Why is Pluto not a planet?

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Short answer: Pluto has only two of the three necessary characteristics to be called a planet. Pluto has not cleared its neighborhood, or orbit. But, of course, there are additional details.

The simplest reason that Pluto is not a planet is that planet experts say so, and this is their job. But you may be looking for a more detailed explanation.

Let’s look at what defines a planet. This could be a very long and tedious discussion, because “planet” is an ancient concept used long before scientists knew very much about them. Also, frankly, in many areas of science the definition of a thing, perhaps counter-intuitively to non-scientists, is often pretty irrelevant to its study. Definitions that change over time that are never quite in line with the phenomenon being observed, etc. may seem like an impediment to science, but they often are not. The definition of a “gene” has changed dramatically as we’ve learned more about them, but this shifting description has not hampered genetic research. To some extent this may be the same with planets. A “planetologist” who studied Pluto back when it was still counted as a planet would not have to find a new job when our solar system went from 9 to 8.

The International Astronomical Union has settled on a set of definitions of solar system bodies, which includes planets, dwarf planets (which are mostly minor planets), small solar system bodies, trans-Neptunian objects which also might be called plutoids (those are also minor planets) and some small solar system bodies (including some comets) and satellites, and satellites are, of course, things that go around things that are not the Sun. Confused? Probably, but that is not a big problem because while these various identified flying objects have complex overlapping categorical status, one type of object does not. Planets are planets and they are not anything other than planets.

To be a planet, you have to be in orbit around the Sun. This would rule out the Moon, which, if it was in orbit around the Sun instead of the Earth, could well be a planet.

To be a planet the object has to have sufficient mass to have been shaped by gravity to be (mostly) a globe. This depends on various things so at the low end of the mass spectrum there will be different masses and different sizes of things that don’t quite make it to globular status.

To be a planet the object has to have cleared its orbit. In other words, as an object orbits around the sun, it is likely to bump into other objects. Over a period of time, the object has finished bumping into everything it is likely to bump into, and thereafter has only a low probability of bumping into something. That does not rule out something bumping into the object, of course.

A globe shaped object that goes around the sun but that has not cleared its orbit is classified as a “dwarf planet.” This is of course historically contingent. In the early days of a solar system, perhaps there would be large star-circleing round things that have not yet cleared their orbit. This speaks to the strangeness of definitions alluded to above. The definition we use today to classify our solar system’s objects applies to a solar system developed to the extent ours is developed. The IAU nomenclature would probably need revisions if applied to all planetary-star systems in the Universe.

This scheme is not without its critics and there is indeed debate. Some of that debate is a bit nitpicky but still interesting. For example, Alan Stern, with NASA, notes that many planets have not really cleared their orbit, noting in relation to the Pluto controversy, “If Neptune had cleared its zone, Pluto wouldn’t be there.” Yes, apparently heavenly bodies have irony.

Anyway, as implied, Pluto is not classed as a planet because it has not cleared its orbit. Therefore it is a Dwarf Planet. Since it is far away (farther than Neptune) it also gets classed as a Trans-Neptunian Object. Furthermore, it is a Plutoid. That is simply a newer term applied to Trans-Neptuina dwarf planets.

The term Plutoid, then, refers to a dwarf planet, which for various reasons is apparently always specifically an ice dwarf, which is a trans-Neptunian body (orbiting most of the time beyond Neptune) that is sufficiently massive to be shaped like a globe. This term, plutoid, is officially adopted A plutoid or ice dwarf is a trans-Neptunian dwarf planet, i.e. a body orbiting beyond Neptune that is large enough to be rounded in shape. The term plutoid was adopted by the International Astronomical Union’s Committee on Small Bodies Nomenclature, but not by the working group on Planetary System Nomenclature. So you can use Plutoid or Dwarf Planet, or Ice Dwarf, depending on whom you wish to annoy.

Pluto, Eris, Haumea, and Makemake are the only known Plutoids. They are small enough and far enough away that more could be discovered.

