Sciency Words: Planet (An A to Z Challenge Post)

Today’s post is a special A to Z Challenge edition of Sciency Words, an ongoing series here on Planet Pailly where we take a look at some interesting science or science related term so we can all expand our scientific vocabularies together. In today’s post, P is for:


In 2006, the International Astronomy Union approved a new, official definition of planet, and Pluto didn’t make the cut. Word has it Pluto took the news well.

The I.A.U.’s concern at the time was that more and more small, Pluto-like objects were being discovered, making Pluto seem less like the ninth planet and more like the first of some new class of thing.

To be fair, the I.A.U. did try to come up with a planet definition that would include Pluto while excluding the dozens or perhaps hundreds of other objects potentially out there. But it just didn’t work out.

So to meet the official, I.A.U. sanctioned definition, an astronomical body must meet three requirements:

  • It must orbit the Sun.
  • It must be spherical, due to the pull of its own gravity.
  • It must have cleared its orbital path of debris (this is the part of the planet test that Pluto failed).

Of course, if a definition can be changed once, it can be changed again. Recently, a group of six NASA scientists—specifically, six scientists from NASA’s New Horizons mission to Pluto—put forward a new proposal, which reads:

  • 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 triaxial ellipsoid regardless of its orbital parameters.

In other words, if it’s round, and it’s not a star or wasn’t a star at some point in the past, then it’s a planet. Under this new definition, Pluto’s back in the planet club! And so is the Moon, weirdly enough, along with many other moons elsewhere in the Solar System. In fact, the new definition would reclassify over one hundred Solar System objects as planets—possibly more than that.

The next I.A.U. general assembly meeting will be held in August, 2018. If they’re going to change the definition of planet again, that’s when they’ll do it. But I very much doubt it’ll happen.

Even though this is probably a lost cause, I want to say something in defense of the New Horizons team’s proposal. The strongest objection seems to be that moons should not be planets. I get that, but in my mind any world that I can picture myself standing on or walking on… I don’t know, that just feels planet-y to me.

I frequently catch myself calling Titan and Europa planets, even though they’re moons. Same for Pluto, Eris, Ceres, and all the other objects currently in the dwarf planet category. And I can’t help myself, but I keep calling Endor from Star Wars a planet, even though it’s specifically referred to multiple times in dialogue as a “forest moon.” All of these places—even fictional moons like Endor—feel planet-y to me.

And yes, even the Moon—the most quintessential moon of them all—has a certain planet-esque quality to it when I imagine myself living there, walking around, going about my daily business. I could get used to the Moon being a planet.

Next time on Sciency Words: A to Z, we’ll shrink from planet-scale to the scale of subatomic particles, and we’ll find out what’s so quantum about quantum mechanics.

37 thoughts on “Sciency Words: Planet (An A to Z Challenge Post)

  1. I still had a problem with this definition when it was proposed. There are two groups of objects, those that have cleared their orbit and those that haven’t. It would be useful to have a term for each group, and so we have planets for the first and dwarf planets for the second. The problem is that by convention, the dwarf prefix indicates a subclass, which was not the case for dwarf planets and planets. Since that’s where the problem is, then that’s where the solution needs to be applied. Come up with a different term other than dwarf planet and you’re done. The term planet wasn’t broken, or involved at all with the problem, so there is no reason whatsoever to get it involved. When you need a name for something, it makes far more sense to come up with a term for it, rather than steal a term that already refers to something else and then force everyone to come up with a new term for that. Just apply the new term to the group that needed the term in the first place. Sorry for the rant, but it really bothers me that people are still upset about Pluto and trying to make things work as they see fit rather than following the logical course of action.

    Liked by 1 person

    1. I’m really glad to see someone sticking up for the I.A.U.’s definition. I have to say that initially, I was upset about the Pluto thing because Pluto had always been a planet when I was growing up. But then as I learned more about space and astronomy, I came to understand why the I.A.U. did what it did, and for a long time I accepted their definition.

      There really is something different about Pluto and the other dwarf planets, and I see the value in creating a new term for them. It would be nice if we could consider dwarf planets to be a subcategory of planets, like you said. I don’t really get why the I.A.U. insists that dwarf planets aren’t planets when dwarf stars are still stars. But other than that, I’ve been willing to accept the I.A.U.’s definition.

