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Planetary scientists fight back: “Pluto is a planet!”

A group of eminent and active planetary scientists have just published a new peer-reviewed paper documenting how moons and asteroids were routinely referred to as planets from Galileo until 2006 when a very small number of scientists at an International Astronomical Union (IAU) meeting decided arbitrarily that the definition must be changed.

That IAU definition, which required an object to have a solar orbit and the vague ability of the object to clear that orbit, somehow made Pluto a non-planet. It has also never been accepted by planetary scientists, who consider it inconsistent, vague, and useless in their research as well as in teaching students about planetary science. I know this attitude is real because of what planetary scientists have told me consistently in many interviews since 2006.

The new paper appears to be part of a new aggressive campaign by planetary scientists to get that IAU definition dumped, and replace it with the definition planetary scientists have been using forever, which is that if the object is large enough for gravity to shape it into a spherical shape, it is a planet. This is still the definition they routinely use when discussing large moons like the Moon or the large Galilean moons of Jupiter or the larger moons of Saturn or Pluto itself.

It also appears, based on information at the link, that this campaign is beginning to make headway. To that I say, Hallelajuh!

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  • Phill O

    The terrestrial planets end at Mars after which come the gas giants, then the class of Plutanoids which appear to compose a number, each with similarities to Pluto.

    We know the number of terrestrials and gas giants. The number of Plutanoids is yet to be determined.

    Pluto’s biggest fault was that it was discovered by an American, something the Europeans can not stand! (IMHO)

  • Jay

    About damn time! I give my colleague at work grief over his Solar System coffee cup not having Pluto on it.

    In relation to the New Mexico story wanting to tax Virgin Galactic earlier, they did pass one law in the past to recognize Pluto as a planet whenever it is seen in the night skies of New Mexico. Of course they will next pass a telescope tax for people looking at the planet Pluto!

  • David K

    I still think that Pluto is not a planet, but not for the reasons commonly mentioned. The center of gravity in the earth moon system is inside earth. The center of gravity in the Pluto Charon system is between them. Charon does not orbit Pluto, they orbit each other, so IMHO it is a binary planetary system and not just a planet.

  • John

    Pluto will always be a planet to me! Well, double planet with Charon. It’s a nice Plutanoid, I mean planet.

    But Ceres? Heck no, screw Ceres! Nobody likes Ceres. Ceres needs to stay in its asteroid lane!

  • BtB’s Original Mark

    I read somewhere that the the Pluto/Charon system does not have any stable Lagrange points. Consequently any hypothetical NASA mission to park an orbital satellite at either of them would not be possible since it would require active station-keeping to compensate for perturbations in the orbit.

    I would have thought that you could theoretically park a satellite at the center of gravity that lies between Pluto and Charon system.

    Can anybody explain this to me?

    FYI, my understanding of Lagrange Points at this point is limited to what is on the NASA website.

  • BtB’s Original Mark

    As a point of clarification, I do understand that there are stable and non-stable Lagrange points. For example, for the Sun-Earth system L4 and L5 are stable, and L1, L2, and L3 are all unstable.

    The JWST is at L2, so NASA needs to manage the Webb’s orbit.
    And while L2 is defined as a non-stable Lagrange point, it is not very unstable, and takes relatively little effort to stay near these points.

    So I guess I don’t understand what makes non-stable Lagrange points Very Unstable.

  • The Moon is half again the size of Pluto. Jupiter’s even-larger moon Ganymede has considerably more than twice Pluto’s radius and diameter. I see nothing wrong with referring to such smaller bodies of the solar system — in solar orbit(s) — as “minor planet(s).” But if it were me, on the other hand, I’d call not only Ceres but Vesta not just a “minor planet” but a planet (as indeed both were so regarded during the first half-century after their discovery). Vesta — one of the few surviving “protoplanets” left over from the earliest days of the solar system — is fundamentally a differentiated terrestrial planet much like the big-boys of inner solar system like Earth and Mars (with iron cores and rocky mantle and crust), and moreover is basically spherical — significantly departing from same only in its far southern region (i.e., Rheasilvia Crater) which was excavated by a gigantic impact a billion or so years back.

    So are the (IAU) astronomers via their newfangled “planetary” definition really saying or implying that Vesta was indeed a (“minor”) planet back before that impact, but then suddenly — despite being basically physically intact while continuing in its solar orbit much like before — nonetheless suddenly ceased to be a (minor) planet? It’s nuts.

    But if Pluto is to be considered a (full-blown not just minor) “planet,” then Eris — basically exactly the same size as Pluto, but with a quarter-again its mass and orbiting up to twice as far away from the sun — surely must also be regarded as a planet.

    The upshot it is, however you manage it, there aren’t going to be nine planets in the solar system ever again.

