Scientists try to restore Pluto’s planet status

From the “Make Pluto Great Again” department, new definition raises number of planets in solar system to about 110

The Long Range Reconnaissance Imager aboard NASA’s New Horizons spacecraft captured this photo of Pluto’s surface in 2015 IMAGE CREDIT: NASA / JHUAPL / SOUTHWEST RESEARCH INSTITUTE

Johns Hopkins University scientist Kirby Runyon wants to make one thing clear: Regardless of what one prestigious scientific organization says to the contrary, Pluto is a planet.

So, he says, is Europa, commonly known as a moon of Jupiter, and so is the Earth’s moon, and so are more than 100 other celestial bodies in our solar system that are denied this status under the prevailing definition of “planet.”

The definition approved by the International Astronomical Union in 2006 demoted Pluto to “non-planet,” thus dropping the consensus number of planets in our solar system from nine to eight. The change – a subject of much scientific debate at the time and since – made no sense, says Runyon, lead author of a short paper making the pro-Pluto argument that will be presented next week at a scientific conference in Texas.

Icy, rocky Pluto had been the smallest of the nine planets, its diameter under three-quarters that of the moon and nearly a fifth of Earth. Still, says, Runyon, who is finishing his doctorate this spring in the Department of Earth and Planetary Sciences, Pluto “has everything going on on its surface that you associate with a planet. … There’s nothing non-planet about it.”

Runyon, whose doctoral dissertation focuses on changing landscapes on the moon and Mars, led a group of six authors from five institutions in drafting a proposed new definition of “planet,” and a justification for that definition. Both will be presented at the Lunar and Planetary Science Conference’s poster session. The poster will be on view for a full day on March 21 at the conference sponsored by the Lunar and Planetary Institute, and Runyon will be on hand for at least three hours to answer questions about it.

The other authors are S. Alan Stern and Kelsi Singer of the Southwest Research Institute in Boulder, Colorado; Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona; Will Grundy of the Lowell Observatory in Flagstaff, Arizona; Michael Summers of George Mason University in Fairfax, Virginia. All the authors are science team members on the New Horizons mission to Pluto, operated for NASA by the Johns Hopkins University Applied Physics Laboratory. In the summer of 2015, the New Horizons spacecraft became the first to fly by Pluto, some 4.67 billion miles from Earth, passing within 8,000 miles and sending back the first close-up images ever made of Pluto.

Runyon and his co-authors argue for a definition of “planet” that focuses on the intrinsic qualities of the body itself, rather than external factors such as its orbit or other objects around it. They define a planet as “a sub-stellar mass body that has never undergone nuclear fusion” and that has enough gravitational heft to maintain a roughly round shape. (Even if it bulges at the equator because of a three-way squeeze of forces created by its own gravity and the influence of both a star and a nearby larger planet.)

This definition differs from the three-element IAU definition in that it makes no reference to the celestial body’s surroundings. That portion of IAU’s 2006 formula – which required that a planet and its satellites move alone through their orbit – excluded Pluto. Otherwise, Pluto fit the IAU definition: It orbits the sun and it is massive enough that the forces of gravity have made it round.

Stern, the principal investigator for the New Horizons mission, has argued in the past that the IAU definition also excludes Earth, Mars, Jupiter and Neptune, which share their orbits with asteroids.

The proposed new geophysical definition omits stars, black holes, asteroids and meteorites, but includes much of everything else in our solar system. It would expand the number of planets from eight to approximately 110.

That expansion is part of the appeal of the new definition, Runyon says. He says he would like to see the public more engaged in solar system exploration. As the very word “planet” seems to carry a “psychological weight,” he figures that more planets could encourage that public interest.

The new definition, which does not require approval from a central governing body, is also more useful to planetary scientists. Most of them are closely affiliated with geology and other geosciences, thus making the new geophysical definition more useful than the IAU’s astronomical definition.

He has some reason to be optimistic, as the new definition has already been adopted by Planet Science Research Discoveries, an educational website founded by scientists at the University of Hawaii at Mānoa.

“I want the public to fall in love with planetary exploration as I have,” Runyon said. “It drives home the point of continued exploration.”




189 thoughts on “Scientists try to restore Pluto’s planet status

  1. The IAU didn’t make Pluto a non-planet but a minor planet, along with other objects in its zone, like Sedna, and with asteroid Ceres.

    • Exactly. I thought it was re-classified as a “dwarf planet”.

      “There’s nothing non-planet about it.” Well , apart from all the qualities like not clearing its orbit, that the IAU had decided to use as criteria. Maybe there is more to astronomy than landscapes.

      • Anyway, the main point of demoting Pluto was to ensure that there were still nine planets when they get around to identifying the large TNO whose existance is becoming more and more accepted. We would not want to have to admit that there really is a “planet X” would we?

      • Greg,

        There might be more than one planet-sized object out there in the far reaches of the solar system:

        For that matter, there are interstellar planetary mass objects, orbiting the barycenter of the galaxy rather than a star. Probably lots of them. They might constitute a big chunk of the ordinary (baryonic) dark matter.

      • I should have written dwarf planet.

        It’s estimated that 200 dwarf planets probably inhabit the Kuiper Belt, with up to 10,000 possibly lurking farther out, scattered around toward and in the Oort Cloud.

        Only about a half dozen dwarf planets are currently known, with more likely candidates awaiting further study.

      • Well, if clearing an orbit is part of the definition of a planet then Jupiter fails because it has about 5000 Trojan asteroids in its orbit at the Lagrangian points. Neptune & Mars also have some Trojans and even the Earth has one (2010TK7). So the Earth is not a planet either following the IAU definition I would have thought! Pluto is a planet, not a dwarf planet.

        But I do think moons are different from planets…planets orbit stars, not other planets. Moons can orbit planets and asteroids and maybe comets(?).

      • Think of the natural resource possibilities on all these rocks in our solar system. All we have to do is survive going to get them.

    • But they also said a dwarf planet is not a planet. (Think of it as a two-word noun instead of an adjective and a noun.) Yes I agree it’s nuts. Pluto has two moons, an atmosphere (I helped discover it), seasons and weather. Something odd about saying that’s not a planet. And their definition involving clearing its path gravitationally means that exactly the same body would be a planet if it orbited at one distance, but not at another. That’s really odd.

      • Ron,

        Having an atmosphere doesn’t matter. Ceres might have not just an atmosphere but an ocean. Many moons have atmospheres, some dense, and probably oceans.

      • PS: Pluto presently has five known moons, but that still doesn’t make it a planet. Asteroids have moons.