      However over the last few years, the current definition has been strained somewhat by the growing numbers of exoplanets, some of which have rather strange orbital characteristics, as well as the discovery of rogue planets. And then there’s also the search for Planet Nine, which may really challenge the planet vs. dwarf planet distinction if/when we find it. Planet Nine is expected to be a super-Earth, or possibly a Neptune-sized body, but because of its distance from the Sun, it might not have cleared its orbital path. I’m not sure it makes sense for an object that dwarfs Earth to be classified as a dwarf planet.

      Also, as I said in this post, there are certain large moons like Titan and Europa that feel planet-y to me, and I keep slipping up and calling them planets. And I’m not the only one, apparently, because I’ve seen terms like “planetary features” and “planetary geology” are used in astrophysical papers about those sorts of large moons. That’s something the New Horizons team specifically mentions in their proposal: that professional astronomers already use a lot of planet-like terms to describe what we currently consider non-planets.

      I think the I.A.U. did the right thing, scientifically speaking, back in 2006 based on what they knew at the time. I don’t want to make some sort of special exception for Pluto based purely on nostalgia. But because of discoveries made since 2006, and also because of the kinds of discoveries astronomers anticipate making in the near future, I think a broader, more inclusive definition of planet may be appropriate.

      Liked by 1 person

      1. Yes, although if one were so inclined you could probably, with effort lead them to the point where they had to agree it…

        This is not my area of expertise, but isn’t one of the things about a moon that it rotates about a planet, whereas a binary pair of planets would rotate around each other?

        Liked by 1 person

      2. If I remember correctly, there was a proposal back in 2006 to define binary planet that way, which would have made Pluto and Charon binary planets. The I.A.U. rejected that proposal.

        Liked by 1 person

  2. So 2018 then… Yeeeeeessssss….. they have to do something for Pluto and all the other little lost planets yet to be discovered. Different classifications are fine, like animals classifications, same for Moons… Maybe it is a moon but supports life… Endor Class Moon for example 🙂

    Comics make me laugh to thanks for the post.

    Liked by 2 people

  3. It’s often forgotten today that the sun and the moon actually were once considered planets back in the time when “planet” meant wanderer, any celestial object that moved against the firmament background, a category which under the geocentric view of things excluded Earth itself.

    I personally don’t have strong feelings about the Pluto / planet debate. Pluto is a very different kind of object from Earth, but Earth is very different from Jupiter or Neptune. I do think it’s kind of silly to call Pluto, Eris, Ceres, and similar objects “dwarf planets” while insisting that they’re not planets.

    If it was up to me, we’d make a distinction between major planets and dwarf planets, and move on.

    Liked by 2 people

    1. I’d forgotten about that: that the Moon used to be considered a planet. I still think making the Moon a planet again feels a bit weird, but I could get used to the idea.

      Regarding Pluto specifically, my feelings aren’t as strong as this post might make it seem. I wouldn’t want to make some sort of special exception for Pluto just because of nostalgia.

      But the new definition of planet proposed by the New Horizons scientists appeals to me, because it’ll let me call Titan a planet… something which I just can’t help myself from doing sometimes.

      Also I think the definition they’re currently using is already becoming strained by new exoplanet discoveries, and the possible discovery of Planet Nine could make the planet/dwarf planet distinction seem really silly, if Planet Nine turns out to be a Neptune-sized object that hasn’t cleared its orbital path.

      If all of this happens to reinstate Pluto as a planet, I think that’s a nice bonus.

      Liked by 1 person

    1. Thanks, I’m glad you like them! We’ll have to wait and see what happens in 2018, but at least there is a proposal on the table that would let Pluto back in the planet club.


    1. Yeah, I know what you mean. I was in a school recently and saw a poster about the Solar System that didn’t show Pluto at all. Even though I get why the I.A.U. did what it did, that still made me sad.


  4. I still think of Pluto as a planet, even if it got kicked off of the Planet Log. Maybe it is a Junior Planet but it is still a planet. I remember an episode of Doctor Who that was set on Pluto. It was called the “sunmakers.” Pluto had been terraformed and people lived there. Unfortunately, they were being taxed to death by a corrupt company. Life on Pluto had become very taxing.Why would Doctor Who and his companions and K-9 battle villains on a fake planet?

    Liked by 1 person

    1. Oh, that was a good episode. I saw it again just recently. Yeah, it’s a little strange now, watching “The Sunmakers” without hearing one reference to Pluto’s new status. Maybe that means the new definition was accepted in 2018, and everyone forgot about the dwarf planet designation by whatever century that episode was set in.


