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Pluto as ‘Planet’

I have never been exactly indignant about the demotion of Pluto to ‘dwarf planet’ status but I do think it’s curious and in at least one respect too arbitrary for my taste. I’ll buy the idea that a planet needs to be round because of its own gravity, and I’ll sign off on the notion that to be a planet, an object has to be in orbit around the Sun (even though we do have apparent wandering planets in the interstellar deep, far from any star). But the International Astronomical Union also decided in its 2006 deliberations that a planet has to ‘clear’ its neighborhood of debris, thus sweeping out its orbit over time. That one, of course, is controversial.

Assuming the Earth is a planet, why are we worried about things like Near Earth Asteroids (NEAs)? Our planet clearly hasn’t swept out its neighborhood, not when we can number problematic asteroids in the thousands. Jupiter is estimated to have about 100,000 trojan asteroids in its orbital path as well, and Alan Stern, principal investigator for New Horizons, has pointed out on more than one occasion that if Neptune (obviously a planet) had cleared its orbit, Pluto itself wouldn’t be there, and the whole discussion might never have come up.

For that matter, we would not be having this discussion now if not for continuing discoveries in the Kuiper Belt that more or less force the issue. Eris was found in 2005, a world more massive than Pluto and first thought to be larger as well, leading to the temporary designation of it as the ‘tenth planet,’ and raising the question of what to do about a Kuiper Belt that might be stuffed with such objects. Here the matter seems to be one of simple redundancy. Nine, or eight, planets, is acceptable. Several thousand is not — it seems to devalue the notion altogether.

I can see why working this out is still controversial. Meanwhile, from a purely aesthetic point of view, I have a hard time looking at the image below and seeing it as anything other than a planet. Or, as I have said before, the larger component of a binary planetary system.

Fig1_Pluto-Mountains-Plains 9-17-15

Image: Just 15 minutes after its closest approach to Pluto on July 14, 2015, NASA’s New Horizons spacecraft looked back toward the sun and captured a near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto’s horizon. The smooth expanse of the informally named Sputnik Planum (right) is flanked to the west (left) by rugged mountains up to 3,500 meters high, including the informally named Norgay Montes in the foreground and Hillary Montes on the skyline. The backlighting highlights more than a dozen layers of haze in Pluto’s tenuous but distended atmosphere. The image was taken from a distance of 18,000 kilometers to Pluto; the scene is 380 kilometers across. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

We see mountains here, plains, glaciers, but what really catches the eye is the multi-layered haze in Pluto’s thin nitrogen atmosphere. Here we’re looking at more than a dozen such layers extending all the way from the surface up to 100 kilometers. Mountains falling into shadow at sunset and a bank of low-lying haze near the terminator give us the sense of looking at a place in a constant state of change, a world laden with active patterns of weather. We seem to have the Plutonian version of a ‘hydrological’ cycle here, substituting nitrogen for water ice.

“We did not expect to find hints of a nitrogen-based glacial cycle on Pluto operating in the frigid conditions of the outer solar system,” said Alan Howard, a member of the mission’s Geology, Geophysics and Imaging team from the University of Virginia, Charlottesville. “Driven by dim sunlight, this would be directly comparable to the hydrological cycle that feeds ice caps on Earth, where water is evaporated from the oceans, falls as snow, and returns to the seas through glacial flow.”

Call this world what you will, the imagery is getting better and better. Here are some recent releases.


Image: In this small section of the larger crescent image of Pluto, taken by NASA’s New Horizons just 15 minutes after the spacecraft’s closest approach on July 14, 2015, the setting sun illuminates a fog or near-surface haze, which is cut by the parallel shadows of many local hills and small mountains. The image was taken from a distance of 18,000 kilometers, and the width of the image is 185 kilometers. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Fig3_Overview_reduced-Annotated 9-17-15

Image: Sputnik Planum is the informal name of the smooth, light-bulb shaped region on the left of this composite of several New Horizons images of Pluto. The brilliantly white upland region to the right may be coated by nitrogen ice that has been transported through the atmosphere from the surface of Sputnik Planum, and deposited on these uplands. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.


