solar system.
By some extreme definitions, Earth might not be a planet. Some say the moon is too big to qualify the earth "cleared out its orbit" criteria, making Earth-moon some sort of double planet... like Pluto-Charon.
Setting aside the whole "demote Pluto" and "dwarf planet" aspects, I find it amusing that we use the same term for small rocky Mercury (2,440 km radius, 5% of Earth's mass) and immense gaseous Jupiter (69,000 km radius, over 300 times Earth's mass). Only we don't, of course. Jupiter is a gas giant, while Mercury is a rocky planet, or iron planet; there's a lot of 'planet' terminology.
https://en.wikipedia.org/wiki/List_of_planet_types
It gets even weird when you consider Ganymede which is larger than mercury but not a planet because it's a moon.
Ultimately these sorts of quibbles are fundamentally semantic in nature thus (in general) entirely pointless. It's only the subjects themselves - the myriad celestial bodies - that are interesting.
It's semantics as well as a collision with our inability to work with gradients. We can look at two colours and can name the discrete ones - but when does red stop being red? When does a species emerge?
Yeah, I don't like when a modifier is mandatory. If they chose to label them "dwarf planets" then it should be expected that people will leave off the modifier "dwarf" when being concise.
If they really didn't want people to continue to refer to them as planets, then they should have chosen a completely different noun.
Why is Jupiter in the same category as planets then?
Jupiter does not orbit the sun (the first requirement per IAU definition of planet), the barycenter for the system is not inside the surface of Sol at 1.07 solar radius.
It is not planet then yes ? There is no push to promote it to a sub-brown dwarf or a new category .
Classifications are always arbitrary and only fits some bias one way or other.
A planet is an object that:
Ergo, Jupiter is a planet. It is a certain type of planet—a gas giant—but a planet nonetheless.There is some controversy about when giant planets stop being giant planets and start being brown dwarfs—typically around 30 Jupiter masses—but this is an active area of research, to understand the fusion ignition in these objects. As I mentioned in another comment, classifications are useful. Of course they are arbitrary and they are biased—they were decided by humans. But the entire point of science is to push the edges of our understanding and reduce these biases and make our models more general and less arbitrary.
Their claim is that it fails (a) because the Sun and Jupiter orbit a common barycenter, and hence Jupiter does not orbit the Sun (or any star).
Everything orbits a barycentre. The position of that barycentre within or without the primary is not really relevant within several orders of approximation—and when it does become relevant, then that system becomes interesting in itself. Chances are the planet in question is so massive relative to its primary that we need to ask a more fundamental question: are we even looking at a star-planet relationship, or is it closer to a binary star system? Then we can investigate how that system formed and developed, and generalise and improve our model of stellar/planetary system development.
There are plenty of binary star systems where this orders of magnitude standard is not met .
There is no academic literature or accepted consensus barycenter has to be orders of magnitude away - the accepted consensus is that a body orbiting another means internal barycenter, if they have external barycenter then they orbit each other period.
The fact that Charon and Pluto do not have internal barycenter is why Pluto-Charon (also Eris-Dysonmia) are considered dwarf planet systems ; and that was used partly in support (while not used in definition) of arguing for dwarf status of Pluto .
Yes — their point is that as-written, it orbits a barycenter external to the star: hence doesn’t orbit the star, but they’re in a mutual orbit.
Thus the suggestion for a new dwarf category — precisely because we do label times the barycenter is outside the star differently, as you yourself note.
You were so busy trying to “correct” me, you failed to address the topic.
'In orbit around' is frequently used as short-hand to refer to a secondary with mass << primary, to a first order approximation. In the Solar System, even for satellite transfers, the effect of Jupiter on the Sun can and is ignored.
The topic is pointless hair-splitting and nostalgia. It's like saying we can't ever use F = ma to describe free-body diagrams; we need a full relativistic per-particle QFT simulation to accurately and precisely model every single interation. That is impractical, at least for now. Likewise, we come up with classifications that are useful for the present time. If 'dwarf planet' is a catch all for all the small roundish bodies in our planetary system, then so be it. I suspect the IAU will make things more fine grained in the relatively near future. We even classify the asteroids as more than just rocky lumps.
We have QFT precisely because mechanics improperly categorized events.
We can similarly say that “cold dwarf binary systems behave similarly to planetary systems, due to the proximity of the barycenter to the star”.
And that’s more informative than calling Jupiter a planet.
