Do we need a new planet definition?

I should mention before I start this video that I usually try to avoid planet definition debates at all costs. The biggest reason I do this is because I simply don't think it matters. The universe is under no obligation to conform to our rigid categories. Even something as seemingly simple as counting the days on earth is complicated due to small imperfections, and now we have a system where we add an extra day to the year every four years, except for roughly once a century when we don't, and we also add a leap second on a basically This is true for time, and it will be true for physical objects as well. Ancient people invented the word planet when they still thought planets were just points of light that moved funny, and we've been using that word ever since out of convenience. And as more objects were discovered, it became clear that calling everything a planet was inconvenient, so new terms were created, including asteroid, and more controversially, dwarf planet. The planet definition we have today is a definition born of convenience more than anything else. A line had to be drawn somewhere because people didn't want to memorize the names of a hundred planets. I'll get to that point later, but the point here is that line was chosen to fall before Pluto, and there are a lot of good reasons many people have brought up for why this should be changed. And this isn't talked about nearly as much, but when the decision was made to downgrade Pluto to a dwarf planet, a bunch of other objects were also affected. Ceres was upgraded from asteroid to dwarf planet, for example. Eris, which is more massive than Pluto, was also very briefly considered by NASA to be the 10th planet, but it was also classified as dwarf planet. The current planet definition affected a lot more than just Pluto, and that's not even counting moons. Some moons act more like planets than actual planets. As I'll get into soon, you can interpret the current planetary definition to classify Mercury as a dwarf planet, but if Ganymede wasn't orbiting Jupiter, it would meet the criteria. The current planet definition is messy, and everyone seems to have a problem with it. Again, I personally don't think this matters that much as I care about the physical properties of an object rather than what category of object it is. But my opinions on it aside, this is a topic that's constantly being argued over in pretty much every online astronomy space, some arguments more annoying than others, and a lot of people do seem to care about it. So I might as well weigh in. I usually avoid this topic, but my personal ideal planet definition is very different from the include 36 planets and counting in the solar system. First, we'll go over the current definition of planet and some common problems with it, before proposing some alternative replacements. If you aren't currently aware, the most widely used definition of what a planet is, the one used by the International Astronomical Union, is that a planet must orbit the sun, must be round, must not be a star, and must have cleared its orbital neighborhood of other large objects. This has been the most popular planet definition that has been in use for the last 20 years, but many of these criteria are ambiguous. So before we get into criticizing this definition, let's try to get a better idea of what exactly it's saying. Must orbit the sun is obvious. To be a planet, it can't orbit another planet. Planets orbiting stars that aren't the sun are called exoplanets, and planets that don't orbit anything are called rogue planets. Must not be a star is also pretty clear. It can't be doing any kind of nuclear fusion in its core. Main sequence stars do hydrogen fusion, and any objects that do fusion that aren't hydrogen are either non-main sequence stars like red giants or brown dwarfs. Objects that used to be stars but don't do nuclear fusion, like white dwarfs, neutron stars, and black holes, are stellar remnants rather than planets. Must be round is the first category where we run into problems. To be more specific, the IAU says that a planet must be in hydrostatic equilibrium. This is just a fancy way of saying it has to be large enough to form a roughly spherical shape under its own gravity. However, there's an ambiguity in this. Some materials are easier to put into hydrostatic equilibrium than others. Let's look at two objects that will be important for the rest of this video, the asteroid Vesta and the moon Enceladus. Vesta is the second largest asteroid in the asteroid belt, and notably not in hydrostatic equilibrium. Vesta is both larger in radius and more massive than Saturn's moon Enceladus. But as you can see, Enceladus is rounded under its own gravity. So is Saturn's moon Mimas, which is even smaller. This is because icy material is softer and more bendable than rock, meaning less massive objects can reach hydrostatic equilibrium easier. So if Enceladus and Mimas were orbiting the sun instead of Saturn, they would be called dwarf planets despite being smaller than Vesta and asteroid. There's also another problem though. Not all rounded objects are in hydrostatic equilibrium. For example, Mercury is not. It's round because in the distant past when it was still forming, Mercury was in equilibrium, but as it cooled down and the rock solidified, it no longer is. The definition of what being in hydrostatic equilibrium actually means is pretty ambiguous, and you could really interpret it in two ways. If you assume the definition means that the object just needs to have been in hydrostatic equilibrium at some point in its life, or that it just has to look round, then you can include Mercury. However, if you assume the definition literally means that an object has to be in hydrostatic equilibrium right now, then Mercury is an asteroid. And things are more complicated when you add in the criteria that, to be a planet, an object must be massive enough to clear its neighborhood. Everyone seems to have a different idea on what this actually means. The IAU did not include any