Here’s the Answer That Will Finally Settle the “Is Pluto a Planet?” Debate for Good (Yeah, Right)

By Corey S. Powell | June 5, 2018 12:43 pm
Pluto is a beautiful world, with ice mountains, nitrogen glaciers, and methane dunes. But that doesn't make it a planet. (Credit: NASA/JHUAPL/SwRI)

Pluto is a beautiful world, with ice mountains, nitrogen glaciers, a haze-layered atmosphere, and methane dunes. But all that complexity does not necessarily make it a “planet.” (Credit: NASA/JHUAPL/SwRI)

I love Pluto. I grew up entranced by this strange little world: What could you be, you rebel that doesn’t seem to follow any of the rules? I even wrote a childhood letter to a local astronomer, offering my homespun hypothesis that Pluto might be a captured fragment of an exploded star. When the New Horizons spacecraft finally revealed the true face of Pluto, I was right there at mission control in Langley, Maryland, to watch the images as they came in.

So I have a lot of sympathy for the Pluto-lovers who were wounded when the International Astronomical Union declared that the 9th planet was not exactly a planet after all, but something called a “dwarf planet.” I also appreciate the sweet irony that the fuss over Pluto’s reclassification stirred up even more interest in the New Horizons encounter. But really, the endless effort to restore Pluto’s planetary status and relegislate the definition of a “planet” is getting tedious. Time to settle this thing.

The problem, as I see it, is that people are asking multiple questions while somehow expecting only a single answer. I’m going to be a little presumptuous here and claim that there are really two answers—and that making sense of those two answers requires breaking them down even further. For you TL;DR types, here are the top-level answers to “is Pluto a planet?”

Answer one: It depends.

Answer two: It’s irrelevant.

It depends. I know, this is an unsatisfying answer, but it’s also a truthful one. There are at least three different things that people (even professional astronomers) mean when they use the term planet. There are historical planets, dynamical planets, and geophysical planets.

Historical planets are those that have entered the common language of science and science writing as planets. Starting in 1801, the first dozen or so objects discovered orbiting between Mars and Jupiter were originally called planets. Then starting in the mid-19th century, astronomers began calling them “asteroids” or “minor planets.” By the early 20th century, popular astronomy writers like Agnes Clerke had embraced that language, and objects like Ceres and Vesta were no longer considered true planets.

Pluto went through a similar evolution. It was classified as a planet about as long as Ceres was, but with some differences. For more than 60 years Pluto seemed like one of a kind, whereas astronomers realized that Ceres was clearly part of a larger group of objects almost right from the start. And Pluto was considered a planet well into the modern era of space exploration, which gave it a special status. Historically, then, Pluto was and remains a planet—one of the nine “classical” planets circling the Sun.

Dynamical planets are defined by how they formed and how they interacted with their surroundings. This is where the IAU’s formal criterion that a planet should have “cleared the neighborhood around its orbit” comes from. The eight major planets are each the dominant objects in their regions, formed by sweeping up the smaller bodies around them. That is obviously untrue of the asteroids. At first, some scientists speculated that the asteroids were the remains of a planet that exploded. By the mid-20th century, people realized that they were actually the leftovers of a population that never accumulated into a planet in the first place.

Here, too, our thinking about Pluto went through a parallel evolution. The discovery of other objects in the Kuiper Belt, starting in 1992, confirmed a growing suspicion that Pluto belongs to a vast population of similar objects orbiting beyond Neptune. Some of these objects, such as Eris, are very similar in size to Pluto. These Kuiper Belt objects are drastically different from Neptune and the other (classical) planets, both in how they formed and in how they got to their current orbits. Dynamically, then, Pluto is definitely not a planet.

The Kuiper Belt and its surrounding region, the Scattered Disk, make up the largest zone of the planetary solar solar system (Credit: NASA)

There is a lot of stuff out there around Pluto in the Kuiper Belt and its surrounding region, called the Scattered Disk. Collectively they make up the solar system’s third zone, after the rocky and gas-giant planets. (Credit: NASA)

Geophysical planets are defined by their size, complexity, and activity. This relates to the IAU’s other criterion that a true planet should be “nearly round.” In academic conversations, planetary scientists routinely refer to any large, round, geologically varied object as a planet. They commonly refer to the moons of Jupiter this way. They commonly refer to our own Moon this way.

