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.
> ED138 joins the rapidly growing sample of solar system objects discovered by Rubin since it began collecting early data in preparation for the Legacy Survey of Space and Time (LSST), which began in June 2026. In April, before LSST formally began, the Rubin solar system team announced observations of over 11,000 new asteroids, which included roughly 380 new TNOs. At the time, the team called this large submission “the tip of the iceberg” and expressed their excitement for the discoveries to come once LSST began. With the discovery of ED138, those hopes are coming to fruition.
My point was that the LSST survey was looking in those areas as part of its "mapping the Milky Way"
Pluto has a 248 year orbit... which is close to the 282 years for ED138. 100 years ago, maybe Tombaugh would have found ED138 that if he was looking in that part of the sky.
I've always only ever seen diagrams of the Jupiter Trojans from a top down perspective so I never realized how much they stretched beyond the orbital plane of the solar system.
Do you know why they stretch so up and down relative to Jupiter? Is that sort 3d distribution shape typical for L4 and L5 Lagrangian points, and it's just amplified by the size of Jupiter or is there something else at play there?
The Lagrange points are more like "regions" than points in reality. There is some complicated math you can do to work it out, but typically numerical stuff is a bit more reliable once you start to include the rest of the planets, the analytic theory gets pretty complicated the more bodies you include.
The trojans that you see there sort of do a little dance when you plot them in the rotating frame of Jupiter (more technical term is librate). The size of the region has to do with the mass of the parent body, since jupiter is so big, the capturing region is pretty large. Technically there are other trojans around some other planets, but most of them are not long term stable like jupiter is, mainly because jupiter is just big.
Yeah I knew they were regions more than points, I just sort of envisioned them as something more in the same of a tapered tail than something so tall compared the diameter of Jupiter.
I'd like to take the time to tell you that I really appreciate your contributions to this site. I'm always excited when there's a post about asteroids or something similarly sized in the solar system because I know that you'll post about it more often than not.
I've taken a course on the geology of the solar system and a course on exoplanet detection but I definitely consider myself a layman in this field. I've always had a particular fascination with these small tumbling rocks in our solar system so it's always a pleasure to see you talk about them and your work.
it's interesting the further the planets are the more off level their orbit is, is that because most of the planet creation chaos was happening near the sun?
It requires less delta-V to create a large change in inclination when the object is far from the Sun, because the orbital speed in a more or less circular orbit varies as (orbital radius)^(-1/2).
It looks like its orbit is actually within the orbit of Pluto at one point, and at times, the two bodies could come within a very small distance of each other. Wouldn't this disrupt one or the other, possibly throwing them off of their orbits?
Part of why Pluto was demoted is it's so small it doesn't effectively clear it's own orbit. Orbits can look close here but with how small Pluto is it's gravity would barely affect this dwarf planet. It's also possible they've only been at their closest approach a handful of times in their entire life.
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.
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.
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?
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.
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”.
Speaking of lumping and splitting: in the nebular hypothesis planets accreted from the protoplanetary disk. Out in the Kuiper belt objects were too far apart to properly accrete, and remain separated.
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.
> In fact, near the end of that paper he proposes not two, but five classifications of planets: subdwarf, dwarf, subgiant, giant, and supergiant.
Yes, “classifications of planets”. So Pluto would still be a planet. Then if you wanted a more specific modifier you could add it, but it’s still a planet.
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.
That's exactly my point. The Moon orbits the Earth in the same way the eight planets are what they are - because enough people say it's that way.
By what 'conventional definition' does the Moon orbit the Earth? Barycenter inside the parent? By that definition Jupiter doesn't orbit the sun. Gravitational pull? The sun exerts twice as much pull on the Moon than the Earth does; the earth just slightly wobbles its path.
« 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. »
Major impacts on Earth on a regular basis would indeed upset the applecart of planetary taxonomy.
> because one thing we all agree is a planet is planet Earth!
Of note, the ancient planets did not include Earth. I say this as someone who respects Alan Stern so much that my son is named after one of his projects.
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.
And to paraphrase Douglas Adams, the stuff is basically just a rounding error in the not-stuff. And insofar as there is stuff, it's all stars, with everything else being a rounding error. This is the case in our solar system, which, mass-wise is the Sun, with Jupiter as a rounding error - everything in the Solar System that isn't the Sun is mostly Jupiter.
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?
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.
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.
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.
> I think the backlash was fed by insistence dwarf planets are not just a subtype of planets[...]
I'm 99.9% certain that the backlash would have been far greater if our solar system suddenly had dozens extra planets.
> 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[...]
Because otherwise the definition would differ greatly from our historical classification. Since it is necessarily arbitrary, sticking close to our history IMO seems like the obvious choice.
The historical IAU term such bodies was already "minor planets", the public was not in backlash about it despite there being over 100,000 of them by the time the "dwarf planet" category was created. "Minor planets" was even historically a subcategory of planets for a long time.
But, as said above, it'd still have went over fine if, instead of considering dwarf planets a subcategory of planet, the IAU had just not used "planet" in the category of things they didn't want people to consider planets. E.g. like they did for breaking "small solar system bodies" out of "minor planets".
Yeah, and people clearly didn't count them as "real planets", except for Pluto. So what's the issue? How are "dwarf planets" less planet-y than "minor planets" were?
Had the IAU said "Pluto is now a minor planet, which is a type of planet different than major planets" it would have gone over really well, been clear, historically accurate, and scientifically accurate.
Had the IAU said "Pluto is now a dwarf body, which is not a type of planet" it would have still been unpopular, at least clear, no more historically inaccurate than coming up with "dwarf planet", and scientifically accurate.