Image: Ice (probably frozen nitrogen) that appears to have accumulated on the uplands on the right side of this 630-kilometer wide image is draining from Pluto’s mountains onto the informally named Sputnik Planum through the 3- to 8- kilometer wide valleys indicated by the red arrows. The flow front of the ice moving into Sputnik Planum is outlined by the blue arrows. The origin of the ridges and pits on the right side of the image remains uncertain. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.


Image: This image covers the same region as the image above, but is re-projected from the oblique, backlit view shown in the new crescent image of Pluto. The backlighting highlights the intricate flow lines on the glaciers. The flow front of the ice moving into the informally named Sputnik Planum is outlined by the blue arrows. The origin of the ridges and pits on the right side of the image remains uncertain. This image is 630 kilometers across. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.


Comments on this entry are closed.

  • David S. F. Portree September 21, 2015, 9:40


    I see our habit of regarding Pluto as a planet as an historical accident. I explore that concept here: http://spaceflighthistory.blogspot.com/2015/09/pluto-alternate-history.html


  • Mike Scott September 21, 2015, 9:55

    Jupiter and Neptune have cleared their orbits because the smaller debris in or crossing their orbits (the Trojan asteroids and Pluto respectively) is gravitationally bound to them — the Trojan asteroids are obviously in a 1:1 resonance with Jupiter, and Pluto is in a 3:2 resonance with Neptune. If you have evidence that any of the near-Earth asteroids that cross Earth’s orbit is in a stable orbit that’s not extremely elliptical and is not in a gravitational resonance with the Earth then you should let the IAU know, since that would in fact be evidence that the Earth is not a planet.

  • Ron S September 21, 2015, 10:46

    Labels tend to be generational, based on what we’ve each become accustomed. It’s only controversial to those who have reached a certain age. Here is a cute, and true, story from a house party I attended in another city several years ago.

    Among the crowd of adults there were a few children wandering about being rather bored. Meeting one young girl (9 at the time, I believe) I pulled out my smart phone thinking to let her play some games to relieve her boredom. Her mother mentioned she had other interests so I showed her an app that would display a sky map corresponding to where the phone was pointed. It’s a guide to the night sky, and a useful sky watching tool.

    I let her wander off with the phone while I returned to chatting with the other adults. Some time later she returned and handed back the phone. But she had a frown on her face. When I asked her if she enjoyed playing with the app she sadly informed me that the program was wrong.

    There was an option to display various objects of interest, one of which was the planets. When she enabled this feature the positions of 9 planets were displayed on the sky map. But, she informed me, Pluto isn’t a planet.

    I was delighted. Here was this child who knew enough about the subject to know about Pluto’s current status. I explained to her what this was all about, which seemed to satisfy her, although I could tell she found the debate about nomenclature confusing.

    The surrounding adults then got into a brief discussion and argument that was, to be polite, less well informed. I think the next generation will handle this issue just fine, no matter what some of our generation might want.

  • Carl Bergmanson September 21, 2015, 12:12

    Very Nice.

    The one point I would disagree with is this:

    “Nine, or eight, planets, is acceptable. Several thousand is not — it seems to devalue the notion altogether.”

    for two reasons:

    1) The number of planets is simply that, the number of planets, whether it is 6, 9 or several thousand. The number of stars is uncountable (at least to our abilities at this time). They are still stars, regardless of what any “authority” might decree. The number is unimportant, we should endeavor to learn as much as we can about as many of them as we can.

    2) While there is much “talk” about how the Kuiper Belt may be full of objects the size of Pluto or larger, and I will concede that it is possible, the reality is that none even close in size has been found (Eris is not a Kuiper Belt Object – it is a Scattered Disk Object – which opens up a whole different can of worms regarding its planetary status, although I regard it as a planet).

  • ljk September 21, 2015, 12:31

    It is so sad that so many children have to grow up and become misinformed (or uninformed) adults.

