The Solar System's Forgotten Planets

Would you believe there are 40 planets are in our Solar System? If you ask the IAU - the International Astronomical Union - there are only 8, but that might not be the best possible answer. If we take a more physics-based definition of what makes a planet a planet, we might have at least 40 planets in our Solar System, and maybe many more! Learn about these forgotten planetary bodies and what makes them so amazing - with some help from New Horizons principal investigator Alan Stern. Let's explore!
Chapters:
0:00 Intro
0:20 What is a planet?
2:46 The planetary bodies
3:44 The big asteroids
4:42 The icy dwarfs
6:37 The 7 big moons
8:15 The other round moons
9:24 Conclusions
Media Credits:
Sagittarius A*: EHT Collaboration, CC BY 4.0
Kuiper Belt: by European Southern Observatory, CC BY 2.0
21 Lutetia: ESA / MPS / OSIRIS Team / Kevin Gill, CC BY 2.0
10 Hygiea: ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS)
Rhea Silvia Sclupture: Sailko, CC BY 3.0
Makemake: Hubble ESA, CC BY 2.0
Haumea Ring: Kevin Gill, CC BY 2.0
Planet Nine: Kevin Gill/Elizabeth Gill, CC BY 2.0
Jupiter and moons: Jan Sandberg
End theme: "Jupiter" from Gustav Holst's "The Planets", performed by the United States Air Force Band (Public Domain)

[@Corium1]

I love the moons that that would be big enough to be considered planets if they weren't captured by a gas giant.

[@SignoreGalilei]

They are pretty cool places. I'm excited for JUICE, Europa Clipper, and Dragonfly to tell us more about them soon.

[@ENDI8089]

Size doesnt even matter. (For planets) the reason pluto isnt considered a planet its because its in the kuipers belt (he hasnt cleared his orbit from similiar sized objects)

[@ENDI8089]

Exepct it being round it doesnt need to be too big

[@Corium1]

@@SignoreGalilei I hope they're successful. always afraid of these probes failing.

[@Tubaplayer2]

@@Corium1Same

[@demeurecorentin]

Imagine the IAU desperately trying to convince aliens that they're not _actually_ from a planet but need to go back and do weird large-scale rituals like "clearing the neighborhood" first.

[@SignoreGalilei]

So if future humanity went out and reshuffled the Kuiper belt, could we make Pluto an IAU planet again?

[@andrewparker1622]

Who's to say an alien civilization doesn't have the same or different standards? I'm sure we can change the definition IF we ever actually find extraterrestrial life

[@TIMEtoRIDE900]

Every nine Months Earth captures a "small" asteroid that becomes a temporary Moon for a while. Therefore Earth has not "cleared" it's orbit - - and is a Planetoid !!

[@thatdognotthepuppy5809]

​@@SignoreGalileiI don't know if that would fit their wording, because their terms are that the body itself has to clear the orbital path. Human intervention would give em something to argue over for a while.

[@person8064]

To be a planet, it has to orbit our Sun. So literally nothing outside the solar system can be a planet

[@hyun-shik7327]

I like the idea that "planet" and "moon" are not necessarily mutually exclusive.

[@SignoreGalilei]

Charon especially feels like it ought to be both, if not all the round moons

[@marcoasturias8520]

If anything moon is a more coloquial name for [natural] Satellite, and, if you think about, all planets in the Solar system are Satellites... Of the Sun!

[@stevenclark2188]

Honestly everything with a moon orbits around a barycenter. Just usually that's underground.

[@Jellyman1129]

“Moon” just means “satellite”. There are satellite asteroids, satellite stars, and satellite galaxies. Satellite planets also exist. They don’t cease to be asteroids or stars or galaxies or planets just because they orbit something else.

[@-_.._._--_.-.-_-_-_-...-.-]

I'm in the process of becoming a moon.

[@alchemiccolored1907]

i had a book as a kid that was presumably made during the whole "is pluto a planet" controversy that named eleven planets. the main eight, pluto, ceres, and I believe makemake. very weird to have one of my introductions to the solar system be so contrary to everything I learned afterwards. great video!

[@SignoreGalilei]

Glad you enjoyed the video! I remember seeing one that included Eris under the name "The Tenth Planet". Eleven is new to me, though.

[@TlalocTemporal]

We had a card game with the 8 major planets, pluto, sedna, and the sun, for a deck of 11 per suit.

[@Chubby_Bub]

But the Tenth Planet is Mondas.

[@josephjohnson6849]

I had that but it had eris

[@ahhongng9916]

The tenth planet iss..... makemake.

[@airiannawilliams3181]

Because Charon doesn't exactly orbit Pluto, it can be said that Charon and Pluto is a double Dwarf Planet system, and together they have 4 natural satellites orbiting them.

[@SignoreGalilei]

That's a pretty good description of the system

[@shaansingh6048]

and even together they don't have enough mass to clear the orbit 😒😒

[@Jellyman1129]

@@shaansingh6048Neither does Earth. Should we demote Earth too?

[@shaansingh6048]

@@Jellyman1129 Earth does. There’s a difference between a few asteroids and the Kuiper Belt.

[@Jellyman1129]

@@shaansingh6048 What defines “a few”? Earth has over 10,000 asteroids in its orbit. Jupiter has hundreds of thousands of asteroids in its orbit.

[@EdbertWeisly]

1:15 Eris: the responsible one Pluto: the rebellious one Makemake: middle child Haumea: the weird/unique one Ceres: the cousin Quaoar: the one that defied expectations Sedna: the distant one/ the loner

[@SignoreGalilei]

I might flip Pluto and Eris but otherwise I'd agree

[@ymodnar]

@@SignoreGalilei Eris was the reason why Pluto became a dwarf planet and many people still think Pluto is a planet.

[@SignoreGalilei]

@@ymodnarTo me, that makes Eris feel like the one that's causing trouble

[@Novusod]

It was more appropriate when Eris was named Xena (the god slayer). After Xena (Eris) was discovered Pluto was slain and ceased to be a planet.

[@EdbertWeisly]

@@Novusod"know your place pluto" -Eris

[@samuelmelcher333]

THIS! This is exactly what I've been trying to tell people for years! The discussion of "What could be considered a planet and why" is so much more interesting than "Pluto is a planet because I feel bad for it 😢❤" vs "Nuh-uh you must be stupid because Science™️ said it isn't #sciencenerd 🤓". Thank you for an excellent video on the topic

[@SignoreGalilei]

You're welcome! I definitely agree.

[@Jellyman1129]

People often miss the forest for the trees. They talk about bringing Pluto back as a planet and ONLY Pluto, but ALL the dwarf planets should be planets. What about Eris and Sedna and Gonggong and Quaoar? Although I can’t entirely blame them. School curriculum has failed all of us. Most people don’t know these objects even EXIST! So how can they root for them? It’s a big science communication fail.

[@ankharahallstrom1580]

​@@Jellyman1129 I agree 100%. All of the dwarf planets becoming official planets is far more exciting. Especially given that the man who coined the term meaning it to be a third category of planet and that he's upset his term was used to create a new category of insignificant space rock. I think the idea of keeping the planet club exclusive and tiny (both the "Pluto isn't a planet" and "Pluto's a planet because I feel bad for it" crowds fall into this camp) is born out of a sentimentality of how we were told planets are rare and special and there's so few you can count them on your fingers. It'd be like deciding retroactively everything outside of the 13 original colonies isn't a state, giving Vermont to New Hampshire and giving Maine back to Massachusetts while relegating the other 35 states into a territory group called "Western Territories" and adding a third criteria to "state" being "must have been established by a British crown charter".