What the IAU actually decided was "Pluto is now a dwarf planet, which is not a type of planet at all". Since the sole positive attribute of this is it is scientifically accurate, it will have some level of unpopularity and be the cause of endless confusion even after everyone who knew Pluto as a planet is long dead.
I'm very supportive/glad they made it so Pluto wasn't the exact same classification as the remaining 8 planets. I'm continually facepalming over the absolute insistence to name the category dwarf planets but not have it be a type of planet.
> Had the IAU said "Pluto is now a minor planet, which is a type of planet different than major planets" it would have gone over really well, been clear, historically accurate, and scientifically accurate.
I disagree on multiple levels:
1. It would still have gone over very poorly, because many people explicitly oppose the demotion of Pluto. Demoting it to a different category would not change the issue in those people's eyes.
2. I doubt that "[...] a type of planet different than major planets" would improve clarity much for the general public - if you've seen studies or similar, a reference would be appreciated. But changing the meaning of an established term would definitely be far less clear on the academic side (since every mention would be ambiguous before the publishing year and/or individual stances of the authors/journal are known).
3. "Historically accurate" depends on what type of accuracy we're talking about, because IMO this approach is less historically accurate because we'd be changing the definition of an existing term, without a meaningful difference aside from that (since both are literally named X planets).
4. I'd need an argument for why it'd be more scientifically accurate. Since we were unaware of most of the objects in our solar system when the previous classifications were created, it's again IMO much more scientifically accurate to create a new classification.
> Had the IAU said "Pluto is now a dwarf body, which is not a type of planet" it would have still been unpopular, at least clear, no more historically inaccurate than coming up with "dwarf planet", and scientifically accurate.
This one would have been even less popular! It's definitely historically less accurate when talking about Pluto and previous minor planets, though at least I can agree regarding the scientific accuracy.
> What the IAU actually decided was "Pluto is now a dwarf planet, which is not a type of planet at all". Since the sole positive attribute of this is it is scientifically accurate, it will have some level of unpopularity and be the cause of endless confusion even after everyone who knew Pluto as a planet is long dead.
It's definitely less unpopular than "dwarf body" and much more clear than "minor planet (no, the new one)"! And as explained, I'd argue it's historically the most accurate one.
Pluto didn't really fit in with the other planets even before the discovery of further dwarf planets, that discovery was just the final nail in its coffin as a "proper" planet: it was the smallest planet by far, smaller even than Earth's moon, it has a highly unusual orbit (not in the ecliptic plane, so elliptical that it's sometimes closer to the Sun than Neptune - https://en.wikipedia.org/wiki/Pluto#Orbit) etc.
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.
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.
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.
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.
> 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 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.
> any classification is constructed to fit some of our preconceived notions
That is not the concern here. It seems to me that you're failing to account for intent. Classifications exist to serve a useful purpose. What I object to is mental gymnastics used to gerrymander a particular outcome. These things should conform to something resembling occam's razor - minimalism in pursuit of a generalized goal.
> These things should conform to something resembling occam's razor - minimalism in pursuit of a generalized goal.
That is literally what happened - a definition for "planet" that is as minimal as possible, while staying as close to our historical categorization as possible.
Now, I know you'll object to the "while staying as close to our historical categorization as possible" part, but that is literally how scientific fields always approach arbitrary classifications. Without taking our historical use into account, there is no "objective" definition you can derive.
If I'm wrong, please give me an example of what kind of definition for "planet" you'd be happy with. But remember, it must not take into account what we today consider "planets" to be.
> that is literally how scientific fields always approach arbitrary classifications.
That's a big overstatement. It's how some soft science fields sometimes do it. You can't get away with that in, say, physics and still end up with useful theories, because your theories quickly break down due to inconsistencies. You can't design working scientific theories by committee, because votes don't affect fundamental physical reality.
Much of astronomy is a hard science, which is why this definition came in for pushback even from astronomers themselves. Here's astronomer Phil Plait's take:
> "For some time now I, and many others, have stated that the IAU definition is silly for many reasons, including a line that says that planets can only orbit the Sun. If that’s true, then exoplanets — worlds orbiting other stars — aren’t really planets!"
-- https://www.syfy.com/syfy-wire/mea-culpa-are-exoplanets-plan...
The article goes on to clarify that the IAU explicitly limited its definition to solar system planets - in fact, the resolution is titled ""Definition of a Planet in the Solar System."
As such, it's not even a universal scientific definition for "planet".
Another quote:
> "At [the IAU] 2006 international meeting in Prague, they drafted a series of defining characteristics for planets and put it to a vote. The definition passed, and the outcome was entirely predictable: They made things a lot worse."
> "...the more proximate problem is that these definitions are, um, silly."
> That's a big overstatement. It's how some soft science fields sometimes do it. You can't get away with that in, say, physics and still end up with useful theories,
Sorry, but this objection reveals that you either don't understand the topic of discussion at all, or you're arguing in bad faith. No theory in physics will break if you change the criteria of an arbitrary classification.
I'll pose the same challenge to you: please give me an example of what kind of definition for "planet" you'd be happy with. But remember, it must not take into account what we today consider "planets" to be.
> Sorry, but this objection reveals that you either don't understand the topic of discussion at all, or you're arguing in bad faith.
Nice false dichotomy and binary thinking.
You're right that arbitrary classifications are arbitrary, obviously. My point is that you can't get away with arbitrary classifications in the context of hard science theories.
There's an example of a definition of "planet" in one of those Plait articles I linked:
> "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."
The Plait articles are worth reading, because they might help disabuse you of the notion that the IAU definition is a good one, and explain why it would make sense to have a better one.
> My point is that you can't get away with arbitrary classifications in the context of hard science theories.
Arbitrary classifications don't factor into hard scientific theories, so I have no idea how this is relevant. Or what "hard science theory" are you thinking of that relies on whether we classify an object as a planet?