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41 thoughts on “Why is Pluto not a planet?

  1. I would say, “the only known Plutoids that are accepted as dwarf planets by the IAU”. More already have been discovered, even recommended for classification as dwarf planets, but aren’t officially there yet:

    Orcus, Quaoar, Sedna, Salacia, Varuna, Ixion — and those are just one that have non-numerical names…

    Then there is the first dwarf planet to be discovered — and it’s not Pluto: Ceres, the planetino whose namesake gave us “cereal”. It was discovered in 1801, before even Neptune, and was initially regarded as a planet; it’s now officially just another dwarf planet. (And it was visited with a spacecraft ahead of Pluto.)

    The practical problem that led to the need to come up with this definition of a planet is one of sheer number: If we were to accept Pluto as “The Ninth Planet”, it would immediately have to take 10th place, as Ceres would be forced to return to planetary status as #5, displacing Jupiter. Then, joining Pluto as #10, would be the above-mentioned plutinos, plus, eventually, the other existing discoveries, and then future discoveries.

    Anyone up to memorizing the names of the 537 “planets”?? No, me neither. We need a manageable number, and 8 is it.

    The problem with poor Pluto is that it is indistinguishable from all its other Trans-Neptunian brethren. Sentimentality does not make it special, unfortunately. Nor would it in being the first discovered, which it also isn’t. Too bad Clyde Tombaugh didn’t keep looking, but he was simply lucky to have spotted Pluto in the first place…

    In short, Pluto is not a planet because it never was. A case of mistaken identity. (Hello, Alan Stern?) Still, paying it a visit is very cool, and this issue doesn’t detract from that.

  2. If the International Union of Heart Surgeons announces that your clavicle is a sousaphone…

    Alan Stern:
    “Meanwhile, in planetary science, we just call it a planet. We don’t care what the astronomers say.”

    Elsewhere he refers to Pluto as an “unter-subdwarf” planet.

    As for moi: Call me anything you want, just don’t call me late for dinner.

  3. “If the International Union of Heart Surgeons announces that your clavicle is a sousaphone…”

    …they’d be collared for a boner that blows confusion on what instruments they’d need to conduct surgery, he noted flatly.

  4. We already had one Red Planet, no need for another one. Imagine all the confusion over the names of science fiction novels. Now we have a Red Dwarf!

  5. Since everyone is entitled to an opinion, I’ve always thought these definitions lacking any meaning. Earth and the Moon: Terra & Luna, the double planets, those big gas balls? Dark Brown Dwarf Stars (so what if they don’t use fusion to power their light emissions… they still give off light in the infra-red)… and those “satelites” around those Dark Brown Dwarfs? Those are terristrial planets. That’s my system, and I’m sticking to it!

  6. We can call it a Rimmerworld!

    “My concerns are slightly more meaningful than what coloured stupid smegging cardboard hat I’m wearing!  I’m trying to decipher this! This is science, laddie!” AJR

  7. [Ceres] was discovered in 1801, before even Neptune, and was initially regarded as a planet; it’s now officially just another dwarf planet.

    And it’s not the only dwarf planet to have been discovered before Neptune. Pallas, Vesta, and a few other big rocks in the asteroid belt would also qualify.

    At Old Sturbridge Village, a recreation of a New England town circa late 1830s, the actor who plays the part of the schoolteacher mentions that (at the time) there are (were) 35 (known bodies that were considered) planets. No, I couldn’t name all of them.

  8. Brainstorms:
    “The practical problem that led to the need to come up with this definition of a planet is one of sheer number: If we were to accept Pluto as “The Ninth Planet”, it would immediately have to take 10th place, as Ceres would be forced to return to planetary status as #5, displacing Jupiter. Then, joining Pluto as #10, would be the above-mentioned plutinos, plus, eventually, the other existing discoveries, and then future discoveries.”