  2. OK, I support returning Pluto to planetary status but I don’t see is as an improvement to create a definition which ignores the common sense category of ‘moon’. We should recognize the planetary character of large moons, asteroids and rogue wanderers based on their intrinsic qualities but the neighborhood in which they are situated is also important, distinctive and should be acknowledged even if it could be transitory.

      • Pluto is certainly not a planet. Pluto – Charon makes some case for being a binary planet (based on an external barycentre). Pluto on its own is not.

        The Moon being a planet? The idea is absurd. Being spherical doesn’t make a planet. Never has. Never will.

      • Andrew, one of the proposed new planetary definitions I heard would indeed turn several moons into planets. For some odd reason, that particular definition did not include “orbits the sun”

      • If you suggested that having a moon should be *required* for a planet, it’s a rather radical idea. After all, Mercury and Venus – two of the more “core” or characteristic planets – don’t have any moons. Some people really seem to be irrationally obsessed with Pluto. It’s just a round of a rock that is very far and there are numerous rocks like that which weren’t known. On the other hand, Mercury, Venus, and especially the Moon are things we can see rather easily on the sky. Changing *their* status is an invitation for a civil war.

  3. Getting my kids to memorize the order and names of 8 or 9 planets wasn’t too hard, but for 110 would be Herculean.

      • Jeff,

        Except that they sometimes switch places. Pluto’s orbit is so out of whack (for a planet) elliptically that sometimes it’s inside of Neptune’s.

      • I guess that would make it a Cis-Neptunian Object, even though it was the first Trans-Neptunian Object to be discovered.

        A real switch-hitter, definitely in need of a multi-sex bathroom.

  4. I agree that “planet” should be defined in terms of an object’s intrinsic properties for two reasons.

    Firstly, now that we can detect exoplanets and measure some of their properties we need to use terminology that will function consistently across all stellar systems. If object X is classified as a “planet” in system A but identical object Y in system B is classified as something else because of “external factors such as its orbit or other objects around it” then that will only create confusion. It makes it harder to compare the prevalence of particular types of object across different systems and makes it harder to describe them clearly.

    Secondly, the use of “external factors” means that an object might be classified as a planet at some points during its existence but not at others, depending on changes in its orbit or the conditions around it.

    However, we also need to recognise that the word “planet” is obsolete. It originally referred to objects that appeared to move in regular cycles relative to the “fixed” stars. It was invented by people who could only see a few of the objects that actually exist in the solar system, and who therefore assumed that these few must represent a particular and important category. But we now know that there are a huge number of objects in the solar system, with sizes ranging from vastly bigger than the Earth down to tiny rocks. There is also a huge variation in composition and structure, even among those objects that have been traditionally known as “planets”.

    Therefore, it becomes increasing difficult to draw any meaningful diving line between “planets” and other objects, and there is less and less value in trying to preserve a category that originally meant no more than “we can see it and it moves”. The same argument can be applied to “asteroid”, “comet” and other popular terminology. It’s obsolete because it originated as a description of how things appeared to the human eye and through small telescopes before we were aware of the full range of objects that actually exists in the solar system.

    So, what we really need is a system of classification for non-stellar objects that can be applied consistently across all stellar systems and to objects that do not orbit any star. It would therefore need to define them in terms of their intrinsic properties and would give them a classification code rather than a name. I’m imagining something a bit like a Hertzsprung–Russell diagram for discrete non-stellar objects.

    It would probably still be necessary to retain archaic words like “planet” as a description of particular types of object within that classification system. But it would no longer matter if the definition of the word involved an arbitrary combination of characteristics, because it would just be an arbitrary term that was used to assist in mapping the terminology used in previous scientific literature to the new system and as an aid to understanding in popular science. The word itself would no longer carry any weight as an analytical category.

    • AndrewZ

      “If object X is classified as a “planet” in system A but identical object Y in system B is classified as something else because of “external factors such as its orbit or other objects around it” then that will only create confusion.”

      I think we have to admit that what astronomers think and say is not what ordinary folk think and say and there is lots of room to allow the regular people their voice. Why, for example, should there not be a popular vote on what to call Planet X? It should not fall to a couple of astro-nerds.

      I am with the groups above who what large objects that shine, not twinkle, to be called planets and the things orbiting them to be called moons. Objects that orbit moons should be called satellites.

      I don’t mind giving the asteroid belt members names but they are not planets because the whole lot of them is in fact a broken up planet that has no name, yet. It is unlikely to re-assemble much of itself in the lifetime of humanity.

      Pluto is a planet with a couple of moons. Perhaps we can go as far as naming Planet X, but Oort Cloud members? Put a minimum orbital time on the definition and they are separated into Oort Cloud Objects and Planets. The public can live with that.

  5. Planets orbit stars, moons orbit planets, so I don’t get the point of calling Europa a planet. That’s just silly, so everything else he says can be dismissed.

    • That apparently is too simple a definition. If asteroids happen to share the orbit, are they planets? However, increasing to 110 planets is far too complex. I’m sure there’s a better way.

  6. You know he is actually right about Earth, Mars, Jupiter and Neptune sharing their orbits with asteroids.
    That means that Earth, Mars, Jupiter and Neptune are not actually planets, yet.
    But they will be when they sweep their orbits clear.

    He has a good point that this definition is inadequate.

    But a definition that lets everything in – 100 plus planets – still misses the mark.

      • No it’s not. Pluto’s internal structure is differentiated, with a rocky material at the core surrounded by a mantle of water ice and a crust of nitrogen ice. The diameter of the core is approximately 1700 km, 70% of Pluto’s diameter.

        Climate Heretic

  7. I always thought that for celestial object to be a planet it has to be orbiting a star. So something like Moon could never be a planet cause it doesn’t orbit a star, no matter how large it is. But what do I know?

      • The sun’s gravitational field deflects the earth/moon system about 1° per day. The earth’s field deflects the moon about 13°. I’d say the earth wins that one.

      • If the barycenter falls within the the mass of the larger object, the smaller object is a moon. If the barycenter falls betweeen the two masses then it is a double planet.

        The earth/moon is a planet/moon system.

        (my definition only. made up by me. not approved by anybody else.)

      • DonM, according to your definition: “If the barycenter falls betweeen the two masses then it is a double planet” that would make the Sun-Jupiter system is a “double planet.” “Jupiter’s mass is 2.5 times that of all the other planets in the Solar System combined—this is so massive that its barycenter with the Sun lies above the Sun’s surface at 1.068 solar radii from the Sun’s center.” reference:

      • David,

        I would say my made up definition of dual planets, based on barycenter, doesn’t include suns (or black holes) as planets.

