Maybe Pluto Is a Planet After All

Two planetary scientists so argue, and there's a legal hook, but it's really just an excuse for me to ....


David Grinspoon & Alan Stern (in the Washington Post) argue that Pluto is indeed a planet (thanks to Stephen Green at InstaPundit for the pointer); an excerpt:

We use "planet" to describe worlds with certain qualities. When we see one like Pluto, with its many familiar features — mountains of ice, glaciers of nitrogen, a blue sky with layers of smog — we and our colleagues quite naturally find ourselves using the word "planet" to describe it and compare it to other planets that we know and love….

We find ourselves using the word planet to describe the largest "moons" in the solar system. Moon refers to the fact that they orbit around other worlds which themselves orbit our star, but when we discuss a world like Saturn's Titan, which is larger than the planet Mercury, and has mountains, dunes and canyons, rivers, lakes and clouds, you will find us — in the literature and at our conferences — calling it a planet. This usage is not a mistake or a throwback. It is increasingly common in our profession and it is accurate.

[UPDATE: Reader Robert Woolley points to a similar recent statement by S. Alan Stern, the principal investigator of the New Horizons mission to Pluto, who condemns "the sad development of 2006 when a few hundred astronomers—mostly non-experts in the study of planets—declared that Pluto and the then burgeoning list of other small planets that had been discovered beyond Pluto were not planets, largely to prevent schoolchildren from having to memorize their names. (I wonder if those same astronomers—also non-experts in chemistry—believe that there are too many elements in the periodic table for the same reason.)" It is a small object, but there are those who love it.]

This is an interesting argument, and it helps illustrate how even scientific terms are often a matter of definition, with each definition yielding close cases along the edges, and with the definitions often chosen based partly on utility (what helps most with better understanding certain aspects of the world?) rather than simply based on objective scientific fact. That's a helpful lesson for incoming law students, and I've at times used the planet controversy as an analogy for them. Such definitions aren't completely arbitrary, because they are aimed at better categorizing real phenomena; but they aren't completely dictated by the real phenomena, either—likewise with many legal definitions.

But, really, this is all just an excuse for me to repost one of my favorite humorous songs, the author of which is regrettably shrouded in the mists of time:

ALWAYS A PLANET TO ME (to the tune of Billy Joel's "She's Always A Woman to Me")

He can orbit the sun, he can look like a moon
He can leave the ecliptic from April to June
He'll be just a faint smudge, magnitude twenty-three
He hides in the sky, but he's always a planet to me

Ohhh … a potato-shaped ball …
He can drift where he wants
He's a relic of time
Ohhh … if he's made of pure ice
Or of vapor and dust
It's the same to my mind

If he zooms in near us, would he show us a tail?
Was the Kuiper Belt once the great home whence he sailed?
And if he gets demoted, who'll be next, Mercury?
And the most he can do is cast shadows, it's true
But he's always a planet to me

NEXT: Symposium on Ganesh Sitaraman's "The Crisis of the Middle-Class Constitution"

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  1. The legal lesson for law students is define and debate and define and then Congress declares some small lake somewhere a “Great Lake” so it can take advantage of Great Lakes spending packages.

    And crypography is munitions or something.

    And the EU declares carrots fruits so carrot jelly makers can continue to call their product “jelly” as someone decided jelly can only be fruit-flavored.

    1. Carrot jelly? I learned something new today.

      1. “Something new” has a null hyperlink. What did you have in mind?

        1. Well, that’s weird. It worked when I first linked it. I was trying to point to www (dot) carrotmuseum (dot) co (dot) uk (slash) jam (dot) html

          I wonder what the censors found so objectionable on that page.

          1. Let me try linking to Carrot Jam History and Recipes at World Carrot Museum. Does it work? What if I just use the bare URL like this: … They both work fine in preview.

  2. How about this:

    “A planet is:

    “(a) (insert scientific gobbledegook)


    “(b) a body orbiting the sun, which was regarded by astronomers as a planet in the year 2005.”

    Grandfather it in.

    What’s next, correcting our pronunciation of “Uranus”?

    1. Lol. I wonder when the first astronomer will be fired over this. I’m shocked it hasn’t happened yet.

      1. Why would anyone get fired over this — whatever “this” is? Scientists use scientific terms like “planet” in whatever way works within their science, which may change in different scientific contexts. That scientific usage may or may not be “gobbledegook” to the rest of us is neither here nor there.

        1. I may not have your fancy book learnin’, but I knows a planet when I sees it. It orbits the Sun, and you can land one of your fancy spaceships on it without’n it floatin’ off.

          Plus the smarty-pants scientists admitted it was a planet until they decided to change the rules.

          And if the smarty-pants scientists changed the rules once they can do it a second time.

          So there.