When the New Horizons spacecraft flew past Pluto in 2015, it revealed a remarkable world with a haze-layered atmosphere, active glaciers, floating mountains, and slow-churning chasms full of squishy frozen gases. Pluto may be less than 1/5th the diameter of Earth, but it is fully rounded by its gravity and it is wildly complex in its terrain and its processes. Geophysically, then, Pluto certainly qualifies as a planet—a point frequently made by members of the New Horizons team.

So now we have two definitions that say yes, Pluto is a planet and one that says no, it is not. How do you resolve a split verdict like this? The only sensible answer is that you don’t, because trying to force a simple answer onto a complicated question is not just contentious…

It’s irrelevant.

Arguing about the “correct” classification for Pluto will never reach a meaningful resolution, because there is more than one meaningful answer.

Historically and emotionally, Pluto is a planet. No scientific argument will ever change that.

Dynamically, Pluto is not a planet, and there’s no truly convincing way to dispute that, either. Some of Pluto’s defenders argue that the dynamical argument can’t be applied to planetary systems around other stars, but that objection doesn’t hold up. If we find a group of small objects that share similar orbits around another star, we would immediately recognize those as analogs of our asteroid belt or Kuiper belt.

The issue of orbit-clearing is where I find the whole “is it a planet?” debate most counterproductive, because fighting to elevate Pluto back to planetary status only obscures what is most important about it. Pluto is not a loner, but the largest, brightest member of the Kuiper Belt. It is the archetype of an entire class of objects, the Rosetta Stone to the solar system’s third zone.

Now that we have seen Pluto up close, we have a sense of just how rich and complicated those objects can be. Pluto is unusual in having a huge moon, Charon, that is more than 5o% its diameter, along with four smaller satellites. Pluto is also one of the closest, warmest of the Kuiper Belt objects. For these reasons, it may be something of an outlier, more geologically active than the others…but that’s just a guess.

We need to look at more of the little worlds out there to know. When we finally get a clear look at Eris and some of the other intriguing bodies out there (Quaoar, Haumea, Makemake, and so on), we may find that they are active as well. One of the most exciting lessons from New Horizons is that even at temperatures just a few dozen degrees above absolute zero, icy bodies can be geologically active. “Dwarf planet” sounds a little clunky but it’s a useful term for Pluto and its ilk: complex, spherical worlds with a compelling but non-planetary origin story.

Based on their colors and shapes alone, the denizens of the Kuiper Belt appear to be extremely varied. Learning more about that diversity will tell us a lot about how these objects formed, and by extension how the solar system as a whole came together. And all of those variations, all of those different parts of the story, will be measured against Pluto, Kuiper Belt Object #1: not a planet, exactly, but in many ways something even more wonderful.

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  • Rixware

    From my elementary school education: Many Very Early Men Ate Juicy Steaks Using No Plates.

    Pluto will always be a plate — er, planet — to me.

    • StanChaz

      Hmmm. Seems that you’re at the center of your universe,
      and everything revolves around you. Good luck with that :)

      • Jennifer Anne Bangstrom

        JEEZ! What a jive turkey answer. Get over yourself, Stan! (And sorry, guess I’m being a jive turkey too.)

    • Dirk Diggler

      My Very Expensive Motorboat Just Sank Under Navy Pier.

      That’s how I remembered them…made it up myself in 8th grade, 1981 during a contest in science class.

  • http://science20.com/robertinventor robertinventor

    The dynamical definition works well so long as the non dynamical planets are small. But what happens if we find an exoplanet the size of Jupiter that is not able to clear its orbit around a blue giant? They do have accretion disks, but these start at 100 au from the star and even Jupiter is not a dynamical planet in such a system unless at the very innermost edge of its accretion disk. Though very young, they are old enough for planets to accrete so it’s a discovery we could make any day.

    What also if we find a non dynamical planet object the size of Mercury or Mars beyond Pluto? There’s also the possibility of a sub-brown dwarf in the Oort cloud – formed like a star to make a binary system, it could be orbiting Earth, be the same mass as Jupiter, and yet wold not count as a planet beyond 1000 au, well within our solar system.

    I think the most likely reason it falls apart is the discovery of a Mercury sized or larger object maybe in a Sedna type orbit that is not a dynamical planet. Those in favour of the IAU definition would continue to say it is not a planet, but the public are not likely to pay much attention to that. This is a reasonably likely discovery according to Mike Brown who speculates on how the IAU would get into a tizzy again if this happens.