  • Hello
    For the Pluto issue.
    I don’t think that Euopeans don’t accept the fact that an American astronomer has discovered a planet, it’s the Americans that don’t accept that Pluto has been designated as a Dwarf Planet.
    Once again,in the 2000’s , or before, there was an american astronomer who has discovered 2 small bodies that are Make Make and Haumea, if i’m right.If Pluto is the ninth planet, he said that he wanted to be the discover of the 10th and 11th planet.
    But there have been other small bodies discovered few months or years later, that have about the same size as Pluto.Wich meant that we could have an 12th,13th,14th,15th, etc..planet
    So in 2006 the president of the IAU has decided that they need to give a definition of what a planet is by using 3 points :

    1.”The object must be in orbit around the Sun.”
    2..”The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape defined by hydrostatic equilibrium.”
    3.”It must have cleared the neighborhood around its orbit.”

    “Pluto fails to meet the third condition.[56] Its mass is substantially less than the combined mass of the other objects in its orbit: 0.07 times, in contrast to Earth, which is 1.7 million times the remaining mass in its orbit (excluding the moon).[57][55] The IAU further decided that bodies that, like Pluto, meet criteria 1 and 2, but do not meet criterion 3 would be called dwarf planets. In September 2006, the IAU included Pluto, and Eris and its moon Dysnomia, in their Minor Planet Catalogue, giving them the official minor-planet designations “(134340) Pluto”, “(136199) Eris”, and “(136199) Eris I Dysnomia”.[58] Had Pluto been included upon its discovery in 1930, it would have likely been designated 1164, following 1163 Saga, which was discovered a month earlier.”

    I use Wikipedia for the last points for helping
    Thank you for reading me
    Have a nice day

  • Just adding that the president of the IAU in 2006, was not a writer, a politrical activist, but a french astronomer, Catherine Cesarsky, and that the debate about Pluto have been between scientists also.
    Of course Pluto is an interesting planet, like other objects in our Solar System.
    i have no problem with Clyde Tombaugh, neither that Pluto is a Dwarf Planet.

    Like the article shared however

  • pawn

    In my youth, I was visiting Lowell observatory during a Martian opposition and they had the 24 inch Clark available to the public for viewing the planet. One of the guides happened to be a real astronomer and overheard some of the remarks I made and started talking with me. Well, as a kid interested in astronomy who was starting college, this fellow decided to give ma a personal tour of the observatory that housed the 13 inch astrograph that took the photos that Tombaugh used to discover Pluto. He even pulled up some plates and ran the blink comparator on the plates along with some other of objects like Barnart’s Star. You can’t imagine what a thrill that was and even today it is one of my most cherished memories.

  • sippin_bourbon

    John. You said Ceres should stay in it’s asteroid lane…

    But Ceres WAS a planet for almost 50 years, and was then demoted.

    Then printed to the new class of dwarf planet.

    Really, one of the themes of this blog is the uncertainty of science. As our knowledge grows, understanding changes, we must allow for those changes. In this case we created a new category, and re classified based on the new paradigm. In 50 years it may shift again.

    Are we going to lock our selves in to old thinking? Are we that certain that this science is settled?

  • sippin_bourbon

    Arg… Small keyboards. Ceres was promoted, not printed.

  • John

    Oh fine, Mr. Bourbon, that’s just what we need, a Ceres supporter. Probably in the Vega camp too.

    When was it, the 1800s when everyone was discovering new planets in the now recognized asteroid belt? Now technology has improved and we’re discovering all the new planets, I mean dwarf…objects in the Kuiper belt.

    Does it change anything for poor Pluto that found his friends?

    I wouldn’t want to subscribe to outdated thinking that objects found in a belt are planets (oldest), demoted (old), dwarf planets(oldish). Never mind, I’m so confused.

    Long live planet Pluto!

  • It’s been pointed out that Pluto and Charon orbit a common center of gravity which is not inside Pluto’s diameter, thereby making them a binary system.

    I say we go with that. Not only is Pluto Planet #9, but Charon is Planet #10. (Or even Planet #9b.)

    That’s show those lame Planet Karens in the IAU.

  • Star Bird

    It is Planet Just because some liberal Party Poopers are trying to Claim its not its a Planet no matter what the nay sayers tell us

  • Edward

    BtB’s Original Mark,
    You asked: “I would have thought that you could theoretically park a satellite at the center of gravity that lies between Pluto and Charon system. Can anybody explain this to me?

    Try this explanation:
    At the L1 point, the satellite orbits Earth with the period of the Moon, but because it is lower than the Moon it is traveling too slowly to remain in the “circular” orbit by itself, but with the tug of the Moon it is able to stay in place between the two bodies without falling toward the Earth (into a different elliptical orbit with a shorter orbital period).

    At the L2 point, the satellite orbits Earth with the period of the Moon, but being higher it is traveling too fast to remain in the “circular” orbit by itself, but with the tug of the Moon it is able to stay in place without drifting away from the Earth (into a different elliptical orbit, or perhaps slipping into a solar orbit).

    It isn’t just the center of gravity between the two bodies because the satellite has its own angular momentum that must be considered. The concept is not intuitive and can take a while to understand. In my class, I learned enough to satisfy the professor, but every once in a while I learn another subtlety or nuance to these orbits.