    • Charon and Pluto form a dual planetary system, since their instantaneous point of rotation in between both bidies. They are orbiting each other.
      I agree that the moon is a satelite of Earth, but that doesn’t necesarilly exclude it from being its own planet. This new definition only states that satelites can also be planets if they are massive enough. I see no problem with that.
      The argument about learning all the planets, is stupit, our kids shouldnt have to learn all 100+ planets, just the classical list is plenty.

      • Sebmagee

        The problem with that approach is that the barycentre of the solar system sometimes sits outside the radius of the solar surface.

        I see no problem with calling the larger member ‘planet’ and the smaller one ‘moon’. If there is to be a ‘rule’ let it be that the moon has to be more than 50% of the mass of the planet to be counted as a binary planet. The reason this is a better approach is that a modestly sized moon that orbited at a good distance away would place the barycentre outside the planet’s radius. Our moon, moved away not too much more, would achieve this. At the moment it is 1000 km down from the Earth’s surface.

  8. You know scientists have no meaningful research to do when they start publishing papers arguing about the definition of things like this. The definition is subjective. It isn’t like these things are part of an equation in which the exact units of measure are critical.

    There’s some small rocks and some big rocks circling the sun. Draw the line where ever you want. But padding one’s resume with “research” papers funded by the public dime to argue where the line should be? This is the stuff of PhD’s now?

  9. Earth and Moon are in binary-orbit; both Earth and Moon orbit the Barycenter. The Barycenter is located about 4,671 km from the Earth’s geocenter-mass and center-of-figure, within the upper mantle (Earth’s ellipsoid-figure radii are 6,378.137 km [major – equatorial] and 6,356.7531342 km [minor – polar]). There is an animation of the Earth and Moon orbiting the barrycenter:

    The Barycenter-orbits of Earth and Moon with axial rotations (Moon is in a locked orbit) and their orbit around the Sun produce the Equation of Time, a component of the Analemma, as viewed from Earth.

    What this means, conjecture, is that the NASA Apollo Program, which was advertised and marketed as a “Voyage To The Moon” and grammatically correct was in fact a voyage to another Planet! Therefore, NASA need not do and Congress and OBM need not fund a “Voyage To A Planet (Mars) Beyond Earth”, because it was already done, almost 50 years ago! Ha ha! Now this is the way to “lower the bar” and cut Federal spending on wasteful “pie in the sky” programs.

    For ref.

  10. A silly Through the Looking Glass Humpty Dumpty like poster.
    Pluto was downgraded to planetoid for two very solid reasons. 1. Orbital plane is tilted 17 degrees from the orbital plane of the 8 true planets, which means Pluto did not form from the solar accretion disk process that places all true planet orbits in a single plane. Orbit is elliptical, which means it is a Kuiper Belt object. Kuiper also contains four other Pluto like (size, orbit) objects: Haumea, Makemake, Sedna, and Eris. All these planetoids are in off plane elliptical orbits. Just like the myriad of smaller elliptical orbit Kuiper Belt objects. Comets are of Kuiper Belt origin.

    • Ristvan claims, “Orbit is elliptical, which means it is a Kuiper Belt object.” According to that logic, the Earth is a Kuiper Belt object, because the Earth’s orbit is elliptical with an eccentricity of 0.0167.

    • Also Mercury is tilted 7 degrees from the orbital plane. Does that mean Mercury did not form from the solar accretion disk ?
      PS… Mercury has an elliptical orbit with .208 eccentricity, also making it a Kuiper Belt object.

      • DD, some astronomy science responses to your two refutatiinal comments. Perhaps will also help educate other WUWT readers on science nuances versus picayune objections. I reseached all this before initially posting. is but one apparently intelligent and reliable resource.
        First, a star’s accretion disk is always thickest closest to the Star (basic math about angular momentum and gravitational attraction vectors– math challenged can google physics of accretion disks to,learn themphysics basics), so Mercury being 8 degrees out of ‘plane’ (none are in a literal plane) of the other 7 true planets is not surprising, rather it is likely. 8 degrees tilted closest to the Sun is not 17 degrees tilted farthest.
        Second, I perhaps should have said Pluto has ahighly elliptical orbit. Of course Earth’s orbit is not a perfect circle. No planet’s is. But there are times when Pluto’s orbit is inside Neptune’s. Earth never goes outside Mars or inside Venus, and those orbits are much smaller, closer ‘circumferencial’ spacings. QED onnyhe geometry.
        A science fail on pedantic semantic debating points. Please do better next time, as you are up against a former regional champion high school debater, later further honed at HLS.

      • I apologize for catching your errors. You made the claim that “Orbit is elliptical, which means it is a Kuiper Belt object” which is false. Being sloppy in your verbiage is not conducive to good science. Anyone that knows anything about astrophysics knows that no planetary orbit is circular. They all are elliptical. Lack of precision in language is common among non-scientists. Your 2nd error was posting: ” that places all true planet orbits in a single plane” (emphasis mine.) When someone in a debate makes a statement with the word “all” in it, the only thing a good opposing debater has to do is provide a single counterexample to destroy your claim. Mercury does this with regards to what you said about accretion disks. Lastly I could care less about your high school antics, the passage of time has not been good to you with regard to your debating skills.

      • “I apologize for catching your errors. You made the claim that “Orbit is elliptical, which means it is a Kuiper Belt object” which is false. ”

        Dont forget Rud was a champion debator.

        he only forget that science isnt a debate.

        yes yes sometimes scientists argue. but science is not a debate.

      • The eccentricity of Pluto’s orbit is extreme, more like a comet than a planet. Its eccentricity is about 15 time greater than earth’s, as I commented previously, and around three times that of Mars, which is highly eccentric for a planet, due to its proximity to Jupiter.

        Kepler used Tycho’s observations of Mars to determine that orbits are elliptical.

        Rud is right.

    • A good example of the futility of terminological disputes. Clearly we’re now learning that the Solar System is not just a neat little paradigm of 9 planets and an asteroid belt, but a collection of hundreds of odd objects of varying sizes, some so distant that they are not observable until they sneak in close enough (e.g. comets). And there is no obvious evolutionary tree on which to structure a taxonomy. But we could use one, as we are now starting to investigate other stars and their ‘stellar systems’, or whatever they’re called. As AndrewZ says (March 19, 2017 at 8:50 am),

      . . . So, what we really need is a system of classification for non-stellar objects that can be applied consistently across all stellar systems and to objects that do not orbit any star. It would therefore need to define them in terms of their intrinsic properties and would give them a classification code rather than a name. I’m imagining something a bit like a Hertzsprung–Russell diagram for discrete non-stellar objects.

      But absent such an agreed-upon classification (which will probably always be fuzzy), it is certainly more convenient for popular consumption, if not technical purposes, to refer to the ‘planets’ as those big objects that orbit the Sun (or a sun), and ‘moons’ as those (generally smaller) that orbit around other objects (usually planets) and not the Sun (or a sun).

