Minor planets, major thoughts

By Phil Plait | August 22, 2008 10:25 am

I link to Emily’s blog at The Planetary Society quite a bit, but that’s because a) she’s really good, and b) we agree on a lot of things.

For an example of (b), read her post about the definition of "planet". I’m still not swayed by a lot of what I’ve read of others’ opinions, but Emily seems to be on the right track. She says this:

Frankly, I think it’s much less important for a student to be able to name all the planets than for the student to understand the basic structure of the solar system: that Earth is one of many worlds that orbit around the Sun at different distances. Each of these other places has similarities to and differences from Earth. Studying these other worlds is both fascinating in its own right and also helps us understand our own planet. And the way we study other worlds is by looking at them from Earth with telescopes and by visiting them with the spacecraft that leave Earth (most of them leaving Earth forever) to wander our solar system, serving as our eyes, ears, hands, and sometimes even noses.

Even after discussions with Alan Stern, I’m still struggling to figure out if this is merely a semantic argument, or a truly scientific one. I may need to buy Alan a drink and talk this over some more. If I do, I’ll let you know. I do need to watch the videos of the planet definition debate first, though.

But back to Emily: for an example of (a), she has posted an astonishing series of pictures about all the smaller worlds (asteroids and comets) visited by spacecraft… and then put up an incredible image showing their relative sizes. The difference between the comet Itokawa and even a smallish asteroid is really rather shocking. If you sat Itokawa on the Earth’s surface, it would be a fair hill, maybe a little difficult to bike up, but that’s about it. Whereas even small asteroids are bigger than Mt. Everest.

And those things sometimes hit us. Brrr.

CATEGORIZED UNDER: Astronomy, DeathfromtheSkies!, Science
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Comments (34)

  1. dre

    I read Emily’s post earlier today, and I wished that I could comment there on how great it is. I will have to comment here about how great her post is, though: it’s a great post and made my head spin a little. When you watch the animation of Itokawa, you can almost feel the material that is glommed together to form it.

  2. When I talk to Girl Scouts about astronomy (I have 2 groups of them coming up at McCormick Observatory, what fun!) I make a point of talking about how we remember the planet names. “My Very Earnest Mother Just Served Us Nine Pizzas” is now “My Very Earnest Mother Just Served Us Nachos”. I tell them how hard it’d be to remember “Very Earnest Mother Just Served Us Nine Pizzas, S(edna)even E(ris)dible Q(uaoar)uacking M(akemake)arshmallows, O(rchus)range I(xion)cky V(aruna)ictuals, L(ogos)umpy C(haos)hicken, R(hadamanthus)otten D(eucalion)ucks, B(orasisi)loated H(uya)ippos, T(eharonhiawako)en C(eto)ookies…

    OK, I know I have the order of the TNOs wrong, I didn’t find a list of them by mean orbital distance, but you get the idea. I tell the kids that in 10 years the mnemonic will get -VERY- long indeed and be impossible for them to memorize. So dropping Pluto makes sense, they can actually take a test in school and peace is restored to the Universe.
    Sorta…
    😀
    Rich

  3. Cheyenne

    Wow- I always thought comets were much bigger. Didn’t Holmes sort of “blow up” to be bigger than Jupiter not too long ago? I know that it’s basically just gas and dust when that happens (very, very, oh extremely very thin gas and dust) but I still thought the comet had to be pretty big originally to puff up to something like that. Learn something new every day with this blog!

    So we know that asteroid and comet impacts can basically wipe out life on Earth (and have in the past) – so we’ve decided to put a program into place that has less full time employees than is needed to run a McDonalds (I think I head that on NOVA)?! What is wrong with us?

  4. Drew

    Well, she has Haley’s comet listed as 16x8x8 km, and Everest is an 8000 meter peak, so I would say at least that comet is quite huge. Also, she has Itokawa listed as an asteriod?

  5. madge

    @ Richard Drumm
    I completely agree. I think we are better off demoting Pluto to a sub- catagory of “dwarf planet” along with Ceres and Eris and add as many new ones as and when we discover them. But please lets ditch the name “Plutoid” it sounds too oogey :)

  6. baley

    To me is pure semantics.

    I don’t see any problem with calling pluto a planet or a dwarf planet. It doesn’t change the fact that it’s an interesting “planet/plutoid/thing/whatever” to study.