  • Brett September 21, 2015, 12:57

    “Minor planet” is fine for me, and helps to distinguish it from smaller objects that can’t pull themselves into a differentiated sphere. Even aside from the “clearing the orbit” factor, Pluto really is odd compared to the Major Eight Planets – an orbit not in the plane of the solar system, a mass and size much smaller than the Moon, and so forth. It’s always been the odd planet out even before they discovered Eris.

  • tchernik September 21, 2015, 13:18

    Sadly this decision was made before Pluto was more than a smudge of light in a telescope. A decision motivated more by the comfort of having just 8 planets and some debris, than by reason. Alas, the Solar System and the Universe doesn’t have to follow whatever is convenient for us to remember.

    Besides, I think that after seeing this world in all its beauty and wonder, we would have never accepted such mislabeling.

    But in the end, labels don’t matter. This is another world, and its existential wonder remains no matter what we chose to call it.

    And I think we will come to regret this decision, eventually, because some suspect there is a lot more that we can imagine in the outer reaches of the Sun’s gravitational domain. One day, when we figure out there are maybe hundreds of Pluto size of greater worlds from here to the Oort cloud, what would we call them then?

  • Francois Belanger September 21, 2015, 13:20

    Your readers may enjoy this article which contains more information and data (see table 1) on the “clearing the neighborhood” criterion: http://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf

    The difference, in orders of magnitude, between the official planets and dwarf planets on this parameter, looks rather convincing to the lay person that I am. Obviously the cut-off value between the two status could be totally arbitrary, and I wonder if there is any official value for roundness in the IAU definition of a planet.

    Please keep up your excellent work, by the way :)

  • Hans Bausewein September 21, 2015, 14:05

    From a viewpoint outside the Solar system Pluto would be a dwarf (or minor) planet, just like Mars, Earth, Venus and Mercury.

    Maybe we, humans, still have to grow up ;-)

  • ljk September 21, 2015, 14:56

    Another parallel between Mariner 4 at Mars and New Horizons at Pluto half a century later: The atmospheric pressure recorded from Earth was higher than what the probes found at their respective worlds.

    See here for the details:


    Conclusion: Nothing like actually going to an alien world directly to find out what is really going on there.

  • andy September 21, 2015, 15:24

    Io, Iapetus, Europa, Enceladus, Triton, Titan… the solar system has many fascinating worlds that are not planets. Pluto is yet another member of that list.

  • Tom Mazanec September 21, 2015, 15:47

    Ironically, I always disagreed…when Pluto was a planet I thought it shouldn’t be, and now I think it should!

    Here is the best answer I know of:

  • Astronist September 21, 2015, 16:42

    Paul, the idea that Pluto was “demoted” is surely a myth. The IAU decided that it is a dwarf planet, which is perfectly sensible. It is a planet, and it is of a size that is a dwarf compared to other planets. The Sun is a dwarf star, and no less a star for that. Jupiter is a giant planet, and no less a planet for that.

    The IAU then tried to say that a dwarf planet is not a planet, which is manifest nonsense. Why did they do this?

    The underlying reason was explained in Alan Boyle’s book in terms of the conflict between the planetologists and the orbital dynamicists. The former regard a planet as you do, as a body with interesting surface phenomena which is not itself in orbit around an even larger one. The orbital dynamicists are more interested in the process of accretion of planetesimals.

    Take Earth: it embodies more than 98% of all the matter in its orbital zone (a fuzzy concept, by the way, and possibly a fatal vulnerability to this viewpoint). Almost all the rest is embodied in the Moon, which closely orbits Earth, and the rest is insignificant crumbs by comparison. Ceres, by contrast, holds only one third of the mass in the asteroid belt. It should therefore be clear that Ceres, and likewise Pluto, are from the viewpoint of an orbital dynamicist members of populations of similar bodies, while the eight major planets are unique in their “orbital zones”.