    Sort of. First, the other Trans-Neptunian Ice Little Planets would have to be included. Then, once they are in the club, since Ceres is not a planet because it is the asteroid belt (the opposite of a cleared neighborhood) it would have to be added.

    At that point, in my opinion, you could take a whole nuther look at planets. If we take all the planetoids and planets together and have a fresh look at them, we would probably be more likely to put Mercury together with Ceres than Mercury and Jupiter. I mean, really, they are way different.

    Type one planets:
    Rocky planets
    Rocky planets with molten cores

    Type two planets:
    Gaseous polanets

    Type three planets:
    Icy planets

    Then, like diamonds, they could get purity measures for their glob-osity and their neighborhood clearance.

    If the above was done with a term other than planet, say, “celestial body,” then we could later decide that some grouping of types would be called “The planets”

    Here we can go back to the origional definition of a planet: A star that moves. Adding simple earth-based telescopes, if it is a star that moves that we can see most of the year, it is a planet, as long as it is round and going around the sun.

  9. Candice: “Since everyone is entitled to an opinion, I’ve always thought these definitions lacking any meaning. Earth and the Moon: Terra & Luna, the double planets, those big gas balls? Dark Brown Dwarf Stars (so what if they don’t use fusion to power their light emissions… they still give off light in the infra-red)… and those “satelites” around those Dark Brown Dwarfs? Those are terristrial planets. That’s my system, and I’m sticking to it!”

    Yes, that does point out the craziness of it all. But, there is clean definition I think between things that orbit the central star and things that orbit the things that orbit a central star.

    Until something goes wrong of course.

    This brings up another question .What if a star passed through our solar system and removed, say, Mars from the sun’s orbit. A recent study indicated that “most planets” in the galaxy may in face be rogue planets like this.

    But how can a rogue planet be a planet if it does not go around a star. I imagine a rogue planet doesn’t really have a cleared orbit either.

  10. “…rogue planet…?”

    I suppose orbiting a sun and clearing a path could be considered criteria based on origin.

    It all suggests to me that perhaps classifications are going to be provisional for a long time and may depend on what we learn from other “planetary” systems before better organizing principles are discovered.

    As you suggest, ‘planet’ could even one day become one of those antique terms that people occasionally stumble across and smile at for their quaintness.

  11. Indeed. I look forward to being able to understand the variation across sun systems. Well, not me, but somebody.

  12. I think we should use the Holstian System for Planetary Determination:

    Other planets beyond Earth are defined as such on the basis of whether or not Gustav Holst wrote a movement for them in his orchestral suite.

    He explicitly declined to write a movement for Pluto (or Ceres). That seals it for me.

  13. @Brainstorms (#12):

    > [Holst] explicitly declined to write a movement for Pluto

    Not surprising, as in 1916, Pluto hadn’t been noticed by humans yet.

  14. Why is Pluto not a planet?

    I) What’ d’ya mean not a planet -it is a planet even if only a dwarf one justliekErath 9s arck dwarf not a gas giant!

    II) Because the IAU made a horrible mistake or fifty in their illogical definition deliberatley chosen to stop a whole category of planets esp. Pluto from counting somehow.

    III) Politics overcoming logic and reason -see above.

    I say this for a whole lot of reasons starting with the fact that a dwarfs star is still very much considered a star so why should a dwarf planet then be considered anything other than a proper planet too?

    Then there’s the small fact that the IAU definition is multiply flawed failing the Copernican test (our solar system is defined as the only one that can have planets? Really C’mon!) the “reductio ad absurdum” test (Earth couldn’t clear its orbit at Pluto’s distance either!) plus the Occam’s razor / unnecessary raising of extra questions test too! (Orbital clearance? how clear is clear? Does Neptune fail because Pluto crosses its orbit? Does Jupiter fail because of all the Trojan asteroids? Do all planets fail because of comets sweeping out from beyond Pluto to inside Mercury and crossing all orbits? etc ..)