  7. Dave Hall

    What? No Uranus jokes yet?
    Sheesh!
    As far as the Pluto = planet/nonplanet debate, it seems the only really het up parties are those who have some sort of personal interest in the outcome:
    Notaplanet folks seem fixed on changing the status as some new iconoclastic act against the old regime. The Yesaplanet people are either worried about revising all their books and articles and–yes, maybe being considered wrong about something. There is a third group: The talking heads on the tee-vee news who make such a big issue of this on a slow news day. Most could not tell a diddly from a squat–so they don’t really count.

    And about: “And those things sometimes hit us. Brrr.” Isn’t there a book on that subject due out sometime soon?

  8. dusty59

    I was absolutely flabbergasted at Emily’s description of the “Think of the children” argument… an odious tactic itself.
    Her points knocking this down are exactly spot on. I would also comment that- if all I’d learned was dictated by the minimum standards of curricula- I think my life would be much more drab. People on this website are likely to understand this better than most: kids (or adults) who only learn 7 hours a day 5 days a week- don’t learn much.
    What ever the name/title/designation selected, the whole discussion is a tremendous opportunity for learning.

  9. Dave Hall

    Oh, and my opinion? It doesn’t really matter to me what its defined as–I am adaptable. Just don’t go renaming it something like “Louise.”

  10. dkary

    That is a fantastic composite image. I’m going to ask if I can use it in my classes: it’s great!

    As for Pluto, I really think being a member of a swarm vs. being the big cheese in the area is the important distinction. On the other hand, I like the name “worlds” for all of the things Emily was talking about: planets, moons, dwarf planets, etc.

    I agree with Dave Hall, we don’t need to change Pluto’s name to Louise. On the other hand, I’m still pushing for 1930 P/Tombaugh (OK, the period is slightly longer than 200 years, but I think we can make an exception here).

    Dave

  11. Ryan

    I’m really neither here nor there on the definition of “planet” since astronomy isn’t my field. So long as whatever boxes the field wants to put around the worlds out there are useful and facilitate the thinking that gets us to the next question, then cool; Pluto’s a planet, Pluto’s not a planet. Pluto was what it was before we knew it existed; it was what it was when a bunch of humans called it planet Pluto; and it will be what it will be when a bunch of humans call it superfun planetesimal Pluto.

    Whatever the definition, if it gets in the way: chuck it. If it helps someone wrap their mind around the universe a little bit and gets them to ask the next great question: cool.

    Per science education, I agree with Emily. Science education is about teaching whatever our current understanding of the universe is, the methods by which we arrive that those understandings and some history of how we got to that understanding. Being able to recite the names of the planets (and in the correct order!) is a trite detail. Put a picture of Jupiter up next to a picture of Earth (preferably somewhat to scale), and then some real science education starts to happen. What’s going on there? How could we figure that out?

    And I know I’m stealing this from someone else so don’t mistake this for anything resembling and independent thought but, Pluto’s “demotion” is a great lesson in the scientific process itself!

  12. I suppose what I’d like to hear from Stern is actual examples of how science is harder to do because of this definition. I’m kind of betting he can’t come up with any. If science needs a word to describe the thing he insists is called a planet, it will come up with one.

    Are scientists incapable of understanding each other’s academic papers because they aren’t sure what definition of planet is used? Are proposals for observing time rejected because they fail to use IAU-approved terminology? Give me a break.

  13. When I talk astronomy with my twin 7-year old girls, I don’t ask for a listing of “planets.” I ask things like “name the x” where “x” is “gas giants,” “ice giants,” “rocky worlds,” “asteroids,” etc. I am just trying to get them to learn the major named objects of the solar system and understand what it means to be whatever type of object it is. I think it is time for professional astronomers to stop using the emotionally- and historically-laden word “planet” and find a new, scientific term to play with.

  14. Sarcastro

    The difference between the comet Itokawa and even a smallish asteroid is really rather shocking. If you sat Itokawa on the Earth’s surface, it would be a fair hill, maybe a little difficult to bike up, but that’s about it. Whereas even small asteroids are bigger than Mt. Everest.

    Itokawa is an Apollo asteroid. The smallest comet on there is Wild 2 (or 81P/Wild) with a short axis of 3.3 Km. That’s a tough hill to pedal.

