Pluto's big Hill to climb

By Phil Plait | September 29, 2008 10:26 pm

In a post yesterday, I talked about the Moon orbiting the Earth, and the Earth’s gravitational sphere of influence, called its Hill sphere. If you have an object orbiting the Sun (like a planet, say), anything inside that object’s Hill sphere is more affected by that object than it is by the Sun.

Is Pluto a planet?I knew about this tidbit of physics, but was reminded of it by Dr. Alan Stern, the head guy of the New Horizons Pluto probe. Alan and I have mixed it up a bit before on what the definition of a planet is. He likes the idea that one criterion of planethood for an object is that the object is massive enough to modify its shape into a sphere. In other words, its gravity can overcome the tensile strength of the material making up the object, and it molds itself into a rough ball. I will admit to being swayed by this argument, though still somewhat unconvinced, because the density of the object comes into play. A smallish (say, 200 km across) ball of slushy water will have enough gravity to cause the semi-liquid water to flow, forming itself into a sphere. By this definition, therefore, it would be a planet. But if the same object were frozen solid, it wouldn’t have the gravity it needs to modify its shape, because ice is so much stiffer than slush. It wouldn’t be a planet!

Still, this is a borderline case, and Alan and I both agree that on the borders you get some room for argument. That’s where the fun is, but it’s also part of the problem with trying to define what a planet is in the first place!

Anyway, by this definition, Pluto is a planet since it’s spherical, and several of the larger asteroids and iceballs orbiting out beyond Pluto would also fit the planetary status bill.

With me so far? OK, let’s look at a different definition.

A current definition of "planet", handed down by the International Astronomical Union, is that a planet can sweep up most or all of the material that orbits the Sun near it. That means the Earth is a planet, but Pluto is not, because Pluto is too small to have the gravity needed to clear out its orbit. The Earth does have what it takes, and almost every rogue object that orbits the Sun near the Earth has long since been cleaned out by Earth’s gravity (with the exception of the Moon, which is bound to us, and the occasional asteroid, which is important in the sense of life-sterilizing impacts, but rare enough that no one will argue with you as far as planet definitions go).

If the IAU ruled the worlds…
If the IAU ruled the worlds…

This definition, though, is silly. If the Earth were out at the distance of Pluto, it would have a hard time sweeping clear the material out there, too. The volume of space that far out from the Sun is vast, and the Earth tiny. It would be like trying to sweep your house with a tiny paintbrush.

The thing is, in your gut, you know that distance from the Sun shouldn’t play into the definition of a planet. If you have two objects the same size, and you call one a planet, the other one should be too, no matter where they are. Orbital dynamics shouldn’t really play a role. If the Earth is a planet where it is now, it should still be called a planet even if it’s 30 times farther out from the Sun at Pluto’s distance. Again, Alan and I agree. This "sweeping out" rule is a bit silly.

And this brings me back to the Hill sphere, the sphere of influence of a massive object. Alan emailed me the other day with an interesting little tidbit. The Hill sphere of an object depends on its mass, and how far it is from the Sun. A massive object (like a planet) can be closer to the Sun and still hold on to an orbiting object (which I’ll call a moon for simplicity). The farther an object is from the sun, the less mass it needs to hold on to a moon, because the Sun’s gravity is diminishing as well.

Hubble image of Pluto and its moons
Pluto and its moons.
The moons are all within Pluto’s Hill sphere.

If you do the math, the Earth’s Hill sphere has a radius of about 1.5 million kilometers. The Moon is only 400,000 km away, so it’s well inside our Hill sphere, and can rightly be said to orbit the Earth.

But what about Pluto? It’s a lot less massive than the Earth, but it’s also very far from the Sun. When you do the math, you find its Hill sphere is 5 times the size of Earth’s! That means Pluto’s sphere of gravitational influence has over 100 times the volume of Earth’s!

When it comes to planets, it sometimes pays to be a small fish in a big pond.

So, does this make Pluto a planet? No, and I don’t think Alan is arguing that either. The point here is that you can’t really use orbital considerations when trying to define what a planet is. Even though Pluto misses out by one orbital-based definition (sweeping up material), it seems like it should pass by another (the Hill criterion). That’s a pretty big hint that orbital considerations should be dropped, and planets should be judged on their own merits.