      So in this regard, it seems to me that poor ol’ denigrated Pluto ought to be restored to ‘planetary’ status, or perhaps Honorary Planet, since after all it was discovered by investigating the perturbations it caused in the orbit of Neptune, and that cannot be said of any of its putative congeners, like Sedna or Eris. Indeed the name ‘Pluto’ was considered particularly apt because the first two letters were the initials of Percival Lowell, and he should rightfully be allowed to retain his then-hypothetical planet, especially since his other famous hypothesis, about the canals of Mars, has been, sadly, disconfirmed.

      /Mr Lynn

    • Ristvan

      I looked at the planetary orbits in the article we had on chaos the other day. Fantastic animation showing how the orbit of Venus sometimes moves out to be very similar to that of the Earth, and how much the Earth is jerked around by the planet’s. Mars also comes much closer to Earth than it is now.

      My point is that we have no idea if Pluto is a true planet that formed in the same plane as the others. It may well have been and was yanked about and winged into its current orbital plane. Much stranger things are possible. It crosses quite near a huge object that could easily fling it about. If both were off their ecliptic at the time they passed near each other.

      I think the primary school version of planets keeping their positions neatly is not a suitable framework for a discussion of the chaotic movements and large changes in orbits that actually occur.

  11. Another meaningless argument about nothing. Does it really matter what Pluto is called? Sticks and stones will break your bones, but names will never hurt you.

  12. The main real objection to Pluto as a planet, I believe, is it is the only one discovered by an American. The element ‘Columbium’ honoring Columbus’s discovery of America (okay, the the Norse beat Chris to it and today they’ve all been trumped by the ‘native’ people) but another international institute bullied in Niobium as official. Dang Yanks then went and created a couple of dozen more elements, so there!

    In any case, a 100 more is a bit ridiculous. The idea that this will create more interest in planets is silly. Daffynitions by bean counter boring mucky mucks who never discovered anything (I call them ‘performance’ scientists like in music – in 350 yrs the great composers used up all the best note ((there are only 12notes and a few octaves)) and harmony combinations by the early 20th Century, same timing as scientific discovery).

      • Sheri March 19, 2017 at 10:38 am
        We’ll know when it sends us a message and tells us.

        Well, since we’ve only observed (discreetly) and haven’t actually heard from it, I’ll hazard a guess it is not a female.
        If it was a female or a very confused male-non-male, we would have heard plenty.
        However Man (or Femailman) decides to define it, it is the rock it has always been.

  13. “I want the public to fall in love with planetary exploration as I have,” Runyon said.

    When you’re in love, you can’t get enough of it. QED

    Still, restoring Pluto’s planet status sounds good to me. Rock on!

  14. They are all HB’s (heavenly bodies). We can distinguish between HRB’s (heavenly round bodies) and others (asteroids, comets, etc.). I could see calling all HRB’s planets; that’s OK, even though it would include our Moon. I am glad Pluto is now again a planet, at least in this reckoning.

  15. What seems to be most illustrated here is the extreme difficulty of trying to build a heirarchy for the various natural phenomena that works and makes sense. The same problem occurs in all sciences—as soon as an item is found that possessess characteristics of multiple catagories (it seems to be a planet, but it also seems to be an asteroid/it seems to be a mammel but it lays eggs) the system is threatened and/or falls like a Jenga tower. Trying to catagorize such complexity may be an exercise in futility. Humans just can’t imagine or think at that level yet.

    • There is no doubt that the platypus is a mammal. Mammals have three different ways of reproducing, but monotremes, marsupials and placentals are all members of Class Mammalia. Their ancestors all laid eggs.

      Taxonomy isn’t hierarchical, although Linnaean categories still have some utility.

      The IAU’s definition of planet is clearcut. That asteroids and other objects cross the orbits of the planets doesn’t obviate the definition.

      Pluto has some traits of a planet and some of a comet, but is neither. Dwarf planet is a useful category.

      It would never have been elevated to planetary status had it been known in 1930 how small it is. It looked bigger to Tombaugh because his telescope couldn’t separately resolve Pluto’s moon Charon. They are close enough to be tidally locked. The barycenter of their system lies outside Pluto. Smaller moons orbit this center of gravity farther out.

      • Pluto is not a comet. A comet is a celestial object consisting of a nucleus of ice and dust and, when near the sun, a “tail” of gas and dust particles pointing away from the sun. Pluto the planet has a solid core of rock 1700km approximately in diameter.

        In addition the IAU’s definitions states that, in the Solar System, a planet is a celestial body which:

        a) is in orbit around the Sun,
        b) has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
        c) has “cleared the neighborhood” around its orbit.

        Definition c) Is Not Clear Cut. Jupiter has the Trojan and Greek asteroids in its path. Therefore according to the IAU definiton of a planet, Jupiter is a dwarf planet.

        For clarification I agree with the conditions a) and b) for the definition of a planet as listed by the IAU. Condition c) I vehemently disagree with.

        Climate Heretic

      • What does “cleared its neighborhood” actually mean?

        The situation in the Kuiper Belt and asteroid belt is fundamentally different from the orbits of the eight planets, including both the four little inner rocky ones and four big outer gas and ice ones.

        Adapted from “Universe Today”, 2008, and updated:

        As planets formed, they became the dominant gravitational body in their orbit. As they interacted with other, smaller objects, they either consumed them or slung them away with their gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.

        Any object that doesn’t meet the IAU’s third criterion is considered a dwarf planet. Thus Pluto is a dwarf planet. There are still many objects with similar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarf planet. Eris, Sedna and the other two dwarfs in the Kuiper Belt all suffer from the same problem.

        I hope this clarifies the issue of “clearing”.

    • Sheri, IMO a very insightful comment. Taxonomic biology has gone through a similar revolution from Linnean to Cladistic. Does it solve all the species/evolution complications? No. But it simplifies they many that remain. As I wrote in The Arts of Truth, Veritas is elusive and in most cases unapproachable. But we can do better or worse in the approach. You post a classic example.

      • Ristvan,

        There is little confusion about what constitutes a mammal. To lumpers such as myself, any amniote with the mammalian jaw joint is a mammal. Splitters might consider those with both the mammalian and “reptilian” (also amphibian and fish) jaw joint is mammiliformes or a proto-mammal, like Morganucodon.

        All living mammals, indeed since the Triassic or Early Jurassic, have just the mammalian joint, so there is only a small number of Triassic forms which could go either way. The discovery of Morganucodon and its ilk came as a shock to creationists, BTW. Students are also sometimes surprised to learn that the key distinguishing derived trait shared by all mammals is the jaw joint, not hair or milk. Luckily for paleontologists, the jaw joint fossilizes well.