  5. In a way, the moon does circle the sun (wobbling around a little bit because of the gravitational influence of the earth). Earth, on the other hand, wobbles a bit because of the gravitational influence of the moon. So why not include all those moons in the planet concept? If, during the formation of a planetary system, a planet is ejected from the system, does it cease to be a planet? It is all a matter of definition, so these are not problems of reality but of language.

    Liked by 1 person

    1. Exactly. The linguistic challenge is trying to make our terminology fit with reality as best we can. As we learn more about planets orbiting other stars, and also as we learn more about our own Solar System, I’m not sure the current definition will still fit with the reality that we know.

      Liked by 1 person

  6. Like just about everyone, I grew up believing Pluto a planet. And like most everyone, I was affronted when Pluto got demoted.

    But in wanting to understand the other side of the matter, I thoroughly read Neil deGrasse Tyson’s narrative on what happened to Pluto. I am getting ready to write a blog post on this, but in quick summary, I am very convinced now that Pluto is not a planet.

    Pluto’s story goes beyond astrophysics jargon. It has a lot of lessons on humanity and our flaws. It is fundamentally about our own failings to reconcile ignorance into knowledge.

    This may sound like hyperbole, but I hope to explain later why it is, I believe, right on the mark. Pluto’s story is fascinating; it’s too bad nobody knows how to tell it.

    Liked by 2 people

    1. I look forward to reading your posts on this. I’m glad some people are sticking up for the I.A.U.’s definition, because I do think there are good reasons to acknowledge that Pluto is different from the eight currently recognized planets.

      The proposal from the New Horizons team appeals to me, though, and I think that as we continue to learn more about the Solar System and other star systems, the I.A.U. may run into some problems with the definition they’re currently using.

      Liked by 1 person

  7. Great post; this whole series has been. I’ve really liked them all.

    It’s surprising to me that this topic keeps going. I mean, it’s great from a PR perspective, right? Anything to keep people interested in astronomy, but it seems like the sort of thing no one other than astronomers would really care about.

    This space taxonomy stuff is an odd thing to get so into. Would anyone care if it were decided that the sun were no longer considered a star, but something else that’s thought to be somehow less than a star? Nostalgia, though…

    I’m guilty of it, too. I often have to correct as I talk and write, and remind myself that Pluto’s not a planet. I’m not sure how I feel about the IAU’s current definition of planet, but I think I mostly agree with it. Either way, I agree with the notion Pluto’s different, an outlier, and not really in line with the Big Eight. This doesn’t mean it’s a bad thing, or there’s anything wrong with Pluto. It’s just a thing. Not much different than saying the Beatles a different type of band than Depeche Mode. Not better, not worse, just different (no, I kid, they’re better).

    I don’t quite agree with the notion that any round object should be a planet. That seems too inclusive. The idea that something needs to directly orbit the sun seems very important. Ganymede is bigger than Mercury, but I’d have a hard time buying that it’s a planet.

    I keep getting confused by the different proposals out there, but the one I like, is any round object that directly orbits the sun (so, essentially two thirds of the current IAU definition) is a planet. The planets are then broken out into groups Major Planets (the big eight), Minor Planets (Pluto, et al), and on it goes.

    Kind of the important thing, and a big, amazing thing, is scientists realized something wasn’t right by including Pluto among the rest, and they corrected for it. That says a lot. It’s a big deal. Was that Dr. Tyson who said that? I think so.

    (wow. that’s a long reply)

    Liked by 2 people

    1. Dr. Tyson certainly gets a lot of credit/blame for Pluto, but I get the impression the issue had been rumbling among professional astronomers for a while before Tyson got involved.

      That two-out-of-three definition makes sense to me too, with planet as a broad category and different subcategories for different kinds of planets. As a personal preference, I’d really like to be able to call Titan and Europa planets, and according to the New Horizons proposal I’m not the only one who feels that way, but that might be asking a little too much.

      Liked by 1 person

    2. Would anyone care if it were decided that the sun were no longer considered a star, but something else that’s thought to be somehow less than a star?

      It would matter, because to assert the Sun is not a star would be to imply that the Hertzsprung–Russell diagram, the most widely used method of classifying stars, is no longer valid. The Sun is a middle-of-the-roadish star and fits within the band of stars considered most numerous in the universe.