I agree with this picture as the barycenter argument seals it for me. The only leap left is for me to believe Ol'Jove as a failed star (it still emits heat in excess of what it derives from Sol) and we find ourselves cozily herded about by a binary star system.
Nah, what we need to do is to just extend our heads to be 2 categories and not think of one as lesser or demotion.
Like, in high fantasy, right, we have humans and elves and dwarves and orcs and all that. Are dwarves 'less' than elves? Are orcs 'less' than humans? No, they exist on a spectrum or line that is not less or more. Maybe less or more evil, maybe less or more aged or fancy. But in high fantasy you can't demote a elf to be a dwarf or a human into an orc. |
So, my proposal: Include all the dwarf planets in all the science exhibits, the kindergarten posters, the rhymes and meters. Just like with the upper and lower cases of the Latin alphabet. Sure you can forget that the dwarf ones exist, but if it's on everything, then that'll be hard.
Here's a quick one that I put like no thought into:
>Mercury, Venus, here comes Earth!
>Mars is next, let's shout with mirth!
>Jupiter's big, Saturn has rings,
>Uranus and Neptune make eight amazing things!
>Ceres, Pluto, Haumea too!
>Makemake and Eris join the crew!
>Sedna, Quaoar, Orcus, Gonggong,
>Dwarf planet dancing all day long!
So, again, sure, dwarves, fine. But just don't make being a dwarf planet a 'thing', celebrate them too!
I initially assumed from your comment that all of those objects are larger than Pluto, but none of them are, so what exactly is "obvious" about the decision? They haven't resolved any inconsistency by demoting it.
Eris has more mass. And our Moon, the Galilean moons of Jupiter, Titan and Triton are all both larger and more massive.
Ganymede and Titan are both larger than Mercury.
I would preferred we require children memorize all 10,000+ satellites of the Sun. More planets the better. Count the moons, why not.
> Eris, Haumea, Makemake, ...
Don't forget about Gǃkúnǁʼhòmdímà! Everyone forgets about Gǃkúnǁʼhòmdímà.
I'm just salty about only getting served "Noodles" instead of "Nine Pizzas" from my very educated mother.
You merely need to define the term "NinePizza", which is a collection of nine pizzas—and then have your very educated mother serve that. This doesn't present any problems philosophically, since we already have terms like "Solar System" for collections of disconnected objects.
Maybe we haven't gone far enough. We should refer to Uranus and Neptune as Trans Saturn Objects.
Uranus and Neptune already are a two-member class (“ice giants”).
Does Saturn have enough density to count, though? I mean, from a certain point of view, it's more of a spheroid of hydrogen with a bit of helium. Maybe we need a new term for it, like "Planetary nebula"? ;)
Ceres by itself doesn't really challenge Pluto's old classification. Pluto has 14 times the mass of Ceres, and five moons. Ceres is the only "dwarf planet" classified so far that has zero moons. There are still good arguments for a definition that doesn't consider Ceres a dwarf planet.
This helps illustrate how arbitrary these classifications are. They depend entirely on what properties are included in the definition. The updated IAU definition is essentially reverse engineered to fit how they wanted to classify the planets.
So, a factor of 14.4 is enough to put the line between Ceres and Pluto, but a factor of 23.0 is not enough to put it between Pluto and Mercury?
Yes, those classifications are arbitrary, but then any classification is. If you want to consider Pluto as a planet, you have to include quite a lot of other bodies, which are all qualitatively and quantitatively different from the 8 (which themselves are in 2 qualitatively and quantitatively different groups. Or 3, depending on who you ask).
In the grand scheme of things, it does not matter and astronomers won’t care about the general public’s feelings. It is utterly pointless to get worked up about these things.
> So, a factor of 14.4 is enough to put the line between Ceres and Pluto, but a factor of 23.0 is not enough to put it between Pluto and Mercury?
Why not? It's an arbitrary line, and there has to be a lower limit somewhere.
In the IAU definition, hydrostatic equilibrium loosely defines a lower limit for all planets including dwarf planets, but at the limits that's a fuzzy property that gets debated for small bodies. In fact Ceres is less massive than what some studies have found to be required for hydrostatic equilibrium, e.g.: https://iopscience.iop.org/article/10.3847/PSJ/ad55f3 . The abstract of that paper starts out with, "The current IAU definition of 'planet' is problematic because it is vague and excludes exoplanets."
If we had more small planetary candidates in the solar system, they would have had to come up with some additional constraint to separate planets from non-planets.