    L1, L2, and L3 are visualized as saddles, making them unstable (picture an upside-down bowl), the farther from the “sweet spot” the less stable, so some amount of stationkeeping propellant is necessary. L4 and L5 are visualized as right-side-up bowls (though somewhat kidney shaped), making them relatively stable, and some amount of distance from the “sweet spot” (the bottom of the bowl) the more it looks like it orbits the sweet spot, when viewed from a reference frame that rotates with the orbit of the Moon. Keep in mind that all five Lagrange points are in motion, turning with the same speed as the Moon. If the satellite gets too far away from L4 or L5, then it is once again unstable, goes beyond the bowl’s lip, and drifts away.


    Neptune also has not cleared Pluto from its neighborhood around its orbit. There are enough near Earth objects that make some people wonder why the Earth is considered to have cleared its own neighborhood. What is more, the definition of planet means that exoplanets do not qualify as planets, as they violate part 1 of the definition.

  • BtB’s Original Mark

    Edward – thanks for taking the time to answer my question and providing a link. I took two years of High School Physics but that was a long time ago. (I wish I still had my HP-33E Calculator from back then)

    Your visually based descriptions were particularly helpful.

  • Hello Edward, and other people that could read this comment.
    You are right when you say that an exoplanet violates part I of the definition, because it doesn’ orbit the Sun, but by its definition, an exoplanet doesn’t belong to the Solar System.
    Of course, we have to change and probably be more precise when we define what a planet or an exoplanet is.
    I looked back to wikipedia, and found an “Euler Diagram”in the IAU definition of “planet”, as i can see , there are a lot of relations and entanglement between celestial bodies, that tend to complexify the definition of a planet and a dwarf planet.
    I’m adding also that since 2006,there have been 6 presidents for the IAU ,with a turn over every 3 years, and there is an general assembly every 3 years.What i meant by this, is that they could have the opportunity to change the definition in 15 years.
    I had the chance to listen 4 times the conferences of Andre Brahic(, who told us about the Pluto debate between 9 astronomers that have to decide if Pluto has to be considered as a planet or not, and he was leading the debate, and he told us that 3 were for Pluto, 3 were against Pluto,the other 3 weren’t decided.
    I’m not aware about the vote for 2006, and the circumstances linked to this vote.
    But i clearly understand there’s a difficulty to create a true definition, but i don’t appreciate what people say that Pluto has to be a planet , because it was a planet, the 9th for 76 years., or because they lear,nt it at school.
    This makes me thinking about the General Relativity and The Quantum Physics.
    I read some publications and articles about these theories, and often , it’s presented in a simple form to be understandable for the public,
    but it’s more complicated from a mathematical point of view, like using the “Field Equations” to explain the G.R.f,or example.

    So I guess we need a complex , scientific definition of “Planet” to be agreed by the scientific community, and a more simple definition, that could be understandable for the public.
    What do you think?

    (P.S: i just read the process for the 2006 vote)

  • Edward

    Try reading the Icarus paper that is referenced in the essay. For convenience, this is the link:

    It is a long paper, but it presents a history of the definition of a planet. Be sure to check out Figure 14, as it is the most complicated Venn Diagram that I have seen or ever want to see (there are better ways of displaying the information).

    What should constitute a planet? This is an interesting question, and as the essay (not the Icarus paper) noted: “When the IAU adopted its 2006 definition, a strong motivation was keeping the number of solar system planets small, supposedly so children can memorize them.” Later, she notes: “Today, memorization makes little sense and teaches nothing about the actual planets.” Should the solar system have only a few planets so that they can be memorized or should there be a lot of planets based upon a consistent definition?

    What do I think? I’m currently ambivalent. Having nine planets and looking for planet ten makes astronomy interesting, but an inconsistent definition of a planet is not scientific at all and teaches the wrong lessons. Changing definitions in order to suit the politics of science also teaches the wrong lessons, and continually changing definitions, say every decade or two, demonstrates the confusion of the scientists, when science is supposed to eliminate confusion, superstition, and the demands of politics.

    On the other hand, I like Pluto as a planet for a variety of reasons, mainly emotional rather than scientific. For similar emotional rather than scientific reasons I don’t care whether Vesta and Ceres are planets or asteroids. Emotional responses don’t make for good consistency, and in science they are likely to result in confirmation bias.

  • Hello again,
    I have read the Icarus paper and it’s a very interesting paper, i also love the Venn diagram
    I have seen that it’s not new that planetary scientists don’t agreed with the definition of “planet”, the geophysical definition as written and introduced is complex, everyone can see it
    I have no problem for any of the small or dwarf objects,whatever it is Eris, Pluto, Ceres, Vesta, i love 67P Chury, (486958)Arrokoth, Dione,Epimetheus,Mars,Io, Bennu,or (162173) Ryugu.
    I can see a large variety of celestial bodies in our Solar System and i like that.
    Now, as previously, they should include the difference between stars and planets in the definition, and i guess that giving a true definition for “planet” will probably something quite complex, that have to keep and refer to all types of objects(not the asteroids and comets).
    That’s my opinion, and as i said, i’m not shocked by Pluto dismiss.

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