    No, sorry, Greg laden but I gotta really strongly disagree with the idea that Pluto is anything but a proper – albeit small, strange and wonderful – planet deserving of full planetary status here and ditto the other ice dwarfs that are round enough to qualify for planetary status.

    Whatever we call it, this is definitely Pluto’s day in Pluto’s year and it is super-luminous (beyond merely brilliant) that the people from on this dwarf binary planet around our dwarf star have sent a dwarf spacecraft (piano-sized) to fly past another dwarf binary planet. One of these dwarf worlds is bigger than the other, the other is more a binary than the other. But both are remarkable and marvellous worlds and both count as planets and deserve respect and awe and discovery.

  15. Aaargh. Typo. Fix for clarity :

    I) Pluto is a planet even if only a dwarf one just like Earth is a rock dwarf not a gas giant!

  16. My preferred definition that I think is reasonably clear and easy to determine – a planet is an object that :

    I) Is rounded through its own gravity thus not a comet or asteroid.

    II) Has never been self-luminous due to in internal nuclear fusion thus not a star or brown dwarf.

    III) Is not directly in orbit around another planet thus not a moon.

    I think that is at least one far better definition of planet that avoids the multitudinous problems and superfluous questions raised by the horribly flawed IAU definition and the “orbital clearance ” rubbish in particular.

    If that means we have twenty or thirty or fifty or more extra planets in our solar system (making ice dwarfs incidentally average planets not odd ones and Pluto actually larger than your average planet) then so be it.

  17. Well, that clears that up! Now, how do we classify a moon that’s shaped like Felicity Kendal’s bum?

  18. “Not surprising, as in 1916, Pluto hadn’t been noticed by humans yet.”

    True. And since he was still alive & composing in 1930, he was encouraged to then write an eighth movement for it. He declined to do so.

  19. “deliberatley chosen to stop a whole category of planets esp. Pluto from counting somehow.”

    Pluto is NOT special (except of course for romance & sentimentality). It wasn’t the first dwarf planet discovered. It wasn’t the first dwarf planet visited. It’s not the only dwarf planet out there. It’s not the only one we’ve given a name to. It’s not unusual for being a TNO or icy plutoid, etc.

    If we adopt a definition that includes Pluto, we must include a HUGE number of other bodies (some of which are “more special” than Pluto). This is problematic… Hence the current definition which solves that problem.

    And romance & sentimentality have their nose out of joint… So sorry.

  20. Does Micky know Pluto is now a dwarf? I thought there were only 7 of them. Is Pluto now the 8th? Is 8 the Magic Kingdom number? And the magic planet number?

  21. “Experts” DO NOT say that Pluto is not a planet–never mind the fact that science is not decided by decree of an “authority.” If you’re referring to the four percent of the IAU that voted in 2006, they are not “experts” because most are not planetary scientists but other types of astronomers. Their decision was opposed by hundreds of planetary scientists in a formal petition led by New Horizons Principal Investigator Alan Stern.

    There are not “three criteria” required for an object to be a planet unless one chooses to view a very flawed, controversial resolution by non-experts as some sort of gospel truth.

    Our solar system never went from nine to eight planets because a self-appointed tiny group of people with ulterior motives said so. That is how a church works, not how science works. Just repeating a statement over and over by fiat does not make it true.

    Dwarf planets are a subclass of planets just like dwarf stars are a subclass of stars, and dwarf galaxies are a subclass of galaxies.This was the intention of the person who coined the term “dwarf planet,” none other than Stern. And he is far from alone. Many planetary scientists today use the term “dwarf planet” with the understanding that they are referring to a subclass of planets.

    The term “plutoid” is viewed as a joke by many planetary scientists and is rarely used.

    Some planets gravitationally dominate their orbits while others do not. The latter are dwarf planets. To say otherwise is to ignore objects’ intrinsic properties and instead classify them solely by their location. This makes no sense. Planets further from their parent stars have larger orbits to clear. If Earth were in Pluto’s orbit, it would not clear that orbit either. This means the IAU definition results in the absurdity of the same object being classed as a planet in one location and as not a planet in another location.