        But the main point is that modern cladistic, phylogenetic taxonomy is not hierarchical. It’s based upon shared, derived traits and genetic similarity, not any imagined hierarchy. It’s bushy, not hierarchical.

        The problem with celestial bodies is that classifying them doesn’t have the benefit of evolution, so astronomy is stuck with a system more akin to the Linnaean, based on “anatomical” similarities.

      • I should add that even in ostensible “mammiliformes” like Morganucodon, the vestigial “reptilian” jaw joint has already been coopted for hearing, while the mammalian joint is the main bite force connection. Its former jaw bones are well on their way to becoming the little bones of the mammalian middle ear.

  16. Mother very easily made a jam sandwich using no peanuts. An obscure reference, I left off the mayonnaise and glue part, but it fits. Now, who groks the reference?

  17. Pluto never stopped being a planet to me. Like we needed a self defined group of “scientismists” to guide the ignorant masses about the razor thin dividing line between planet and unplanet.

    9th planet…. was, is, and always will be.

    (as will starFISH remain a fish, Arabic numerals are Arabic, guinea pigs remain pigs) etc

  18. “I want the public to fall in love with planetary exploration as I have,” Runyon said. “It drives home the point of continued exploration.”

    Getting to the heart of the matter: Continued exploration = continued funding.

  19. Can’t make Pluto a planet without also making the larger Eris into a planet. Eris has a moon too, maybe more. Once you’ve done that, then the question is where you draw the line between them and the smaller round planetoids. It obviously makes most sense to draw that line on this side of Eris, so leaving things as they are. But it’s educational to discuss the topic once in a while.

    • IMO the IAU did the right thing. There are other Trans-Neptunian Objects bigger than Pluto, too, and more will be found. Limping them in with spherical objects like Ceres but excluding them from planetary status makes perfect sense.

      Pluto is significantly smaller than Mercury, but more importantly much less massive. It’s basically a big comet, composed of ice and rock rather than rock and metal, like Mercury, which resembles a planetary core. As I said, it would not have been rated a planet in 1930 had its true size and mass been known, any more than the largest asteroid belt object Ceres was considered a planet.

      • No the IAU did not do the right thing. See my comment later on in this thread and Pluto is not a comet. See the definition of comet.

        Climate Heretic

      • I never said that Pluto was a comet. I said it had characteristics of both comets and planets, but is clearly not a planet by any rational definition. It would not ever have been considered one if discovered in 1980, when its true size could have been seen

        Its eccentricity is extreme, as I also commented, about 15 times greater than earth’s.

  20. This effort was so ridiculous that it does not benefit from your poking more fun at it. Sorry, I know in my little mind there are nine planets. My elementary school papier mache models said so. And I stand by them.

  21. The problem is that someone thinks there needs to be a definition based on criteria. If the IAU had sense the definition of a planet would just be – one of the 9 we all know as planets- end of discussion. If that seems arbitrary one can add the caveat “for historical reasons.” Is anyone proposing that we rename the Sun because it’s been discovered to actually be a star? No, for interesting historical reasons the Sun is called the Sun and the stars are something different. This is a good thing because it means that every budding scientist sometime early in their education goes through a realization or about the true nature of the Sun. The other problem of course is that the debate has become whether to call Pluto a planet or something else – not well specified. Since there is no really well know alternative to the term “planet” that has caught on, people have started calling Pluto a “minor planet” or a “dwarf planet.” Doing this wins the debate for the “Pluto is a planet” crowd because of the way adjectives work. A minor planet is a planet just as much as a giant planet is a planet. And a dwarf human is certainly a human despite the “dwarf” adjective.

    • The definition of dwarf or minor planet is straightforward. It’s massive enough to form a sphere, but hasn’t swept other large objects out of its orbit.

      The planets now recognized as such aren’t just for historical reasons. We now have exoplanets to consider, so an agreed upon definition is required. Science needs precise definitions.

      • Exoplanet, minor planet, giant planet, gas planet, secondary planet, protoplanet, traditional planet, : Use any prefix or adjective you like with the word “planet” and what you have is a planet. For now *our* planets are the ones we all learned in elementary school and that were given the names of classical pagan gods. When there are new discoveries it can be decided what kind names to give them. I’m not even sure I want to call any of the traditional planets merely a planet. Mars is mars. Coincidentally it’s something that roughly fits in with the notion of a planet. Although it is quite different from the rest of them. It’s these differences that makes them worth knowing about, so I’m not sure what the need for precision is if it’s the differences in them that make them unique and interesting. Mars does wander across the sky though – in the way that stars don’t. This I think, is the original and best starting point for the definition of a planet. If the word seems imprecise just substitute “satellite” with appropriate modifiers. Hmmm on the other hand maybe we should take a cue from Prince ….How about just, ♇ formerly known as Pluto.

  22. As I recollect the demotion of Pluto was rather a dodgy stitch up late on the Friday pm session of the IAU when the proponents of the change knew most of the delegates would be on their way to catch flights home – it is questionable if their motion would have succeeded otherwise. And several people have commented rightly that on the grounds they stated for the exclusion of Pluto other planets too should therefore be disqualified. JBom correctly points out the interwoven orbits of the Earth and Moon.
    But more than that this silly exclusion of Pluto caused a lot of unnecessary disappointment to lots of young people interested in Astronomy for virtually no real gain. So what if professional astronomers have to grapple with complex definitions of small planets and minor bodies? The public didn’t need to be involved unless they took a deeper interest.
    In any case the demotion merely looked what it was – a churlish act – once we discovered how interesting and active Pluto actually is.

    • There is real gain, since without the dwarf planet status, more and more tiny “planets” will be discovered in the outer reaches of the solar system. IMO the IAU did the right thing.

  23. So the “gain” is we don’t have to acknowledge a potential host of minor planets which in fact tell us the solar system is much more extensive than once realised. I think professional astronomers are robust enough to cope with that without messing about with the status of Pluto (whatever reservations they might have about it) . No gain, much irritation? I suspect the general public will cheer loudly if Pluto is restored.

    • It’s not just Pluto. A host of new “planets” would have to join the big ones already recognized.

      As of February 2017 over 2300 Trans-Neptunian Objects appear on the Minor Planet Center’s List of TNOs. By late last year, 242 of these have their orbits well-enough determined that they have been given a permanent minor planet designation. So you can see the problem.

      Pluto was demoted not just because we discovered how small it really is, but because the list of “planets” would eventually have to reach hundreds if not thousands in the solar system if it were still designated a planet.

      We’ve known the size of the solar system for quite a while now.