specific numbers when creating this definition. However, this is really the first criteria where we can see a good strong dividing line between planets and dwarf planets. All eight solar system planets have cleared their orbits by orders of magnitude more than any of the dwarf planets. Essentially, the reason Ceres is a dwarf planet is because its orbit is inside the asteroid belt. The entire mass of the asteroid belt combined is larger than Ceres, meaning it's not a planet. Pluto is not only inside the Kuiper belt, but its orbit crosses paths with Neptune, an object that's nearly 8,000 times more massive than Pluto itself. Because Pluto is not large enough to clear Neptune out of its orbit, it's not a planet. However, under this definition, Neptune is a planet because Pluto is 8,000 times smaller than it, meaning it's not large enough in comparison to Neptune to compete with it for gravitational dominance. This is also why Jupiter, despite having thousands of asteroids crossing its orbit all the time, is still a planet. Jupiter is so much larger than all of its Trojan asteroids that they're insignificant. If Jupiter crossed orbits with Saturn, this would be another problem, but because it doesn't, it's a planet. However, the gravity of the planet in question isn't the only factor you need to consider when talking about whether or not an object has cleared its orbit. If you put an object the size of Mars in the Oort Cloud, it would not be large enough to clear its orbit, because the Oort Cloud is so spread out and objects inside it almost never come close enough to one another to gravitationally interact. Essentially, if you leave Mars where it is right now, it's a planet, but if you move it to the Oort Cloud and change nothing else, it becomes a dwarf planet. Similarly, Mercury's orbit is only clear because it's close to the Sun. Like Mars, move Mercury further away from the Sun, and it becomes a dwarf planet. Clearly, this definition, while used for simplicity, falls apart under a bit of scrutiny. Distance from the Sun should not determine whether or not an object is a planet or not, at least in my opinion. Mars should be a planet whether it's in the Oort Cloud or not. In fact, this may seem a bit controversial, but I don't think an object's orbit should matter when determining whether it's a planet or not at all, because there are objects in the Solar System that change categories just depending on where you put them. Again, you can change nothing about Mars except its distance from the Sun, and it suddenly goes from planet to dwarf planet. Similarly, moons like Ganymede are only moons instead of planets because they orbit other planets. Personally, I'm not a big fan of the current planet definition. I use it as a default because it's the one most people are familiar with, and again, whether or not something is called a dwarf planet or not just doesn't really matter to me, but if I were in charge of the IAU, I would want to replace this current definition with a new one. Specifically, there's one proposal I like a lot, the geophysical planet definition. Under these criteria, whether or not an object is a planet is determined solely by its physical properties, not its orbital ones. I like definitions like this because I think it gives a much stronger reason to even have the category of planet at all, because as you've hopefully seen, based on the current definitions, objects can literally just switch categories based on where they are in relation to the Sun. Under geophysical definitions, a planet is a planet no matter where it is. However, there is a caveat with geophysical definitions. Under the one I'm about to explain, the total number of planets in the solar system would jump from 8 to 36, with the potential for further expansion. Personally, I don't think this is a problem at all, and I'll explain why once I explain the definition itself. Under the geophysical definition of a planet, there are only two criteria an object must meet to be a planet. It must have never, at any point in its history, done any kind of nuclear fusion in its core. This excludes stars and brown dwarfs, as well as stellar remnants like white dwarfs, which used to be stars doing fusion before they ran out of fuel. And two, it must be massive enough to have a spherical shape due to its own gravity in less time than the age of the universe. This means that, to be a planet, an object must have reached hydrostatic equilibrium at some point in its life. Whether or not it stayed in it is not important. The real factor here is if it stayed in it for long enough to have a rounded shape in the present day. This solves all the problems with the previous definition relating to hydrostatic equilibrium. Mercury is a planet because, even though it may not be now, it is mass enough to have been in Hg at least some point in its life. This does not, however, resolve the odd situation where, under this definition, there would be some asteroids larger than planets. We'll get to this in a minute. Under this new definition, there would be 36 currently known planets in the solar system. They're all planets not because of where they are, but what their properties are. The full list of all geophysical planets in the solar system that meet those two criteria, in order from the sun, is as follows. Mercury, Venus, Earth, the Moon, Mars, Ceres, Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Uranus, Miranda, Ariel, Umbriel, Titania, Oberon, Neptune, Triton, Pluto, Charon, Orcus, Haumea, Quaoar, Makemake, Gongong, Eris, and Sedna. This is every currently known object in the solar system that meet both the criteria to be classified as a geophysical planet. There are also likely even more because we haven't discovered every large object past the Kuiper Belt yet, and there are currently many objects on the edge we haven't confirmed whether or not they have rounded shapes. So the true number of planets is almost certainly larger than 36, these are just the ones we currently know. I enjoy this definition because it shows the true diversity of