[@alveolate]

i don't mind pluto being a planet or a dwarf planet. i do mind that the dwarf planets in general aren't getting enough attention. pluto got a lot of attention because of that flyby and a previously presumed boring rock turned out to have a goddamn heart shaped crater! what else could be on the other dwarf planets? and some of them aren't even that far out! imo the categorisation is purely functional, especially for scientists. amateurs and laypeople don't really need to be that concerned about pluto's classification at all. you can count pluto as your personal 9th planet; it's not like the IAU will send agents to arrest you or anything. it does seem like many astronomy channels are refocusing on all the other dwarf planets and expanding their scope beyond pluto tho, which i think is great for sci-ed! the expanse used ceres as a major location, and i believe all 32-40 bodies mentioned in here could also be RL major locations in the future.

[@ankharahallstrom1580]

@@alveolate I'd love to see attention to dwarf planets (should be a third category of planet and not of small solar system bodies IMO). It's so cool that we have Orcus for example, a counter-Pluto. I see Pluto as the 10th planet after Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, and Neptune. Orcus would be the 11th planet or maybe 10a. Then we have Haumea, MakeMake, then Eris, Quaoar, Sedna.

[@CoolAsFreya]

I have a solar system model that has 5 dwarf including Pluto and all the major planets have moons, I thought that was pretty cool since most people don't talk about them!

[@SignoreGalilei]

I agree, that is pretty cool! There's a lot more to the solar system than people sometimes realize.

[@CHaRLieBiTMe85]

The Alan Stern at the end got me lol

[@SignoreGalilei]

Nice, glad it worked haha

[@memewhile6242]

@@SignoreGalilei I had to scroll down to see if the video wasn't sponsored or made by Alan Stern

[@alpheusmadsen8485]

Several years ago, I did a deep dive into why Pluto was demoted, and after examining everything for myself, I concluded that the IAU's definition for planets is wrong -- that a planet should be anything big enough to be pulled into a sphere by its own gravity, but too small to generate fusion (or, for things like black holes, neutron stars, and white and black dwarfs, be the remains of something that used to be massive enough to generate fusion). When I concluded that, I concluded that the solar system has a *lot* of planets! And, of course, that Earth/Luna is a pair-planet system, along with Pluto/Charon. Later on, I encountered an article titled "Pluto could be a planet, but you wouldn't like the results!" and it turned out that the results I shouldn't have liked was that we'd have too many planets, and that children would no longer be able to memorize the names of all the planets. I have no idea why people think we'd be better off in a solar system with less planets, rather than more! (I'm also not thrilled with the notion that our definitions should be able to fit into the world-view of 3rd-graders.)

[@Jellyman1129]

DING DING DING!! Flawless answer! The IAU definition is completely bogus and was specifically engineered to limit the number of planets, which is beyond stupid. Our Solar System has dozens and dozens of planets, that’s so exciting! This video perfectly demonstrates that. I read that article you mentioned and it’s from a non-expert, someone who doesn’t study planets. I find there’s a strange pattern: nearly all articles about the definition of a planet that talk about having “too many” planets are written by scientists who don’t study planets. Why should they care how many there are? That’s like if a planetary scientist got upset over having more than nine galaxies in the universe. There is no scientific argument that would exclude Pluto from being a planet. EVERYTHING about Pluto SCREAMS “PLANET”. It’s baffling to me that PhD astronomers, who are full-grown adults, would make such an asinine and senile argument that we can’t have “too many” planets. It makes them look so juvenile and it’s honestly embarrassing. Ironic that ASTRONOMERS would be afraid of an ASTRONOMICAL number of objects. So unprofessional. The Solar System has way more planets than anyone can memorize and that’s got SOME people in a tizzy, but they need to get used to it. That’s the data. A vote doesn’t override that and never will. Well said! 👍🏻👍🏻

[@minutemansam3122]

​@@Jellyman1129no

[@minutemansam3122]

The Earth-Moon system (not Luna, that is not the Moon's name) is not a binary system as the barycenter is within the Earth. It's not like the Pluto-Charon system.

[@Jellyman1129]

@@minutemansam3122If you have a problem with an astronomical number of planets, than astronomy isn’t for you. 😂

[@catpoke9557]

​@@minutemansam3122To be honest that's a loss in itself. A pair of BINARY PLANETS is something we literally CAN'T have anymore because of the definition of a planet, which is deeply upsetting. That would be an absolutely awesome planetary system if it were just allowed.

[@fernbedek6302]

There’s going to be so much confusion in the future with Titan, Titania, and Triton.

[@SignoreGalilei]

I'm imagining a Niger/Nigeria or Dominica/Dominican Republic situation

[@Too-Odd]

We've known them for so long, that it shouldn't be a problem.

[@Too-Odd]

@@SignoreGalilei Sure, but Niger is pronounced nee-ZHAIR, and more people get that wrong than which is which.

[@professoryeetus8955]

ed edd n eddy

[@rianfelis3156]

I remember NDT using the term "world" for round bodies, when you don't care about whether it is a planet. I think he credited Mike Brown for it, but I do like having a term for all of those bodies that you listed that doesn't conflict with the IAU definition, but also doesn't demote these bodies of particular scientific interest. I also like changing it to "major planet", "minor planet", and "moon" when these distinctions are important. It keeps the short list worthy of memorizing, and longer lists for people who actually have an interest in it. It also makes it much easier to discuss the history of Triton, which probably changed from minor planet to moon.

[@SignoreGalilei]

That's fair. I personally don't see why non-round bodies like Phobos and Deimos shouldn't be worlds, but it does make sense to distinguish different categories of object. For Triton, I think I would just say it used to orbit the Sun directly but now it orbits Neptune. I agree there is merit to the short list, so people know where in the solar system different things are. On the other hand just because people don't know, like, every dinosaur, that doesn't mean the less-known ones aren't dinosaurs too. It's a tough one.

[@AstroChara]

When it comes to astronomy, worlds are used for basically any objects we have clear pictures to see all the surface features, which means it’s problematic.

[@iluvuradio2021]

@@SignoreGalilei u could put oneil cylinder in them and give them a pop. of a few million people at least. so u can at least make little worlds how of them. other than that i agree.

[@BrettonFerguson]

They really need to redefine what makes a moon a moon. People are discovering asteroids in Jupiter and Saturn's orbit and some websites now list hundreds of moons around Jupiter and Saturn. Small asteroids. Many only a few meters. A lot of these will likely fall into Jupiter in the next couple hundred years and be replaced with new asteroids. If anything orbiting a planet is a moon, then the Earth has hundreds of thousands of moons too with all the space junk. But defining artificial vs natural is the easy part. Maybe they need to define it by stable orbit and size. Perhaps size relative to the planet it is orbiting. I'm sure scientists smarter than me could come up with a definition better than they have now. Before every rock in Saturn's rings gets identified and Saturn has 100 million moons.

[@Jellyman1129]

@@BrettonFergusonA moon is any natural object in space on its own independent orbital path around another object in space. This counts any asteroid or planet orbiting an asteroid or planet as a moon while excluding ring particles.

[@maxtonuponry]

This was really nicely put together - the IAU definition makes a lot of sense and is clearly necessary to draw a line in an important place in this list. But it's wonderful to really go through why all these others are individually interesting.

[@SignoreGalilei]

Thanks! I agree, the IAU planets are notably different from the rest, but the other planetary bodies are interesting for their own sake too. I'm glad that came across.

[@dreamingissleeping]

I actually think their definition is a lie... they may not like ~500km planets being classified as such, because of their size, however, scale isn't really a factor I don't believe. NO planet in our solar system has "cleared" it's orbit. Earth passes through a section of meteors every single year... there are asteroids scattered throughout from Mercury all the way to the Kuiper belt/Oort cloud. The definition is bullshit and factually a lie.