    Dwarf planets have all the same processes and structures as their larger counterparts.The only difference is they are smaller. Worlds like Ceres and Pluto have geology and weather and are differentiated into core,mantle,and crust. Lumping them in the same class as shapeless asteroids is simply bad science.

    A far better definition, favored by many planetary scientists, is that a planet is any non-self-luminous spheroidal body orbiting a star, orbiting another planet, or free floating in space. If the object is large enough and massive enough to be squeezed into a round or nearly round shape by its own gravity, it is a planet. Spherical objects orbiting other planets are simply satellite planets. They have all the same features as planets except they orbit other planets instead of orbiting stars directly.

    One has to wonder why the IAU view is yet again being imposed as the only “truth” on the very day New Horizons is flying past Pluto. This mission has already shown Pluto to have the complexity and processes common to planets–as Dawn has shown for Ceres. It seems some people are afraid that real data from a team that has actually visited Pluto could–and likely will–trump a bad definition imposed on the world by 424 people more interested in safeguarding their “authority” than in safeguarding the science of astronomy.

    Ceres, Pluto, Haumea,Makemake,and Eris are all small planets. Sorry to disappoint you, but four percent of the IAU, largely NON-experts when it comes to planets, do not get the last word just because some people keep repeating what they say.

  22. Laurel,

    Good points. I sort of suspected that ‘dwarf’ was the modifier and not ‘planet’. Those wacky anglophones and their word order, eh?

    Re classification in general, you don’t have to be just descriptive, which can be weak and arbitrary when you start looking at formation (all that cause and effect stuff science types go on and on about). As for imbalance of numbers of objects per category, that’s kind of an argument based on symmetry. While it does put a check in the manageability column, it also puts a question mark in the false balance column. Most disturbing is that I’m coming up dry with Red Dwarf references.

    Anyhoo, good to see you commenting here, Laurel!

  23. Actually even Earth does not qualify as a planet since there is at least one near Earth objects in the same orbit or plane.

    2010 TK7 at Lagrange 4. Perhaps more will be discovered. Same goes for other planets.

    So this definition of planet is very weak. Moreover, the Earth has a huge Moon that has not been cleared… and perhaps never will be.

    The argument of keeping the number of planets as low as possible for memory reason is unscientific.

  24. I’m with StevoR and Laurel.

    It’s interesting to ponder what happens in the distant future when a “dwarf” planet may have effectively “cleared” its orbit. Does it then graduate to full planet, even if it’s still dwarfish in size?

    This is the problem when a definition is based on a (relatively) transient phenomenon…

    And if there are more than 8 planets, so what? We don’t have to remember them all. After all, I don’t remember all the elements…

  25. Laurel Kornfeld – I appreciate your very cogent comments, but it sounds like under your definition, all moons of substantial size would be considered planets, and you haven’t made that totally explicit because you know nobody will be happy calling Luna a planet.

  26. Serge — the “clearing the orbit” criteria actually doesn’t mean emptying it. Which is probably good because even *Jupiter* would not qualify. There are hundreds of things swept up into its Lagrange points.

    The criteria is actually more nuanced. If the object has dictated the orbits of anything in that region, then it counts. Sweeping stuff up into the Lagrange points is, in other words, a trait of a planet. (According to the IAU.)

    Truthfully, the part I liked the least about the decision was the elimination of the “minor planet” category. Now they’re called “small bodies” (although dwarf planets still get minor planet numbers, even though the IAU doesn’t recognize the category anymore — even Pluto was belatedly issued one).

  27. Incidentally latest findings like these :


    Plus :


    Summed up visually in this :


    neat fly over animation albeit one with a few blanks in need of filling. (Also, wow! that was quick both to put together and have names for these features already.)

    Show that Pluto has clearly got some big geology and remarkable features for a small planet!

