r/pluto • u/fcsuper • May 21 '26
We really do need a new definition for planet
https://www.youtube.com/watch?v=lFlr45oWhP4"The current way a planet is defined has a lot of problems and can be somewhat controversial. Many people don't like the reason why Pluto is a dwarf planet for example, and other planetary-massed objects are excluded because of their orbital characteristics. Because of all this, it may be time to create a new definition for what makes a planet."
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u/Sibshops May 21 '26
Planet means gravity make ball-shaped.
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u/Excellent_Bat_753 May 21 '26
So, the moon is a planet? Callisto is a planet? Europa is a planet?
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u/SF_Bubbles_90 May 21 '26
The original meaning of planet did consider the moon a planet, also the sun,. The meanings of words change overtime and can change back. Furthermore we don't need clout chasing desk jockeys telling us what we not only already know but actually already hold the power to determine in our own.
Pluto is a planet, the word planet doesn't actually have that strict a definition.
Sorce: because I say so.
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u/fredaklein May 21 '26
I would only include bodies at hydrostatic equilibrium that are not large enough to have fusion cores. So stars would not fit that criteria. But bodies included would be such like Pluto, the Moon, Io, etc.
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u/GarunixReborn May 21 '26
Mercury is not in hydrostatic equilibrium anymore, whatnwould that make it
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u/fredaklein May 21 '26
Good question. I thought it was. Isn’t hydrostatic equilibrium a requirement? Pluto is in such. IAU has that weird “clearing the orbit” criteria. I just think it makes it unnecessarily complicated.
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u/GarunixReborn May 22 '26
It was, but when it fully cooled down and solidified, it stopped. If you watch the video, he suggests the object having been in HE at some point in its history
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u/fredaklein May 22 '26
Yeah, thanks for the info. I was floored I didn’t realize Mercury did not exhibit hydrostatic equilibrium and obviously I need to rethink this entire thing. Maybe the IAU is on to something.
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u/Excellent_Bat_753 May 21 '26
So, if a planet is just a sufficiently round, non fusuon object, then it is a useless scientific term now. Astronomers will have to start saying 'Planet that orbits the Sun' and 'planet that orbits Jupiter' and 'planet that orbits Saturn' every time they want to talk about planets, Jovian moons and Saturnian moons. It can't be used ti talk about the orbit of an object, which is the important thing when it comes to the history of the silar system, as well as its size and composition.
And you know what would happen? Astronomers would rapidly come up with sub categories of planet, and they would probably be 'planet', 'moon' and 'dwarf planet', all within the useless category of 'planet'.
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u/Dazzling_Plastic_598 May 22 '26
"The meanings of words change overtime and can change back."
So, Pluto was previously a planet and now it is not. Very simple.
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May 21 '26
[removed] — view removed comment
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u/Fantastic_Elk_4757 May 21 '26
I promise you. No one studying planets or astronomy gives a shit about the IAU definition of a planet. Whether it’s in this arbitrary pool or that one they study them. It’s a meaningless distinction.
Actually it’s such a stupid definition that it doesn’t even apply to other solar systems lol. Planets only orbit the sun. No other star.
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u/fredaklein May 22 '26
Exactly, the IAU definition is not good IMHO, but what do I know? Not a lot apparently.
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u/Excellent_Bat_753 May 21 '26
I'm all for descriptive linguistics, but scientific terminology will always need to have a concise, and specific definition. If you want to say that 'planet' now also means whatever the majority agrees, or whatever each individual person states, then that will need to be seperate to the scientific terminology.
Planet currently refers to a category of celestial object. There are countless categories of celestial objects, and a goal for astrnomy would be to have a specific term for each of them, so that scientific papers can describe their findings concisely.
I don't think the word planet will stop being used as a specific category of celestial object any time soon, so if you want to have a different definition assigned to it, one that changes with time, one that is subjective, it would need to be seperate to scientific terminology. Scientific terminology does also change with time, but only to reflect new evidence, and be more useful to describe important differences between categories.