We can modify arbitrary classifications without any theories being affected, because the theories don't rely on arbitrary classifications. We arbitrarily decided what "one meter" is, yet we use "meters" all over physics. We could change the definition of "one meter" tomorrow, and all our theories would stay valid, because they don't rely on arbitrary classifications.
> There's an example of a definition of "planet" in one of those Plait articles I linked:
>> "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."
How does one arrive at this definition, without taking into account what we today consider "planets" to be? What principle of physics tells you that a planet must be sub-stellar mass, or that it has never undergone nuclear fusion?
You'll have to actually explain how this definition is any less arbitrary, but I don't think you'll be able to, because every useful definition falls under your objection.
> but that is literally how scientific fields always approach arbitrary classifications
No, not always. What you are describing there are shoddy classifications. I already addressed this exact point in my previous reply when I stated that classifications are crafted to serve a useful purpose. It is sometimes useful to hew close to the historical norm; other times it serves no purpose.
> But remember, it must not take into account what we today consider "planets" to be.
An arbitrary restriction that I do not accept.
> please give me an example of what kind of definition for "planet" you'd be happy with
I already did this in my original comment. I proposed an earth centric system that bins mass by powers of ten. My proposed range for planets was 10^23 <= x < 10^26. Hydrostatic equilibrium works well as a constraint to eliminate things that are clearly not earth like (such as black holes) but alternative formulations such as stipulating an acceptable density range are possible (such cross cutting subcategories might even be preferable).
Note that this would result in ganymede being classified as both a planet and a moon. I think that makes perfect sense given how similar in size it is to mars. In fact ganymede being excluded as a planet is precisely the sort of gerrymandering I object to.
As it turns out my proposed criteria also relegates pluto to the status of dwarf planet. I think that's sensible given that it's both smaller and lighter than the moon. However I'd be willing to entertain a set of criteria that acknowledged both pluto and the moon as proper planets - the important thing to me is consistency.
Hydrostatic equilibrium is the most defensible of the requirements to be a planet.
The IAU wording is, "...has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape."
In this case it just means that the planet's mass and therefore gravity is sufficient to give it a spheroidal shape. (Technically the equilibrium is with the opposing push from the planet's material, which resists gravity's attempts to compress it. The result is a sphere, made spheroidal in real planets by rotation.)
Less massive objects like asteroids can have weird shapes, like the asteroid Toutatis which has a sort of dumbbell / unshelled peanut shape. The hydrostatic equibrium requirement prevents such objects from being classified as planets, as well as small objects that just happen to be round for other reasons.
This isn't really social history but simply classifying the objects according to intrinsic properties, which seems fine to me. This classification could be applied to objects anywhere in the universe without any problem.
The other two requirements are more problematic and arbitrary. One is that it has "cleared its orbit". Earth, let alone Mercury, would be unlikely to qualify for this if it were at the same distance as Pluto, for example. Rogue planets also can't satisfy this one, and we've observed those.
The other is that it has to be in direct orbit around the Sun, not a satellite of some other object. This excludes exoplanets as well as rogue planets. The rationale for this is that they didn't have enough observational info to say much about exoplanets, but that seems like a flimsy excuse. A good definition would apply to exoplanets by virtue of being theoretically sound, and not having been tuned to fit the way they were trying to classify solar system planets.
> Less massive objects like asteroids can have weird shapes, like the asteroid Toutatis which has a sort of dumbbell / unshelled peanut shape. The hydrostatic equibrium requirement prevents such objects from being classified as planets, as well as small objects that just happen to be round for other reasons.
Okay, and how did you arrive at the hydrostatic equilibrium requirement? What law of nature told you that planets must be spheroidal?
You're only adding this requirement so the outcome fits your pre-existing desired results, since you're literally arguing for the requirement based on what objects it excludes. And what are the consequences of such arbitrary decisions, according to you? "You can't get away with that in, say, physics and still end up with useful theories[...]"
Do you get the issue with this line of critique? There is literally no useful definition of "planet" that is non-arbitrary.
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?
I think that's an example of coming to a useful realization while working through the process. It should serve to illustrate the importance of not attempting to engineer a particular outcome. If the earth were wandering the galaxy clearly it should still be considered a planet, no? And were we living on a body the size of mars that orbited a gas giant we would still be said to occupy a planet. At least assuming the goal is to classify things of a vaguely earth-like nature.
I think the more difficult question is what exactly qualifies as a moon. Anything orbiting a planet, including a smaller one? That seems perfectly reasonable to me. (Yes yes I know the barycenter matters.)
And yes, I think ganemede should be considered both a planet and a moon. That thing is on the same order of magnitude as mars. What useful purpose is served by excluding it as a planet?
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.
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"? ;)
Certainly, Mercury, Venus, Earth, Mars, and Ceres are rocky planets. And Pluto, Eris, Makemake, Haumea etc are icy planets. But the idea that there are gassy planets is just silly.
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!
People can't help ranking things. You say there are two categories and before you've even finished your sentence the audience has ranked one category as "better" than the other. This is a source of a lot of our problems.
When I was a child it was commonly taught that the dinosaurs were all extinct and birds were not dinosaurs, though it was known that birds evolved from that lineage.
Even as a child I could figure this was BS - Cretaceous period dinosaurs evolved from Jurassic period extinct dinosaurs, so bird are not dinosaurs simply because someone decided dinosaurs were all extinct?
As an adult I find this has been rectified, but we're going backwards when it comes to planets.
The main reason it's controversial is because Pluto was discovered by an American and they want their rock to be in the top category.
I don't really know how the category helps anything, though. No astronomer would use the category in any work, they'd use the actual properties of the planet. The only people who might care are those publishing paedagogical material. Is it really that important that kids around the world all learn the same number of planets?