    It also boasts anatnmosphere, weather and more moons than the entire inner solar system put together.* Really how can we say it isn’t a planet when it has all this and yet currently recognised planet Mercury doesn’t?

    What next demote Mercury? State that Mars is just a big red asteroid too – Mike Brown actually suggested the latter in a recent Aussie radio interview or something like that..

    * Pluto has Charon, Nix, Hydra, Kerberos & Styx, inner solar system has zero moons for Mercury and Venus Earth has our Luna and Mars just Phobos and Deimos for a grand total of three. Pluto is described in some reports as a mini-solar system of its own which may be going a trifle far but still. If there’s a checklist of planetary features Pluto actually ticks more boxes than many others!

  28. @ ^ Typo fix :

    It also boasts anatnmosphere = Pluto boasts an atmosphere ..

    Sorry, afraid I suck at typing. I know what I mean to type, often I even think I’ve typed it typographicals-free – until after the submit button has been clicked & I see that I’ve stuffed up. .

    Wish we could have an editing capability here if possible Greg Laden – even if on a time window eg. 5 ~15 minutes to edit out errors then fixed forever.

    @ Calli Arcale : Yeah, the shift to the clunky small solar system bodies and removal of the old asteroid / comet / minor planet distinction and the idea of calling Pluto -like objects “plutoids” an ugly word which – thankfully – hasn’t caught on. I think the currently IAU naming system just a mess all round.

    I would suggest and hope they opt for something like :

    Planets are defined as in #17 then that broad classification (sort of at the animal,mineral, vegetable level) gets split into gas giants, ice dwarfs, gas dwarfs (perhaps incl. some exoplanets sometimes described as SuperEarths & suggested by planet hunter Sara Seager), rock giants, rock dwarfs, ice dwarfs (The current dwraf planets Pluto, Ceres, Haumea etc). Then have minor planets would be split into comets, asteroids and centaurs plus meteoroids then dust /ring particles. Sort of the continuum analogous to stars in terms of supergiants, giants, subgiants, dwarfs etc ..

    Interestingly just as most stars – including our daytime one -are dwarf stars, most planets are ice dwarfs like Pluto – indeed smaller than Pluto, it sort of makes sense and is easy to recall and also describes the general nature and its characteristics relative to other planets clearly..

    Furthermore we can also further divide each class based on its other properties so that, for instance, we have gas giants being listed as Superjovians, Hot Jupiters / Roaster / Pegasids, Cold Jupiters, eccentric orbiters etc .. Ice dwarfs can be asteroidal (eg. Ceres) or TransNeptunian, Rock Dwarfs can be Earths, Super-Earths, super-Mercuries (Mustafar!?) Martian, carbon planets, silicon planets, etc ..

    Plus we can have rogue planets, colliding & binary planets and forming planets currently actually excluded from planetary status by the current IAU definition which also limits planets to our solar system only by absurd definition!

    I’m not claiming to have originated this system and I do think its one that’s well worth considering for IAU or other official adoption.

  29. http://laurelsplutoblog.blogspot..com
    @Obstreperous Applesauce You’re welcome!
    @Jane Spherical moons of planets can be considered secondary or satellite planets. This terminology has been around as far back as the 19th century. In terms of their intrinsic properties, these worlds ARE planets–they have all the processes planets have; they just happen to orbit other planets instead of orbiting the Sun directly. Some, like Europa and Enceladus, are actually prime locations for hosting microbial life. Complex life could even exist on an exomoon (a spherical moon of a giant exoplanet) as in the film “Avatar.”
    @Calli Arcale The term “minor planet” is a synonym for asteroids and comets, objects not large enough to be rounded by their own gravity–those now known as Small Solar System Bodies. Pluto should not have a minor planet number because it does not fit this category, and neither should any dwarf planet.

  30. Pluto-Charon is actually a binary planet system because the barycenter, or center of gravity they orbit, is outside of Pluto. The IAU definition makes no allowance for binary planet systems because each object in a binary does not clear its orbit of its companion.

