Can Pluto Be a Planet Again Already?

New enhanced-color image of Pluto from New Horizons (NASA/JHUAPL/SwRI)
Enhanced-color image of Pluto from New Horizons (NASA/JHUAPL/SwRI)

Ever since the discovery of Pluto by Clyde Tombaugh in 1930 our Solar System was known to have nine planets orbiting the Sun. “My Very Eager Mother Just Served Us Nine Pizzas” was a popular mnemonic in my elementary school days to help remember the order of major planets from Mercury outward. But in 2006, a controversial decision by the International Astronomical Union—spurred in part by Caltech astronomer Mike Brown—changed the specifications on what officially classifies a planet in the Solar System, thereby stripping Pluto of its 76-year-old designation. The reclassification, done by an in-person vote at a meeting in Prague (at which only about 400 of over 9,000 IAU members were in attendance) has been a topic of debate—often fierce—in the astronomical community ever since, and now some scientists are demanding to have it redefined again.

The new definition, based on a 2017 proposal by six planetary scientists, would classify “at least 110” known objects in the Solar System as planets—including Pluto.

While the IAU put its 2006 General Assembly vote out to astronomers, this new classification would be based on criteria determined by planetary scientists, who are arguably more well-versed in what constitutes a planet. Although the distinction may not be obvious to the average person, deferring to astronomers for science on planets is, according to New Horizons principal investigator Alan Stern, “like going to a podiatrist for brain surgery.”

“Even though they’re both doctors, they have different expertise,” Stern said. “You really should listen to planetary scientists that know something about this subject. When we look at an object like Pluto, we don’t know what else to call it.”

nh-apluto-mountains-plains-9-17-15 detail
New Horizons MVIC image of Pluto’s surface. (NASA/JHUAPL/SwRI/J. Major)

The New Horizons spacecraft made the first close pass of Pluto in July 2015. It discovered a world with a dynamic atmosphere, a surprisingly youthful surface, a complex internal structure, and basically an object that much more resembles a planet than not. Just because of its size and the circumference of its orbit—and that it may share the designation with over 109 other objects—should not exclude Pluto from being a full-fledged planet, the scientists argue.

“For an analogy, there are 88 official constellations and ~94 naturally occurring elements, yet most people are content to learn only a few,” Stern et al. state. “So it should be with planets.”

Read the rest of this article on Science Alert here: NASA scientists have proposed a new definition of planets, and Pluto could soon be back.

HT to Dr. John Barentine for the article. 



  1. SCL says:

    I understand that you want to keep Pluto as a planet. But it just isn’t. It can’t clear it’s orbital disk of debris. That large body is constantly hitting dust and debris, and does not have the mass to clear these obstacles out. Thus, it is a dwarf planet, not a planet. The link you shared explains it exactly.

    This is the danger of treating science like religion. Pluto being a planet is a dogma that people have accepted. When we have better evidence, the scientific method states that we should modify our theories to match the evidence. But instead, the Pluto lover rejects the evidence, and chooses the dogma. That is not science, it’s religion. Science is supposed to be better than this. Pluto is not a planet. Sorry.

    Liked by 1 person

    1. Nobody is going to say, “Well if Pluto can’t clear its orbital disk of debris, obviously I have been wrong my whole life.” It probably resonates with astronomers and astrophysicists who already accept Pluto’s official status, but it will not change popular opinion.

      And taking an abrasive tone that Pluto’s status is a science vs. religion issue won’t help either. There is no Church of the Sacred Ninth World worried that its very existence is in jeopardy. Pluto’s popularly-accepted status as a planet is not religion or dogma; it is an ingrained cultural notion that may no longer be valid since the paradigm of our solar system changed with the discovery of the Kuiper Belt.

      In terms of that cultural notion, it’s pretty trivial for the masses. Pluto’s designation as set forth by prescribed guidelines is not a law of gravitation nor anything else that has practical meaning in our lives. How objects within our solar system are classified is the realm of the scientific elite.

      If your assertion that Pluto is not a planet is the valid one, then it will withstand the test of time. Those that “believe” in Pluto the planet will diminish through attrition. Textbooks will convince children and university students why Pluto is discrete from the eight objects we all accept as planets. Patience always separates sound science from contemporary politics.