[@dreamingissleeping]

Pluto has an atmosphere, core, changing surface, mountains, cryovolcanoes, four moonlet sized satellites. It is most definitely a planet. Earth is not the center of the universe or the final judge.

[@pedrosegundo8109]

It is still arbitrary. Why we call planets that don't have a star a "planet"? I mean, their definition requires a star, so why we still call rogue planets "planets"?

[@eldrago19]

I think the IAU definition is certainly useful for astronomers but I think given it would mean most of the bodies planetary science covers are not in fact planets, it is a slightly odd one.

[@Ithirahad]

Ceres still seems a bit salty about being demoted from planethood. Someone should go check on her. (salt-deposit puns aside, seriously we should send a mission there; Ceres is a funky place)

[@SignoreGalilei]

We had the Dawn mission at least. It would be good to get a lander, too.

[@Ithirahad]

Lander/rover is what I mean by "there".

[@zimriel]

@@Ithirahad Lander's possible. Unsure about piloting a rover from, what, 2.8 AU

[@SignoreGalilei]

It's probably not much harder than piloting one on Mars - you've still got a large time delay.

[@morgan0]

can you do a planet on titan’s lakes and general hydrology? i find it so fascinating, how even though it’s so much colder, literally flowing on ice, the general principles of hydraulic erosion look to still be there, with the fractal-y shapes visible at the waterline.

[@SignoreGalilei]

Titan's lakes would be cool to talk about. I don't know how much is known versus conjecture at this point, though.

[@navilandinator4479]

Yeah

[@macsnafu]

It's amazing how little we still know about our own solar system. But then again, maybe not, since we're really still at a very early stage in space exploration.

[@SignoreGalilei]

We have a long way to go!

[@Jellyman1129]

We actually know a decent amount about the inner and middle solar system. It’s the outer solar system that we know very little about.

[@hugotheimpecileone]

Makes me wonder if our solar system would be concidered a very large and packed solar system. Or if many other solar systems are just as complex but we just can't see it.

[@dreamyrhodes]

It's correct that planet originally means "wanderer", but the meaning was everything that's "wandering" over the sky in front of the fixed star background not just "wander over the sky" because fixed stars "wander" over the sky too but they are in the same spot every night while "wanderers" change their constellation constantly. Thus even Moon and Sun been planets back then. And if the ancient Greek had telescopes and been able to see Pluto, Eris, Ceres, Vesta, and all the other bodies despite their shape, they'd be called "Planets" too and we might have way more than 40 planets by that original definition.

[@Jellyman1129]

True, but nobody uses that original terminology because of the advancements of our knowledge. Nobody calls asteroids “star-like” anymore because we know they’re nothing like stars despite “star-like” being the original terminology of the word.

[@ibiuld443]

YES i'm glad to see this topic getting attention. i posted a comment about something like this on another video recently. i think that it makes our solar system seem SO much more interesting and lively if we look at "planetary bodies" instead of the IAU definition of planets. i always used to be sad that our solar system was so empty with only 8 planets. but there are absolutely more than 8 planets, there's a whole lot out there to explore. i think it should be taught this way in schools. like, how cool is it that when u look up at the sky, you can see the surface of a planet so close it's only 3 days away with your naked eye?

[@SignoreGalilei]

Yeah! It's kinda crazy we can see surface features on another world just by looking up.

[@Jellyman1129]

The Solar System is teeming with planets, and most of them are like Pluto! That’s so exciting! I’m glad the word is being spread, many people sadly don’t know an alternative planet definition exists. I love looking up at our closest planetary neighbor, the Moon, and seeing mountain ranges on its surface with just a pair of binoculars! You can even see Jupiter’s four satellite planets!

[@minutemansam3122]

​@@Jellyman1129the solar system has eight planets, many hundreds of dwarf planets, and an incalculable number of planetoids.

[@Jellyman1129]

⁠@@minutemansam3122No, the Solar System has hundreds of planets. Some are huge, some are Earth-sized, and many are small. “Planetoid” isn’t a scientific term.

[@tardiscommand1812]

I’m fascinated by the knowledge that you could read a newspaper there as it’s bright as the full moon on earth is. I always thought it was a dark, lonely place. Cheers to Pluto, the world with a heart.

[@EdbertWeisly]

4:05 i remember when people's best image of ceres is a white pearl

[@SignoreGalilei]

Pluto, too. 2015 was a great year for Dwarf Planet exploration.

[@EdbertWeisly]

@@SignoreGalilei hubble's pluto image was weirdly green

[@kisjohannita7070]

I love the part when he says "there are more exiting planets, including of course Alan Stern"

[@wendigockel]

Yet given that "Stern" is the German word for "star", he should be classified as a brown dwarf.

[@Naturenerd1000]

The Dwarf Planets are cool.

[@SignoreGalilei]

Yeah!

[@Jellyman1129]

Yeah, I love them! They’re so mysterious and hold a lot of secrets.

[@michaelchance6125]

I think the easiest way to make a unified definition for planet and moon is: if it orbits a star and is round, it's a planet. If it orbits a not-a-star, it's a moon.

[@Jellyman1129]

Planet = any gravitationally rounded object in space that hasn’t undergone nuclear fusion. Moon = any natural object in space orbiting another natural object in space. Earth is a planet, but not a moon. Deimos is a moon, but not a planet. Ganymede is both a planet and a moon. Ida is neither a planet nor a moon.

[@catpoke9557]

Doesn't work. Objects orbiting brown dwarfs are considered exoplanets (not moons) and many objects which are round and orbit stars are not planets or exoplanets. Plus there's exoplanets that don't orbit anything. I think this definition is close though. I think a planet should be an object which is rounded by its own gravity and has at no point in its life cycle started nuclear fusion. This allows us to exclude stars, black holes, and leftover cores of dead stars. The only problem is that this includes brown dwarfs. So there needs to be something else added. It would also mean many moons are planets, which I think is fine since moons are sometimes also asteroids and nobody has any problems with that. Also this definition allows us to include exoplanets as planets so now we don't have 100% of the universe's planets in our orbit.

[@OnTheThirdDay]

9:50 You know who would like this video? . . . . Alan Stern

[@andyghkfilm2287]

“Science once thought there was nine planets-but now, there are NINETY planets.” - Tim and Eric’s Awesome Show Good Job

[@johns_jokes]

Its so cool seeing how you're channel has grown i know how hard it is keep up the good work

[@SignoreGalilei]

Thanks! There are more of these videos to come.

[@EdbertWeisly]

7:35 its so interesting that Huygens needed to be engineered just in case it landed on liquid

[@SignoreGalilei]

Yeah! NASA is planning on sending a helicopter to Titan next, since the Mars one was so successful.

[@ChoccoGlx]

3:38 Ceres is a dwarf planet. 7:13 The Moon’s name is Luna. 9:18 They are binary planets. They orbit eachother.

[@styrofoam4637]

if the moons name is actually Luna that is so confusing in some languages. in russian we call the moon Luna scientifically and if it’s visible in the sky BUT only if it’s a full moon. a crescent moon used the same word as “month” in russian 😭

[@minutemansam3122]

Luna is a poetic name. In English the official name of the Moon is the Moon.

[@Jellyman1129]

@@minutemansam3122Scientifically, the Moon’s name is Luna.

[@noob12455]

I do agree all the dwarf planets should be dwarf planets, but our schools need to actually teach children about these dwarf planets, maybe not all of them but at least the more major ones.

[@CarnelianClout]

I think Alan Stern's definition is more sensible. There is no such thing as "too many" planets, as there are too many mountains, lakes, rivers, to count - but that doesn't change the fact that if it's a mountain, it's a mountain, doesn't matter how many there are.