  15. gopher65

    Exactly Scott G. For some time now I’ve been saying (and, I suppose, other, more important people been as well (that’s a lot of commas in a row:P)) that “planets” should be defined by the *physical characteristics of the object itself*, not by some religious argument about “importance”. If there is no physical difference between 2 different objects, then they are in the same class. If there is a physical difference, then draw a line between them — with a little Romulan Neutral Zone of ambiguity surrounding the border, because that’s important to have.

    There are ice planets, rocky planets, gas planets (Neptune), gas giants (any gas planet that is at or approaches the maximum diameter that it can have in its environment (because a Jupiter-like planet will have a smaller max diameter than a hot-Jupiter)), gas-supergiant (can fuse tritium). If it can fuse deuterium, it is a Brown Dwarf (with subclasses for the method of formation (planet/star), and if it can fuse hydrogen, it is a main sequence star.

  16. Torbjörn Larsson, OM

    If it’s important to have a definition, they will come up with something. If the number of objects decides, it is putting the cart in front of the horse – so it can’t be that important.

    I now sort of understand the geophysicists that points out that hydrostatic round bodies are more interesting due to ordering/potential for atmosphere. But as Emily point out all bodies are interesting in some aspect. And if I go and look at the analogous problem in biology, a definition shouldn’t be important, it’s the processes that are interesting. (So I could go with “bodies”.)

    Instead it seems that brown dwarfs are a class of their own, as they don’t form as just ‘smaller stars’. If that’s true it’s telling.

  17. Torbjörn Larsson, OM

    At least NASA puts Itokawa among Apollo objects. If it was found as late as 1998 I doubt it’s a late captured comet – isn’t it more likely that Apollo’s have been disturbed out of the asteroid belt?

    Btw, that comes nicely back to processes that establishes populations (as in biology). If Apollo’s can be identified from that, I bet one can similarly identify planets if one wish.

  18. BMcP

    I found it interesting that what defines a planet seem to have been in some flux for a long time now. Ceres, when first discovered in 1801 was classified as a planet, along with Pallas, Juno, and Vesta but then in 1854 they were redefined as asteroids. I know now Pluto has been “demoted” because of similar reasons Ceres was originally (before being “promoted” from asteroid to dwarf planet in 2006), because of the discovery of so many other similar worlds. It almost seems that people do not want so many worlds classified as planets, as that dilutes what it means to be a “real planet” or something, heck maybe they worry it makes the Earth less special if every small round body that orbits the sun is called a planet, I don’t know.

    Even though Pluto is still a planet in my bias heart, I don’t really put much thought or worry anymore to what is a planet versus a dwarf planet. Instead I am just fascinated by how many bodies there are in the solar system, so many which were recently discovered. Just this last few years alone out solar system became so much more “populated”. Imagine what else is there yet to be found still?! Better yet, this is just our solar system! Now we know of extra-solar worlds, just imagine how populated the neighboring systems are likely to be with all sorts of strange worlds!

    Now that’s exciting!

  19. Chris A.

    IIRC, the estimated diameter of Hale-Bopp’s nucleus was 40 km. That puts it in the same neighborhood as Ida/Mathilda.

  20. rob

    we could all listen to shakespeare who wrote:

    What’s in a name? that which we call a rose
    By any other name would smell as sweet.

    it seems to me there has always been a problem with the classification of planets. after all, how useful can a description be that lumps jupiter and pluto together? if jupiter were 50 times more massive, it could be a *star*.

    ultimately the definition of a planet should be one that helps clarify the similarities and differences between the types of objects in orbit around the sun. it should not pander to those who want to retain an easy to remember pnemonic, or to those that want to freeze themselves in some scientific status quo.

    pluto will still be pluto, whether it is a planet or not.

  21. Hi everyone, I this post inspired what may be a silly idea, but indulge me for a second. Since childhood I’ve been fascinated the Nemesis theory, which attempts to answer why mass extinctions are cyclical (I know the idea of cyclical extinctions is controversial). But what if instead of a dark companion star disturbing the oort cloud, there was actually an enormous earth crossing comet that is on a 26 million year orbit. And by enormous, I mean Pluto sized. I’m guessing that if there were a comet like this, it would have long ago broken up and stretched into a long string of smaller comets, some of which hit the Earth as they pass. Could this work?

  22. We don’t have strict definitions that separate stones from rocks from pebbles from boulders. We don’t have strict definitions that separate villages from towns from cities from hamlets from metropolises. But we remember the important ones: Plymouth Rock, rocks from the Moon, New York, London, Tokyo.

