In today’s piece, although we are going look at the dwarf planet Pluto, this serves mainly as a background to a discussion of the naming conventions around what planets, dwarf planets and asteroids and comets, also known as minor planets. There are different usages of the word planet, but I wish to be upfront here in the written piece with regard the official divisions. According to the International Astronomical Union, as of 2006, dwarf planets are a separate category of object from planet, they are not a sub-category of planet. That is to say, dwarf planets are not planets, no more so than minor planets, asteroids and comets, are. Unfortunately, each of these labels contains the word planet, which leads to some understandable confusion.
This confusion may be compounded by the one objects that has changed definition, which is Pluto. Pluto is so far the only object who’s definition has truly changed. Ceres is given the dual definition of being both a dwarf planet and an asteroid, and was considered a planet before the category of asteroid was conceived. Pluto on the other hand was considered a planet after the category of asteroid was created, but was subsequently reclassified as a dwarf planet. There is no real alternative, although Pluto is also considered a Trans-Neptunian Object, it was also one of these when is was a planet, as it was still beyond Neptune. It is not officially considered to be a planet and a dwarf planet at the same time. There is a problem with these terms, Officially, the word planet means a body which orbits the Sun, is large enough to become round under its own gravity, or reach hydrostatic equilibrium, and also is in control of its orbit. Pluto is big enough to be round, it has achieved hydrostatic equilibrium and it orbits the Sun (which isn’t really a good part of the definition for reasons I will get to in a moment). However, Pluto isn’t really in charge of its orbit, although some objects do orbit Pluto, Pluto also shares its orbit with objects that do not orbit Pluto. Indeed, the orbit of Pluto crosses the orbits of other objects which are, in the case of Neptune, considered planets.
Being in control of the other objects inn your orbit isn’t quite the same as saying that any object that is considered a full planet needs to have all the other objects that orbit the Sun at the same distance to then orbit it. That would be a very cumbersome definition and it would exclude things like the Trojan asteroids that occupy the Lagrange point of Jupiter. Although those objects do not orbit Jupiter, it is the balance between Jupiter’s gravity and the Sun’s that allow them to remain in that location in space.
Taking a look at Pluto, we’re currently seeing it at 35 AU from the Sun. Pluto isn’t at opposition here, it’s close to 34 AU from us, almost an AU less, meaning it’s almost at opposition but not quite there. By following Pluto through the sky as we move around the Sun, we will see some changes in Pluto’s distance to the Sun as well as its distance to us. As we pass the point where Pluto would form a triangle with the Earth, we’ll see it’d distance to us and the Sun as almost the same, about 35 AU at the moment. We can keep pushing forward until we see the distance to the Earth go up to about 36 AU, at which point it will be pretty much directly behind the Sun with respect to the Earth. This principle is true for any outer planet, as shown last time with Neptune and Saturn. However, we’re also seeing that Pluto’s distance from the Sun varies quite a bit over the course of its orbit. This is because Pluto’s orbit is quite elliptical. All of the planets have elliptical orbits, but for the vast majority of the planets the difference between their perihelion and aphelion, is slight. For Mercury, the most eccentric planet, the difference is about 0.1 AU, whereas with Pluto the difference is 20 AU. With Pluto’s aphelion at 49 AU and perihelion at just 29 AU, Pluto’s eccentricity is such that Pluto’s closest position to the Sun is closer than Neptune’s closest position to the Sun, but it’s furthest is also significantly further away. Thanks to Pluto’s incredibly long year, 248 earth years, it takes a long time for the full variation between Pluto’s perihelion and aphelion to elapse. even over the course of a few years however, we can see that the difference in distance to the Sun just continues to change.
It is an incredibly elliptical orbit, but that on its own isn’t enough to disqualify Pluto as a planet. Planets can have elliptical orbits, and we’ve discovered planets orbiting other stars that are unequivocally, undeniably planets, but whose eccentricity is quite great. Some of these exoplanets are as big as Jupiter, including some that orbit quite close to their Sun, and can have highly eccentric orbits as well, as eccentric or even more eccentric than Pluto’s. Continuing to move through a few years, from 2024 all the way up to 2028, we will see that the distance to the Sun will tick over and go all the way up to 36 AU. That’s a difference of a full AU in just 4 and a bit years, not even covering the entirety of Pluto’s orbit. The full difference is even more extreme, but takes about 124 years to see, from aphelion to perihelion, and another 124 years to go back around and complete one orbit.
More importantly, Pluto is sharing its orbit with other Trans-Neptunian Objects, including other plutinos that share a similar resonance with Neptune. These objects orbit the Sun at the same distance as Pluto, but completely independent gravitationally. Of course, all of Pluto’s moons are orbiting Pluto, but even some objects categorized as asteroids have smaller objects orbiting them like moons. This is the reason we call Pluto a dwarf planet, but I think it would almost be better if there was a different word for these objects that didn’t contain the word planet.