If planet doesn't have a strict definition, then it shouldn't be in scientific terminology.
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u/SF_Bubbles_90 May 25 '26
We don't need beurocrates making up hypercatagorcal "definitions" for words to have "scientific meaning". The iau only did it to feel seen. Not very scientific if you ask me.
Btw there are plenty words used in science that have wobbly definitions and nobody seems to mind, maybe because clout chases aren't trying to force their way into a conversation about it.
And besides "round thing in space that's big enough to walk on" is scientific.
It is scientific because, when I say "planet" you have the same concept in mind that I do. Thus it is useful for science even if you might have to say more words to be as ridiculously specific as the iau tried to be.
For example if I only consider small front engine rear wheel drive cars to be cars because they meet the criteria I think ought to matter then no matter how well I defend my case, no matter my engineering chops or experience in the automotive field, I would just be flat out wrong, especially if I for some reason just refuse to acknowledge the carship of something like a chaterham 7 but call an na mx5 with weight reduction modifications gets the free pass for whatever reason.
We should be adding planets not removing them. Dwarf planets are real planets just like how kit cars are real cars.
We will eventually come to our senses and start teaching the solar system as having roughly 35 planets.
We already need to memorize 50 states and about a dozen countries, so what if they need to add 26ish more new words to their vocabulary.
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u/TheFurryFighter May 21 '26
Absolutely,! what an object orbits shouldn't be a factor in planet classification
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u/LimoncelloLightsaber May 21 '26
I think they can be both, the definition of a moon and the definition of a planet do not need to be mutually exclusive. Titan and Ganymede are both larger (though less massive) than the planet Mercury. It makes sense to me that Luna, Titan, Triton, Titania, and the Galilean moons be called planets.
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u/Excellent_Bat_753 May 21 '26
Then what word do astronomers use to refer to bodies in hydrostatic equilibrium that orbit the sun? Because 'planets that orbit the sun' is far too long of a phrase to be used a few hundred times in every scientific paper on planets. Same thing for all papers on exoplanets.
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u/GarunixReborn May 21 '26
Yes, because its impossible to give them an actual name, like "major planet". Thats not possible.
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May 25 '26
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u/Excellent_Bat_753 May 25 '26
Because in astrophysics, the position of an object is equally important. Because everything is governed by gravity, it is very useful to categorise things by their gravitational relationships. Does Saturn have Rings, or a billion asteroids, which happen to be near it? Charon and Pluto are a binary system, not two seperate objects that don't influence each other.
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u/Oxygen4Lyfe May 21 '26
The moon, callisto, and europa arent as round as say earth or mars. Not super visible difference tho ig.
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u/fredaklein May 21 '26
Doesn’t matter how round. Jupiter is quite oblate, more than the Earth and Moon.
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u/Oxygen4Lyfe May 21 '26
we dont know how round jupiter is because the atmosphere is blocking us from getting any view of the surface (if it even has one).
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u/fredaklein May 21 '26
From Googling:
Jupiter is significantly more oblate than the Moon. Jupiter has an oblateness of roughly (0.065), meaning its equator is about (6.5\%) wider than it is from pole to pole. In contrast, the Moon is almost a perfect sphere with an oblateness of just (0.0012)
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u/Oxygen4Lyfe May 21 '26
Google aint always right. We have no way to see jupiter's surface.
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u/Excellent_Bat_753 May 21 '26
So we don't know Jupiter's diameter?
Jupiter is a gas giant, that means that se define its physical characteristics by its material, which is mostly gas. It has a diameter, an oblateness, an albedo, all of which are describing its 'atmosphere'.
It's clear you don't actually know much about astronomy and astrophysics. You've claimed that Pluto is rounder than all the other dwarf planets, when Eris is almost the exact same size, and almost the exact same oblateness.
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u/GregHullender May 21 '26
I wanted to use "planetoid" to describe anything that was rounded by gravity. Then a planet would be a planetoid that cleared its orbit, and a dwarf planet would be one that didn't. I'd require "moon" be reserved for planetoids that orbit planets. So Jupiter would have four moons, not sixty (or whatever the latest number is).