    Why can’t it be the same with plan… err, all the things orbiting the sun? Remember the important ones. Be familiar with them. Jakarta, where’s that? Indonesia, let me tell you about it… Pluto, what’s the deal with that one? Well it’s icy and part of a triple system with Charon and Nyx.

    I think that’s what Emily might be saying and I agree with her.

  23. Tyler Durden

    “I’m guessing that if there were a comet like this, it would have long ago broken up and stretched into a long string of smaller comets, some of which hit the Earth as they pass. Could this work?”

    No, no it wouldn’t.

    We detected Shoemaker-Levy (which I assume is where you’re pulling this theory from, other than out of thin air) long before it impacted Jupiter. This was in 1994, and since then our detection techniques have advanced considerably.

    The only reason Shoemaker-Levy broke up into a string of smaller comets is because it passed Jupiter’s Roche limit (the point at which the gravitational pull of the larger object is so strong that the smaller object can not remain coherent).

    What would you suggest would have pulled apart this mystery comet of yours? Any object large enough to do that would also have pulled all the pieces into its gravity well and the pieces would have impacted, spectacularly. Just as happened on Jupiter in 1994 with the Shoemaker-Levy fragments.

    http://en.wikipedia.org/wiki/Shoemaker-Levy

  24. What would you suggest would have pulled apart this mystery comet of yours?

    Well, I was thinking of the sun.

  25. amphiox

    Jose, I may be wrong on this, but I suspect that a pluto-sized earth crossing orbit comet that made a close enough pass to trigger mass-extinctions every 26 million years would have perturbed the moon’s orbit enough for its existence to have been inferred already.

    If the sun broke up such a comet, it is likely that the pieces would have fallen into the sun soon afterwards, either on the same orbital pass or one soon after.

    The 26 million year interval of mass extinctions is not that certain and is likely not to be that regular. For the majority of the ones for which there is enough evidence to hypothesize a cause, terrestrial events are more likely for most. In fact the only one with solid evidence for an extraterrestrial cause is the KT dinosaur snuffer. (Some have argued for the end Permian, but that is a scientific debate that a non-expert is probably best advised to steer clear of. I’ve heard speculation that the end-Ordovician was the result of a gamma-ray burst, but have seen no evidence, and in fact have difficulty conceiving of any kind of evidence left by such an event that could still be found)

  26. amphiox

    The only reason to have a very specific definition for a term like “planet” is to have a categorization that is useful for research in the field. As such, if planetary astronomers feel such a definition is necessary, I suggest that they be free to make one, as arbitrary or non-arbitrary as they like, with the only criteria being its usefulness for their particular field.

    The rest of us non-experts may then be free to continue to use the word “planet” in a non-specific context any way we choose, including or not including whatever we want.

  27. Jose

    Thanks Amphiox,
    Let’s forget I said Pluto sized, and go back to really big. I don’t know everything that goes into calculating the Roche limit of an object, but my thinking was that the incredible speed an object with such a long orbit would increase this limit, and might allow much of the comet to escape the suns gravity.

    And I’m certainly not convinced that there’s a 26 million year extinction cycle. I’m just throwing an idea out there.

  28. Wildride

    @carey: That’s why the casting time of the “Transmute Rock to Stone” spell is so long. It takes forever to figure out if it’s done anything.

    I think any argument that bases itself on getting a required number as a result is doomed to failure. That includes both the idea that we don’t want less because it makes us seem less enlightened as well as we don’t want more because they’ll be hard to memorize. If there’s 4, 8, 9, 12 or 50, that’s fine, just so long as we know how they are defined so that we can count them up ourselves and revise the count as new things are discovered.

    Whatever the cutoff point, I just want to know the reason. If “clearing the area” (the definition of which obviously needs to be better explained, too) is important to planet formation, I’d like to know why. Why is spherical shape important? Heck, why is orbit important (although that should be obvious)? What do these constraints tell us about the objects that we a studying that make it important that are or aren’t included in a given group.

  29. If it has its own moons, how can it not be a planet?

    Do we stop calling them “elements” just because we’re getting to many for middle schoolers to comfortably memorize?

    I like the way ScottG and gopher65 want categories of planets.

    The solar system has too many planets to memorize all their names: that’s actually cool.

  30. Jose

    @The Ridger
    There are only 4 elements. Fire, Water, Earth, and Dr. Franks Pain Relief Spray.