We’ll still argue, of course. I don’t like the idea of a hard-and-fast rule of what should be a planet and what shouldn’t. But I do think a general rule would prove useful, as long as — like the definition itself — it doesn’t shape our thinking too much. Definitions tend to restrain us, and our minds like to follow the path of least resistance.

And that tends to flow downHill.


Comments (87)

  1. Max Fagin

    With regards to that hypothetical “ice ball” border case planetoid: Imagine a ball of ice in an elliptical orbit, at times close enough to the Sun to be a liquid, and at times far enough away from the sun to be a solid. Imagine what the IAU would say to such an object that is in hydrostatic equilibrium some of the time!

  2. I propose the Phil Sphere – the limit of a clump of material, arbitrarily chosen by Phil Plait, above which that clump may be considered a planet.

  3. Randall

    I’m personally strongly in favor of the definition imposed by the IAU (with a bit of quibbling about how “sweeping out its orbit” doesn’t clarify that Trojans and 3:2 resonance objects don’t stop something from having swept out its orbit.) Here’s my case to you: If you found a Mars-sized object floating randomly in interstellar space, would you consider it a planet? No, of course not, it isn’t orbiting a star. Now, what if it were orbiting a star, but from so far away that as far as anything else in that solar system is concerned, it might as well not exist? I believe that “planet” doesn’t just say something about you, the object which is a planet, but about your relationship to the solar system you inhabit. Planets should have an effect on their solar systems. If you only affect the system around you because chaos theory says everything affects everything else, I don’t think you’re “important” enough to warrant the label “planet.” Only if large swaths of the system would be dramatically and instantly changed were you to suddenly be teleported to the galactic center, do you deserve the title “planet.”

    Viewed in another light, one way to look at this definition is, “If I were an alien viewing this system for the first time, and I were trying to make a model of what’s happening in this system, what are the objects I’d definitely have to include, if I want any sort of accuracy?” A Mars-shaped object in a near orbit is important, because it affects us, Venus, possibly even Jupiter. But if it were out in the Oort Cloud, who would care? It’s debris, and you can just write “asteroid belt” on that area and be done with it.

  4. Brian

    Yeah, but if we adopted that as our definition, then our sun would probably only have four planets.


    The #1 criterion for the definition of a “planet” should be that it orbits within the planetary plane of the Solar System, which Pluto certainly does not — it’s an oddball!

  6. In fact, in my opinion, an object Mars sized floating in interstellar space is still a planet.

    It probably formed as a planet did, and orbited a star as a planet, and got ejected by a gravitational encounter with another planet.

    So when did it stop being a planet?

    This is my problem with hard definitions. I can always come up with some counter-example, either pro or con. I think that tells us something all by itself.

    A definition should be a guide, but not a law. When it becomes a law, it boxes in your thinking.

  7. I disagree with your quibble over the sweeping-out definition. I think you’re missing the Kuiper belt for the minor planets.

    That is, if Earth were father out and in an area where it would be likely to share its neighborhood with a lot of other chunks, then the whole thing — the Earth and the other chunks — would constitute an important unit. The term “planet” is on a par with “belt” or “cloud” or other large-scale terms describing not only the one bug chunk, but all the dynamics in the orbital neighborhood. So yes, distance from the Sun makes a difference in whether we’re talking about a planet or something else.

    We talk about the asteroid belt between Mars and Jupiter as a whole, with some note taken of particularly interesting individual chunks (asteroids). We talk about Trojan and Greek asteroid clouds as wholes, with some note taken of particularly interesting individual chunks (more asteroids). And we talk about the Kuiper belt as a whole, with some note taken of particularly interesting individual chunks (like Pluto).

  8. So when did it stop being a planet?

    Under my viewpoint: when it was ejected from a planetlike orbit. It’s not moving like a planet, it’s moving like a… I want to say “wanderer”, but surely you’re smart enough to see the obvious comeback.


    Dr. Phil Plait: “In fact, in my opinion, an object Mars sized floating in interstellar space is still a planet.”

    That would make it a “rogue planet”, then.