    Or to put it another way, in the asteroid and Kuiper belts the process of accretion has stalled with a number of bodies of broadly similar mass coexisting, while in the case of the eight major planets the process of accretion has reached its conclusion, with almost all the mass incorporated into a single body, and most of the remaining leftovers orbiting it or resonant with it.

    The obvious solution is to specify the orbital characteristics and body characteristics separately, as I tried to do earlier (http://www.astronist.co.uk/astro_ev/2015/ae109.shtml).

    Meanwhile, several people have now pointed out that the new vision of the solar system is: four giant planets, four terrestrials, and dozens of dwarfs. This surely makes sense? (John Davies, Alan Boyle, Alan Stern, precise refs on the webpage given above.)

    Oxford, UK

  • Randy McDonald September 21, 2015, 16:48

    Pluto was misidentified in the first place. Tombaugh et al believed that they had found a roughly Earth-mass world responsible for tugging on the orbits of the ice giants. Even at the time, there was some suggestion this was premature. As time progressed and astronomers learned more about Pluto, discovering first how small it was, then noting it was one ofany like bodies, the more it became clear Pluto did not fit.

    Definitions change, science learns more. Reclassifying Pluto as a dwarf planet hardly makes it less worthy of attention. What about Titan, or Europa, or Ceres? Are they intrinsically less interesting because they are not planets?

  • ljk September 21, 2015, 16:52

    How about calling them all Plutoids after their most famous member by far?

    That way future colonists can call themselves Plutoidians and be the envy of the Sol system.

  • Frank Smith September 21, 2015, 17:38

    Getting away from the planetary debate, I find those images fascinating.

    That nitrogen “ice” must be very fluid to flow in Pluto’s 1/12th earth gravity. I would think Sputnik Planum is a old basin now nearly filled with flow material.

    Would the “ice” flow mimic pahoehoe, where the top of the flow is solid and the warmer liquid nitrogen stays liquid for large distances?

  • Schwartz September 21, 2015, 19:24

    Ironically, it was Alan Stern himself who came up with the “cleared the neighbourhood” criterium (http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf), and pointed out the huge difference between the major players and the wannabes. Steve Soter (see Francois Belanger above for the paper’s link) showed it empirically.

    If you think Earth hasn’t cleared its orbit, you don’t understand the concept. As Mike Brown put it, you can clean a room, but it immediately gets dirty again. Things keep falling into Earth’s orbit, but Earth keeps kicking them out. Re trojans and Pluto-Neptune, Mike Scott, above, explained the concept of being captured into a resonance; it’s similar to being a satellite.

    On the other side, there is no sharp division between round and non-round, just a slow transition. Cleave nature at the joints.

    Historically, Ceres was de-listed as a planet despite being then-assumed round and having a then-estimated 3000 km diameter (larger than Pluto). Similarly, the Gallilean moons are large enough and round enough to be planets, but they weren’t long considered so because of dynamic considerations. “Being the local boss” is the traditional understanding of a planet, and the IAU just formalised it. Pluto was only ever considered a planet because of massive PR by the discovering observatory (there were early objections), combined with ignorance of its in-belt situation.

    Pluto may be an unter-planet (see Stern’s paper), but it is uber-cool.

  • Christopher L. Bennett September 21, 2015, 19:28

    I think the main problem with the definition is the paradoxical notion that a dwarf planet is not a planet. That doesn’t make any sense, linguistically or otherwise. A dwarf star is still a star (unless it’s a brown dwarf, but that’s another debate); a dwarf galaxy is still a galaxy; a dwarf palm tree is still a palm tree. Let’s just call dwarf planets another category of planet alongside giant planets, superterrestrial planets, and terrestrial planets. Instead of obsessing over whether Sol system should have 8 planets or thousands of planets, let’s just say it has four giant planets, four terrestrial planets, and a large and as yet undetermined number of dwarf planets.

    After all, there are massive differences in characteristics between a giant planet and a terrestrial planet. If we hadn’t been saddled with the label “planet” from the ancients and had discovered those different types of object from scratch, we probably wouldn’t even have given them the same name to begin with. So let’s break free of the antiquated notion that the taxonomy reduces to the binary of planet vs. not-planet, and get smarter about it, defining a continuum of distinct planet size classes as we have for stars.