Asteroids and comets are often called minor planets and the asteroid tracking and detection groups are often called minor planet groups, the most famous being the Minor Planet Center. The Minor Planet Center Database takes care of tracking asteroids and comets and meteors. This means we could call everything that orbits the Sun directly, without orbiting something else in between, a planet. This can then be broken down by object, it will be either a minor planet like an asteroid or comet (which are very similar objects except for their eccentricity), a full planet or major planet, whereby it reaches hydrostatic equilibrium and is in control of everything that orbits at the same distance as the Sun, and then we have dwarf planets. These are in between, almost like planets, they orbit the Sun, just like minor planets and major planets, they have hydrostatic equilibrium just like major planets, but they’re not in control of their orbit gravitationally, like a minor planet, Dwarf planets are then be in between minor planets and major planets, but we would get a whole different name, not planet, for objects that do not orbit stars. Right now rogue planets are objects that would be planets if they were orbiting stars, but they don’t. They probably did, rogue planets almost certainly formed around a star.
There are also other extrasolar objects like `Oumuamua. I was delighted to find that if that `Oumuamua is in the Stellarium database of objects, and I didn’t even have to type an `okina to find it. What I have typed here, before the O of `Oumuamua and `okina, is not actually and `okina, but a backtick. An `okina properly is its own letter, and although it can look like an apostrophe, it is not. Instead, it is the Hawai’ian for a glottal stop, or the Hawai’ian grapheme for a glottal stop. These things get very awkward, as most fonts don’t include an actual `okina. In the same way that the letter “h” is called “haitch” or “aitch” in English, both for the sound and for the written letter, and `okina is how you write a glottal stop and the name for the sound. This means that the name of this object is not Oumuamua, it’s `Oumuamua. I find it hard to pronounce a glottal stop deliberately, as in most English dialects every word that starts with a vowel really starts with a glottal stop. There is no contrast between having a glottal stop at the start or not, so it is unwritten. This is similar to the sound “ng” that English has, but only at the end of words. This can make it hard for English speakers to pronounce words that begin with these sounds, such as Nguyen, or Nguyễn.
According to the Stellarium data, which may be out of date, it is currently 42 AU from the Sun and 41 AU from the Earth. This is the object that passed through our solar system a couple of years ago, but did not actually orbit our Sun. It entered our solar system, passed through, and is currently on its way out again. It was a visitor, it is not a member of our solar system, it’s an interstellar object. Based on its size and shape, it would have been a minor planet if it was orbiting a star, like an asteroid or a comet. However, it is not orbiting a star, and so it is called an interstellar object instead. This is in contrast to rogue planets, who also do not orbit stars, but are still called planets.
This leads us to different usages of the word planet. A planet for most people means one of the big things that orbits the Sun, and dwarf planets are usually seen as a subtype of that, there’s planets including dwarf planets. On the other hand we could say a planet is something with orbits the Sun ,and we have minor planets, asteroids and comets, we have major planets, like Jupiter and Saturn and the Earth, and then we have dwarf planets like Pluto. If we see that the word planet as really just meaning something that goes around the Sun, then its easy to see that the major planets and minor planet all different categories of planet, and that everything that orbits the Sun is a planet. Unfortunately, most people don’t call minor planets minor planets, but say asteroid ort comet instead. This leads to the usage of planet, whereby planet is major thing that orbits the Sun, asteroids and comets aren’t, and then dwarf planets are just a type of planet. This feels incorrect, because dwarf planet, planets, and minor planets, they all occupy their own position, their own class of object orbiting the Sun. Most importantly, as distinct classes from each other, there are minor planets, dwarf planets and major planets. I think that if we’re going to say that planet is a major object orbiting the Sun and then we have asteroids and comets for the smallest objects, these objects like Pluto, that are not a planets, that are not comets, that are not asteroids, they deserve their own word. Currently, we use dwarf planet, but that, I feel, leads to people conflating dwarf planets and planets, in a way we usually do not do with planets and minor planets.
Pluto of course is the most famous dwarf planet. It was the first object to be considered a dwarf planet having been considered a planet when it was first discovered. Pluto does deserve its own special position, in history at least, but it is just one of a class of similar objects. The Earth gets a special position as a planet because it’s our one, but it is just a planet among other planets. Halley’s Comet is probably the most famous comet but it is just a comet among a class of comets, we would never call it a dwarf planet just because it is famous. Then we have Ceres, the Queen of the Asteroid Belt, which certainly a special thing as well, having been considered a planet, asteroid and now dwarf planet over the course of its history. I think that Pluto, calling it a dwarf planet is fine as long as we remember that being a dwarf planet by definition makes you distinct from a planet. I also think that Pluto, being named after a god, the god of the underworld rather than the god of death, Pluto should be the King of the Dwarf Planets. As the first one declared to be a dwarf planet, even if we discover other bigger ones, the same way that Ceres is the Queen of the Asteroid Belt, Pluto should be the King, King of the Dwarf Planets. Or the King of the Trans-Neptunian Objects at least, maybe we’ll find another monarch or leader for the more distant, scattered objects, the Oort Cloud objects, we do still have a lot to discover out there. Perhaps Sedna could be President of the Scattered Disk Objects, who knows.
All in all, that’s only a little bit about Pluto, a little bit about its eccentricity, and more on the differences in definition and how these definitions, how the word planet, can mean different things in different contexts and when it’s associated with these other modifiers. I will almost certainly make another video speaking more about Pluto itself and its composition. I do hope you enjoyed this article, and feel free to let me know in the comments what your opinions are. Although there is an official definition of these objects, some of what I have said here is my opinion, I believe that the current system can be a little bit confusing and you can let me know what you think in the comments here or on the video itself. If you enjoyed this article you can like it and if you like this kind of content, you can help ensure it continues and grows by subscribing to this website and my YouTube channel. Hopefully, I’ll see you back here for the next piece.
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