  10. Brian

    I’d tend to agree with John Armstrong here. Like that old riddle: When is a meteor not a meteor? When it becomes a meteorite. We name things based not just by their nature but also by their function and present state. All in all, I’m comfortable with a definition for planet that would demote Earth to nonplanetary status if it were suddenly moved to the Kuiper Belt.

  11. kuhnigget

    Planets are like pornographic movies. I can’t define one, but I know one when I see it.

    Um, hypothetically.

  12. IVAN3MAN

    @ kuhnigget

    How to define a pornographic movie: If the love scenes are out of focus, it’s a “romantic drama”; if the love scenes are in focus, then it is a pornographic movie. :-)

  13. RoaldFalcon

    “The point here is that you can’t really use orbital considerations when trying to define what a planet is. [….] orbital considerations should be dropped, and planets should be judged on their own merits.”

    If we take out orbital considerations, then shouldn’t we add Titan and Ganymede to the roll of planets?

  14. Wildride

    If an object that would otherwise be a planet is ejected from a system is a planet, then what about an object that would have been a planet, got ejected from its orbit only to get trapped by the gravity of another, larger planet. Is it still a planet, or is it now a moon?

  15. Unspeakably Violent Jack

    Can’t see these arguments ever going away. Space is full of “things” and “stuff” & has no respect for our liking for boxes with names. Wouldn’t have it any other way :)

  16. If Pluto is no longer a planet, then maybe it needs a new name, to distinguish it from the other Roman God names currently reserved for planets.

    I suggest “Derek”.

  17. This is my problem with hard definitions. I can always come up with some counter-example, either pro or con. I think that tells us something all by itself.

    and that’s the lesser of your problems. The big problem is:

    The thing is, in your gut, you know

    and the commenters come surging in, with their gut feelings which, surprise, surprise, are different.

    If there’s any point at all to having a definition, it’s to override all our gut feelings. Saying “this doesn’t agree with my inner intuition of what a planet is”, is pointless.

  18. Jeeves

    I have to agree with most responses here. I would have thought that orbital considerations were the most important ones for considering a body a planet. Aren’t the first requirements something like “must orbit a star” and “must not orbit another planet”?

    I’m also surprised that Pluto’s weird orbit never seems to be considered important for deciding its planetary status, because that’s what makes it stick out like a sore thumb to me. To answer the question “what is it”, I would first ask “where is it” and “what is it doing”.

  19. BA: “The Earth does have what it takes, and almost every rogue object that orbits the Sun near the Earth has long since been cleaned out by Earth’s gravity”.

    Actually, there’s countless objects that haven’t been swept clear by Earth. Every shooting star you see is an example. Perhaps “cleaned out” referring to objects with sizes or masses significant to a fraction of Earth’s size and or mass would be more meaningful.

    BA: “If you do the math, the Earth’s Hill sphere has a radius of about 1.5 million kilometers. The Moon is only 400,000 km away, so it’s well inside our Hill sphere, and can rightly be said to orbit the Earth.”

    Earth’s moon rightfuly orbits the Earth not because it is within our Hill Sphere (examples can be shown of objects within the Hill Sphere that don’t orbit), but because its eccentricity with respect to Earth is less than 1… because its orbital energy with respect to Earth is a negative number…, because its semi-major axis with respect to Earth is not a negative number…, and as you say in your last blog, because if the sun suddenly disappeared, the moon would remain bound to the Earth.

    Want to play with the Hill Sphere? Here’s a Javascript calculator I made that allows you to quickly enter data into this formula, and it does unit conversion for you:

    It’s been pointed out in the other blog “discussion” that Hill Sphere does not define the volume in which an object can orbit. The moon is pretty close to the edge of Earth’s prograde stability zone, despite residing deep within its Hill Sphere. That’s why Earth can’t pull the moon into an orbit co-planar with its equitorial buldge. That’s why the sun causes the moon’s orbit to precess in one complete cycle every 18 years with respect to its nodes.

    BA: “That means Pluto’s sphere of gravitational influence has over 100 times the volume of Earth’s!”