  • Terry Moseley September 21, 2015, 19:54

    Criteria for definitions can be very arbitrary. For example in terms of sphericity, Pluto is much rounder than either Jupiter or Saturn! Yes, I know that their oblateness is due to their fast rotation and mostly gaseous composition, but they are still not spherical.
    And Mars has not fully cleared its orbit either, but it’s still a planet.
    Another example of an arbitrary criterion is that of the satellites of Saturn – where is the dividing line between the smallest so-called moons, and the largest ring particles? One could say that every ring particle is in fact a small moon!
    Another factor is that Pluto has 5 moons – more than Mercury, Venus, Earth & Mars combined!
    So my head tells me that Pluto is not a planet, but my ‘heart’ tells me that it is! It’s got such ‘personality’ it’s bound to be a planet!

  • David S. F. Portree September 21, 2015, 21:26

    The responses I see here convince me that the IAU planet definition needs to be revised. Folks here are interpreting the definition in a number of different ways which they think make it work. That can’t be good. It’s not meant to be a poem or an abstract painting, it’s meant to be a scientific definition.


  • Marko Amnell September 21, 2015, 22:25

    ” …Pluto really is odd compared to the Major Eight Planets – an orbit not in the plane of the solar system, a mass and size much smaller than the Moon, and so forth. It’s always been the odd planet out even before they discovered Eris.”

    Note, however, that until 1973, Pluto was thought to be about as massive as Mars (which has a mass of 0.107 Earths). So for over 40 years after its discovery, it was *not* thought that Pluto had “a mass and size much smaller than the Moon.” Here is how the mass estimates for Pluto changed over the years:

    Year Mass

    1931 1 Earth Nicholson & Mayall
    1942 0.91 Earth Wylie
    1948 0.1 (1/10 Earth) Kuiper
    1973 0.025 (1/40 Earth) Rawlins
    1976 .01 (1/100 Earth) Cruikshank, Pilcher, & Morrison
    1978 .002 (1/500 Earth) Christy & Harrington
    2006 0.00218 (1/459 Earth) Buie et al.

    The 1973 estimate was interesting:
    “In 1973, Dennis Rawlins conjectured, based on the similarities in the periodicity and amplitude of brightness variation between Pluto and Neptune’s moon Triton, that Pluto’s mass must be similar to Triton’s. This is, in fact, true, and had been argued by astronomers Walter Baade and E. C. Bower as early as 1934. However, because Triton’s mass was then believed to be roughly 2.5% that of the Earth–Moon system (more than ten times its actual value), Rawlins’s determination for Pluto’s mass was similarly incorrect.”

    Pluto’s size was not correctly determined until 1978, when Charon was discovered.

  • Jim Strom September 21, 2015, 23:44

    The semantics debate is a fun sidebar, but the article’s suggestion of possibly thousands of bodies like Ceres or Pluto was my takeaway. Does anyony have some recommended readings on this topic?

  • John September 22, 2015, 7:30

    @ Frank Smith: The ice might well be a mix of N2 CO, methane and organics, so it’s fluidity might not be easy to nail down. Tens of meters below the surface the temperature and pressure conditions might be right for liquid nitrogen, so perhaps the ice at lower levels is like that of many glaciers on Earth; mostly solid with fluid veins and planes that help it slip. N2 ice itslef does some odd stuff though – video of N2 ice undergoing an explosive change from amorphous to crystalline state here with a bit of explanation: http://ancientsolarsystem.blogspot.co.uk/2015/08/the-exploding-glaciers-of-pluto.html or if that doesn’t work, here: https://youtu.be/c0rK2bLTimQ

  • Frank Smith September 22, 2015, 15:42

    Thanks for the blog link! I’ve added it to my bookmarks. My other thought is the utter lack of impact craters in the flows (however they operate) means the nitrogen flows are geologically very recent. I can’t wait for the planetary geologists to publish some papers on Pluto.