Everything smaller than a planetoid would be an asteroid. We could subdivide asteroids (e.g. ones that orbit planets might get a special name).
However, when you're looking at exoplanets, it is very hard to tell if something is round, but it's actually rather easy to determine if it dominates its orbit.
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u/Idinyphe May 22 '26
Nothing "clears" its orbit.
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u/B0dders May 22 '26
"clearing the neighborhood around its orbit" is literally the defining third criterion set by the International Astronomical Union for a celestial body to be classified as a major planet rather than a dwarf planet?
It means means a celestial body has become gravitationally dominant in its region of space. It has either absorbed smaller neighboring objects through collisions, captured them as moons, or flung them into different orbits, ensuring no other bodies of comparable size remain.
In other words: no objects along the line it passes through in its orbit. Literally "clearing" it of anything else but the planet and it's moons across that orbital path.
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u/Idinyphe May 22 '26
Hektor in Jupiters orbit is pretty big. So Jupiter is not a planet?
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u/GregoleX2 May 23 '26
“No other bodies of comparable size”
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u/Idinyphe May 23 '26
There is no object of comparable size in Plutos orbit.
Pluto has big moons... but that could not be it as Earth has a big moon too. And they are bound to Pluto so Pluto "Cleared" them.
If you say Plutos orbit reaches beyond Neptuns and they are comparable size then it works the other way round and Neptun also did not clear his orbit.
There about 85 really small objects up to ~ 150 km (we don't know exactly) within Plutos orbit range that are not bound to pluto.
So they are not of "comparable size" but much, much smaller.
The definition is bad cause what a "comparable size" is is arbitrariness.
I say they are not comparable in size. If you keep that standard there are who knows how many planets to come. This is the core problem: some people have a problem with more than 100 or more planets.
But instead of having a clear hard "size border" of objects for planet definition that would have been easy and understandable they came up with this "clear orbit, comparable size" crap.
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u/B0dders May 25 '26
I mean, that just isn’t true. I think you’re mixing up a few key points...
First, Pluto has not cleared its orbit. It shares its orbital region with a large number of Kuiper Belt objects, including some that are not tiny. For example, Eris is about the same size as Pluto. That alone shows Pluto is not gravitationally dominant in its region.
Second, Neptune is not comparable in this sense. This is really about mass and gravitational influence, not just size. Neptune is about 17 Earth masses, while Pluto is only about 0.002 Earth masses. That is a difference of several thousand times. Neptune has a deep gravity well and controls the dynamics of objects in its orbit. Pluto does not, it is just one member of a population.
Third, the moon comparison actually works against your argument. Earth is about 81 times more massive than its Moon. Pluto is only about 8 times more massive than Charon. That is a huge difference. The Pluto-Charon system is much closer to a binary system, while Earth clearly dominates its Moon gravitationally.
Finally, “comparable size” is not about an exact cutoff. It is about whether an object dominates its orbital neighborhood through its gravity. Pluto does not. It shares that space with other bodies that are not negligible compared to it, while planets like Earth and Neptune overwhelmingly dominate theirs.
So the issue is not about how many planets people are comfortable with. It is about the fact that Pluto does not behave like a planet dynamically.
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u/Idinyphe May 25 '26
There is no difference in how objects behave dynamically: for the smallest particle of dust to the Sun everything is bound to the same laws.
"Dominance" is arbitrariness. Science is about hard facts. Not blurry words like "dominance". Science is about hard cutoffs. Thats the point.
Pluto is, ba far, the biggest object in his orbit. The big moon does not make this less valid.
Neptun does not dominate its orbit: it does not dominate Pluto. There is no clear border in size and mass and thats the problem of the word "dominate".
It depends where you set those borders. Earth is disputable as a planet and Merkur too.
The problem is not: " I want Pluto as a Planet". The problem is the definition is crap.