    Volume is irrelavant. In-plane orbits are more stable than out-of-plane orbits, so this is closer to a 2-D problem. For example, if Earth’s moon orbited in a polar orbit, it would crash to Earth within a decade due to the Kozai mechanism. That’s why Jupiter, although having moons orbiting in a myriad of directions, has no moons with inclinatons greater than about 60 degrees.

    BA: “That’s a pretty big hint that orbital considerations should be dropped”.

    I agree with you here. But keep in mind that we’re allowing Titan and Ganymede into the club if we drop orbital dynamics.

    BA: “If the Earth were out at the distance of Pluto, it would have a hard time sweeping clear the material out there, too.”

    Agreed. But that is a good reason to say that if a Jupiter-mass object were found in a circular orbit at 200 AU, it also would not be a planet.

    BA: “Even though Pluto misses out by one orbital-based definition (sweeping up material), it seems like it should pass by another (the Hill criterion). ”

    Keep in mind that as you get out to about 50,000 AU, still within the sun’s Hill Sphere with respect to Alpha Centauri, or with respect to the Galactic Tide, something a billion times less massive than Pluto has a Hill Sphere larger than Earth’s. So the “Hill criterion” may let countless undiscoverable tiny asteroids and comets into the club.

  20. It’s debris, and you can just write “asteroid belt” on that area and be done with it.

    Here there be dragons.


  21. What’s wrong with a planet classification system based loosely on mass? It works for stars…

  22. I know this sounds like a circular argument, but I think a planet is basically whatever we decide to call a planet, especially if it’s been given a name. It’s like deciding who your best friends are. There is no rock-solid (sorry) universal system for categorising planets as if they are discrete types like sub-atomic particles, so all we have to go with is our own human preferences. The term “planet” is purely a human thing, so let’s keep it on that level and stop trying to measure or rationalise it.

    So on that basis, Pluto is a planet because it’s been as such in the public consciousness for almost eighty years. You can’t go back on that now. So my definition is, if it orbits the sun (primarily) and it’s been given a name, ipso facto, it’s a planet, for the simple reason that we irrational humans have given it an identity.

  23. amphiox

    Elwood Herring,
    But, Ceres was a planet for 8 years before being demoted. So how many years is your cutoff going to be?

    Or do you think that Ceres should be a planet?

  24. amphiox

    I would not necessarily object to a definition of planet that includes the major moons, so long as it was internally consistent.

  25. After reading more of the above posts, I’ll take on board the notion that a planet that’s been ejected from a solar system is still a planet, so strike out “if it orbits the sun” from my earlier post. That simplifies it even more. A planet is whatever we decide to call a planet, and that’s that. There can be no hard and fast rule so why carry on banging our heads against the wall trying to come up with one? It’s like trying to segregate lakes from ponds, streams from rivers, islands from continents. Whatever rule you come up with, someone will disagree. So all we can fall back on is human “poetic licence”. It’s a planet because we’ve noticed it and named it.

  26. amphiox: yes, I consider Ceres to be a planet. Any reason why it shouldn’t be thought of as such? All the other objects in the asteroid belt that we have given names to are planets in my book. Minor ones, true. But they were considered important enough to be named, weren’t they?

  27. I don’t intend to hijack this thread but I’ve written a longer reply on this subject and posted it on my Myspace page.

    I’ll shut up now.

  28. Lao Tzu

    Here’s another idea:
    A planet is any object massive enough to modify its shape into a sphere. That’s it.

    From there on, we could build sub-categories:
    If it orbits another planet, it’s called a moon planet, or simply a moon.
    If it does not orbit a star, it’s a rogue planet.
    If it orbits the sun, it’s a solar planet.
    If it orbits any other star, it’s an exoplanet.

    No need to invoke any other exotic criteria based on its orbit or on its sweeping abilities. Of course, it would be more difficult to say how many planets there are, and maybe additional more subcriteria would be needed to make things clearer (e. g. Kuiper Belt Planets).

    But the basic definition for a planet would be simple and easy to understand.