  • John Freeman September 22, 2015, 17:33

    Me either Frank!

  • Laurel Kornfeld September 22, 2015, 22:34

    Alan Stern is the person who coined the term “dwarf planet,” but he intended it to refer to a third class of planets in addition to terrestrials and jovians, not to refer to non-planets. The four percent of the IAU who voted in 2006 misused his term.

    Furthermore, Stern never in his study said that “unter planets” should not be considered a subclass of planets, or that clearing its orbit should be a criteria for an object to be a planet. He simply divided the solar system’s planets into two categories, those that gravitationally dominate their orbits, and those that do not.

    Significantly, most of the four percent of the IAU who voted on the controversial 2006 decision were not planetary scientists but other types of astronomers. Their decision was immediately rejected in a formal petition signed by hundreds of professional astronomers led by Stern. In 2009, these scientists requested the IAU reopen the discussion at that year’s General Assembly. The IAU leadership refused, resulting in the petitioning scientists boycotting the GA.

    Ron S, an opportunity was missed with that nine-year-old, specifically the chance to teach her that scientists can disagree with one another and that there can be more than one legitimate scientific position. I write a blog about Pluto and do a lot of community outreach and have not seen any generational differences. Many children, even younger than nine, are aware of the debate and are emphatic in their views that Pluto is a planet. What children are taught at home and at school seems to vary a lot depending on their individual parents, teachers, siblings, etc.

    Those of us who oppose the IAU definition do so from the standpoint of favoring a geophysical rather than dynamical definition of planet. According to the geophysical planet definition, a planet is any non-self-luminous spheroidal body orbiting a star, free floating in space, or even orbiting another planet. If a celestial object is large enough and massive enough to be squeezed into a round or nearly round shape by its own gravity, it is a planet. Spherical moons of planets are complex worlds that have many of the same processes and characteristics as the primary planets; the only difference is they orbit other planets rather than orbit the Sun directly.

    Using this definition, the 19th century demotion of Ceres clearly was an error. Astronomers of the time did not have telescopes powerful enough to resolve Ceres into a disk, so they had no idea it is round and therefore a small planet. Today, we know it is spherical and a complex world that could host an underground ocean, a world shown by the Dawn mission to be very similar to the terrestrial planets, only smaller.

    There really are no “Major Eight Planets.” One has to cherry pick characteristics to lump these eight objects together and exclude other objects. For example, Earth has more in common with Pluto than it does with Jupiter. Both Earth and Pluto have solid surfaces where we can land rovers and landers. Both are geologically differentiated into core, mantle, and crust. Both have large moons formed through a giant impact. Both have nitrogen in their atmospheres. In contrast, Jupiter is gaseous with a composition akin to that of the Sun, almost all hydrogen and helium. It has no known solid surface and has its own “mini solar system” of moons and rings. Yet those who argue for the “Big Eight” put Earth and Jupiter in the same category and exclude Pluto.

    In science, you first do the experiment, then reach a conclusion. Four percent of the IAU voted in 2006 before anyone even had a look at Pluto, in spite of the fact that they knew a spacecraft was on its way there. Ironically, that spacecraft has shown Pluto to be more Earthlike than any other solar system planet. It has a hydrological cycle, flowing ices, an internal heat source, and possibly a subsurface ocean. My head tells me that this world cannot be anything but a planet.

    I encourage anyone interested to check out my blog at http://laurelsplutoblog.blogspot.com

  • ljk September 23, 2015, 10:39

    If Pluto had been found, say, fifty years later than it was, would this planet label controversy still exist?


  • Peter Gilkes September 23, 2015, 11:13

    I’m almost certainly the least academic poster ever, but Pluto is the most excellent (dwarf) planet in the solar system and I hope Icehenge shows up on one of the photos.

    What I really want to know is will tbe data be studied by dwarf planetologist, or by full sized ones?