It could easily be taught a different way: there are planets, you can see these ones tonight with your naked eye, you could see these other ones with a telescope. I wish they'd taught me that instead of those silly pictures of the solar system.
I always feel amazed to know how suns gravity keeps hold of massive planets millions of kilometre away from it and how we are still finding objects in our solar system!
True. My intuition is a really weak force can still have a lot of effect if it isn't competing with any other force. In this case, forces from other stars are many orders of magnitude weaker.
Similarly, mutations are a pretty weak force of evolution. But they add up over a long period in the absence of any other force keeping species stable.
I always feel amazed by the fact that light from the sun takes 8 minutes to reach Earth and is still powerful enough to sustain all the life around us. The fraction of energy hitting us 8 light-minutes away is miniscule.
yea there is so much energy out there outside of our comprehension and we feel and think of things of our scale, the design of nature is so awesome and complex that we barely understand it
> 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?
Here's a screenshot from my locally hosted emotive audiobook narration software backed by Qwen3.6 for checking grammar (and invoking a custom SymSpell implementation for spelling):
The hypothetical Planet 9 is expected to be much more massive and further out than this, and perturbing the orbit of objects like this. I think they'd need to get the full numbers on ED138 before they can plug it into simulations and see if it fits the pattern or not.
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
With the big exception being radio waves. Once electronics are invented and especially anything like radio, the planet should light up. There is nothing particularly human about it, except for how we encode the data and what data we find worthwhile to encode (maybe), but it would still stand out over the background noise.
Assuming tech == electronics is a very human assumption to make. Biology is a way more "advanced" kind of nanotech and it has significantly different characteristics to electronic based tech. We simply don't know and even taking that into account there are blind spots in our blind spots.
even on earth those high-power radio transmitters were relatively short-lived phenomenon; modern 5G cells are dramatically quieter that the kilowatt AM radio (and TV) towers of the past.
Since it's at the outer reaches of our Solar System where it's very cold I'd like to officially propose to name this new dwarf planet Boreas, after the Greek god of the north wind and winter.
But then with the Kuiper Belt and the Oort Cloud you're getting into interstellar space, yes ? As space-based telescopes improve by another order of magnitude or two (one hopes), will we need a new term for drifting objects gravitationally influenced both by our Sol and by (say) the Centauris ?
Speaking of which, TIL there's been more discovered in those systems. There's Alpha Centauri A B & C (C is a.k.a. Proxima Centauri) AND Beta Centauri A B & C. You can't tell your players without a scorecard.
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...
https://www.washington.edu/news/2026/10/27/rubin-observatory...
> ED138 joins the rapidly growing sample of solar system objects discovered by Rubin since it began collecting early data in preparation for the Legacy Survey of Space and Time (LSST), which began in June 2026. In April, before LSST formally began, the Rubin solar system team announced observations of over 11,000 new asteroids, which included roughly 380 new TNOs. At the time, the team called this large submission “the tip of the iceberg” and expressed their excitement for the discoveries to come once LSST began. With the discovery of ED138, those hopes are coming to fruition.
My point was that the LSST survey was looking in those areas as part of its "mapping the Milky Way"
https://rubinobservatory.org/explore/how-rubin-works/lsst
Another recent discovery from Rubin - https://www.washington.edu/news/2026/11/05/rubin-observatory...
You can see that it is looking in those dense starfield areas.
https://rubinobservatory.org/gallery/collections/news-images... is where Rubin detected 12,700 new asteroids... and ED138 is in that part of the sky where Rubin was looking.
Pluto has a 248 year orbit... which is close to the 282 years for ED138. 100 years ago, maybe Tombaugh would have found ED138 that if he was looking in that part of the sky.
https://dahlend.github.io/ketev/#t=1930-02-18.717&sel=2026%2...
In 1925, ED138 was crossing Pluto's orbit (and closer to Earth than Pluto was).
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
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.
I've always only ever seen diagrams of the Jupiter Trojans from a top down perspective so I never realized how much they stretched beyond the orbital plane of the solar system.
Do you know why they stretch so up and down relative to Jupiter? Is that sort 3d distribution shape typical for L4 and L5 Lagrangian points, and it's just amplified by the size of Jupiter or is there something else at play there?
The Lagrange points are more like "regions" than points in reality. There is some complicated math you can do to work it out, but typically numerical stuff is a bit more reliable once you start to include the rest of the planets, the analytic theory gets pretty complicated the more bodies you include.
The trojans that you see there sort of do a little dance when you plot them in the rotating frame of Jupiter (more technical term is librate). The size of the region has to do with the mass of the parent body, since jupiter is so big, the capturing region is pretty large. Technically there are other trojans around some other planets, but most of them are not long term stable like jupiter is, mainly because jupiter is just big.
Yeah I knew they were regions more than points, I just sort of envisioned them as something more in the same of a tapered tail than something so tall compared the diameter of Jupiter.
I'd like to take the time to tell you that I really appreciate your contributions to this site. I'm always excited when there's a post about asteroids or something similarly sized in the solar system because I know that you'll post about it more often than not.
I've taken a course on the geology of the solar system and a course on exoplanet detection but I definitely consider myself a layman in this field. I've always had a particular fascination with these small tumbling rocks in our solar system so it's always a pleasure to see you talk about them and your work.
How do I enable asteroids from the UI? It looks like the information is packed into that `v` param somehow?
Yeah its not the most user friendly, go into the filters tab and check the numbered and unnumbered check boxes.
Did you intend to recommend 2688 Halley in the suggestions over 1P/Halley?
Hah, no that's a mistake. I really need to update that list anyways, there are more interesting orbits.