[@SignoreGalilei]

That's a good point. I guess it comes down to what we want out of a category like "planets". It might all change as we discover more of them, too!

[@Fractured_Unity]

But it is important to not call hills mountains, no?

[@CarnelianClout]

@@Fractured_Unity Obviously.

[@Fractured_Unity]

@@CarnelianClout So therefore it would be too many planets if we start including other things erroneously in the category.

[@CarnelianClout]

@@Fractured_Unity There is no such thing as "too many planets".

[@hjohnson966]

"Another 5 round moons orbit Uranus" That has to be the best line in the video

[@LeetengjingYT]

*DANG THEY THICC ALSO*

[@YOVOZOL]

this video brought me back to my childhood fascination with astronomy... those were good days :)

[@SignoreGalilei]

Glad I could bring you happy memories!

[@blakemesina]

this was very nostalgic for some reason, really well done video man

[@Starry-Nathan]

I love the little bit of the Planets Suite you put at the end :D

[@toweringhorse2054]

I love these kinds of videos because I recently became aware there was other stuff out there such as ceres. I was in elementary school during the whole Pluto reclassification and I just kind of assumed the extremely simplistic models we observed in school were literally everything known in the solar system and never really questioned it or expanded on my understanding until recently.

[@Jellyman1129]

That’s why I really despise the IAU’s approach. They wanted to keep the number of planets low so school kids can memorize them. But doing that excludes a majority of other interesting objects in the Solar System like moons, asteroids, and comets. Most people don’t know objects like Orcus or Gonggong or Quaoar even *exist* because they’re not included in textbook Solar System diagrams or school curriculum. Just like the title of the video, these planets feel forgotten.

[@X3MgamePlays]

I can't unsee them anymore with eyes, mouth and a character.

[@SignoreGalilei]

Planetballs?

[@X3MgamePlays]

@@SignoreGalilei Planetballs

[@unslited]

Loved the video! i like the idea of having geography and space in your channel

[@SignoreGalilei]

Thanks! Glad you enjoyed it - I might do more space videos in the future since people seemed to like this one.

[@NeroDefogger]

Finally a good video on the topic! I was beginning to think no one actually knew a single thing about our solar system bodies, yes those bodies can definitely be considered planets, I have my own definitions because let's be honest everyone that actually knows about the solar system bodies knows that the IAU's definitions are just awful. So with my definitions we know of: 4 gaseous planets 10 planets and 16 semi-planets. I'd like people to try to guess either the bodies category or the definitions for each category.

[@SignoreGalilei]

Gaseous ones seem pretty clear (Jupiter, Saturn, Uranus, Neptune). I'm having a bit of trouble with your other categories because I'm counting 28 other confirmed round objects in the solar system, not 26. Are you not including Makemake and Haumea, or maybe are the Moon and Charon in a special category of their own?

[@NeroDefogger]

@@SignoreGalilei yes the gaseous are clear, but I'm confused because you say you can think of 28 confirmed round yet you are thinking that maybe the moon is not accounted in those 26? shouldn't that 28 have some very small bodies? oh also no the other categories are for asteroids, stars, and stuff like that

[@SignoreGalilei]

What I'm trying to puzzle out is which 2 of the 28 non-gaseous round objects are not in either your "planets" or "semi-planets" category, since you only have 10 planets and 16 semi-planets.

[@NeroDefogger]

@@SignoreGalilei yes exactly, that's why I found extremely weird that out of all of those 28 objects with so many tiny ones you thought that maybe the moon or charon weren't even semi-planets and were just asteroids, understand that the 2 ones that are not there are only considered asteroids in my categories

[@SignoreGalilei]

@@NeroDefogger oh alright. My guess was that The Moon and Charon would have been in a category MORE "important" than semi-planets, not less. So is your definition that semi-planets orbit planets? Or maybe do you use a size cutoff between Triton and Europa?

[@matthewevans1217]

This was a great video! I'm currently a HS senior with aspirations of becoming a planetary geologist so it definitely struck a chord

[@SignoreGalilei]

Awesome! I hope you get to achieve this ambition, it's a really cool field from an outsider's perspective.

[@Jellyman1129]

Planetary geology is so fun! Good luck in pursuing your dreams! 🪐

[@u92element4]

great video as always keep up the good work!

[@SignoreGalilei]

Thanks, there's more to come!

[@shimrrashai-rc8fq]

This is almost exactly what I've been thinking, too - that we should perhaps have _two_ different systems of classification at work at the same time - one is an astrodynamical classification, which is similar to the IAU's, and the other is an intrinsic or physical classification. Basically, what makes something a "planet" vs. "not a planet" is _physical,_ while depending on its dynamical relationship to other objects, we may have a number of _sub-categories_ to which it may belong. One scheme I thought, and based on some alternative proposals to the IAU's, is that one would have "uberplanets" (exactly the current IAU definition of "planet"), "dwarf planets" (also the IAU definition for _that,_ copied), but also then "satellite planets" (i.e. and aka. "planetary moons"), and "rogue planets", i.e. those detached from a star and roaming free in the Galaxy. The term "moon" would then just mean any non-artificial satellite of any sub-stellar object, including asteroids that moon other asteroids, and thus would cross-cut both planets and non-planetary objects. For the physical classification of planets, we would use their usual compositional terms, e.g. "terrestrial planets", "gas dwarfs", "ice giants", and "gas giants", though we might divide further, e.g. I tend to think a volatile-rich but still solid planet like Pluto is kind a different thing than a "truly" terrestrial planet like Mercury. Also, fwiw, if the term "double planet" is to make any sense then a moon has to be able to be a planet. Heh. A planet is something you recognize at a glance, with few exceptions. Looks like a ball thingy, and it doesn't hurt your eyes like a star? (and of course is of cosmic size) Almost surely that should be a "planet". The rest is just slight technicalities to handle edge cases. That said, my idea for the physical definition of a planet was simpler than the one with layering: just that it be a) insufficiently large to have ignited any nuclear fusion reaction at any point in its history, and b) is sufficiently large to be "balled" by its own gravity. Thnking that the Moon is a planet is also much cooler. It's the stepping-stone nearby little planet before we get off into the "serious stuff".

[@Jellyman1129]