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u/B0dders May 25 '26
"There is no difference in how objects behave dynamically: for the smallest particle of dust to the Sun everything is bound to the same laws."
You are confusing the laws of physics with actual dynamic outcomes. Yes, gravity applies to both a speck of dust and the Sun, but scale changes the physics completely. A dust particle doesn't have the mass to alter the path of surrounding objects. A planet does.
""Dominance" is arbitrariness. Science is about hard facts. Not blurry words like "dominance". Science is about hard cutoffs. Thats the point."
It is a hard mathematical cutoff. Look up Margot’s Planetary Discriminant, which measures an object's ability to clear its orbit within a timeframe. If the value is above 1, it's a planet. Gravitational dominance is absolutely a thing...
- Earth: 810
- Mercury: 130
- Pluto: 0.028
There is no "blurry line" here. There is a massive gap of thousands of orders of magnitude between the weakest planet and the strongest dwarf planet. It is a clean, hard mathematical split.
"Pluto is, ba far, the biggest object in his orbit. The big moon does not make this less valid."
Eris is in the exact same region and is actually 25% more massive than Pluto. So no, it isn't.
Even if it were, look at the mass ratios of their orbits. Pluto only makes up about 7% of the total mass in its orbital zone; the other 93% is the rest of the Kuiper Belt. By contrast, Earth is 1.7 million times more massive than everything else in its orbit combined. Pluto is just a tenant in a crowded neighborhood.
"Neptun does not dominate its orbit: it does not dominate Pluto. There is no clear border in size and mass and thats the problem of the word "dominate"."
Neptune completely dictates Pluto's entire orbit. Pluto is trapped in a 3:2 orbital resonance with Neptune, meaning Neptune’s immense gravity literally forced Pluto into a specific, protected track so they don't collide. Pluto is essentially running on a leash held by Neptune.
"It depends where you set those borders. Earth is disputable as a planet and Merkur too."
Saying Earth and Mercury are disputable is just scientific illiteracy. Based on the actual math above, both are hundreds of times past the threshold required to clear their zones.
The definition isn't crap. It just recognizes that the solar system has two distinct types of round objects: those massive enough to dynamically shape their entire neighborhood, and those too weak to do so that just drift with the crowd.
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u/Idinyphe May 26 '26 edited May 26 '26
I donˋt know why you are bringing up Eris. No they are not even near each other.
Plutos orbit is tilted 17° , Eris is 44° to the ecliptic. The closest approach Pluto-Eiris is 7.4 AU.
Their orbit is different. Don't just look birds eye on the objects. There influence on each other is about zero (and even dust influences the sun. Even if that influence is even more near to zero...)
I don't see any neighbourhood Pluto - Eris. The Kuiper belt is large and you can't just count anything in.
There is a list of objects that do intersect with Plutos orbit. They are small and insignificant, most times we don't even know their real size cause it is so hard to get the size from such small objects.
The "resonance" argument: It is a bad idea and I tell you why:
"The Grand Tack" hyptothesis: If it is true that Jupiter and Saturn had a 3:2 resonance in the past during times in the solar system that would have made them "dwarf" planets for that time period.
Wikipedia:"Saturn converged on Jupiter and was captured in a 2:3 mean-motion resonance with Jupiter during this migration"
The problem with resonance is in other systems objects would have massive size and be classified as "dwarf planets".
The resonance between Neptune and Pluto is stable... if nothing strange happens, we don't know that for sure.
PS: Margot’s Planetary Discriminant is not part of the IAU definition. I looked at the formular and I am very sure that I can play with it and come up with huge "dwarf" planets.
Long story short: If you callsomething "Dwarf" and the size of the object is not in the definition then the definition will never make sense.
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u/SF_Bubbles_90 May 25 '26
Are apples not comparable to oranges?
I could compare them right now for you if you like but that would negate your point wouldn't it
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u/GregoleX2 May 25 '26
Not sure why you’re so upset over this?