    When there are several definitions you don’t have to say it’s uncertain what it is. Any individual can just adopt one of them as the one they prefer and many planetary geologists already just call all the objects rounded under gravity planets. They often call rounded moons planets too.

    The biggest gripe I have with the IAU definition is not so much deciding the word planet has to mean “dynamical planet” which is just an executive decision and not something built into the ordinary English language use of the word.

    It is their decision to call them dwarf planets. This is distorting language too far. If it is not a planet, say i tis not a palnet and use some other word for objects rounded under gravity, not the word planet at all.

    But to say it is not a planet, but IS a “Dwarf planet” is a serious distortion of the English language. And using the word “dwarf” which is normally associated with size, to describe “non dynamical” which is only accidentally associated with size, is a serious distortion of our language. This will be shown up most of all if we find a non dynamical subbrown dwarf Jupiter. That’s a sub brown dwarf star yes but it would be so confusing to call it a dwarf planet. It would then be a dwarf planet gas giant non planet according to the IAU. Even without discovering such an object, just the fact that we can talk about it as a hypotheitical object that we could discover is so confusing.

    We could also find dwarf planet Neptune sized non planets just a little further out than “PLanet 9” which if it exists, and so long as it is just a single object, would just sneak in as a dynamical planet. That also would be a dwarf planet gas giant non planet according to the IAU.

    I don’t see any of this as a big deal. Anyone is free to call these objects planets. But I think the IAU definition probably has a “use before date” as there are many discoveries we could make that would mean fewer and fewer people use it and the general public stop using it. We’d then need another definition.

    Why not call them all planets and the ones picked out by the IAU dynamical planets as you call them in your article?

    My afticle on this topic is here:

    http://www.science20.com/robert_walker/how_a_dwarf_planet_gas_giant_could_challenge_iau_definition_pluto_ceres_haumea_etc_can_all_be_planets-224965

  • http://laurelsplutoblog.blogspot.com/ Laurel Kornfeld

    I don’t have a problem with there being two answers, one dynamical and one geophysical, but I do object to the notion that Ceres, Eris, and Pluto are just part of vast populations or belts. This puts their location ahead of their intrinsic characteristics when it comes to defining and classifying them. Doing that essentially amounts to taking the side of the dynamicists. Ceres and Pluto would geophysically be planets no matter where they are. Saying they’re not planets but “something better” is a cheap cop out and is not fair and balanced. What is wrong with classifying small planets that are spherical but don’t gravitationally dominate their orbits as the dwarf planet subcategory of the broader umbrella category of planet?

    • http://science20.com/robertinventor robertinventor

      Yes, well – except that though at present they are dwarfs, in future there could be non dynamical giants.

      I think the word “dwarf” should be based on size. Non gravitationally dominating planets should be labeled in some way that makes it clear what they are. Otherwise you still get the issue that any day you might find a non gravitationally dominant gas giant. Even a non gravitationally dominant Mercury would be an issue if you call it a dwarf when it is way beyond Neptune but not a dwarf when it is in Mercury’s orbit.

      I think we should use the word dwarf to characterize a planet according to some criterion by which if you know its size that is all you need to know to tell if it is a dwarf.

      • Corey S Powell

        That’s a sensible argument, but it runs afoul of both the dynamical and geophysical criteria. You’d lose the utility of “dwarf” telling you something about an object’s formation history, and you still run into weird boundary cases.

        Is dwarf based on diameter? Then a spherical ice ball and an irregular rocky object might both qualify as planets. Is it based on triaxial symmetry? On mass? All of these standards lead to their own weird contradictions akin to your giant planet example. It’s a broader problem of any ironclad definition.

        • David Platts

          Thank you! If size were the only determinant then cutoffs would have a magical point which has no basis in scientific determinants. Oort Cloud, Kuiper Belt and the Asteroid Belt have spherical objects that do not have the dynamics to clear a defined area within those cluttered areas.

          If (or rather when noting some of the work coming out of CalTech followed up by research from other noted institutions) Planet 9 is located, we’ll be able to observe a size that may clear a elliptical periphery beyond the Kuiper Belt and inside of the Oort Cloud (a probable guess). I don’t know how it couldn’t be true. If so, it will confirm the proper designation for dwarf planets.