You hit the nail on the head! Most taxonomy in science is done by taking a general concept and describing it more specifically using adjectival prefixes. Similarly, planetary scientists define planets as gravitationally rounded celestial bodies that aren’t massive enough to undergo nuclear fusion. ANY object in space that meets these two criteria is a planet. It’s a very broad term that covers a variety of different characteristics a planet can have. It’s also intuitive. Alan Stern calls it “The Star Trek Test”. When the Starship Enterprise visits a large round object in space that isn’t a star or black hole, everyone on the crew and everyone in the audience immediately knows “That’s a planet” without having to say “Let me survey the entire solar system for all objects that are here, integrate their orbits, calculate if this object is gravitationally dominant in its orbit, and then determine if it’s a planet or not”. Nobody ever needs to do that, it’s not that hard. While determining the gravitational dominance of a planet may be an interesting thing to study, it doesn’t DEFINE what the object IS. As an analogy, learning about someone’s heritage may be interesting, but it doesn’t determine whether that person is a human or not. There are numerous adjectival prefixes that categorize the planets. You can organize them by their role within a system. Planets between the Sun and the Asteroid Belt are called the inner planets. Planets between the Asteroid Belt and the Kuiper Belt are called the middle planets. Planets between the Kuiper Belt and the Oort Cloud are called the outer planets. Planets that orbit the Sun are called solar/primary planets (like Earth). Planets that orbit other stars are called extrasolar planets (like Proxima b). Planets that orbit other planets are called satellite/secondary planets (like Titan). Planets that orbit nothing are called rogue planets (like PSO J318.5−22). You can also categorize the planets by their composition and size. Earth-sized rocky planets are called terrestrial planets (like Venus). Large gaseous planets are called gas giant planets (like Jupiter). Small icy planets are called ice dwarf planets (like Pluto). I’m fine with Pluto being a dwarf planet as long as dwarf planets ARE planets, especially since dwarf stars are stars and dwarf galaxies are galaxies. Planets that orbit each other are called binary/double planets (like Pluto and Charon). The final step is to combine these adjectival prefixes. Earth is a solar terrestrial planet. The Moon is a satellite terrestrial planet. Pluto is a solar dwarf planet. Triton WAS a solar dwarf planet, but got captured by Neptune and is now a satellite dwarf planet. TRAPPIST-1 e is an extrasolar terrestrial planet. Kepler-1625 b I is a satellite giant planet. There’s also more categories like hot Jupiters with evaporating atmospheres, super-Earths much more massive than our planet, puffball planets with extremely low densities, hycean planets with oceans that reach thousands of miles deep, and many more. Planets can fit into multiple categories and often do. Earth is located in the inner solar system (inner planet) and orbits the Sun (solar planet) and is rocky (terrestrial planet). So it’s an inner solar terrestrial planet. The more adjectives you include, the more specific and descriptive the object gets. You can choose how vague or descriptive you want to be depending on what characteristic of the planet you want to highlight. If you only want to describe the regions of the Solar System, calling Earth an inner planet is good enough. If you want to highlight Earth’s composition compared to Saturn, calling Earth a terrestrial planet is ideal. If you want to talk about what objects each planet orbits, calling Earth a solar planet is perfect. You can use all the categories to be as descriptive as possible or you can use only one category to highlight one characteristic of your choice. That’s the beautiful part. No matter how a person chooses to preset the data, the data itself is always the same: m.kzbin.info/www/bejne/rKjKoJmFbr1of6c&pp=gAQBiAQB This system of categorizing planets is incredibly useful as it describes what the object is, what it’s like, and its context within a system, all at the same time. Instead of saying “Io is one of Jupiter’s moons, and it’s intermediate in size with a rocky surface”, you can say “Io is a satellite terrestrial planet of Jupiter”. The same applies to stars. Instead of saying “Proxima Centauri is a small red star that orbits the Alpha Centauri binary”, you can say “Proxima Centauri is a satellite red dwarf star”. It’s very efficient, describing multiple characteristics in just a few words. The Pluto-like objects in the Kuiper Belt beyond Neptune are planets. They’re small, but they’re planets. It’s ridiculous to think they shouldn’t be just because kids won’t be able to memorize their names. Our solar system has hundreds of planets, most of which are dwarf planets. That’s exciting! We should be sending more missions like New Horizons to the outer solar system to go explore Sedna and Haumea and Quaoar and the hundreds of other worlds that are waiting to be revealed. That’s what NASA is all about! Fantastic comment! 👍🏻👍🏻

[@catpoke9557]

I agree. Keep the term dwarf planet but have it be considered a TYPE of planet. Also remove the 'in orbit around the sun' part so we can apply our definitions to exoplanets and rogue planets as well.

[@ellenbryn]

Hear hear! I'm a big Haumea fan, and as far as I'm concerned, everyone was so busy fighting about whether pluto was a planet (arbitrary label!) they missed out on the cool news that there's LOTS of mini worlds out there, many as fascinating as Pluto. my fave is Haumea the red-nosed ovoid (one seems to be redder; it's probably got the same color palette as pluto) the fastest-spinning object in the solar system, which has rings AND moons - I keep hoping against hopr they'll find the car that hit it, as it looks like it's still spinning out from a recent hit-and-run. But yeah, shame on the IAU for demoting Pluto after Alan Stern pioneered the mission to visit the last unrxplored planet in the solar system, a mission he'd fighting for for years. Pluto is heading away from the sun now on its eccenttic orbit, so it was important to catch it before it got any farther away. The real pity is that again, in this soundbite 140-character worlld, a lot more people voiced opinions about Pluto's classification (which is just an arbitrary label) than found out the cool things about Pluto that New Horizons and Alan Stern's tram discovdred. At least he was vindicated by it turning ouf to be far more geologically active and interesting than anyone could've hoped!

[@ellenbryn]

sorry about the typos. I have a choice between arthritic fingers and bad eyesight, or Siri voice, dictation mistakes and bad eyesight or both :-) I'm not nearly as stupid as it makes me look!

[@SomeAT-AT]

I really hope I get to see a good photo of it in my lifetime, it's the most interesting object we don't have a good photo of.

[@SignoreGalilei]

Haumea's moons and ring might be the clue to what hit it, "big whack" style. I'd love for there to be New Horizons style missions to all the dwarf planets out there.

[@Too-Odd]

Pluto is still a planet; it is a dwarf planet. It is a distinct classification, but it fits better. Imagine the middle school students having to name the "planets" if Pluto and other Kuiper belt objects were included. Then again, they do have to name Pluto, Ceres, Makemake, and Haumea on tests in our local middle school, so they know about big asteroids and Kuiper belt bodies.

[@Jellyman1129]

Haumea is absolutely a planet and it’s fascinating! The IAU should be embarrassed at their public display of politics. Their opinion is irrelevant. But defining objects in space is not an arbitrary label, it’s an important taxonomical decision that helps us better understand the object.

[@EdbertWeisly]

8:28 Tethys and Dione are Twins and Enceladus is the little brother of Dione (orbital resonance)

[@SignoreGalilei]

It's really cool that Saturn has so many of these moons that they end up in resonances like this. Janus and Epimetheus form another fun pair, though they're smaller.

[@alexakalennon]

We're lucky to live in a system where there's so much to learn

[@2Dentz]

This was an awesome watch, thank you

[@SignoreGalilei]

Glad you enjoyed it, thanks for watching!

[@salihkarayel]

Thanks for the video, it was quite interesting.

[@truvc]

If planet 9 is a black hole, it’d still be a planet. It checks the three boxes of the IAU criteria.

[@SignoreGalilei]

Depending on how big it is compared to everything else in its orbit it might even hypothetically be a dwarf planet!

[@truvc]

@@SignoreGalilei the one sad thing is that if it’s a black hole it’ll be a long long time before we find it. But I bet it’ll get the coolest name of all the planets.

[@Jellyman1129]

Even a star orbiting the Sun would qualify as a planet.

[@minutemansam3122]

​@@Jellyman1129it wouldn't as a planet is specifically defined as a non stellar object.

[@minutemansam3122]

​@@truvc if it was a black hole then the sun would likely orbit it. Still wouldn't be a planet since it's a stellar remnant.

[@DeltaHydrixian]

I ran a simulation to see the smallest body that would meet the IAU requirements for planetary status. The smallest was Europa, who barely knocked anything out, but tried. Anything Luna and bigger was successful at clearing an asteroid belt.

[@Jellyman1129]

At what distance was Europa located? The simulations will give you different results at different distances from the Sun, making “clearing the neighborhood” a bad criterion.

[@DeltaHydrixian]

@@Jellyman1129 I put an asteroid belt at 1 AU from the sun, with no other objects, and put them in the middle. But i agree, there should be different criteria than “clear neighborhood” and more like “dominate the mass of surrounding region”

[@Jellyman1129]

@@DeltaHydrixian The concept is still bad. Even if you change the criterion to “dominate the mass of its region”, Earth qualifies as a planet at 1 AU, but when moved to the Kuiper Belt at 40 AU, it ceases to be a planet. Any definition where identical objects classify differently is inherently broken. 👎🏻

[@minutemansam3122]

​@@Jellyman1129it's not bad, just a bit too vague. But if you're not a stick in the mud you'd know it essentially means its the dominant object in it's orbit

[@minutemansam3122]

​@@Jellyman1129if the earth moved to the kuiper belt it would still be the dominant object and would eventually clear the orbit of most debris, either kicking out objects like Pluto and Eris or colliding with them.