          1. The largest of the traditional asteroids is Ceres. It has a surface gravity about 3% of earth’s, and you could land a spaceship on it. But its diameter is under 600 miles. Do you think a rock with a diameter three quarters of the distance between New York and Chicago counts as a planet? And that’s the largest. There are many smaller ones that you could land a spaceship on that are much smaller. Take 100 Hekate, with mass less than a millionth of earth’s mass. Escape velocity is 54 meters/sec, or 120 mph, so if you landed on it, your spacecraft isn’t going to float away.

            1. Well, I’ll be a monkey’s uncle, I reckon I’ll be happy to welcome these asteroids to the great family of planets, especially one with a cool name like Hekate.

              1. I mean, if Pluto coming back means it brings a few friends, well, that just makes the welcoming party all the more exuberant.

        2. To be clear, the “this” refers to the last line of Eidde’s comment. And if someone can be disciplined for saying, “Ladies’ lingerie”, for example, surely soon we will have change that particular planet’s name.

          1. I read in a recent solar-system book that it’s pronunced “Your-uh-nus,” which is actually closer to the original Greek, but the other pronunciation is more fun if you’re in grade school.

            1. (and in spirit I never graduated)

            2. Eidde: “I read in a recent solar-system book that it’s pronunced “Your-uh-nus,” which is actually closer to the original Greek,”

              Which ruins the classic riddle about how the Starship Enterprise is like a piece of toilet paper.

  3. The most sensible definition of “planet” I know of:

    “A body large enough to be forced by gravity to be a spheroid, which does not orbit another such body that isn’t a star, unless that other body is of comparable size.”

    By this definition, of course, not only Pluto, but also Ceres, is a planet. But it allows for two roughly equal bodies orbiting each other to both be planets.

    Of course, “planet” is a taxonomic category, not a natural category, so it’s always going to be arbitrary.

    1. That definition would not just include Ceres, but dozens of other bodies in the Kuiper belt and beyond. Moreover, Pluto and other KPOs are completely different from the 8 planets in that they have completely different orbits, makeups, etc. You can define planets any way you want of course, but any definition including Pluto creates a lot of problems. Pluto looks more like a comet than it does Earth.

      On Twitter yesterday, Mike Brown (who discovered a lot of these KPOs that caused the “demotion” of Pluto) said that the best way to classify things is this: (1) Terrestrial planets (inner planets); (2) Gas Giants; and (3) other assorted stuff. I think that makes the most sense.

      1. “That definition would not just include Ceres, but dozens of other bodies in the Kuiper belt and beyond. ”

        Yeah, so? There are a lot of planets in the universe. I’d like a fairly objective definition, that doesn’t hinge on whether the body in question is smaller than the distance between two cities, or is really far from the Sun.

        Being massive enough to be forced into a spheroid neatly separates out random hunks of rock or cometary stuff. Bodies that orbit planets are already defined to be “moons”.

        It’s all taxonomy anyway, so, largely a matter of taste. But let’s at least tastefully pick criteria that aren’t totally arbitrary.

        1. I see three semantic choices.

          1) A quantifiable definition that expands out solar system to dozens of planets.
          This runs up against our historical conception of the solar system as all wheat, no chaff.
          2) a quantifiable definition with an arbitrary line drawn such that Pluto and it’s many similar bodies are not included.
          This is a middle ground. All wheat, some rigor, but has an arbitrariness and Pluto pays the price.
          3) A nonquantifiable definition based on historical practice.
          This is not scientifically useful, but does recall some pretty fun history.

          I’m all for adopting the arbitrary line as a compromise between how language works and how science works.

  4. Sure, if Titan’s a planet then so’s Pluto. But Titan’s not a planet.

    If once a body is on the list of planets it must never be removed, then why is Ceres no longer a planet? Either restore them both or neither.

    I vote for neither. It makes for a nicely symmetrical system. There are eight planets ? four little ones and four big ones, each set followed by a belt of assorted objects that are not planets. Ceres is an Asteroid belt object, and Pluto is a Kuiper belt object

    1. Titan isn’t a “planet” only because it’s a “moon”; If it were independently orbiting the Sun it would deserve the designation, it’s more than twice the diameter of Pluto.

      But you don’t get to be a “planet” if you’re orbiting around a much larger “planet”.

      1. Why not?

        The obvious and natural classification system would be to ignore orbits entirely. Anything big enough that it underwent fusion is a star, whatever it orbits. Anything too small to undergo fusion but big enough that it winds up in hydrostatic equilibrium due to gravity (that is, anything big enough to make itself round under its own gravity) is a planet, whatever it orbits. Anything too small to be a planet is an asteroid, whatever it orbits (or a meteoroid or micrometeoroid as you get smaller).

        Then, usefully enough, we can clean up the definition of “moon” by using the original Moon as our example — a moon is a planet orbiting another planet. So Earth has a moon, Mars does not (it has satellite asteroids), Jupiter has four (and a bunch of satellite asteroids), Saturn has seven, Uranus has five, Neptune has one. Pluto has none because Charon doesn’t orbit it, but a mutual barycenter outside Pluto’s surface. And you can call a non-moon planet a major planet or something if you need to distinguish.