      1. SCL says:

        “Nobody is going to say, “Well if Pluto can’t clear its orbital disk of debris, obviously I have been wrong my whole life.””
        That’s the point of the scientific method. If evidence disproves your theory, you modify your theory to match the new evidence. A real scientist would of course say that pluto is not a planet.
        As for the masses of people, they aren’t scientists. They accept what theyre told and won’t change. Maybe you don’t like the word dogma, but we are talking about inherited notions that are disproven by later discoveries yet still held for sentimental or cultural reasons. That’s dogma. You can choose which you believe, of course that is your own choice.


        1. You took my first sentence out of context by not referencing my second in the first paragraph. My point was that making cases about orbital debris or planetary mass or the like is a complete non-starter when trying to sway public opinion on the matter (regardless of ongoing discoveries about our solar system).

          People and their ways of thinking change, all the time, just at wildly different rates.

          Is Pluto a planet? I very much doubt Pluto cares how we Earthlings classify it.


    2. Jason Major says:

      Actually, the decision to redefine what makes a planet a planet was the arbitrary part, based on what seems to be a strange fear of having “too many” planets in the Solar System (spurred by the discovery of Eris). Plus all planets hit dust and debris as they orbit the Sun, and as it’s been said even Earth wouldn’t clear its orbit were it to be at the distance of Pluto. Therein lies the problem with the 2006 definitions.

      Planets shouldn’t be defined, say Stern & Co., based on WHERE they are located but rather on their own intrinsic qualities. It’s like saying that a polar bear can only be a polar bear if it lives in the Arctic. If one were to walk down to Mexico, it’d have to be called something else. Or that pandas eat bamboo, but if a panda were found ingesting another plant it’d need a new name—regardless of its panda DNA or taxonomy.

      What it really comes down to is that this IS how science works, as opposed to religion. When new information is discovered (e.g., data from New Horizons during approach and flyby of Pluto and Charon) sometimes it forces a re-evaluation of what’s currently accepted as facts or definitions. Science has room for this; religion does not. Should the IAU outright refuse to take another look at their definition of planet—perhaps this time opening it up to more in-depth discussion from planetary scientists—they would be the ones guilty of following dogma.


      1. SCL says:

        Well said. We should always be able to reassess the definition. But looking at the new definition, I am not convinced.

        “A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a traxial ellipsoid regardless of its orbital parameters”

        AKA, the Moon. By this definition, the moon is a planet. It’s a round object in space, regardless of what it’s orbiting. This definition is not exclusive enough.


        1. Jason Major says:

          While I personally am not a fan of having moons classified as planets, this is not something that bothers planetary scientists at all. And considering that’s their realm of expertise, I guess I can be more okay with accepting moons as planets as I’d otherwise have to be with there only being eight planets allowed.


          1. SCL says:

            I’m not going to argue forever. It just strikes me as a little condescending is all. Like they said “oh, they couldn’t possibly understand the concept of planets dwarf planets and moons. Lets just call them all planets”. To me thats insulting.


            1. Jason Major says:

              At the same time, there’s something also condescending about “oh they’ll never be able to handle a solar system with more than nine planets so let’s just change the definition.” People would have adapted better to there being so much more than there’s actually less, IMHO.

              Plus there’s also the factor of exoplanets, which currently reside in the realm of astronomy but will soon require planetary scientists to determine just what they are really all about.

              Liked by 1 person

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  3. Even if our understanding of what a planet is and how many there are is changing, that adaptation can still include Pluto. The more we learn about what is really out there, the further space science can advance, instead of pretending Pluto doesn’t exist to make it easier to understand.


  4. Alan Stern has a well-earned reputation as a leading planetary scientist. But I suppose this debate is proof that even science leaders can act with emotion rather than intellect. If Pluto were discovered today, nobody would clamor for it to be a planet. There is no groundswell for Eris to be designed a planet.

    I originally resisted the idea of Pluto’s demotion, but it was reading some clear-headed commentary by Neil deGrasse Tyson that got me to stop thinking emotionally and to look at it with a clear head. Perhaps an object that “dominates” rather than “clears” the space around it would be a better definition. But Pluto is simply one of thousands of objects in a swarm. It doesn’t merit being classified with the eight planets.

    The first four asteroids were originally classified as planets, but were demoted when more began to be discovered. Ceres is far larger relatively to other asteroids than Pluto is relative to other Kuiper Belt objects. But nobody is clamoring for Ceres to be called a planet. For good reason.

    Pluto is a wonderfully interesting object to study, and fully worthy of the spacecraft sent to it. That it is not a planet does not alter this in any way. And if there really is a 10-Earth mass object out there, we can even go back to having nine planets.


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