Thank you for an awesome app! It got me to learn the difference between 2688 Halley and 1P/Halley :)
It would help on mobile if you could you make left panel dismissable / reappearable so that the main rendered animation is visible by default
I've built a similar app with the same sources, it works pretty well on mobile: https://spacemap.co/s/54667839/2026%20ED138?at=now,-2.48520,...
Tangential, but what was that job? Sounds interesting.
Worked at caltech on the Near Earth Object Surveyor (NEO Surveyor) space telescope building orbital simulation tools.
Is that an egui app?
Yes its egui. I know nothing of UI stuff, I am more on the "make math go fast" side of the programming spectrum, and less on the pretty button side.
It's not great on mobile, but totally works!
The only thing that bothered me is the sidebar.
Is this really a candidate for planet X.
I thought Planet X was supposed to be calculated to be several earth masses, not a dwarf planet.
Ohh, I love this. I think I need to re-read The Expanse and use that as a reference. Thank you for making it!
it's interesting the further the planets are the more off level their orbit is, is that because most of the planet creation chaos was happening near the sun?
It requires less delta-V to create a large change in inclination when the object is far from the Sun, because the orbital speed in a more or less circular orbit varies as (orbital radius)^(-1/2).
My first thought was 'where is New Horizons?'
That website is cool as
It looks like its orbit is actually within the orbit of Pluto at one point, and at times, the two bodies could come within a very small distance of each other. Wouldn't this disrupt one or the other, possibly throwing them off of their orbits?
Part of why Pluto was demoted is it's so small it doesn't effectively clear it's own orbit. Orbits can look close here but with how small Pluto is it's gravity would barely affect this dwarf planet. It's also possible they've only been at their closest approach a handful of times in their entire life.
> They even found it in photographic plates from 1937
Pretty rad.
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.
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
Nature doesn’t care about what kids can or cannot easily remember.
… Picante?
Fifty is an undercount by many orders of magnitude. Several thousand TNOs have been cataloged, there are probably millions of them.
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.
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?
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.
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.
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”.
I'm amused that you described a group policing terminology right after saying there's no group policing terminology.
You said colloquially. They are talking more formally.
Speaking of lumping and splitting: in the nebular hypothesis planets accreted from the protoplanetary disk. Out in the Kuiper belt objects were too far apart to properly accrete, and remain separated.
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
> In fact, near the end of that paper he proposes not two, but five classifications of planets: subdwarf, dwarf, subgiant, giant, and supergiant.
Yes, “classifications of planets”. So Pluto would still be a planet. Then if you wanted a more specific modifier you could add it, but it’s still a planet.
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...
That's exactly my point. The Moon orbits the Earth in the same way the eight planets are what they are - because enough people say it's that way.
By what 'conventional definition' does the Moon orbit the Earth? Barycenter inside the parent? By that definition Jupiter doesn't orbit the sun. Gravitational pull? The sun exerts twice as much pull on the Moon than the Earth does; the earth just slightly wobbles its path.
« 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. »
Major impacts on Earth on a regular basis would indeed upset the applecart of planetary taxonomy.
Of note, the ancient planets did not include Earth. I say this as someone who respects Alan Stern so much that my son is named after one of his projects.
The Alan Stern's Project, presumably.
New Horizons
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.
And to paraphrase Douglas Adams, the stuff is basically just a rounding error in the not-stuff. And insofar as there is stuff, it's all stars, with everything else being a rounding error. This is the case in our solar system, which, mass-wise is the Sun, with Jupiter as a rounding error - everything in the Solar System that isn't the Sun is mostly Jupiter.
https://en.wikipedia.org/wiki/List_of_Solar_System_objects_b...
> 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?
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.
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.
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.
> I think the backlash was fed by insistence dwarf planets are not just a subtype of planets[...]
I'm 99.9% certain that the backlash would have been far greater if our solar system suddenly had dozens extra planets.
> 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[...]
Because otherwise the definition would differ greatly from our historical classification. Since it is necessarily arbitrary, sticking close to our history IMO seems like the obvious choice.
The historical IAU term such bodies was already "minor planets", the public was not in backlash about it despite there being over 100,000 of them by the time the "dwarf planet" category was created. "Minor planets" was even historically a subcategory of planets for a long time.
But, as said above, it'd still have went over fine if, instead of considering dwarf planets a subcategory of planet, the IAU had just not used "planet" in the category of things they didn't want people to consider planets. E.g. like they did for breaking "small solar system bodies" out of "minor planets".
Yeah, and people clearly didn't count them as "real planets", except for Pluto. So what's the issue? How are "dwarf planets" less planet-y than "minor planets" were?
Had the IAU said "Pluto is now a minor planet, which is a type of planet different than major planets" it would have gone over really well, been clear, historically accurate, and scientifically accurate.
Had the IAU said "Pluto is now a dwarf body, which is not a type of planet" it would have still been unpopular, at least clear, no more historically inaccurate than coming up with "dwarf planet", and scientifically accurate.
What the IAU actually decided was "Pluto is now a dwarf planet, which is not a type of planet at all". Since the sole positive attribute of this is it is scientifically accurate, it will have some level of unpopularity and be the cause of endless confusion even after everyone who knew Pluto as a planet is long dead.
I'm very supportive/glad they made it so Pluto wasn't the exact same classification as the remaining 8 planets. I'm continually facepalming over the absolute insistence to name the category dwarf planets but not have it be a type of planet.
> Had the IAU said "Pluto is now a minor planet, which is a type of planet different than major planets" it would have gone over really well, been clear, historically accurate, and scientifically accurate.
I disagree on multiple levels:
1. It would still have gone over very poorly, because many people explicitly oppose the demotion of Pluto. Demoting it to a different category would not change the issue in those people's eyes.