          Our Solar System remains full of undiscovered mysteries. My favorite planetary scientist/physicist is Mike Brown out of CalTech. He specializes in the Kuiper Belt. He’s the scientist who has angered the Pluto is a planet group. Of course, he’s spent his life studying objects that do not absorbing dust and objects within their orbit periphery. He discovered Eris (more mass than Pluto), Makemake and a host of other large Kuiper Belt objects, far more than any other planetary scientist. He does a lot more than pop in and out of the discussion. It consumes his life in research.

    • Corey S Powell

      But we DO call them dwarf planets. How is that not a subcategory of planet?

      “Dwarf planet” may not the most musical name, but it’s a very apt description for Pluto, Ceres, Eris, etc. They are planets because of their geophysical complexity. They are dwarfs because they are members of a larger population. It makes sense to me.

  • OWilson

    What’s in a name?

    Perhaps solar “satellite” would be more appropriate and scientific nomenclature. It just turns out that there are more solar satellites than we imagined back in the dark ages.

    Maybe it is getting the astrologers and crystal ball gazing confused. Maybe the current trend to gender classification is being extended to heavenly bodies?

    Dwarf is a subjective relative term.

    Imagine the outrage if sociologists demoted dwarfs to lesser persons? :)

    • StanChaz

      Are all “solar satellites” therefore equal,
      or as in Animal Farm are some solar satellites more equal?
      As with many labels this one does not lead to greater clarity.
      And the fact that the term dwarf is relative does not make it necessarily subjective (as is, however, your callous attempt to humorously compare it to “lesser” persons).

      • OWilson

        Arbitrary names do not interest me, as long as they all revolve around the Sun.

        Pluto is to the Earth, what the Earth is to Jupiter.

        We are all god’s chillun’ :)

    • Scales7

      Pluto will however get a participation award.

    • Corey S Powell

      One point I keep trying to express: Calling something a “dwarf” planet doesn’t mean it’s worse than a “real” planet. Pluto is a fascinating object. So are comets, asteroids, even meteorites. The idea that a scientific adjective describing Pluto’s size & dynamical status is necessarily pejorative is the major reason we keep having these silly debates.

  • David Terry Dorais

    The crux of his arguments are not stated. But here it is: planets are objects that are round/spherical/ellipsoidal under the influence of their own gravity. The problem is with the word itself. Without an adjective in front of the word, it is meaningLESS. So we have terrestrial, gas giant, ice giant, KBO, trans-Uranic, maybe even someday Oortal, let alone exo-; but the Pluto is a dwarf due to its size AND orbital dynamics, as discussed in the article. The use now of an adjective describing what KIND of planet or satellite or orbital body it is based on its qualities. IAU needs to step up and state this explicitly to end the arguments over what older adult and children learned that is NO LONGER TRUE in light of new data that has come in from space probes. This is especially true of objects which orbit larger bodies, usually planets, that are called moons. Many of these moons are more properly called natural satellites or captured asteroids because, IMO they are NOT round under their own gravity. The definition of MOON needs tightening up just like PLANET. We need in other words a complete spectrum of size/shape/behavior boundary values and definitions that range from gas clouds/dust all the way to brown dwarfs, defined as failed stars. Then we can then apply these classifications to ALL star systems (notice I did NOT say solar) throughout the galaxy.

  • StanChaz

    It’s like asking whether Trump is truly a President.
    He has the trappings, and yes, from far away he might even look the part in some ways.
    But come closer and you’ll see & hear a traitorous collection of twittering tantrums and flapping hair, as he periodically erupts in chaotic orange-colored bursts, heated exchanges, and destructive lies.
    A poor imitation of a true President indeed, blindly revolving around the poisonous planet Foxnews, as the impeachment asteroid Mueller speedily approaches to seal his fate.
    So sad, so sad…

    • Bill Mayberry

      Uh huh. Still beats Planet Narcissus.
      Named after Barack Obama, which has increasingly become part of the “irrelevant” category. Thank Jupiter.

    • Dirk Diggler

      Wow, butt hurt much snowflake?? Get over it…he IS President and will be President in 2020 thanks to the behavior on the left side.

      And you must really be hating the strong economy, bringing jobs back, the surge in the Dow Jones, tightening security, and cleaning up Washington.

      That fact that you would have rather had a criminal bitch in the oval office speaks VOLUMES of your undeniable stupidity. Keep drinking the Kool-Aid, a55hole.

  • jimgrot

    What is the density of the objects in the Kuiper belt? In your pictures the”haze of objects” is vastly over scaled. If you drew the inner solar system and all of its objects at the same scale of the things typically drawn for Kuiper belt objects then the whole solar system would be uncleared and nothing would qualify as a planet. The clearing argument is “rigged”.