[@DibzNr2234]

Great video, I hate how every Moon is lumped together in the exact same category, Ganymede, Titan and Callisto should not be considered the same type of object as Phobos and Deimos, I personally believe the term "Moon" should be broken up into distinct categories to better represent the true range of scales of these bodies. (E.G. Planet Moons, Dwarf Moons, Asteroid Moons) Also while we're talking about this, need for a planet to "clear its own orbit" is really dumb imo, since the mass needed to clear your orbit scales exponentially with distance to the parent body; Earth would, per the IAU's own definition, be a dwarf planet if it were placed out in the Kuiper Belt, hell even "Planet 9" may be a dwarf planet if it's only a few Earth masses.

[@SignoreGalilei]

Yeah, it's not a great criterion in my view. It feels like they took the list of 8 that they wanted and reverse-engineered a definition to fit.

[@Jellyman1129]

Those moons are lumped together because “moon” simply means ANY object orbiting another object, while “planet” and “asteroid” define the characteristics of the object itself. So Titan is a satellite planet/planetary moon while Deimos is a satellite asteroid/asteroidal moon. And yes, the “clear the neighborhood” criteria biases against distant planets and was specifically engineered to limit the number of planets. All of the examples you gave of planets ceasing to be planets if moved farther from the Sun are absolutely true and is the main reason why the IAU definition doesn’t work. Identical objects will classify differently in different locations. That’s nonsense! If a celestial object is a big non-stellar sphere, it’s a planet.

[@minutemansam3122]

​@@Jellyman1129moon astronomically speaking only refers to The Moon. The rest are natural satellites, which whole colloquially called moons aren't actually moons anymore than Mars is an earth.

[@Jellyman1129]

@@minutemansam3122”Moon” refers to any natural satellite.

[@kgb_active]

I love how the picture shows us as a forgotten planet

[@navilandinator4479]

Forgot Sedna in the round objects list But still good job

[@EdbertWeisly]

8:45 Iapetus has PTSD

[@SignoreGalilei]

Oof yeah, it might be an impact or something but I don't think scientists are certain yet

[@TheButtflyEffectAnimator]

walnut world

[@liamredmill9134]

Great analysis, thanks

[@lucasvanderhoeven3760]

Bro this is quality content, so I hope you will do more unexplored astronomy video’s like this!

[@SignoreGalilei]

Thanks! There's lots of space and astronomy topics that would be cool to do.

[@lucasvanderhoeven3760]

Yeah would be really cool if you cover some unexplored astronomy

[@wizardsuth]

The entire "what is a planet" discussion is just semantics. What we call something has no effect on what it is. A rose by any other name would smell as sweet.

[@SignoreGalilei]

That's fair. Categories are not the be all end all.

[@Jellyman1129]

So the Sun is an asteroid and the Milky Way is a nebula?

[@EdbertWeisly]

Imagine Mimas boasting about being round while Vesta and Proteus are in the background with the rage of a thousand Asteroids

[@SignoreGalilei]

Yeah, I kinda feel bad for not including Proteus since it's bigger than Mimas, too.

[@simiv.s3384]

Totally 39 planets

[@Cahos_Rahne_Veloza]

Gotta Catch 'Em All! Planetmon!

[@Crazy.World777]

😅funny 👍

[@rursus8354]

The IAU definition mixes hard-to-determine observables (the cleanup-orbit criterion) with arbitrary chosen extremely hard-to-determine physics criteria (the hydrostatic equilibrium criterion) that computes differently in the inner solar system from the outer solar system. The IAU definition is awkward, but it indicates that the current confused 'planet' definition should be scrapped and replaced by multiple observational definitions, as well as multiple origin definitions. They could take some inspiration from IUPAC who have smart mechanisms for updating terminology and competence authority to keep chemists in line.

[@Jellyman1129]

The IAU definition is indeed a mess. If you moved the gas giants inward and the terrestrial planets outward, you’d suddenly only have four objects that qualify as planets (Jupiter, Saturn, Uranus, and Neptune). The terrestrial planets wouldn’t be planets anymore. It’s scientifically defenseless and idiotic. The geophysical planet definition that Alan Stern uses is far more consistent and makes way more sense. Plus, you can still use the subcategories we use today. Hot Jupiters, super-Earths, pulsar planets, satellite planets, rogue planets, and more can all be used as subcategories to differentiate planets from each other based on how they’re similar and how they’re different.

[@DxAxMxD]

I would definitely count some of the moons of Jupiter and Saturn to be planets. They have atmospheres and geology and everything!

[@SignoreGalilei]

They definitely behave a lot like the IAU planets in those ways!

[@Too-Odd]

They could have been planets, but they orbit a planet rather than the sun (Sol). I wonder if any of them were planets at one time but were captured by Jupiter and Saturn.

[@Jellyman1129]

They were called “satellite planets” for centuries, so it makes sense. Asteroids orbit asteroids, stars orbit stars, and galaxies orbit galaxies. Planets can orbit planets too!

[@minutemansam3122]

​@@Jellyman1129planets cannot orbit planets. They become natural satellites at that point.

[@Jellyman1129]

@@minutemansam3122They’re still planets. Just because they become satellites doesn’t mean they stop being planets.

[@lyght3043]

Yup so no rogue planets, no exo plants, no definition of how big the neighborhood is. Then you get to location, if you put Earth where Pluto is it becomes a dwarf planet, it's deeply flawed.

[@Jellyman1129]

It’s a big fat mess. The geophysical planet definition is superior in every way.

[@Alice_Sweicrowe]

Thank you so much.

[@JacobJohnsen-nu7nl]

Iau: planets must orbit the sun Exoplanets: 😔

[@Jellyman1129]

Rogue planets: 😭

[@johnfirth6541]

naming in science is a never ending thing. whether its living organisms (are viruses in this group or not - ongoing debate), non living objects like rick types, landforms, etc. There is never complete agreement because all definitions eventually have things discovered or already known that sit on the edge of definitions. We people have to categorize things to make sense of nature. Every solid (oops does that include mostly gaseous objects that might have solid cores?) object floating in space that isn't a star (oops, are brown dwarfs stars, planets, both or neither) could not practically be called a planet because (a) how would you teach that to students? with the asteroid belt, the Kuiper belt and the Oort cloud, our stars system would have a million planets to memorize and learn. Mike Brown began to worry that too many 😅objects named planets would be overwhelming, as he discovered more KBO's. But I do not think 40 planets is too many. How many species of birds are there? or breeds of dog? I think there is a problem with IAU def. because what does "clear its orbit" mean? Jupiter has leading and trailing "asteroids" that are IN its orbit. It hasn't cleared it. Venus has a couple of the same. Neptune not only affects the orbit if Pluto but Jupiter, Saturn and Uranus affect the orbit of Neptune, and Neptune affects Uranus' orbit. That is how Neptune was discovered. I like 'planetary bodies' as a category for all of these you covered. Based on other planetary systems discovered, a Neptune sized planetary object, or planet, could be a moon of a 5x Jupiter sized gas giant. I think Pluto-Charon should be considered a double planet system rather than a minor planet with a moon. I'm a geologist, so I've had to deal with classification terminology for a long time. And debates on classification never end.

[@SignoreGalilei]

Yeah, that's a pretty good explanation of the issue. It's not like we make kids memorize every dinosaur, as much as kids love dinosaurs.