2. I doubt that "[...] a type of planet different than major planets" would improve clarity much for the general public - if you've seen studies or similar, a reference would be appreciated. But changing the meaning of an established term would definitely be far less clear on the academic side (since every mention would be ambiguous before the publishing year and/or individual stances of the authors/journal are known).
3. "Historically accurate" depends on what type of accuracy we're talking about, because IMO this approach is less historically accurate because we'd be changing the definition of an existing term, without a meaningful difference aside from that (since both are literally named X planets).
4. I'd need an argument for why it'd be more scientifically accurate. Since we were unaware of most of the objects in our solar system when the previous classifications were created, it's again IMO much more scientifically accurate to create a new classification.
> Had the IAU said "Pluto is now a dwarf body, which is not a type of planet" it would have still been unpopular, at least clear, no more historically inaccurate than coming up with "dwarf planet", and scientifically accurate.
This one would have been even less popular! It's definitely historically less accurate when talking about Pluto and previous minor planets, though at least I can agree regarding the scientific accuracy.
> What the IAU actually decided was "Pluto is now a dwarf planet, which is not a type of planet at all". Since the sole positive attribute of this is it is scientifically accurate, it will have some level of unpopularity and be the cause of endless confusion even after everyone who knew Pluto as a planet is long dead.
It's definitely less unpopular than "dwarf body" and much more clear than "minor planet (no, the new one)"! And as explained, I'd argue it's historically the most accurate one.
Pluto didn't really fit in with the other planets even before the discovery of further dwarf planets, that discovery was just the final nail in its coffin as a "proper" planet: it was the smallest planet by far, smaller even than Earth's moon, it has a highly unusual orbit (not in the ecliptic plane, so elliptical that it's sometimes closer to the Sun than Neptune - https://en.wikipedia.org/wiki/Pluto#Orbit) etc.
Right, but I learned the 9 planets names; now you want me to learn like 13 names ... my brain does not have that much capacity for 13 ... :(
I don't remember who phrased it, maybe Asimov: When aliens come to survey the Sol system they will report one star, four planets, and debris.
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.
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'm fine with dwarf planets being planets, I'm fine with satellite planets being planets.
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.
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.
> 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 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.
> any classification is constructed to fit some of our preconceived notions
That is not the concern here. It seems to me that you're failing to account for intent. Classifications exist to serve a useful purpose. What I object to is mental gymnastics used to gerrymander a particular outcome. These things should conform to something resembling occam's razor - minimalism in pursuit of a generalized goal.
> These things should conform to something resembling occam's razor - minimalism in pursuit of a generalized goal.
That is literally what happened - a definition for "planet" that is as minimal as possible, while staying as close to our historical categorization as possible.
Now, I know you'll object to the "while staying as close to our historical categorization as possible" part, but that is literally how scientific fields always approach arbitrary classifications. Without taking our historical use into account, there is no "objective" definition you can derive.
If I'm wrong, please give me an example of what kind of definition for "planet" you'd be happy with. But remember, it must not take into account what we today consider "planets" to be.
> that is literally how scientific fields always approach arbitrary classifications.
That's a big overstatement. It's how some soft science fields sometimes do it. You can't get away with that in, say, physics and still end up with useful theories, because your theories quickly break down due to inconsistencies. You can't design working scientific theories by committee, because votes don't affect fundamental physical reality.
Much of astronomy is a hard science, which is why this definition came in for pushback even from astronomers themselves. Here's astronomer Phil Plait's take:
> "For some time now I, and many others, have stated that the IAU definition is silly for many reasons, including a line that says that planets can only orbit the Sun. If that’s true, then exoplanets — worlds orbiting other stars — aren’t really planets!" -- https://www.syfy.com/syfy-wire/mea-culpa-are-exoplanets-plan...
The article goes on to clarify that the IAU explicitly limited its definition to solar system planets - in fact, the resolution is titled ""Definition of a Planet in the Solar System."
As such, it's not even a universal scientific definition for "planet".
Another quote:
> "At [the IAU] 2006 international meeting in Prague, they drafted a series of defining characteristics for planets and put it to a vote. The definition passed, and the outcome was entirely predictable: They made things a lot worse."
> "...the more proximate problem is that these definitions are, um, silly."
-- https://www.syfy.com/syfy-wire/redefining-planets-answer-sea...
> That's a big overstatement. It's how some soft science fields sometimes do it. You can't get away with that in, say, physics and still end up with useful theories,
Sorry, but this objection reveals that you either don't understand the topic of discussion at all, or you're arguing in bad faith. No theory in physics will break if you change the criteria of an arbitrary classification.
I'll pose the same challenge to you: please give me an example of what kind of definition for "planet" you'd be happy with. But remember, it must not take into account what we today consider "planets" to be.
> Sorry, but this objection reveals that you either don't understand the topic of discussion at all, or you're arguing in bad faith.
Nice false dichotomy and binary thinking.
You're right that arbitrary classifications are arbitrary, obviously. My point is that you can't get away with arbitrary classifications in the context of hard science theories.
There's an example of a definition of "planet" in one of those Plait articles I linked:
> "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."
That comes from the following paper: https://www.hou.usra.edu/meetings/lpsc2017/pdf/1448.pdf
The Plait articles are worth reading, because they might help disabuse you of the notion that the IAU definition is a good one, and explain why it would make sense to have a better one.
> My point is that you can't get away with arbitrary classifications in the context of hard science theories.
Arbitrary classifications don't factor into hard scientific theories, so I have no idea how this is relevant. Or what "hard science theory" are you thinking of that relies on whether we classify an object as a planet?
We can modify arbitrary classifications without any theories being affected, because the theories don't rely on arbitrary classifications. We arbitrarily decided what "one meter" is, yet we use "meters" all over physics. We could change the definition of "one meter" tomorrow, and all our theories would stay valid, because they don't rely on arbitrary classifications.