  29. TheWalruss

    I agree with Elwood – to an extent. Of course, any label we use is purely an invention by the mind and used to categorize our experiences and facilitate reasoning and communication. That being said, in order to maximize its usefulness and expressiveness we must decide on a common definition, which should be designed to be as precise and concrete as possible, while being general and understandable enough to still allow us to use it to describe a useful set of phenomena. This is why discussions (or arguments) of semantics are important and why handwaving and saying “dude, whatever you want it to mean” is unhelpful. What Elwood said about a planet being whatever we want it to be is right on the money! We just need to agree on what we want!
    Cognitive psychology has found that part of how we organize our internal ontologies is by linking a set of member objects to a concept, so perhaps a good way for us to proceed in this discussion is by making a list of “these are definitely planets”, a list of “these are definitely NOT planets”, and of “these might be planets, but we’ll have to clarify when using the term here”.

  30. You could make it so that the “planet” had to have at least point X number of Earth gravities to qualify.

  31. MarshallDog

    I remember reading the Alan Stern supported classifying Eris and Ceres as minor or dwarf planets but still considered Pluto a planet. I probably read this on Wikipedia but I can’t find it now. Does anyone know if this is true? If it is, can anyone think of a reason for Pluto to be a planet that would not also elevate the status of Eris and Ceres?

  32. Vagueofgodalming said: If there’s any point at all to having a definition, it’s to override all our gut feelings. Saying “this doesn’t agree with my inner intuition of what a planet is”, is pointless.

    Hear, hear…

    We have these definitions for reasons that help us understand the nature of objects we study. Just as the definition of “planet” has been retooled recently, so too are the definitions of what is and is not a star or a galaxy. I mean, look at how we are adding to stellar classifications with the L and T objects; and the numbers of objects that are now galaxies as opposed to say, massive globular clusters or other amalgamations of stars and galaxy “stuff” moving through the universe. The current definitions for all of these take into account an ever-flexible set of dynamical and evolutionary characteristics that are incorporated as we learn more about the objects themselves. Not gut feelings.

    Admittedly it takes a little “decoding” to unpack all the meaning behind these definitions, it’s worth the work. So, I don’t have a problem with the idea of setting definitions up to help us understand what we’re seeing and how it got to be the way it is… what I do have a problem with is the “inner definitions” we all seem to carry around and if what science decides doesn’t jibe with that, then there are fireworks.

    One of the things that seemed so illogical in the debate a couple of years ago was the battle cry of “Think of the children who learned nine planets and now have to unlearn one of them!” and similar sentiments. To which I often replied, “Since when did what children learn become a guiding principle in determining scientific taxonomy?” — a question that went unanswered. Kids are pretty good at learning what’s changing and what’s hot. If they weren’t, we wouldn’t see them grasp new technology faster than adults seem to do. I think kids (and adults) should be able to adjust to the fact that the definition of planet (or star or galaxy) is one that depends on certain criteria that help us understand what those things are…

    (wild-haired lady science writer now steps off of soapbox) 😉

  33. Sion

    How about this for a general rule?

    Big, round thing orbiting a star, may have moons.

    I suspect that in the minds of 99.999% of the population this is what they use as a definition.

  34. Philip

    though minor discussions still persists, biology has its taxonomy worked out quite nicely,
    and they have to cope with, at this point of discovery, many more differentiated organisms than astronomy has differentiated planets.

    Planet is any body, wherever it may wander, not causing thermonuclear fusion.
    Regardless of size, composition and location.
    And then, differentiation may take place.
    So the Moon is as much a planet as Mercury or Ganymed, Ceres, Halley or Pluto.

    The never ending discussion shows that the word”Planet” is not very conductive to a descriptive definition. Much less so for a scientific classification.
    Like “Animal” or “Plant”.

    Could anyone shortly describe why a scientific definition of “Planet” is necessary? Taking into account that any subcategory will happily provide the needed description of its properties. Like gas-p, asteroid, comet, terrestrial-p, minor-p, dwarf-p, trojan, etc.

    And don’t tell me that the only reason for definition is that a certain planetarium needs one single word for the objects they hang under their roof and they don’t have the space to add KBOs, Asteroids and Comets.

  35. I never understood the commotion that followed Pluto’s demotion. It was always an oddball, and always recognized as such. When I was learning about the nine planets in school classrooms, it always seemed odd to me (there’s that “gut feeling” thing again) that this crazy, tiny thing in a radically different orbital plane was considered a planet – but at the time, I figured, well, we consider it a planet because it’s there.