        1. Yup. And your definition is more logical than one that includes the 8 planets plus Pluto.

        2. So Pluto gets promoted from dwarf planet to major planet, and Charon gets promoted from “satellite of dwarf planet” to – major planet?

          Just guessing Rev: Are you related to Jim Christy, or do you have a grandfather named Percival?-)

          (More seriously, I was going to criticize the complexity of this proposal before I saw that the IAU taxonomic scheme is pretty complex in its own right. For an Euler diagram depicting 10 categories of objects in the solar neighborhood and their relationships, see e.g. “Asteroids” article.)

          1. The “Asteroids” article of Wikipedia, that is.

          2. arch1

            You were responding to Right-Wing Bigot Mini-Me, the Volokh Conspiracy’s latest partisan plaything (or maybe Prof. Heriot was the latest?).

            The confusion is regrettable, but Prof. Volokh seems to believe it promotes movement conservatism, so he allows it.

            Rev. Kirkland

            1. Again, if he’s Mini-Me, you would be Dr. Evil, right?

              1. Perhaps I should say Dr. Ivil.

              2. To right-wingers, I suppose I might be seen as Dr. Evil.

                Much as some yahoos see Pres. Trump as an economic savior and an evangelical church as a source of guidance and wisdom.

                1. “To right-wingers, I suppose I might be seen as Dr. Evil.”

                  No, that’s what you implicitly called yourself, I prefer the term Dr. Ivel.

            2. The impersonator again? Yeesh. Every time I read a smart-sounding comment by Kirkland, it turns out to be the impersonator.

              1. I suppose next you’re going to claim that the comment over at the post blaming Megan McArdle for her own beating is the impersonator too, eh?

            3. Ahh that would explain why he had something meaningful and relevant to contribute to the discussion. I was shocked for a couple of minutes there that AK could be capable of anything besides content-less vitriol.

            4. Ahh that would explain why he had something meaningful and relevant to contribute to the discussion. I was shocked for a couple of minutes there that AK could be capable of anything besides content-less vitriol.

            5. Well, that clears that up. Glad to see you still haven’t posted a single intelligent comment yet.

              And here I thought you were starting to slip.

        3. But, but, but… You can’t ignore orbits because gravity or something….. 😉

    2. Milhouse: “If once a body is on the list of planets it must never be removed, ”

      So you’re all for consistency. Well, tell that to the Brontosaurus, I mean Apatosaurus, I mean Bronto … oh forget it!

  5. I heard a slightly different (but not contradictory) explanation from another astrophysicist about a year ago (over beers) – the Pluto decision was done the last day of a conference, and almost every active scientist who had real work to do had already left. The few who stayed because they actually study Pluto or other planets were wildly outnumbered by random members that had nothing very useful to do back home. He didn’t quite call them “over the hill” or anything like that, but clearly let our imaginations go there.

    1. Umm, that’s absurd and is not what happened at all. Read Mike Brown.

  6. Who are all these science deniers?
    Pluto is a dog; a cartoon dog.

    1. I thought Pluto was the cousin of Bureau and both were Crats.

  7. Science has proven that the category of “planet” is illusory. Therefore the category can be applied to anything. I wish to call the mote of dust floating before my eyes a planet; how dare you challenge my absolute right to do so. Pluto was assigned the term “planet” at its birth; since there is no such thing as planets, it may be transition to become a rock, a penguin or a goo-gaw if it wants to. And you should be prosecuted if you dare deny its trans-planetary nature.

  8. Is this a proper republication of what seems to be an entire lyric? Attribution is sketchy. A license seems nonexistent, especially in the context of a for-profit use. Fair use?

    Authors and creators (should) have rights, too.

  9. I think this Slate Star Codex post may be relevant. (The first mention of Pluto is in Section II if you want to skip ahead.)

  10. Never mind that, what about Bluto?

  11. It seems to me that if we call it a “dwarf planet,” we are conceding right then and there that it’s a planet. Isn’t a dwarf star a star? A dwarf rabbit a rabbit? A green onion an onion? A stupid astronomer an astronomer?

    Putting an adjective in front of a noun doesn’t negate the noun.

  12. Just a bunch of astronomers arguing about the number of angels who can dance on the head of a pin.

  13. “…largely to prevent schoolchildren from having to memorize their names.”

    Or the so-called astronomers are more like the schoolchildren and wanted the last planet to be UrAnus!

  14. My dilettante’s understanding is that the reason for Pluto being called a planet is purely historical. That any set of physical indicia that includes Pluto will necessarily include many more objects that we just didn’t happen to see back in the day.

  15. I feel bad that Ceres got demoted from planet to asteroid. If Pluto is a planet, so should Ceres be.

  16. By statute, Pluto IS a planet when it’s in the sky over Illinois. The legislature said so.

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