> There's an example of a definition of "planet" in one of those Plait articles I linked:
>> "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."
How does one arrive at this definition, without taking into account what we today consider "planets" to be? What principle of physics tells you that a planet must be sub-stellar mass, or that it has never undergone nuclear fusion?
You'll have to actually explain how this definition is any less arbitrary, but I don't think you'll be able to, because every useful definition falls under your objection.
> but that is literally how scientific fields always approach arbitrary classifications
No, not always. What you are describing there are shoddy classifications. I already addressed this exact point in my previous reply when I stated that classifications are crafted to serve a useful purpose. It is sometimes useful to hew close to the historical norm; other times it serves no purpose.
> But remember, it must not take into account what we today consider "planets" to be.
An arbitrary restriction that I do not accept.
> please give me an example of what kind of definition for "planet" you'd be happy with
I already did this in my original comment. I proposed an earth centric system that bins mass by powers of ten. My proposed range for planets was 10^23 <= x < 10^26. Hydrostatic equilibrium works well as a constraint to eliminate things that are clearly not earth like (such as black holes) but alternative formulations such as stipulating an acceptable density range are possible (such cross cutting subcategories might even be preferable).
Note that this would result in ganymede being classified as both a planet and a moon. I think that makes perfect sense given how similar in size it is to mars. In fact ganymede being excluded as a planet is precisely the sort of gerrymandering I object to.
As it turns out my proposed criteria also relegates pluto to the status of dwarf planet. I think that's sensible given that it's both smaller and lighter than the moon. However I'd be willing to entertain a set of criteria that acknowledged both pluto and the moon as proper planets - the important thing to me is consistency.
Hydrostatic equilibrium is the most defensible of the requirements to be a planet.
The IAU wording is, "...has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape."
In this case it just means that the planet's mass and therefore gravity is sufficient to give it a spheroidal shape. (Technically the equilibrium is with the opposing push from the planet's material, which resists gravity's attempts to compress it. The result is a sphere, made spheroidal in real planets by rotation.)
Less massive objects like asteroids can have weird shapes, like the asteroid Toutatis which has a sort of dumbbell / unshelled peanut shape. The hydrostatic equibrium requirement prevents such objects from being classified as planets, as well as small objects that just happen to be round for other reasons.
This isn't really social history but simply classifying the objects according to intrinsic properties, which seems fine to me. This classification could be applied to objects anywhere in the universe without any problem.
The other two requirements are more problematic and arbitrary. One is that it has "cleared its orbit". Earth, let alone Mercury, would be unlikely to qualify for this if it were at the same distance as Pluto, for example. Rogue planets also can't satisfy this one, and we've observed those.
The other is that it has to be in direct orbit around the Sun, not a satellite of some other object. This excludes exoplanets as well as rogue planets. The rationale for this is that they didn't have enough observational info to say much about exoplanets, but that seems like a flimsy excuse. A good definition would apply to exoplanets by virtue of being theoretically sound, and not having been tuned to fit the way they were trying to classify solar system planets.
> Less massive objects like asteroids can have weird shapes, like the asteroid Toutatis which has a sort of dumbbell / unshelled peanut shape. The hydrostatic equibrium requirement prevents such objects from being classified as planets, as well as small objects that just happen to be round for other reasons.
Okay, and how did you arrive at the hydrostatic equilibrium requirement? What law of nature told you that planets must be spheroidal?
You're only adding this requirement so the outcome fits your pre-existing desired results, since you're literally arguing for the requirement based on what objects it excludes. And what are the consequences of such arbitrary decisions, according to you? "You can't get away with that in, say, physics and still end up with useful theories[...]"
Do you get the issue with this line of critique? There is literally no useful definition of "planet" that is non-arbitrary.
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?
> orbits and rogue planets
I think that's an example of coming to a useful realization while working through the process. It should serve to illustrate the importance of not attempting to engineer a particular outcome. If the earth were wandering the galaxy clearly it should still be considered a planet, no? And were we living on a body the size of mars that orbited a gas giant we would still be said to occupy a planet. At least assuming the goal is to classify things of a vaguely earth-like nature.
I think the more difficult question is what exactly qualifies as a moon. Anything orbiting a planet, including a smaller one? That seems perfectly reasonable to me. (Yes yes I know the barycenter matters.)
And yes, I think ganemede should be considered both a planet and a moon. That thing is on the same order of magnitude as mars. What useful purpose is served by excluding it as a planet?
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.
Maybe we haven't gone far enough. We should refer to Uranus and Neptune as Trans Saturn 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"? ;)
Certainly, Mercury, Venus, Earth, Mars, and Ceres are rocky planets. And Pluto, Eris, Makemake, Haumea etc are icy planets. But the idea that there are gassy planets is just silly.
Uranus and Neptune already are a two-member class (“ice giants”).
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!
The comparison falls flat though if they were called "dwarf humans". Such a name would obviously invite comparison.
People can't help ranking things. You say there are two categories and before you've even finished your sentence the audience has ranked one category as "better" than the other. This is a source of a lot of our problems.
> Eris, Haumea, Makemake, ...
Don't forget about Gǃkúnǁʼhòmdímà! Everyone forgets about Gǃkúnǁʼhòmdímà.
Whoa! Click consonants!
I would preferred we require children memorize all 10,000+ satellites of the Sun. More planets the better. Count the moons, why not.
When I was a child it was commonly taught that the dinosaurs were all extinct and birds were not dinosaurs, though it was known that birds evolved from that lineage.
Even as a child I could figure this was BS - Cretaceous period dinosaurs evolved from Jurassic period extinct dinosaurs, so bird are not dinosaurs simply because someone decided dinosaurs were all extinct?