    For most of the time Ceres was never even mentioned, so it came as something of a surprise to me when I first heard the story about Ceres’ discovery, planetary status, and ultimate removal of that planetary status. And it certainly got me thinking about Pluto, that tiny little wanderer billions of miles away.

    Now that we know for certain that not only are there other objects in Pluto’s eccentric orbital plane, but there are bigger ones – and very likely many more that wait to be discovered – it seemed completely obvious to me that Pluto wasn’t a planet. We shouldn’t just label things planets just because we find them. It in effect cheapens the word, cheapens the idea. If this is all about educating the uneducated (the primary argument, it seems, on both sides), then the uneducated should be given more of an explanation about what a planet is than “it’s an object that is round and orbits a star.” Considering that in our star system alone that definition may include hundreds of objects, how does that help the uneducated? What’s interesting about using the same term for a whole group of hundreds of things that are all radically different? One could argue that as we’ve found hundreds of moons in the Solar system of all types, the whole term “moon” has lost a lot of its interesting meaning, and perhaps a new classification system is in order.

    Pluto, like Ceres, deserved a demotion. If anything, Ceres has more right to be called a planet than Pluto, considering that at least it’s in the Sun’s orbital plane.

    Personally, I think it’s much more interesting to say, “Here’s the inner, rocky terrestrials, here’s the asteroid belt, here’s the outer gas giants, here are the outer ice giants, here’s the kuiper belt objects, and here’s the oort cloud. Let’s look at the similarities of these objects, and what it all means.” That’s a lot more relevant than “Pluto is a planet because dangnabbit I want it to be one!”

  36. Tim H

    The word “planet” is, more than anything else, a description of the way things look from Earth. One might argue that Mars and Jupiter are not particularly similar objects, but humans spent thousands of years staring at the sky without telescopes, and if you don’t have a telescope there is not much difference. Even now, though people try to think of astronomy from a “universal” perspective, it’s not surprising that the real definition of “planet” gets mixed up with what we care about here on Earth. Gas giants are interesting because they are big, have lots of interesting moons, and because extrasolar gas giants are relatively easier to detect. Big rocks flying around are not really interesting except by analogy to Earth, which means they have to be big enough, close enough to stars, and in a stable enough orbit for us to really care.

    If we want to have a really universal perspective I suggest we stop attempting to build a “planet” category and go with “gas giant”, “big rock”, etc and shift the definition argument to where it belongs: what objects can be thought of as “Earth-like”.

  37. Torbjörn Larsson, OM

    Interesting to see how astronomers analyze this, especially when you have no stake in this.

    This definition, though, is silly. […] The thing is, in your gut, you know that distance from the Sun shouldn’t play into the definition of a planet.

    But this is analogous to an argument from personal incredulity, it is silly (false) because of personal conviction. Better to say that it is convenient to have a non-contingent (albeit fuzzy) definition.

    The point here is that you can’t really use orbital considerations when trying to define what a planet is.

    My understanding is that you can, because the specific criteria selects a natural population of the traditional planets (pre Tombaugh). But moreover, I thought the criterion’s contingency, inconvenient as it may be, revolves around selecting geologically mature planets. They are no longer subjected to events that, say, blasted off light crustal material, changed Earth composition and created the Moon.

    On this point I can but recapitulate what I’ve noticed so far from other threads.

    By conveniently looking at an analogous problem in biology, defining species and recognizing/naming them, my understanding is that biologists looks at the process, both as regards biology and as regards scientific recognition.

    Thus, while there have been at least 26 species definitions considered at one time or other, and AFAIU no one standardized by a scientific body, there is a standardized way of recognizing new species. And it relies ultimately on homologies and differences as resulting from the evolutionary process, whether or not the species are definable as biological (sexual populations) or, say, ecological (non-sexual populations).

    Thus it seems to me the “species definition” is sorted out later, as a result of cladistics placement. Seems the public is on board to let the biologists decide whether or not they actually want to define species for some specific purpose, as long as the public “know one when they see one”. If astronomers did the same, they could name every object they like, and sort out what they want it to be later.