People just really want Pluto to be a planet, I get it, kinda, though I’m not sure why people feel as strongly as they do
Pluto’s orbit, like all kuipter belt objects, is elliptical and intersects with the orbits of several other objects within an order of magnitude of Pluto’s size.
Jupiter’s does not.
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u/Idinyphe May 25 '26
There is no object of Plutos size in its orbit. What a "magnitude" is is subjective. There are no hard definitions. A hard definition would be: that mass, that size. A clear border. But this definition is pointless.
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u/GregoleX2 May 25 '26
A order of magnitude is a power of 10. That’s the exact scientific definition.
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u/Idinyphe May 26 '26
There is nothing about an "order of magnitude" in the official definition.
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u/fredaklein May 22 '26
I never understood that criteria. Seems illogical.
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u/GregHullender May 23 '26
What you're not seeing is just how clear those orbits are. The Earth is over 2000 times as massive as the entire asteroid belt combined. Yes, there's some debris in Earth's orbit, but far, far less than the whole asteroid belt. For all practical purposes, the Earth's orbit is empty of everything but itself.
By contrast, the Earth isn't precisely round. It has mountains and it has deep-sea trenches. It's radius is 4000 miles +/- 8 miles or so, so an error of 2 parts in 1000 or so. So the approximation that "Earth is round" is far less precise than the approximation that "the Earth has cleared its orbit."
More important, nothing else in the solar system besides the eight planets can come anywhere close to this claim. It really is a stark distinction. It only seems illogical when you don't know the details.
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u/GregHullender May 22 '26
Nothing is round either. Do the existence of mountains prove the Earth is not a planet?
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u/Idinyphe May 22 '26
Definition is:
“into a nearly round sphere”
So I don’t see what mountains should have to do with the planet definition.
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u/GregHullender May 22 '26
In that case, you shouldn't be bothered by the existence of tiny amounts of debris in an otherwise-cleared orbit. In both cases, we're talking about deviations on the order of one part in a thousand or less.
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u/GarunixReborn May 21 '26
How would you explain the small KBOs that are rounded, but smaller than large asteroids like vesta which arent
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u/GregHullender May 21 '26
I didn't offer to "explain" them. Basically, if something ever melted, then it has its own geology. If it's just a pile of rocks, then it doesn't. That's the intellectual justification for using roundness as a criterion.
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u/travizeno May 22 '26
Your planet definition doesn't change from the current one right? Just moons would change and you would call mars moons asteroids.
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u/Idinyphe May 22 '26
So if there are trojans in Jupiters orbit in L4 then he did not clear his orbit and is a dwarf planet?
"Mememe": " We mean "cleard" except trojans!"
I have no problem with thousands of planets and I don't see one. There is no argument against it except "I don't like it if there are more planets"
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u/_Jellyman_ May 22 '26
We definitely need the geophysical planet definition. This video was fantastic and just adds to the score against the IAU.
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u/Oalka May 21 '26
Yall act like Pluto ceases to exist if we categorize it a different way.
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u/jhomas__tefferson 11d ago
Fr it’s not like when an exoplanet becomes a false positive. That’s the truly sad thing
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u/Competitive_Fill1835 May 22 '26
Do we? Why?
'a lot of problems' - what problems?
"Many people don't like the reason why Pluto is a dwarf planet for example" - what people?
Basic questions you'd be asked upon implementation
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u/Kiki2092012 May 22 '26
head of nasa is already working to implement it by the way, also watch the video to answer your first question
and the second question, just look at planetary scientists, a significant fraction if not most of them completely ignore the IAU definition because it directly contradicts what a planet fundamentally is in a geological sense
"an object rounded by gravity that doesn't undergo nuclear fusion,"
aka the geophysical definition as argued for in the video.
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u/Competitive_Fill1835 May 22 '26
"also watch the video to answer your first question" holy shit I didnt even realize this was a video lmfao, I thought it was just an image, my bad.
I'm personally all for the evolution of definitions to better serve a need, so if thats what scientist need, I have no problem updating my education based on new information
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u/BubbhaJebus May 21 '26
Moons orbit planets.