worlds in the solar system. There are interesting objects all over the place that are often skipped over because many people assume they're just moons or just dwarf planets, and so are uninteresting. However, there are two main problems people bring up opposing the geophysical planet definition that I'd like to address. Size, and the fact that having this many planets would be hard to memorize for people learning about the solar system. As I've already mentioned, under this definition, there would be asteroids in the solar system larger than planets. Mimas, which would be the solar system's new smallest planet, would be smaller than not only the asteroid Vesta, but the asteroids Pallas and Hygiea, the second largest moon of Neptune Proteus, and several other trans-Neptunian objects. Some people have a problem with the idea that asteroids could be bigger than entire planets, but honestly, I don't. There's no reason that planets have to be large, they just have to be large enough to have rounded shapes. The idea that planets have to be bigger than everything else is a byproduct of the old IAU definition, and not necessary under the geophysical one. I don't see anything wrong with there being a few asteroids larger than geophysical planets, I actually think it's pretty interesting. The idea that the biggest objects in the solar system are the most interesting or most noteworthy isn't necessarily true. Under this new definition, planet is a geologic term, and a planet can be any size as long as it's round. Ultimately, I don't care that there would be asteroids bigger than some planets, and I don't really think it matters when you're operating under the assumption that planet is a geophysical term rather than a definition based on physical size. If you really want to have a separate category for the largest planets, calling them major planets works just fine. This ties into the second counter argument. We shouldn't have this many planets because it's hard to memorize them all. Kids in school would have to memorize like 50 planets instead of just 8, and that's bad. But I have a question regarding this point. Why exactly are we making kids memorize the solar system in the first place? It's been proven time and time again that simple memorization isn't really an effective way of making learning actually interesting. I agree that we should not have kids memorize all 36 planets in the solar system, but I believe that's a problem inherent to the American education system rather than any problems with the geophysical planet definition. There's an analogy about this I really like. I think the solar system should be taught like the periodic table. Nobody expects you to memorize every element on the periodic table. However, you are expected to know broad strokes about what specific elements do, how they interact, and what sets them apart from others. The solar system should be taught like this. You don't need to know the names of 36 geophysical solar system planets, but you should know roughly how the solar system is structured, which planets are where, and how they're different from all the others. You should know that planets get icy the further out you go, and the difference between icy, rocky, and gas giant planets. But you shouldn't have to memorize the name of every planet, as that's simply an ineffective way to teach the solar system. The periodic table is already a great model of how complicated systems should be taught, and it should be applied to astronomy as well as chemistry. You can also subdivide the geophysical planet into multiple categories. As I've already mentioned, you can use the currently in use IAU definition to separate major planets from everything else. Similarly, you can categorize any planet orbiting another planet as a satellite planet for example. You can use the term binary planets for two planets that orbit one another, like Pluto and Charon. We already do this with the planets we have. Rocky planets, ice giants, and gas giants are all three different categories for different types of planets. Just like the words noble gases, halogens, and radioactive elements are all different categories for types of atoms. If you're worried about teaching this new model of the solar system to people, just divide it up into manageable tasks, as that's how other concepts in science are already taught. Some people also have a problem with moons being planets, but again, I don't see why. Ganymede and Titan would be planets if you moved them away from Jupiter and Saturn. The only reason they aren't is because they don't orbit the sun. Titan has an atmosphere, geologic activity, and seas of liquid methane. It's a geologic environment just as active and interesting as Earth or Mars. Why shouldn't it be included in the same category as them? Because there happens to be a larger object nearby? I just don't think that makes sense. Some of the most interesting worlds in the solar systems are moons. Io is the most volcanically active object in the solar system, and Triton also has a thin atmosphere thick enough to form clouds. But they're often ignored when teaching about the solar system simply because they orbit other planets. Which is another reason why I think these objects should be planets. It truly shows the solar system for what it's actually like, a complicated, dynamic, and ever-evolving system made up of hundreds of worlds, not just 8 and sometimes 9. As I've said on this channel many times before, names hold power. It's one of the reasons I always use the official proper name of an exoplanet if it has one. The word planet is already ingrained in popular culture as an interesting alien world. An object with the title of planet is immediately seen as more important to the average person, so we should call every object that meets the geophysical requirements of a planet a planet no matter where it is in the solar system. I simply think it's a better way to communicate the solar system to the average person. Just like how I know from personal experience that people are more likely to remember