[@Jellyman1129]

You’re right. Taxonomy can be difficult and there are some objects that are on the borderline. The platypus is not quite a mammal, but not quite an amphibian, even though it lays eggs. Brown dwarfs are almost stars, but not quite planets. It can be tricky, but that doesn’t eliminate the need for a classification system. That also doesn’t mean people should vote on these things. The IAU definition is hot garbage and is nowhere near an accurate representation of the planets in the Solar System. Mike Brown, despite discovering many objects in the Kuiper Belt and having admirable dedication to his career, is a fool. It’s baffling to think ANY scientist seriously thinks we need to limit the planets in our Solar System so school kids can memorize their names. That’s not a scientific rationale. The IAU created the “clearing the neighborhood” criterion specifically to exclude the planets beyond Neptune from qualifying as planets. That’s ridiculous. We already know of binary gas giants with no host star in the Orion Nebula. We know of hot jupiters, super earths, puffball planets, pulsar planets, hycean planets, and many more. A good definition of planet should encompass ALL of these objects, and that’s exactly what the geophysical planet definition does.

[@DQBlizzard_]

Thanks for the entertaining video

[@terrellscaife2411]

Fascinating

[@PlutoDaPlanet14]

Finally someone remembers me ❤

[@jaxmatthews2748]

I hope I'm not the only person in this comment section who still considers Pluto a planet.

[@SignoreGalilei]

You're definitely not the only one.

[@Jellyman1129]

There are a LOT of people who feel the same way. We’re with you. 🤜🏻🤛🏻

[@AndDiracisHisProphet]

That was a really really nice video

[@SignoreGalilei]

Thanks!

[@EdbertWeisly]

I've watch this video 7 times already 👍👍

[@SignoreGalilei]

Awesome!

[@Karlen-wd1xp]

I love planet moons they sound amazing

[@HipixOFFICIAL]

It's always been eleven to me! Eris and Ceres, along with Pluto!

[@Jellyman1129]

There’s many more as well!

[@danhaas9730]

Very cool!

[@SignoreGalilei]

Thanks!

[@izusoda]

alan stern is everywhere.

[@Maurice-Navel]

One day we'll wake up and discover that Earth is only a Dwarf Planet.

[@SignoreGalilei]

I think the IAU would switch to calling the bigger things "superplanets" or something first, haha. But logically, you're not wrong.

[@EdbertWeisly]

9:51 the main Protagonist of the video

[@SignoreGalilei]

Haha, yep

[@ChunkyHandCream]

Once common inter planetary travel is achieved, we'll find every single planet moon and dwarf planet in the solar system

[@ss16gokou]

Thank you Allan Stern for giving the Middle finger to those scientists who voted for Pluto to be demoted.

[@tjpprojects7192]

I heard somewhere that our currently defined "planets" is somewhat based on astrology. If that's true, then I suggest we need to change our current astology infected definition of planets. My personal definition would be the classic, "if it's big enough to collapse into a spherical shape, then it's a planet". Then we can add modifiers onto them. Like how Jupiter is a planet and ALSO a gas-giant. Pluto is a planet, but ALSO dwarf. Earth is a planet but ALSO terrestrial. The Moon is a planet but ALSO a moon. I don't get why astronomers can be so anal about making things mutually exclusive. Titan could be a planet that just so happrns to ALSO be a moon. Hell, Earth is ALREADY a moon of The Sun, so they've already broken their own rule. A planetary object doesn't magically become "just" a moon if it's simply orbiting another planet instead of a star. If we somehow ignited Jupiter into a brown dwarf, would Titan magically become a planet then? I CLEARLY know nothing about what I'm talking about, but the whole planet shit seems to just be astronomers being petty. "tHeSe TwO oBjEcTs ArE *EXACTLY* ThE sAmE, bUt ThIs OnE iS a PlAnEt, AnD tHe OtHeR oNe Is NoT!" To point out more how absurd "clearing its orbit" is, there would be a hypothetical point at which a Jupiter sized object WOULDN'T be considered a planet because it has too much material floating around in its orbit. By that logic, one could also MAKE a planet out of a dwarf ppanet by REMOVING material from the dwarf planet, and using that material to blast all debris in said dwarf planet's orbit, thus magically turning it into a planet despite it becoming SMALLER.

[@SignoreGalilei]

Your last situation is interesting. It's not likely to happen naturally, but a similar thing that might happen is a planet migrating to a different orbit, thus acquiring a new "neighborhood" that it wouldn't yet have cleared.

[@Jellyman1129]

Since the IAU is full of astronomers who don’t study planets, they’re not educated on the history of the word. Their definition was indeed based on astrology. You can learn more about that here: kzbin.info/www/bejne/qJXMZIiZYrGenJIfeature=shared The planetary experts (like Alan Stern, among others) all ignore the IAU’s planet definition and use the geophysical planet definition instead, which states any gravitationally rounded object in space that hasn’t undergone nuclear fusion is a planet. It’s more massive than an asteroid, but less massive than a star. So your intuition about a planet being a big spherical object, regardless of orbit, is correct. Alan Stern calls it “The Star Trek Test”. If the crew of the Starship Enterprise looks on the viewfinder and sees [insert spherical object here] (like Makemake or Titan or Callisto), everyone on board and everyone in the audience immediately knows “That’s a planet”. They don’t have to say “I’m not sure what that is. Standby, let me survey the entire solar system for all objects that are here. I’ll have to integrate their orbits and calculate the gravitational clearing parameters to THEN determine if that is a planet or not. I’ll get back to you in the morning.” It’s not that hard. All objects in space are defined ONLY by their intrinsic properties. Planets should be no different. Plus, when do we VOTE on things in science? That’s not a scientific concept and it violates the scientific method. Voting doesn’t change facts. The IAU definition was specifically created to limit the number of planets. That’s ridiculous! That’s like if we defined galaxies as being giant formations of stars, gas, and dust, but needing to be located within the Local Group and needing to be the dominant gravitational force. So there’s only eight galaxies. Anybody with a brain would say “What about satellite galaxies? What about galaxies in other galaxy clusters?” Because it only focuses on gravitational dominance, it ignores the intrinsic attributes of the object. So by the IAU’s planet definition, a black hole orbiting the Sun would qualify as a planet, but a rogue gas giant with no parent star wouldn’t qualify. Even though the rouge gas giant is OBJECTIVELY a planet and the black hole OBJECTIVELY is NOT. That’s a problem. In fact, image our Solar System consisting of ONLY Earths in the orbits of the classical nine planets. The Earths closer to the Sun would qualify as planets, but the Earths farther away wouldn’t qualify. Identical objects classifying differently SIMPLY because of where they’re located is a fallacy. “Satellite planet” has been a term used for centuries to describe large round moons. “Satellite asteroid” has been used to describe tiny moons like Phobos. Technically, Proxima Centauri is both a star AND a moon, a satellite star. The Small Magellanic Cloud is a satellite galaxy. I don’t know why astronomers think “planet” and “moon” are mutually exclusive terms when that’s not the case with any other object anywhere else in astronomy. It’s the astrologers that excluded moons from being planets because they wanted a small number of planets to fit into horoscopes. Planetary scientists like to classify planets by their composition and context within the system. Earth is a solar terrestrial planet. The Moon is a satellite terrestrial planet. Saturn is a solar giant planet. Pluto and Charon are binary dwarf planets. Triton WAS a solar dwarf planet, but is now a satellite dwarf planet. Poltergeist is an extrasolar terrestrial planet. PSO J318.5-22 is a rogue giant planet. By describing planets this way, people immediately understand what the object is, its composition, and its role in the system, in just three short words. Saying “Enceladus is a satellite dwarf planet” immediately tells you multiple fundamental properties of the object and condenses it in an eloquent and efficient way. It saves you the breath of saying “Enceladus is a tiny moon of Saturn that’s spherical and similar in composition to Pluto.” This entire debate is just non-expert astronomers being terrified of astronomical numbers of planets. How ironic! Sorry for the long rant, I’m very passionate about this topic. Don’t sell yourself short, you definitely know your stuff. Very well said! 👍🏻👍🏻

[@magical_catgirl]

I ignore what the IAU says about the planet topic. Less then 2% of IAU members voted for that definition.