  31. OK, maybe Neptune shouldn’t be a planet either, since it hasn’t cleared its orbit (because of Pluto crossing it). But how can it be that the two haven’t collided, after all these eons. I realize that both Neptune and Pluto take very long times to complete an orbit, yet…. I suspect it has something to do with Pluto having an orbit that is not on the same plane as the true planets, but this isn’t a very satisfying answer in itself.

  32. Stephen, Neptune & Pluto are in orbital resonance, with 3 orbits of Neptune completed in the time it takes Pluto to orbit twice. This resonance prevents them from ever being near the crossing points at the same time.

    This resonance “passes the test” regarding clearing an orbit (otherwise no planet would be a planet). Bodies with objects orbiting around Lagrange points also pass the test.


    1. Thank you, Brainstorms. That certainly answers the question. And I don’t want to belabor this, but can you tell me if there’s any scientific reason why there are 3 Neptune orbits vs. 2 Pluto orbits, precisely? By “precisely,” I mean it certainly can’t simply be a coincidence. I wouldn’t be surprised if there’s no well-defined answer to this.

  33. Stephen, Take a look at the link I pasted into my reply. It’s not exactly 3:2, but varies slightly. (It actually wavers back & forth around 3:2 so that it averages 3:2.)

    The way the physics works, Neptune controls Pluto’s orbit & period, causing it to speed up or slow down slightly when it wanders too far away from the 3:2 ratio. This ability to control the other objects in its neighborhood qualifies Neptune as a planet. (It doesn’t have to “eliminate” all objects around it.)

    This same principle applies to the “Shepherd Moons” of Saturn that keep some of the tiny rings formed. Over time, they seem to play tag with each other.

    This is a common thing with large orbiting bodies, so there are other planetary resonances, and most of the moons of Jupiter & Saturn also have resonances, etc.

  34. How to define what a planet is:

    Step 1: Define the minimum dominant mass (MDM) of a solar system

    The mass of the smallest rounded celestial body in orbit around a star (or stellar remnant) that has cleared the neighborhood of [or is dynamically dominant in] its orbit using Jean-Luc Margot’s planetary discriminant (where ? ? 1).

    For example, for our solar system Mercury is the MDM; in the solar system known as Kepler-37, the MDM is Kepler-37b (which has a diameter slightly greater than Earth’s moon). See the following link for a more technical explanation of Margot’s practical planetary discriminant:


    Step 2: Define the term planet

    A celestial body that…

    (1) orbits one or more stars or stellar remnants;
    (2) is a gravitationally dominant member of its solar system, defined as follows:
    (a) has cleared the neighborhood of [or is dynamically dominant in] its orbit (e.g., Margot’s ? ? 1)
    OR (Note: Skip 2b if 2a is already fulfilled, for example, to expedite exoplanet classification.)
    (b) has a mass >= the MDM of its solar system;
    (3) has a mass below 13 Jupiter masses, a nominal value close to the limiting mass for thermonuclear fusion of deuterium.

    With this definition, as long as Earth and Jupiter orbit the sun directly, they will always remain planets regardless of their hypothetical location in the solar system (e.g., a “remote” Jupiter that orbits in the Oort Cloud or a “remote” Earth” that orbits at 100 AUs from the the sun). And with this definition, you don’t set an arbitrary cut-off point for planethood for all other solar systems at Mercury but instead use a contextual cut-off point for planethood unique to each solar system (as in the case of Kepler-37).

    Everything less than the MDM will be a dwarf planet or small solar system body (SSSB, or sub-planetary mass object) so there won’t be any more Jupiter-like planets potentially mislabeled as “dwarf planets” because of their given location within a solar system. Rogue planets and large rounded satellites will remain separate categories under the classification of planetary mass objects (PMOs).

    In summary, planets are by all rights the dominant players of any given solar system (after their parent star(s) of course). Dwarf planets, large rounded satellites, rogue planets, and SSSBs are, for various reasons, not dominant players of solar systems.

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