The problem is not experts having precise terminology in technical settings; the problem is the army of “experts” who will descend upon you if you use the incorrect arbitrary term in colloquial settings. The same sort that will descend on you if you dare refer to a planet orbiting a different star as a planet, not an exo-planet.
A lot of comments here explicitly complain about the precise terminology of experts in technical settings.
> The updated IAU definition is essentially reverse engineered to fit how they wanted to classify the planets.
I wholeheartedly agree with this take. I have something of a personal hatred of poorly executed retconning thus I present the following proposal.
For the sake of simplicity we should use even powers of 10 with earth as the primary reference point. Note that the earth is 10^24, mars is 10^23, and the earth's moon is 10^22 (pluto is also 10^22 and less massive than the earth's moon). It seems "obvious" then that 23-25 are planets, 22 or less are relegated to dwarfs, and 26 and over are giants (jupiter is a 27 and the sun a 30 for reference).
By that logic, if we want to consider pluto a planet then the earth's moon also has to qualify. I don't really care one way or the other but that just doesn't seem sensible to me.
Following this logic, a black hole with earth's mass would be a planet.
You could add a hydrostatic equilibrium requirement, which is what the IAU definition does. It doesn't really affect anything else the GP was saying.
Why did we add this requirement? To fit how you want to classify planets? That retcon was not executed well.
It shows that the line of reasoning makes no sense, because any classification is constructed to fit some of our preconceived notions. Choosing a classification that builds off our social history is perfectly fine, as long as the rules for classification are applied consistently.
It's easy to exclude the moon, like the IAU does, by requiring that planets orbit the Sun. (Although you might want to avoid the IAU's parochialism, which excludes exoplanets, and say "a star" instead.)
> By that logic, if we want to consider pluto a planet then the earth's moon also has to qualify. I don't really care one way or the other but that just doesn't seem sensible to me.
Ganymede is larger than Mercury, so a definition that doesn't specify orbiting a star would have to include some moons, or exclude Mercury.
This exposes another problem with this kind of definition - it seems to make more sense to define a body in terms of its intrinsic properties rather than whatever location it happens to be in.
For example, a rogue planet is not a planet by the IAU definition. You'd have to call it something like a free-floating planetary-mass object. ("Free-floating" is not the right term either, but I don't have the research funds to come up with a better one.)
> I have something of a personal hatred of poorly executed retconning
Totally agree. Wanna join me outside George Lucas' house to protest the Kessel Run?
> They even found it in photographic plates from 1937
Pretty rad.
This is one of the reasons I think it's too early to talk about the Fermi paradox. We still know so little about what's around us in the universe
We've barely confirmed the existence of 10_000 exoplanets in our galaxy, and the methods we use to detect them are heavily biased toward certain ranges of planetary mass and orbital distance
As far as we know, everyone could very well be right here
Is this Planet X that they have been talking about for a while now?
No, all dwarf planets are too small to affect orbits the way that hypothetical planet is supposed to do.
Planet Nine is the new Planet X https://en.wikipedia.org/wiki/Planet_Nine
Glad that artist's impression was there so I could get a good idea of what Planet 9 may or may not look like.
there are a few different “Planet X”s theorized
https://en.wikipedia.org/wiki/Planets_beyond_Neptune
> The new dwarf planet candidate, called 2026 ED138, orbits on a plane tiled nearly 25 degrees
I've noticed similar glaring typos in 2 recent articles. In both cases very early in the article. Is it possible that they are adding typos intentionally to mask the AI authoring?
No. It's understaffing. Magazines used to have robust copy desks. Very few still do.
Could this be Planet X? https://science.nasa.gov/solar-system/planet-x/
No
Why not?
It's too small to affect other orbits as much as the hypothetical planet X.
Shouldn't we be giving dwarf planets names like Dopey and Sleepy?
OK but if we decide to make it an official planet, it must start with the letter P so that all the classic mnemonics work again.
They should call it Twitter.
(Character limit 280)
so there's potentially undiscovered planets in our own neighborhood ?
Yep. Who knows, maybe there's a sub-dwarf of 10 Jupiter masses faintly emitting in the infrared out there.
Why not? Blindsight by Peter Watts got one, Big Ben.
Nibiru!
Maybe the Anunaki will return and teach us how to use the Pyramids!
So, this is another new candidate, different from the one from a week ago?
No it's still 2026 ED138.
then we need all the previous posts about it linked