[@SignoreGalilei]

Yeah, it was controversial even among astronomers. It's not like the other 98% voted against though, most just weren't at the vote.

[@Jellyman1129]

It baffles me when textbooks adopt the IAU planet definition and teach it in schools like it’s fact when it’s the opinion of a handful of non-experts at one meeting in one room in one country at one time. It’s minority opinion that should be regarded as nonsense.

[@Jellyman1129]

@@SignoreGalileiThey weren’t at the vote because they didn’t know a vote was happening. It shouldn’t have happened, it violated the union’s own bylaws. Most of the astronomers had no notice and therefore were unable to vote, leading to a bad sample size and biased results.

[@jovetj]

9:58 Cute musical cue a the end. 🙂

[@StevenGesolgon]

Awesome

[@tigronya3354]

i've been into this idea for a long time, so i'm glad to see a video on it. I especially think that Ganymede and Titan should be considered planets because they're the size of mercury and have definitively planetary features all around

[@Jellyman1129]

Large moons are underrated. Titan is more of a planet than Mercury will ever be.

[@nikkacostavicedo3034]

Yrs6w6😊

[@zackakai5173]

But they're in orbit around other bodies, not in orbit around the sun directly. Thus, despite their size, the classification of being moons or satellites rather than planets in their own right.

[@Jellyman1129]

@@zackakai5173 So what? Objects can have two classifications at once. Dactyl is both an asteroid and a satellite. It doesn’t cease to be an asteroid just because it’s orbiting a more massive asteroid. Sirius B is both a star and a satellite. It doesn’t cease to be a star just because it’s orbiting a more massive star. The Small Magellanic Cloud is both a galaxy and a satellite. It doesn’t cease to be a galaxy just because it’s orbiting a more massive galaxy. Being consistent with this logic, large spherical moons are both planets and satellites. They don’t cease to be planets just because they’re orbiting more massive planets.

[@zackakai5173]

@@Jellyman1129 "so what" is that the term "planet" is widely understood to apply to the major objects directly in orbit around any given star. As opposed to "moon" or "satellite," which are generally understood to apply to objects in orbit around other objects (like planets) which are in orbit around a star. I can't think of a single good reason to start classifying moons as planets in their own right.

[@eldrago19]

I like this definition better. Increasingly what we are studying in our solar system is planetary science not astronomy and it seems reasonable to let planetary scientists define planets.

[@SignoreGalilei]

That's a good point. I'm a bit surprised the IAU has defined planets and not exoplanets.

[@Jellyman1129]

I absolutely agree! The IAU overstepped their boundaries by defining an object they don’t study. I know for a fact nobody would accept a galaxy definition from a planetary scientist, so the same should be true the other way around. The Solar System has dozens of planets, and most of them are like Pluto! That’s exciting! 💫

[@v1e1r1g1e1]

Thumbs up for finishing with Holst's Jupiter movement!!

[@SignoreGalilei]

Thanks! It felt appropriate.

[@JustAcrylicArts]

i love the inclusion of holsts “jupiter” at the end!

[@SignoreGalilei]

Thanks!

[@HomewaveA]

Imagine if titan broke its orbit and self centered it’s own orbit would it be classified as a planet?

[@Jellyman1129]

Yes, of course it would! It doesn’t matter where an object is in space. If it’s a gravitationally rounded sub-stellar celestial object, it’s a planet. Titan absolutely qualifies.

[@minutemansam3122]

​@@Jellyman1129unfortunately it's not a planet.

[@Jellyman1129]

@@minutemansam3122 Says you. 🙄

[@squishybrain]

I still hold that the Pluto/Charon system is a double planet since they're spherical and both orbit outside their barycenter.

[@SignoreGalilei]

A lot of astronomers would agree.

[@Jellyman1129]

That’s right. They’re currently the Solar System’s only binary planet.

[@xanderunderwoods3363]

I completely agree with this video

[@SignoreGalilei]

Awesome!

[@TheJuiceDidNineEleven]

I love how you pronounce Uranus correctly

[@hamelconsultancyllc]

Pour one out for my boy Allan

[@SignoreGalilei]

Nothing but respect for Alan Stern

[@j.l.glover4037]

Pluto is a planet🗣️

[@AlastorTheNPDemon]

I like the ancient criteria for a planet.

[@thescooshinator]

Weird thing is, Saturn technically has three tiny moons that are also round. But they are always excluded from the planet list due to how tiny they are, despite fitting both requirements. They are called Pallene(4.46km), Methone(2.9km) and Aegeaon(0.66km). Aegeaon is actually the smallest moon around any planet (not including moonlets). They are theorized to be made of dust, which is why they can pull themselves into a sphere, despite being so tiny.

[@adriennegormley9358]

Back i. The late 60s, when i took adtronomy in college, one of the books our prof recced to us talked about the earth- moon system actually being a double planet, simply because of how large the moon is in relation to earth.

[@SignoreGalilei]

The Earth-Moon system is second only to maybe Pluto-Charon for being a double planet. You could see Earth's Moon as a separate object from the surface of Mars.

[@minerman60101]

A reasonable differentiation is whether the two spin around a point inside one of the bodies or not. Earth-Moon's barycenter is inside the Earth, while Pluto-Charon's is outside of but closest to Pluto

[@Too-Odd]

Little Luna gives Earth tides and helps plate tectonics. Without a moon, Earth could look a lot like Venus.

[@samshort365]

I wouldn't be at all surprised if one day we find an "Earth-like" moon circling a gas giant at just the right distance from its star. The seasons on that world would be dictated by a combination of relative orbital position and orbital tilt, if any, and need not be restricted to our 4 seasons, their duration or their sequence.

[@brantley-xc5np]

0:48 every time i see this photo all i imagine is Jupiter having a 1 on 1 standoff with the sun

[@eeriekekashi419]

I think it’s pretty cool for moons to be full on planets.

[@SignoreGalilei]

I think so too!

[@Jellyman1129]

That’s my favorite part about Stern’s definition! It helps us to correctly view Ganymede and Titan and Europa as equally valuable worlds, not insignificant satellites. They were called “satellite planets” for centuries. Asteroids orbit asteroids, stars orbit stars, and galaxies orbit galaxies. Planets can orbit planets too.

[@danwylie-sears1134]

I would rather have a classification based on how the objects formed. Planets are (or would be, if I had been the one to define the word) things that started out as protoplanets and then grew. All spherical moons in the solar system probably are "planets" in this sense, except Earth's moon. It coalesced from the wreckage of a collision between Earth and a Mars-sized planet.

[@SignoreGalilei]

That would make sense. It's sometimes hard to know the formation history of something you just discovered, though.

[@danwylie-sears1134]

@@SignoreGalilei True. You can assign objects to categories provisionally, and you can have the categories themselves be provisional. Or you can use a set of categories that's not the one I would prefer.

[@Jellyman1129]

Defining celestial objects by formation is likely unstable. New formation theories pop up all the time. Instead of defining them by circumstantial ideas, we should define them by objective data (the characteristics of the object in the present moment).

[@minutemansam3122]

​@@Jellyman1129you mean like the objective data of dominating it's orbit?

[@Jellyman1129]

@@minutemansam3122 That “data” you speak of is a criterion for defining a planet that was specifically engineered to limit the number of planets in our solar system so school kids can memorize them. It’s a criterion that isn’t even used when studying planets in planetary science.