As an adult I find this has been rectified, but we're going backwards when it comes to planets.
The main reason it's controversial is because Pluto was discovered by an American and they want their rock to be in the top category.
I don't really know how the category helps anything, though. No astronomer would use the category in any work, they'd use the actual properties of the planet. The only people who might care are those publishing paedagogical material. Is it really that important that kids around the world all learn the same number of planets?
It could easily be taught a different way: there are planets, you can see these ones tonight with your naked eye, you could see these other ones with a telescope. I wish they'd taught me that instead of those silly pictures of the solar system.
Why couldn't Pluto have been Grandfathered into the planets just because.
Just because it isn't a planet doesn't mean it isn't in our hearts.
Does science seek hearts or truth?
Pluto still exists. Re categorizing something isn't truth, its just renaming.
Human Categories aren't truth.
Well in that case Pluto is a god. Recategorizing it as a planet isn't truth.
You wouldn't be wrong.
"The name Pluto came from the Roman god of the underworld; and it is also an epithet for Hades (the Greek equivalent of Pluto)."
This might help
https://www.google.com/books/edition/On_Truth/fBheDwAAQBAJ?h...
I always feel amazed to know how suns gravity keeps hold of massive planets millions of kilometre away from it and how we are still finding objects in our solar system!
True. My intuition is a really weak force can still have a lot of effect if it isn't competing with any other force. In this case, forces from other stars are many orders of magnitude weaker.
Similarly, mutations are a pretty weak force of evolution. But they add up over a long period in the absence of any other force keeping species stable.
I always feel amazed by the fact that light from the sun takes 8 minutes to reach Earth and is still powerful enough to sustain all the life around us. The fraction of energy hitting us 8 light-minutes away is miniscule.
yea there is so much energy out there outside of our comprehension and we feel and think of things of our scale, the design of nature is so awesome and complex that we barely understand it
> 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.
It's time for websites to have a "proofreader contribution" button. Enough people report it and it auto-updates.
Imagine all the ways this could be abused using bots or a sufficiently large group of people
Edit: Except maybe when limited to singe character edits, but those could still pose a partial attack vector for a subset of words.
Proofreader McProofreaderface
Here's a screenshot from my locally hosted emotive audiobook narration software backed by Qwen3.6 for checking grammar (and invoking a custom SymSpell implementation for spelling):
https://i.ibb.co/2LLVZ0N/keenlore-proofread.png
It took about two hours to code using Opus 5.5. Finding and fixing grammar mistakes is quick. Not sure why magazines aren't doing similar checks.
Is this Planet X that they have been talking about for a while now?
there are a few different “Planet X”s theorized
https://en.wikipedia.org/wiki/Planets_beyond_Neptune
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.
This artist’s impression is optimized for plausible deniability.
No, all dwarf planets are too small to affect orbits the way that hypothetical planet is supposed to do.
The proposed Planet X would be something the size of Uranus or Neptune. This is 1000km across, +/-50%.
Does this new object strengthen the argument for 'Planet Nine' or weaken it?
The hypothetical Planet 9 is expected to be much more massive and further out than this, and perturbing the orbit of objects like this. I think they'd need to get the full numbers on ED138 before they can plug it into simulations and see if it fits the pattern or not.
Yes. I can't remember what the clustering of objects was that weakly imply orbital resonance, though I think this page has some info:
https://en.wikipedia.org/wiki/Effects_of_Planet_Nine_on_tran...
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
With the big exception being radio waves. Once electronics are invented and especially anything like radio, the planet should light up. There is nothing particularly human about it, except for how we encode the data and what data we find worthwhile to encode (maybe), but it would still stand out over the background noise.
How far would the radio emissions of Earth be detectable? Could even Martians watch our soap operas?
Assuming tech == electronics is a very human assumption to make. Biology is a way more "advanced" kind of nanotech and it has significantly different characteristics to electronic based tech. We simply don't know and even taking that into account there are blind spots in our blind spots.
even on earth those high-power radio transmitters were relatively short-lived phenomenon; modern 5G cells are dramatically quieter that the kilowatt AM radio (and TV) towers of the past.
Since it's at the outer reaches of our Solar System where it's very cold I'd like to officially propose to name this new dwarf planet Boreas, after the Greek god of the north wind and winter.
Seems reasonable, but how many frigid deities or their minions are there? There are probably more such objects out there.
Already taken: https://en.wikipedia.org/wiki/1916_Boreas
We ran out of Greek names a while ago.
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.
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.
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.
Jupiter-mass objects are vanishingly unlikely to be orbiting the sun, they would have been found by infrared surveys.
Yes. The Kuiper belt is full of massive, yet-to-be-documented asteroids, planetoids, comets, and dwarf planets.
But then with the Kuiper Belt and the Oort Cloud you're getting into interstellar space, yes ? As space-based telescopes improve by another order of magnitude or two (one hopes), will we need a new term for drifting objects gravitationally influenced both by our Sol and by (say) the Centauris ?
Speaking of which, TIL there's been more discovered in those systems. There's Alpha Centauri A B & C (C is a.k.a. Proxima Centauri) AND Beta Centauri A B & C. You can't tell your players without a scorecard.
> But then with the Kuiper Belt and the Oort Cloud you're getting into interstellar space, yes ?
The Kuiper belt is inside the heliosphere, the Oort cloud is outside it.
ANNNNNUUUNAKI - Heckler Fish
They should call it Twitter.
(Character limit 280)
He is taking his sweet time. No rush.
Nibiru!
Maybe the Anunaki will return and teach us how to use the Pyramids!
The 1st image in that article looks like Celestia. It would be nice to have an add-on for Celestia once enough details are known about 2026 ED138.
https://celestiaproject.space/
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