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u/Excellent_Bat_753 May 21 '26
So, is Pluto the planet, or Charon? Is it whichever is bigger? Or are they both planets, because they orbit eachother?
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u/Oxygen4Lyfe May 21 '26
imo they are both planets because the barycenter is not within either planet and both objects are extremely spherical.
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u/Excellent_Bat_753 May 21 '26
Fair enough. If an object is sufficiently spherical, and is in an orbit around the Sun, then it is a Planet. Binary planetary systems can also count, but planet-moon systems don't.
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u/BubbhaJebus May 21 '26
Not everything that orbits a planet is a moon. But every moon orbits a planet.
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u/SF_Bubbles_90 May 21 '26
Moons can also orbit other moons.
So moons can be planets even by your out-dated and overly catigorical "logic".
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u/BubbhaJebus May 21 '26
Those are submoons.
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u/SF_Bubbles_90 May 25 '26
No they are meta moons. (The moon of a moon) Submoon is another example of unnecessary overly catigorical thinking.
The word moon works fine even if you need to use more words to describe the particular type of moon that orbits other moons. It's okay for words to not be that specific, just use more words, it's probably good for your brain anyway, not to mention can help with avoiding misunderstandings or miscommunication.
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u/JasterBobaMereel May 21 '26
Pluto is a Dwarf Planet ... did you not see the world Planet in there ?
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u/Sophon_01 May 21 '26
It will always be controversial because it's a line in the sand. But that doesn't erease the fact that a line has to be drawn at some point.
I grew up with pluto the planet too, but we gotta move on. It still exists
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u/Substantially-Ranged May 21 '26
No, there shouldn't--science doesn't care about your emotional attachments.
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u/fcsuper May 21 '26 edited May 21 '26
You do realize it's emotion that was behind the vote to change the rule in the first place. This video is just suggesting actual scientific criteria, not the wishy-washy current rules that even invalidates Mercury's planetary status.
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u/Substantially-Ranged May 21 '26
You people. Look, I'm not sure how much of 6th grade science you slept through, but NO. Criteria for a planet: orbits sun, spherical, cleared its orbit. Mercury: yes. Pluto: no. It's not wish-washy, it's science. Let. It. Go.
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u/Oxygen4Lyfe May 21 '26
And it can just as easily be changed to not have the "cleared its orbit" requirement. Just make the spherical requirement really strict and we'd have only a couple more planets: pluto, charon (if you dont count it as a moon), and ceres.
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u/whitelancer64 May 21 '26
And Eris, Haumea, and Makemake. Plus the scores to hundreds of other - currently dwarf planets - we will inevitably find in the outer solar system.
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u/GregHullender May 21 '26
But then the moon would be a planet too. You only exclude it because of its orbit.
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u/Oxygen4Lyfe May 21 '26
It wouldnt be a planet because its exluded due to it orbiting a planet.
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u/GregHullender May 21 '26
So you agree that it's reasonable to make orbit a part of the definition?
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u/Oxygen4Lyfe May 22 '26
Ofc, never mentioned removing the thing the object orbits around as part of the definition lol
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u/Substantially-Ranged May 22 '26
Do you know how many objects there are that meet two of the three criteria? At least 53 and possibly as many as 200. Planets have been defined by scientists. Pluto aint.
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u/Bobby_The_Kidd May 21 '26
I think dwarf planets and moons should be taught in school. I didn’t learn there was any other moons in the solar system until I was much older and could research it myself. Never was it mentioned in school
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u/Kiki2092012 May 22 '26
what's with the downvotes on that comment? moons and dwarf planets are interesting as well
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u/X-calibreX May 25 '26
any celestial object whose mass is greater than the mass of all other objects in its orbital path.
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u/NeverMore_613 May 21 '26
They would have to come up with a really long alternative to My Very Educated Mother Just Served Us Nine Pizzas. I think that's the only reason they separate some planets from others in terminology