    • Corey S Powell

      Orbit clearing is a clunky way to quantify a judgement about which objects are unique and which are parts of larger populations…but the nature of taxonomy is that we need to draw boundaries in order to create useful categories, and this is at least a functionally useful way to do that.

      Calling every round object a planet makes the categories less useful and more confusing. And why stop there? The Sun was once considered a planet. Earth creates its own internal energy through nuclear fission. So should stars be planets too? This way lies confusion, not clarity.

    • David Platts

      One of the most interesting topics of study in planetary science is how any planetary system reaches a mature state or falls apart. I’ve taken two of these courses. One of the integral parts of understanding how an object grows by feeding off the dust and objects within their orbital path are essential knowledge. This is not hazy or rigged. The understanding of how this works mathematically explains how planet size and state of existence (rocky, gas, ice) happen. We begin to understand why the Asteroid Belt’s fate was determined (collisions without strong formations caused by the effect of the mass of the Sun’s influence within and the influence of the size of Jupiter outside of that Belt). The Kuiper Belt and Oort Cloud are a bit different but follow a path of inability to grow by clearing material within it’s orbit. If it were true, the Kuiper Belt would be much, much less cluttered with larger planetary masses. It didn’t happen.

      I am interested in the effort to locate Planet 9. Many Kuiper Belt objects have extreme elliptical orbits that suggest influence within (Neptune is known) and an outside influence (not known yet a mathematical probability exists of another large planetary body). Mike Brown (CalTech) and a host of plaentary science researchers are looking for something much more difficult to see than a small needle in a haystack the size of Europe. Brown’s located we’ll over 60 Kuiper Belt objects (the last report I saw over a year ago). I do believe it can be discovered (maybe in a decade or so). If they locate a Planet 9 and it, too, has cleared a orbital path, they’ll be no more conjecture on what a planet is. The IAU would no longer face this criticism. The definition for dwarf planet would be correct.

  • Erik Bosma

    I don’t think that Pluto really cares one iota about what a group of life forms on a rock orbiting the same star thinks it should be named. I can’t believe we would anthropomorphize a planet 5 billion km. from here. Pluto has been described in a certain way, by us, to help us in our classification systems and no matter what we may lovingly call it, it is what it is.
    Could be a financial opportunity here for someone to flood the market with a cute little stuffed pillow saying “When I grow up I wanna be a pwanet too…!”

  • http://www.thoughtcrimes.biz/ Thought_Crimes
    • PremiumMind

      Nice

  • PremiumMind

    Dwarf or Giant, Pluto is still a planet. So should we also start telling anyone who is under 4 feet tall that they aren’t human? The spherical shape and complexity of Pluto’s environment clearly indicates that it is a planet.

    • Corey S Powell

      So if a dwarf human is called a “dwarf human,” why is it wrong to call a dwarf planet a “dwarf planet”? You seem to be agreeing with me.

  • Flain fan

    “The issue of orbit-clearing is where I find the whole “is it a planet?” debate most counterproductive, because fighting to elevate Pluto back to planetary status only obscures what is most important about it. Pluto is not a loner, but the largest, brightest member of the Kuiper Belt. It is the archetype of an entire class of objects, the Rosetta Stone to the solar system’s third zone.”

    This is where the article lost all credibility.

    • Corey S Powell

      How so?

      • Flain fan

        You’re seriously replying to a 2 month old comment?

        • Corey S Powell

          Yes! The new Metzger paper inspired me to go back and look at comments I had missed.

  • Matthew Orante

    Dear NASA,
    Your mom thought I was big enough.
    Love Pluto

  • HollyWeirdIsFascist

    Apparently in America you can choose if Pluto is a planet or not. They also claim you can choose your gender too. What the heck is going on in America?

    • Corey S Powell

      And both issues are best approached by gathering information and looking at the facts, not by making rhetorical attacks.

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Out There

Notes from the far edge of space, astronomy, and physics.

About Corey S. Powell

Corey S. Powell is DISCOVER's Editor at Large and former Editor in Chief. Previously he has sat on the board of editors of Scientific American, taught science journalism at NYU, and been fired from NASA. Corey is the author of "20 Ways the World Could End," one of the first doomsday manuals, and "God in the Equation," an examination of the spiritual impulse in modern cosmology. He lives in Brooklyn, under nearly starless skies.

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