      • All of those tiny insignificant ice specks discovered by Americans. And that above all other facts is the determining factor in this retroactive push for new definitions of what is or isn’t a planetas.

        Bigotry was invented by Europeans after all.

    • PS: It’s unclear how many minor planets would eventually rate as dwarf planets, but the overlap is large.

      Euler diagram showing showing types of bodies in the solar system:

      • Nice diagram. Are the box sizes by number or by mass?
        If by number then dust ought to dwarf everything else. Followed by rocks too small to discern by sweeping telescopes.
        If by mass, where’s the Sun?

      • Euler diagrams represent sets. The boxes don’t denote either mass or number of members, just that there are larger or small numbers of intersections.

      • Thanks. I learn again. Twice in a week you have shown me something new.

        So basically this is showing that planets are defined as being planets only; planets are not anything else.
        The fact that they are not “satellites (natural)” is a special definition. They are actually satellites of the Sun.

        Comets are defined as not being “minor planets”. OK. That feels right. But I can’t quite say why.

        And there ought to be a box for “Stars” to hold the Sun. In some systems it would represent more than one example (as seen on Tatooine).

      • M,

        You’re right, but this diagram is just for bodies in the solar system that aren’t the sun.

        Overlapping sets occur, as for instance with Ceres, which at a minimum is an asteroid and a dwarf planet. Pluto is a KBO, a TNO (most of the time) and a dwarf planet.

        Comets aren’t minor planets because their orbits are highly eccentric and they haven’t cleared out their own paths through the solar system, which is why they have the disturbing habit of colliding with planets and other objects. Think Comet Shoemaker–Levy 9.

      • The point about comets clearing their orbit is not persuasive (the eccentric orbits makes sense).

        You mention Ceres. It certainly feels like the same sort of thing as Pluto. Very Dwarf Planety.
        But look at where it lies. Is the asteroid belt split into two fields? So dwarf planets don’t need to clear their orbit. But nor do actual planets.

        The original article is wrong, as I wrote in my first comment on this thread. Over 100 planets in our solar system misses the point of planets.
        But the argument that “clearing the orbit” is necessary to be a planet is too tight a restriction. Definite planets don’t do that.
        And it means that objects can attain planet status with time and collisions. That seems wrong.

      • Seems a bit confusing.
        We have centaurs, dwarfs, neptunes and even more than one of Santa’s comets.
        Are there more than one dancers? Are they a subset of the comets?
        Shouldn’t they be a subset of the Rudolf’s?
        /sarc, with apologizes. 8-)

      • Hank,

        You hearken back to M’s comment about dust.

        The rings are mostly chunks of water ice, so don’t rate a category. They vary greatly in size, and can incorporate dust and even rock, but range only from dust particle to mountain-size.


        Glad you found the Euler diagram useful. It’s subject to change, of course, but current at from AD 2006.

      • Correction:

        Smallest Saturn ring particles are the size of sand grains, not dust, although they can incorporate dust.

        Jupiter too has rings.

      • M,

        Regarding clearing their orbits:

        Pluto is only 0.07 times the mass of the other objects in its orbit. Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.

        Thus, the odd asteroid crossing the orbit of or centaur cluster coorbiting with a planet just doesn’t matter, when the difference in mass as a share of all mass in the orbit is on the order of ten to 100 million-fold.

    • Now that makes sense.
      A planet is x times larger in mass than all the other things that are regularly in its orbit. And I mean “regularly” literally.
      I would put x at 100. But any arbitrary number would do. It would be measurable and decisive.

      Hmm. This rules out Counter-Earth planets. A same size planet orbiting the same distance but on the far side of the star. It must appear somewhere in the universe.

      Alternatively a planet is just an historical label given to the most culturally important objects orbiting nearest a star.

      • M,

        Originally “planet” just meant “stars” which wander, instead of appearing fixed. Discovery of the true nature of the solar system, galaxy and universe has forced astronomy to come up with new, more accurate nomenclature.

        True planets and Pluto and other dwarf planets are “round” and in direct orbit around the sun. What they lack is “clearing their neighborhood”. As you note, the threshold for “clearing” isn’t defined, but doesn’t really need to be, since in all dwarf planet cases so far, the gulf between planets and dwarf planets is enormous in this criterion.

        Times 100 would be a good threshold, but given the enormity of the difference between present planets and dwarf planets in this regard, even 1000, 10,000, 100,000, a million or ten million times would suffice.

        IMO a KB object of planetary mass, such as Mercury, would have already cleared its neighborhood, thus achieving planetary status. As the outer solar system is explored, such far out planets may be found.

      • If “planet” is a status that is obtained through time, rather than size or trajectory, then it doesn’t seem to be a suitable definition. To me, at least.

        And for “planet” to be useful it must apply to solar systems other than our own. Just as The Sun is a special name in our home system while “star” is general, “Earth, Mars, etc” are special names in our home system while “planet” should be general.
        I refer again to my Counter-Earth example. That can never be a “planet” or two. But if they both evolved humanoids then they would both be more planety than Mercury.

        Frankly, what definition of planet can include lumps of rock, lumps of frozen gas and NOT lumps of frozen water?
        More I think about it, More I think “planet” is an archaic and obsolete term.

      • IMO “planet” in its modern definition works well for other star systems as well as our own. All exoplanets discovered so far meet all three conditions of the IAU for “planet” status, whether rocky like earth or gaseous like Jupiter.

        Under no rational system are Pluto and Mercury equivalent, let alone Jupiter and Pluto.

        The inner rocky planets are in effect equivalent to the cores of the outer gas and ice giants. Pluto, Ceres and other dwarf planets, by contrast, could never even constitute planetary cores. They are, like comets and asteroids, tiny mixtures of ice and rock. Even Mercury is rock and metal, like the cores of the other inner planets and even the outer ones.

  24. Umm.. does it matter?

    Does it matter the name humans call something – it doesn’t change what it is, it exists..

    It’s purely an arbitrary decision – like “x” is a galaxy and “y” is not – no one else in the galaxy cares what humans call something.

    Now – if Plutonians turn up and say they are offended that we don;t call Pluto a planet – sure, redefine it.
    It’s a planet… simple as that… what else does it matter?

  25. The size of the problem isn’t improved by adopting qualifications on what constitutes a planet that in fact should also disqualify some bigger inner planets. It would be perfectly straightforward and meet Chimp’s reasonable concerns atbout the hordes of minor objects awaiting us by just ruling that objects beyond Pluto will not be classified as planets. Frankly astronomers could worry themselves into nervous breakdowns if they are going to be fretting about how many categories of odd, small or strangely orbiting objects they are going to find in the Kuiper belt. And who knows, there may be a few more surprises that will tax whatever definitions we devise.