  • Robert September 23, 2015, 12:30

    It was stupid and short sighted to make an uneccesary controversy over really nothing. They should have just left the designation alone justified by historical precedence and added a footnote. Money for planetary studies is mainly government welfare for scientists so why go out of their way to anger the public? Just stupid!

  • Omnologos September 23, 2015, 19:19

    I second Astronist’s proposal.

    I also keep getting astonished to see people with astronomical knowledge speak of Pluto as being _demoted_ to “dwarf planet”.

    Such language is politically incorrect in a spectacular way. Short people are still people.

    Furthermore the idea that smaller planetary bodies are somehow less interesting and therefore less important is childish in the extreme.

    The sad side of this story is that a generation of children are being taught 8 planets and that’s it about the Solar System. In the meanwhile one of the most fantastic planets has been disappeared from their textbooks. Shame, shame.

  • Randy McDonald September 24, 2015, 20:17


    “Such language is politically incorrect in a spectacular way.”

    What are you talking about? Celestial bodies are not people.

    “Furthermore the idea that smaller planetary bodies are somehow less interesting and therefore less important is childish in the extreme.”

    The only people saying that, it seems, are the people who are saying that Pluto must be a planet because it is interesting.

  • David S. F. Portree September 24, 2015, 20:27

    The debate over Pluto’s status is fascinating and probably heralds many more debates as we press outward. The universe will no doubt stump our most carefully wrought definitions; that is, it will throw us many examples that don’t quite fit any definition.

    It doesn’t help efforts to pigeonhole Pluto that we have some people trying to stir up emotions. Such people have used interest in Pluto’s status to draw attention to themselves. I’m thinking here, for example, of a certain NY astronomy performer who cut Pluto from the family of planets well before the IAU.

    The notion of “dwarf planet” unnecessarily complicates the picture we have of our Solar System and other planetary systems. It just isn’t necessary to invent a new category of body. The definition has been said to be based on dynamics, which is fine, but it’s dynamics that make little sense unless one introduces arbitrary conditions.

    Pluto should be grandfathered in as a planet. That’s not scientific, it’s cultural, but that’s OK. Scientists would still call Pluto a KBO or TNO, since that’s what it is. A KBO or TNO is a subclass of asteroid.

    I suggest that we keep Pluto on as a relic of olden days, much as we’ve kept the term “planet.” Strictly speaking, going by the original definition, Earth, Uranus, Neptune, and Pluto are not planets, because they do not wander in earthly skies. Planet comes from the Greek and mean “wanderer.”

    We overlook that fact nowadays, and no one much cares. We even apply the term to worlds orbiting around other stars.

    Our public definitions don’t have to make sense. The scientists can keep them straight. Most (but not all – some have linked Pluto’s planet status to their own self-image) have no great concern over whether Pluto is a planet. They are too busy exploring it.


  • Randy McDonald September 25, 2015, 11:10


    “It just isn’t necessary to invent a new category of body.”

    I wonder if it might.

    If Pluto had not been found by Tombaugh in the 1930s but later–perhaps in the 1950s, perhaps in the 1980s–what would have happened? There seems even at the time to have been some skepticism that this newly-discovered world was a fully-fledged planet. What would have happened if it was found at a time when Pluto’s mass was better-characterized? If other Kuiper belt worlds were found at roughly the same time, as was possible given precovery methods indicate many were picked up in the 1950s, I would suggest that it would be more likely that Pluto would be seen not as a planet but rather as something like a dwarf planet.

    Public names do matter. The criteria of the IAU, particularly of clearing its orbit, makes some sense. Yes, Pluto is a complex world, but so is Titan, and Europa, and even the Moon. Should we make them planets, too?

    In the end, all of these bodies are worlds, complex and full of wonders. The precise type of world–dwarf planet, satellite, any variety of planet, any variety mentioned before–really does not matter.

  • Francois Belanger November 10, 2015, 20:50

    In case you missed it, Jean-Luc Margot from UCLA recently published a paper on the same topic, highlighting a quantitative method for determining a planet is a ‘regular’ or a ‘dwarf’ one.

    The paper is available here: