The unbearable roundness of being

By Phil Plait | November 22, 2010 11:59 am

[UPDATE: The article discussed below is now online at Discover Magazine’s website, so you can read it there.]

Every now and again I delve back into the ancient art of writing for an actual magazine that has words printed in ink on paper which gets sent to you via the postal service.

Quaint, I know.

discovermag_dec2010coverBut I wrote just such an article for Discover Magazine which is in the December 2010 issue. The article, called "Why Size Matters" is about why defining the word planet is proving to be so difficult.

Funny how writing works sometimes. I got the idea for the article while researching a blog post on a moon in the outer solar system. Curious about its size, I started poking around the web looking for other moon diameters, and then started wondering how big an object you need before gravity crushes it into a ball. I thought I could write the article about just that, but the words apparently had a mind of their own and went in a different direction. I wound up talking about what we think of as planets, and then in the middle of all this I read an advance copy of Mike Brown’s wonderful book How I Killed Pluto and Why It Had It Coming (full review coming soon). Mike spends quite a bit of time on this very topic, as you might imagine (he discovered the Kuiper Belt object Eris, which kick-started the demotion of Pluto). I found his thinking to be very similar to mine, and his writing actually gelled a lot of disorganized thoughts I had about all this.

Anyway, the article was fun to write, and I think anyone who likes my blog will like it. I got the issue in the mail the other day, and it’s on newsstands and at bookstores now. I hope you’ll check it out.


Comments (19)

  1. JM_Shep

    Great article! I enjoyed the topic and a lot of things you discussed. My favorite was when you were talking about how scientists work in concepts and not definitions. I found it interesting very easy to relate to.

  2. QuietDesperation

    Hasn’t “The Coming Plague” been coming for quite a long time now? It’s one of those things like fusion power and true AI that never seems to happen.

    And, actually, if you read that title a little differently, it doesn’t sound so bad. Ooo, I’m naughty.

  3. John Paradox

    Check out the latest by melodysheep.


  4. Matt B.

    Nothing like a little black-and-white fallacy versus argument of the beard to get your mind going, huh?

  5. Utakata

    “Why size matters” is up there with “hung like a telescope”…at least in astronimical terms. :(

  6. Zucchi

    I’ll have to pick up the magazine. Phil, your article sounds exactly like something Isaac Asimov might have written for his monthly science column at The Magazine of Fantasy & Science Fiction, back in my youth. (That’s meant as a compliment.)

  7. Messier Tidy Upper

    Every now and again I delve back into the ancient art of writing for an actual magazine that has words printed in ink on paper which gets sent to you via the postal service.

    Or you can buy them from newsagents which is what I always do. 😉

    I love the astronomy magazines – I regularly purchase ‘Astronomy’ mazine (US published and focused), ‘Astronomy Now’, Patrick Moore’s ‘Sky At Night’ magazine, (UK published and focused mags) ‘Australian Sky & Telescope & I’ve even had a few articles of my own published in (Australia’s) Sky & Space magazine.

    So, yes, I will definitely get a copy of this one. I’m looking forward to it – although there may be a long wait here in Oz which is usually a month or so behind. :-)

    Such magazines certainly have their advantages being informative rounding up and summarisng news, having interviews, star maps and also including the odd extra poster or calendar and hopefully far from being “quaint” or “outdated” will still be around for a very long time yet! 😉

  8. Discover blog, Discover Channel TV show, Discover magazine article. Is there no end?

    – Jack

  9. nobody

    How about us people outside the States, where can we find the article? Don’t tell me to order it by mail!!!

  10. Mark Hansen

    nobody, did you even read the previous comments? As you can connect to the internet from wherever you are, there is a better than average chance that you live somewhere near a newsagent. Maybe you could ask the proprietor if s/he could reserve a copy for you. You get the magazine, meet your local newsagent and maybe some other people, and get some fresh air. Unless you suffer from agoraphobia, what have you got to lose?

  11. Keith Bowden

    I enjoyed the article. I got my copy last week and was pleasantly surprised to see our very own BA in it. :)

  12. Messier Tidy Upper

    @9. nobody Says:

    How about us people outside the States, where can we find the article? Don’t tell me to order it by mail!!!

    Well as (# 10) Mark Hansen has said, there are still good* newsagents that should have copies.

    I can get that Discover magazine from my local one and I live in South Australia so I would expect that it is available in most Western nations.

    If you are not living in a culturally & politico-economically Western country then, firstly, my condolences to you ( 😉 ) and, secondly, I suggest you try searching online.

    @2. QuietDesperation Says:

    Hasn’t “The Coming Plague” been coming for quite a long time now? It’s one of those things like fusion power and true AI that never seems to happen.

    Very true.

    Sadly also true for returning to the Moon, landing people on Mars and having personal Asimovian type household robots. All of which always seem to be just twenty or so years away and have been ever since about 1950.


    * Almost by definition after all – a “good newsagent” will have the mags that you’re after, whereas a bad one, won’t! 😉

  13. Ben

    If its gravity is sufficient to pull it into a ball, and it is in a stable orbit around a star, it’s a planet. If it is in a stable orbit around a star, but isn’t big enough to form a ball, it’s an asteroid or comet. If it is in orbit around a planet (by the above definition), it’s a moon, ball or not. If it is in an unstable orbit around a star, ball or not, it’s an asteroid, unless it is sublimating or will sublimate at non-rock melting distances from the star, in which case it’s a comet.

    Pluto is a planet.

  14. Messier Tidy Upper

    What about planets that end up in unstable orbits – or even thrown out of their star systems altogether becoming free-floating “rogue planets?”

    We already know of a few exoplanets that are in comet-like orbits and that planetary systems can be destabilised and end up with some worlds, even superjovian worlds that are in very odd orbits.

    For example, I’ve heard that computer simulations going into the very distant future show that one day Mercury’s orbital eccentricity might let chaos loose on our inner solar system leading to chain reactions where Venus and/or Mars and/or the Earth may end up on collision courses.

    In that scenario would Earth or Venus cease to be a planet under your definition strictly applied?

    Do you really think that stops them being planets? I don’t.

    I prefer a definition where orbits (“cleared” or otherwise!) don’t come into consideration at all unless the object is directly orbiting another planet in which case it is a moon.

    Otherwise, if an object is massive enough to be round via its own gravity but not so massive that it can ever shine via fusing elements at its core then its a planet. If its too small to be round its a planetoid, comet or asteroid, if its large enough to shine by nuclear fusion its a star or brown dwarf.

  15. Messier Tidy Upper

    See :

    For an example of a planet, HD 80606b or the “Icarus Planet”, in a comet-like orbit.

    See :

    For an exoplanetary system, Upsilon Andromedae, with a world in a very messed up orbit.

    Plus see :

    For a blog post about that study which predicts a chance (slight but still) that all planetary hell may break loose in our solar systems remote future.

  16. Ben

    Messier Tidy Upper has questions, I believe directed at me. Consequently:

    1) What about planets that end up in unstable orbits – or even thrown out of their star systems altogether becoming free-floating “rogue planets?”

    1a) Status changes to asteroid.

    2) Do you really think that stops them being planets? I don’t.

    2a) Yes, I do. Status is everything. If the object falls into the sun and melts, is it still a planet? After all, it’s still there, right? Just in a different status. A diffuse gas. To me, it’s *just* a gas now. So if it comes out of a stable orbit, it’s now an asteroid. If it regains a new stable orbit, it’s a planet again. Another example: If one planet is captured by another, then the captured planet has become a moon. Easy. Simple. Sensible. These definitions — all of them — are about status. Because in the end, that’s how we identify them.

    Now history is something else — it would be interesting to know, for instance, that a moon was previously a planet. Also when, and why, the status changed. But this doesn’t affect its current status. It’s a moon.

    3) “comet-like orbit”:

    3a) Is the object’s orbit stable about the star? In that case, if it is large enough to ball up, and it isn’t sublimating, it’s a planet. I didn’t say the orbits had to be in a particular plane, or really much of anything else about them, just stable and about the star(s) in the system. To me, “comet” means “it’s sublimating.” It isn’t about the orbit — it’s about the emissions. After all, a ball of hard rock in a steep orbit isn’t comet-like, is it? Not in the least. Because that’s not the status that makes us go, “oh hey, look: a comet!” The thing that makes us do that is the presence of ion and/or dust tails.


    I like my definitions because they are based on *why* we call astronomical objects what we do: status. It allows us to easily determine by observation, exactly what it is we’re looking at, and it is flexible in that when things change, as they do from time to time, the status of the object will inform us instead of confuse us.

    Pluto, being roundish, being in a stable orbit about our star — it’s a planet. Some ragged rock in the asteroid belt… naturally enough, an asteroid, like its compatriots. One big enough to pull itself into roundness? A planet, very, very unfortunately located. Not a good place to set up a base!

    If the earth fell into the sun, before the final change of status (into a gas), it would first become an asteroid, and then probably become a comet, assuming it still had atmosphere and water and the like when this happened. It’d have a heck of a tail pair for a short while. And then, like other comets large and small, poof.

    Things aren’t what they are because of what they were; they are what they are right now. You’re not a baby, though you were once; I don’t love my ex-wife, though I did once (yes, insanity can change something’s status); and the moon isn’t a planet, though we think it was part of ours at one time. Current status. That’s what we should be primarily concerned with, keeping in mind that historically status is still interesting, as are status changes.

  17. Frank

    I say we scrap the word “planet” altogether. It originally meant moving heavenly bodies, and we’ve since learned that they ALL move. The word “planet” tells us nothing. Jupiter and Mercury are both “planets,” even though they couldn’t be more different. Pluto used to be a “planet,” whatever that is, but now it’s a “dwarf planet,” whatever that is. It’s a useless, confusing word.

    Instead, let’s refer to the heavenly bodies by more descriptive, accurate terms. Mercury, Venus, Earth, and Mars are “rocky worlds.” Jupiter, Saturn, Uranus, and Neptune are “gas giants.” Pluto is, say, an “ice dwarf.”

    Once we scrap “planet,” “moon” is next on the list.

  18. JediBear

    Here’s the original definition of a planet:

    A planet is any object that is in the sky and is:
    1) Visible to the naked eye.
    2) Can be seen over time to move against the background stars.
    3) Essentially Permanent.

    By this definition, Neither massive Neptune nor tiny Pluto are planets. But the Sun and Moon both are.

    We no longer use this definition, because we decided that orbiting us disqualifies Luna and that being a gigantic ball of fire disqualifies the Sun. In each case, we found a class of objects that a former planet fit into better than that of planet. Luna became the prototype object for “moons” and Sol became the prototype object for “Stars.” Ceres enjoyed a brief life as a planet before we found the class of objects to which it belonged. Pluto enjoyed a similar (if longer) reign due to the bizarre happenstance that led to its discovery. But we’ve now found the class of objects to which it belongs.

    Not only has Pluto (like Ceres) not cleared its neighborhood, it’s in the neighborhood of a colossal object that dominates the shape of its orbit. In this sense, it’s no more important than one of Neptune’s moons. It is, by comparison, merely a bit of debris that managed to avoid either falling into Neptune or being ejected by it only by happily falling into a stably resonant orbit with it. This makes it no more important in its neighborhood than a moon or a trojan.

  19. JediBear

    I must say, though, that I’m annoyed at the rocky planets having lost their “dwarf planet” status.


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