    • Yeah like dyson spheres or the orbits of orbiting-bodies around binary stars! There are more things in heaven and earth, chimp, than are dreamt of in your philosophy. Just Imagine the stuff we’ll see caught in lagrange points. Where is Oilerman going to fit in the exo stellar planetary fuzz.

      • Which is my point.

        Pluto is not a planet. If it is, then all the as yet undiscovered TNOs bigger than it will also have to be planets. Kids are not going to learn the names of 8000 planets. Eight are hard enough for most, even though all but two of those are visible to the naked eye.

        Uranus is alleged to be naked eye visible, but I’ve never been able to see it. Of course, I’m near sighted. Somehow observers with normal vision and even far-sighted never managed to notice it (or identify it as a planet, ie a “wanderer”) for the whole 200,000 years of modern human existence, until Sir William Herschel.

  26. That portion of IAU’s 2006 formula – which required that a planet and its satellites move alone through their orbit – excluded Pluto. – IAU
    Yeah, well, Charon was discovered in 1978, long before the IAU came up with this specious nonsense, and Hydra and Nix came along in 2005, never mind the separate, if eccentric, orbit of Pluto and its (now five) moons. So the IAU’s so-called definition is either baloney. And that brings up Titan, a ‘moon’ of Saturn. It’s bigger than our moon, bigger than Mercury, has an atmosphere, liquid bodies (methane and nitrogen) and seasons. So Titan almost qualifies as a planet on its own, except that it doesn’t have any moons in orbit around it, and doesn’t orbit the central star.
    I think this is another case of much ado about nothing.

  27. Ya know? My first impulse after reading the headline was thank you Dr. Trump for giving us the opportunity to discuss something going on in science besides CO2. What a blessing, this guy is proving to be. imo

  28. This fellow is pushing this because of activism. Sounds dumb, because it is. The early astronomers had it about right. No need to deconstruct everything. They will end up creating classifications similar to those currently in use, only with silly names or numbers. Planet means wanderer, yes? That is a splendid name and meaning.

  29. For those who prefer the TL;DR

    I know, Pluto is a planet, because it has always been a planet and always will be.


    1) Pluto is a planet because of the historical connection first discovered in 1930, by Clyde Tombaugh

    2) Pluto is not a comet, The definiton of a comet is: “a celestial object consisting of a nucleus of ice and dust and, when near the sun, a “tail” of gas and dust particles pointing away from the sun.” Pluto has not produced a “tail”, has not approached the sun sufficiently enough for a tail to form. Also Pluto has a density of 1.86 and “Pluto’s internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of water ice.”[Wikipedia] NB: water ice sould be Nitrogen ice (contradiction in Wikipedia articles)

    3) All planets move in elliptical orbits, with the sun at one focus. This is one of Kepler’s laws. The elliptical shape of the orbit is a result of the inverse square force of gravity.[Google: elliptical orbits]. Yes Pluto does have a higher eccentricity. So What! Therefore you cannot use eccentricity to define a planet.

    4) Every planet has an orbital inclination, which “measures the tilt of an object’s orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object”.[Wikipedia: Orbital inclination]. Yes Pluto does have a higher orbital inclination. So What! Therefore you cannot use orbital inclination to define a planet.

    5) The definition of planet set in Prague, Czech Republic in August 2006 by the International Astronomical Union (IAU) states that, in the Solar System, a planet is a celestial body which:

    a) Is in orbit around the Sun,
    b) Has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
    c) Has “cleared the neighbourhood” around its orbit

    The planet Pluto satisfies a) and b) above. But not c), according to the IAU definition of a planet. However, “he wording of the final draft of the definition has continued to be criticized, primarily in the United States. Notably, Alan Stern, the lead scientist on NASA’s robotic mission to Pluto, has contended that Earth, Mars, Jupiter, and Neptune have not fully cleared their orbital zones, just like Pluto. Earth orbits with 10,000 near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000 Trojan asteroids on its orbital path. Stern has asserted: “If Neptune had cleared its zone, Pluto wouldn’t be there. [Wikipedia: IAU definition of planet]

    Some people have countered with a different opinion. However, as Stern pointed out above, Earth Mars, Jupiter and Neptune have not cleared there orbits, therefore IAU definiton of a planet c) is contradicted. Therefore Pluto is a planet. Because there is contention or controversy surrounding definition part c) of a planet.

    6] The scandal surrounding the number of people who voted 424 (last day of conference) against the total membership of the IAU, is a farce. All members of the IAU should have a vote on something important as the status of the planet Pluto. Regardless of whether they are involved in planetary research areas or not. A consensus (sic) of all 10,000 members should have been taken. Because of this vote, farce, scandal, whatever you want to call it, Pluto is a planet.


    Climate Heretic
    PS Did I tell you that Pluto is a planet?
    PSS I will continue to teach that Pluto is a planet :)

    • Then what about Ceres, Haumea, Makemake, Eris and Sedna? And even bigger dwarfs to be found in the future? Will you also teach that they are planets, too, even when they number thousands?

      As I noted, had Pluto been observed first in 1980 rather than 1930, it wouldn’t have been considered a planet, as it is too small. It was granted planetary status because Tombaugh couldn’t distinguish Pluto from Charon.

      It is true that American astronomers were more supportive of Pluto than those from other countries, it being the only putative “planet” discovered by an American.

      Ceres was briefly considered a planet, but astronomers concluded it was too small to qualify.

      • Chimp said: “Then what about Ceres, Haumea, Makemake, Eris and Sedna? And even bigger dwarfs to be found in the future? Will you also teach that they are planets, too, even when they number thousands?”

        Corrected: “And WHAT IF even bigger dwarfs are to be found in the future? Will you also teach that they are planets, too, even IF they number thousands?”

        The answer to all is YES!. What, you haven’t memorized the Periodic Table of elements yet? Or the names of all the rivers on planet Earth? Surely we need a better way of defining “mountains,” or at the least “mountain ranges,” because no one can remember them all?

        The problem with (c) of the IAU definition is huge: move Mercury to Pluto’s orbit & it is no longer a “planet.” Move Earth to Sedna’s orbit & it is no longer a “planet” either. Suddenly, one realizes that meeting or not meeting (c) involves calculating a fairly complicated, non-intuitive mass/distance/age ratio, rather than observing something definitive or observable about the object itself.

        But this isn’t even the largest, most obvious problem w/the IAU’s garbage “definition.” Point (a) says the object has to orbit our Sun! Really? “Sorry, the rest of the universe: no planets for you?” Worthless.