  31. StevoR

    BA :

    “The difference between the comet Itokawa and even a smallish asteroid is really rather shocking.”

    Surprised you haven’t corrected that yet! Itokawa is an asteroid. 😉

    Plus I thought there were some really small astereoids up to the point where they merged into meteoroids?

    Pluto is indeed a planet. I don’t see that a dwarf planet is any less a planet than a dwarf star is less of a star – 90 % ofd stars incl. our sun as dwraf stars -are they somehow not to be counted “proper” stars – I think not! 😉

    Additionally, just as there are more smaller stars than larger ones it makes sense that there are more smaller planets than large ones.

    The current IAU definition of planet is as Alern Stern said at the time “idiotic.” Orbital clearing? Come on, NO planet’s orbit is entirely clear and the implications of this as I’ve noted elsewhere are just ridiculous. That criterion fails on many counts and was only added to eliminate Pluto – which was a politically motivated and NOT a scientific or logical decision. The IAU mob who voted against Pluto being classed as a planet should quite simply hang their heads in shame – and correct their error ASAP! 😉

  32. StevoR

    Carey said on August 22nd, 2008 at 4:16 pm :


    “We don’t have strict definitions that separate stones from rocks from pebbles from boulders. We don’t have strict definitions that separate villages from towns from cities from hamlets from metropolises. But we remember the important ones: Plymouth Rock, rocks from the Moon, New York, London, Tokyo.

    Why can’t it be the same with plan… err, all the things orbiting the sun? Remember the important ones. Be familiar with them. Jakarta, where’s that? Indonesia, let me tell you about it… Pluto, what’s the deal with that one? Well it’s icy and part of a triple system with Charon and Nyx. … ”

    Actually its a quadruple system – you forgot Hydra! 😉

    Pluto has three moons – Charon, Nix and Hydra.

    Mercury has no moons and neither has Venus.

    Pluto has an atmosphere -and Mercury lacks one.

    True Pluto has a distant and weird orbit near other icy planets like Eris, Quaoar and Sedna but then if Mercury – or Erath or even Jupiter – was orbiting out where pluto is then they’d be in exactly the same situation
    – so NOT calling Pluto a planet just makes no scientific or logical sense at all. :-(

    It is a decison that simply must be changed if the IAU is to have any real credibility -& if the IAU doesn’t come to its senses & restore Pluto to “proper planethood” then the IAU should be abolished and replaced with a reasonable astronomical authority that does.

    Its current anti-Plutonian definition does astronomy & astronomers no favours & brings us into disrepute among the wider community. :-(

  33. StevoR

    Jose Said on August 23rd, 2008 at 7:53 am :

    “@The Ridger
    There are only 4 elements. Fire, Water, Earth, and Dr. Franks Pain Relief Spray.”

    Air?

    Actually it depends on how you define elements! 😉

    The philosophical type have four – the chemical periodic table type has well over 100.

    I agre too with The Ridger who noted :
    “Do we stop calling them “elements” just because we’re getting to many for middle schoolers to comfortably memorize? I like the way ScottG and gopher65 want categories of planets.”

    & I’ll third that! 😉

    Its probably best if we say there are X types of planet – rocky ones like Earth and Mercury, Gas giants like Jupiter and Neptune, Ice dwrafs like Pluto and Eris, asteroidal dwrafs like Ceres and Vesta, Hot Jupietrs like
    “Bellopheron” or 51 pegasi b & “Osiris” or HD 209458 b!

    & quote :

    “The solar system has too many planets to memorize all their names: that’s actually cool.”

    Unquote.

    Definitely! 8)

  34. Joe Meils

    Loved the little catalog of some of the asteroids and comets we’ve been looking at recently. The only thing I wonder about: in her animation of the Eros rotation… how long is the actual period? The video doesn’t seem to be accurate…

    How long before they open up planets for people to name, much the same way they do the Star Registry? Some names I’d like to see for some planets, down the road:

    Tweenis 12
    Limbaugh (preferrably a hot gas giant)
    Bob
    Peanut (after my dog, when I was a kid)
    Slurpee
    Honda
    Death (but it has to be creepy-looking)
    Dryer Lint (and, if it’s inhabited, we’d have to make contact with the Dryer Lintians)

    And, of course, a whole gaggle of worlds named after Japanese kaiju.

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