      H-R classifies stars by their brightness and luminosity. Astrophysicists have used this catalog in part to determine the size and life cycle of stars (e.g. the differences between a red giant and neutron star). More importantly is that this lends clues to how stars behave throughout their life, first by identifying the star’s temperature at a given time, which leads to understanding which elements are being created in the star’s nuclear fission furnace.

      The case of Pluto is a similar matter. For our understanding of our solar system and star systems beyond to advance, it is going to be important to properly identify the differences between the types of possible planets as well as other non-planetary objects we may find in a system such as ours.

      The problem is not science, but scientists. They think they have all the answers, but they don’t. Collectively, they do not know how to wield their power – knowledge – such that it is both understood and accepted by the general populace.

      Liked by 2 people

      1. That was just one example. What’s needed is a fundamental understanding of why a planet is a planet. All these new attempts to re-planet Pluto seem more a sociology experiment to make people feel good again, versus to do what needs to be done (which is to acknowledge Pluto is the king ice ball of the Kuiper Belt, not a planet, because it shares nothing in common with either the inner four rocks or the four gas giants, and does share so much with its lesser Kuiper brethren).

        Liked by 1 person

      2. Yeah, I know about the the H-R diagram. You make a good point. Presumably, given my hypothetical example, the reason the sun needed to be reclassified is because some flaw in the H-R diagram (or, really, in the science supporting it) had been found, for which the scientists were correcting, or they would correct the H-R diagram to account for this necessary reclassification.

        As far as what I was saying about the general “would anyone care…” attitude, I should have been more clear, you’re right, but I really referring more to non-scientists, non-astronomers. In the places I go, when I talk about astronomy, it’s surprising to me how often people want to talk about Pluto. Again, these are non-scientists I tend to talk to.

        I fully agree with you that the science is important and from that perspective, this should be hammered out. No argument there. What I’m saying is I’m surprised when I get chatting with people who can’t name the solar system’s planets in order, but are frustrated or, at times, downright angry that Pluto isn’t among them. I’m not slagging these people for not being able to name the planets. Rather, my point is they’re not scientists, not interested in the nitty-grtty (sorry to be so technical), but are hung up on the cordoning off of Pluto. From a sociological standpoint, a human standpoint, it’s interesting.

        As far as the scientists not having all the answers, isn’t that at least part of the point? I mean, we want to get it as correct as possible as often as possible, yes. It would also seem to me that it’s very important to be human enough to know that we don’t have all the answers, accept that, and correct for the things we’ve stated in the past when new information becomes available. I’ve written about this many times: when I first got into astronomy in the early 80s, the solar system was very neat and tidy; a star in the middle, four planets up front, five planets at the back, a line of rocks in the middle, and, for all intents and purposes, a Plexiglas shell around the whole thing. Over time new data has come in and the scientists have adjusted. This sort of thing happens all the time in other branches of science. Just last year a new human organ was discovered. New species of animals are discovered all the time, and taxonomy changes. New elements are created and discovered and the periodic table is updated. When I was in school, we were taught that there were three biological kingdoms (plant, animals, protists). That’s been changed. It goes on. New information becomes available, and the scientists correct. It’s humanity at its best.

        Liked by 2 people

      3. Science may be humanity at its best but scientists are still human, complete with their own flaws, biases, and weaknesses. Compounded, I’d say, by their belief they they have the answers to “everything.”

        I agree with you on all counts Scott. Yet what I was trying to hint at is that scientists are great at the hard sciences but they are terrible on the less mature soft ones, specifically anything to do with the humanities, including politics.

        Going hand-in-hard with his Pluto debacle is my old friend Dr. Tyson’s latest video pleading with America to believe in science again. I don’t want to dissect it but only to say he’s not going to convince anyone who doesn’t already agree with him. “You don’t have that option,” to not believe…yep way to win them over Neil. He’s probably ideologically very similar to his hero Carl Sagan, but he’s no Carl Sagan when in comes to communicating to the masses. My point is that scientists are really book smart, but they make the same arrogant pitfalls in dealing with the non-science majority.

        Liked by 2 people

      4. Wow… I think in a lot of ways you and I are circling the same point. I’m glad to hear you say that about Tyson, though. I like what he does a whole lot, really, but I find there’s a certain nihilism, and a certain, as I think you’re alluding to, elevation, a certain dismissiveness (that’s not really quite the right word), to what he says that I don’t really go for, and what you say, “you don’t have that opinion,” illustrates that. He’s no Sagan, you’re right. Sagan was more accepting, more inclusive, more understanding of people’s humanity.

        Liked by 2 people

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