        The new definition needs to be modified to recognize that many of these “planetary” objects are also “moon,” and should primarily be referred to in that way. This would reduce the proposed ~110 or so “planets” to a little more than a dozen… & even I can memorize THAT many state capitals!

      • It’s not what if, but when. There are already hundreds of minor planets awaiting more details to determine whether they are dwarfs or not.

        A planet isn’t akin to rivers, or even the elements. They’re more like continents, which are distinct from islands. A few islands are on their own small tectonic plates, so that they’re in a gray area.

      • Smokey,

        If you moved Mercury to Pluto’s orbit, it would rapidly attract enough of the Kuiper Belt objects in its orbit to crash into it, or fling them out of its path, so that it would indeed again become a planet in its own right, since it is about 2.5 times as massive as Pluto.

        In your geological or geographical analogy, I equated planets with continents because they are the highest order subunit in each case, ie solar system (less sun) and earth. Well, most oceans cover more area than most continents, but you get the idea. Equating planets with features like rivers is a false analogy.

      • Chimp said, “In your geological or geographical analogy, I equated planets with continents because they are the highest order subunit in each case, ie solar system (less sun) and earth. Well, most oceans cover more area than most continents, but you get the idea. Equating planets with features like rivers is a false analogy.”

        With respect, Chimp, I think you’ve missed most of the analogy: rivers, mountains, countries, states (& their capital cities) occupy a huge amount of space on Earth’s surface when compared to that which even the “true” planets occupy in the solar system. As such my analogy stands. We don’t need to scale up to continents or oceans because that would be akin to re-labeling the entire “inner solar system” or “the asteroid belt;” we’re not labeling areas or volumes, we’re labeling objects within those volumes… to say nothing of the fact that we already have labels for those. (The Kuiper Belt, the Oort Cloud, etc.)

        Also, you’ve ignored the other problems I brought up with Mike Brown’s the IAU’s deliberately anti-Pluto definition (for no other explanation can reasonably exist for such a flawed, provincial description) to focus once again on the “arbitrary upper limit” objection to Dr. Runyon’s definition. I’ve already stated that I don’t believe “moons” should be included, regardless of size, yet even so: if there end up being 20,000 objects that can objectively be called “planets” in our own solar system by itself, so what? There are already known to be more asteroids & comets than that arbitrary figure in our system, by more than an order of magnitude: should we re-define the those terms as well because there are ‘too many’ of them?

        Finally, to extend my previous analogy, there are mountains, volcanoes, ridges & scarps on many other planets besides Earth; are we going to continue to pretend (as the IAU’s definition does in the case of “planets”) that those features/formations don’t occur via similar processes elsewhere in the universe? For instance, should we be calling Olympus Mons and Tvashtar (e.g.) “exovolcanoes” because only “real volcanoes” occur on Earth? In the same way, planets occur throughout the universe, not just in our own solar system, no matter that the majority-of-the-minority vote of the IAU decided otherwise.

      • As far as the argument that “Mercury would have cleared that orbit by now,” are you sure? have you run the math from the alleged beginning of the solar system to see if in fact that slight mass advantage really does “clear” Pluto’s orbit in the time allotted? Because, just eye-balling it, I don’t think it does, & I KNOW Earth doesn’t clear an orbit much past Eris, let alone at the distance of Sedna (my previous reference).

        Regardless, this idea that a “planet” has to have been around for a certain period of time before it can even be CALLED a “planet” is silly; either it’s a planet or it is not. The amount of time it’s spent in a given orbit is a curiosity, not a defining characteristic of the object (likely saying more about the system in which it formed than about the object itself), nor is it one we can figure with any great accuracy, even for objects in our own solar system (cf. several recent papers alleging that our planets may have shifted or even swapped orbits in the fairly recent past relative to the age of our solar system, i.e., within the last 1 – 3 billion years).

    • The discovery of Eris in 2005 is what prompted the IAU in 2006 to define “planet”. It became obvious that Pluto was going to be eclipsed repeatedly by bigger and better TNOs, possibly in their thousands.

    • Classical planets. The seven classical planets are those easily seen with the naked eye, and were thus known to ancient astrologers. They are the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. … The outer modern planets Uranus, Neptune and Pluto are often called the collective or transcendental planets. Planets in astrology

      I know, but others were thinking it! The real point is context is often important in a definition, how many argued against helocentrism because it would necessarily change the status of Sol and Luna?

  30. Some words don’t need a really great definition. Pluto is a planet because at the time it was given it’s name it seemed so and the people all started calling it such. Other things that seem close we can just call planet-like objects until something better comes along. It’s presumptuous to name something a planet and then try to take it back. It may even be a micro-agression to say that Pluto isn’t a planet because it’s a dwarf planet. Did you clear that one with the IDU (International Dwarf Union)?

  31. Can anyone please tell me what difference calling Pluto a planet or a minor planet will make to the life of anybody?


    • @richardscourtney: It’s not about Pluto, however much the media & the IAU want to make it sound like it is. Pluto is the object “everybody knows,” so that’s how the discussion is framed in the popular gestalt. That’s it.

      Rather, it’s about having a useful scientific term with which to categorize various objects we see in the universe. Whether Pluto happens to be “planet,” “ice dwarf” or “cartoon character” is, in & of itself, immaterial; having a useful, scientifically descriptive label that actually means something is the important thing.

  32. Can I just be the first to say that, since it was pointed out to me, I can’t stop seeing the face of Disney’s Pluto in Pluto’s Southern hemisphere?

  33. I love the contribution by Kwarizmi of a trans residential object . it is the absurd definitions and deviously arranged vote to demote Pluto that caused this needless fuss. Chimp’s defence of the definitions used to end Pluto’ s status are not very persuasive precisely because they apply in equal measure to other planets which no one wants to disqualify. No one is advocating or is seriously going to try to learn 8000 planet like body names. If your definition is rubbish choose a better definition or just set an arbitrary cut off like we stick with the planets most people are comfortable with (as evidenced by many of the comments). And I still don’t see why those of us interested in Astronomy wouldn’t be comfortable with knowing the names of a few extra large bodies beyond Pluto if they turn up – even if we agree not to call them planets.

  34. In my view, there are four planets, Jupiter, Saturn, Uranus, and Neptune. Everything else, including
    Mercury, Venus, Earth, Mars, Pluto, and Charon, is a planetiesimal.

  35. Pluto was named for the god of the underworld because it was beyond Neptune when discovered. It was misnamed. The “real” Pluto has yet to be discovered. The current one should be renamed Salacia and its moon Venilia.

  36. Whenever we were to decide to install outerposts to observe our solar system or the environment of our solar system, we would anchor these outposts on or around the planets.

    With this simple intuitive access, a maximum of 9 so-called planets remain in our solar system.

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