information about the planet Dimidium rather than the planet 51 Pegasi b. Names tell the average person how seriously you should take something, and whether it be the actual name of an object like Dimidium or the name of a category like planet, what you call something has importance. There are also a couple alternate versions of the geophysical planet definition that expand the list even further. I've seen some people say an object should be a planet if it has ever, at any point in its history, had some kind of geologic activity. This would potentially include Vesta on the list of planets, as there is some evidence of it having ancient, long-dead volcanism. However, I think this is kind of ambiguous and wouldn't personally use it, as you can stretch this definition to both say that gas giants aren't planets, and introduce hundreds of comets and asteroids into the category of planet. I do think the best definitions of planet have to be clear, with no room for objects being able to switch sides at random. Which is yet another reason why I like the geophysical definition, as it has very little wiggle room and has some very good hard lines on what is and isn't a planet. That's something the original IAU definition doesn't have, as I've explained many times already. I also like how it can easily be applied to exoplanets. Under the current IAU definition, it can be very hard to determine what exoplanets are planets, and which ones aren't. Exoplanets could be orbiting inside asteroid belts we haven't discovered, have intersecting orbits, which we may have a few examples of, and even have things like Trojan planets sharing the same orbit as them. Under the IAU definition, many exoplanets we know of, some many times larger than Jupiter, would be dwarf planets, but under the geophysical definition, they'd all just be planets. So would all rogue planets for that matter, because the IAU is pretty clear in saying that a planet has to orbit a star, which is yet another thing I disagree with. As you've probably gathered by now, I really don't think a planet's orbital parameters should determine its status as a planet. I also don't care about size, and very small and very large objects can all be planets, if they meet the criteria. The geophysical definition is my personal favorite definition of planet if I had to pick one, but there is another one worth talking about. As I've mentioned, one thing I don't like about the IAU definition is that to be a planet, an object has to orbit the sun. This not only excludes every planet outside the solar system, but also every rogue planet and every large moon. Though to be fair to the IAU, they do have a separate definition for what makes an exoplanet, but we won't be focusing on it for now. Under an alternate definition, planetary objects are divided into two categories, planet and planemo. The term planemo comes from a way of saying PMO, which stands for planetary mass object. The definition is basically just the geophysical one with some added fun names. Any object that is large enough to become round under its own gravity is called a planemo. This means that all rogue planets, exoplanets, and major moons, in addition to the classic planets, are all planemos. However, a planet is specifically a planemo that orbits a star. Using this definition, the number of planets would drop from 36 to 17, which I guess is more manageable, but does run into the same problem of treating moons as less important. It's also worth mentioning that the same people who made up the word planemo will also use the word fuzor to describe any object capable of doing core fusion, covering both stars and brown dwarfs. Using that definition, a planet is any object that orbits a fuzor. Personally, I do like the word planemo, and using it instead of planet could satisfy some of the people who only want 8 planets in the solar system. But again, I don't agree with the orbital constraints. A rogue planet captured by a star can switch categories from planemo to planet, for example. We also know of some planets orbiting stellar remnants like neutron stars that used to be stars, but their mass was stripped off until they became planets. Those supposedly run into that problem with the geophysical definition as well. Take the planet PSR J1719-1438b, a roughly Jupiter-sized object that seems to be the remnant of the core of a sun-like star, which I have a full video about. Under both of these definitions, this Jupiter-massed object would not be a planet because it has done nuclear fusion at some point in its life, though both the terms planemo and stellar remnant both work just fine. Ultimately, I simply think if we were to revise the definition for what makes a planet, I think it should be based solely on an object's physical characteristics rather than its orbit. If you take anything away from this video, it should be that. While there are many alternate proposed definitions and many ways to subdivide the geophysical one to maintain 8 major planets, I genuinely do think a system with 36 planets and counting is the best one we have. Whether you want to call those objects planets or planemos is up to you. Personally, I'd rather just have one term for planetary mass objects, and everyone's already familiar with the word planet, so I'd just use that one. But in the end, the universe is under no obligation to conform to our criteria, and every definition is going to have an odd outlier somewhere that messes it up. But even something as simple as a definition can change people's perspective on the structure and diversity of the solar system, and if I was in charge, I would definitely want to move away from the old system of 8 planets. I don't think we need a new definition, but we could definitely use one. And hey, if enough people like the geophysical definition, or one of its many alternatives, maybe one day it will be officially adopted as the new definition for a planet. I guess we'll just have to wait and see. Thank you for watching. If you enjoyed, check out my other videos about planets and space exploration.