    [Btw, lessons learned: the old naming system is very much Linnean, so AFAIU one runs into problems when trying to adopt modern cladistics. (There, say, birds are cladistically simply “avian” dinosaurs.) So one should use names that doesn’t single out a preferred placement. I hope the naming of the dwarf planets isn’t making such a mistake.]

    My question is then; I can understand why the public wants to keep recognizing planets specifically, but why is astronomers bothered by it? Perhaps Ceres is as interesting as Neptune in some way.

    The #1 criterion for the definition of a “planet” should be that it orbits within the planetary plane of the Solar System,

    Interesting criterion.

    I seem to remember the Science editorial commenting on some new model of planetary system formation that reproduces many features of observed exoplanet systems, such as hot massive planets and eccentricities, but when introducing eccentricities also introduced angles to the plane. IIRC the editorial claimed the jury was still out on that one, observations of eccentric exoplanets was still, and perhaps preferably so, consistent with orbiting within the planetary plane. A mystery unsolved.

    OTOH, what is preferred and especially what is within the tail in a distribution as opposed to outside it can be hard to sort out on scarce data. And do we want to leave potentially unlucky bodies out, because they have “a history”?

  38. Torbjörn Larsson, OM

    D’oh! Should have updated, Philip makes much the same argument and points. But shorter. 😉

    If Pluto is no longer a planet, then maybe it needs a new name, to distinguish it from the other Roman God names currently reserved for planets.

    I suggest “Derek”.

    But this I simply missed this before discussing naming. Of course it could have new names, in the same way that artists needs to change their regular names to something interesting in order to succeed. …, um, no, they tend to keep it even when their careers are in the drain. Never mind.

    enough to modify its shape into a sphere

    Perhaps not the best thread to nitpick, but it should probably be “a spheroid”, seeing that newest dwarf planet member Haumea is one due to its fast rotation.

  39. Gary Ansorge

    Ceres a planet? Last I heard, it was 750 km long by about 450 km wide. That’s hardly a sphere.

    Pluto is (apparently) spherical. That makes it a planet, in my book.

    Besides, I like dogs,,,

    Gary 7

  40. Excellent article, Phil. Bravo. You hit all the nails there were there to hit, even in the reply about a rogue Mars-sized planet.

  41. ruidh

    Your complaint is that the definition for a planet is arbitrary. I maintain it has to be. There’s isn’t a platonic ideal of “planet” out there with it’s own neat definition like the platonic definition of a square. The real question is “Is the definition useful for our purposes?” Does it help scientists organize their work? Does it have pedagogical value? Does it communicate well. the concern that there might ultimately be 35 objects that fit the old definition of a planet makes the category a bit too unwieldy for scientists, educators and students.

    When we categorize things we try to get them down to a few different types. There are vast numbers of species on earth, but we group them in broad ways and then finer ones. Vertebrates and invertebrates and then into fish, amphibians, reptiles, birds and mammals. Each category has a handful of subcategories so that they can be remembered. 9 “planets” served us well for a long time, but as soon as it seemed they were going to go into the double digits some new classification scheme was needed for solar satellites.

  42. MH

    I think there’s a false assumption being made about this issue, which is that there has to be a singular, one-sentence definition for ‘planet’. Why not use the style of definition psychiatrists use for mental health issues?

    A planet is any body displaying N of the following characteristics:

    1. Enough gravity to overcome tensile strength
    2. Can sweep its orbit clear
    3. Orbits a star
    4. Does not orbit another planet*

    Et cetera et cetera. You could also structure it where there’s a list of characteristics the body MUST meet (for example, the object must not itself be a star – definitely want to exclude binary stars, which most definitions suggested thus far would not exclude), and then a second list where it needs to meet 4 of the 6 (or however many) qualities.

    That way the definition can cover most people’s intuition about planethood while still being rigorous and objective.

    *Is there mathematically such a thing as a (stable) binary planet? Pluto-Charon, maybe, I guess?

  43. Cheyenne

    I really don’t understand why we don’t just pick some arbitrary mass (Mercury’s?) and call any singular, compacted object (that isn’t a star or black hole) a planet.

    It could be out in deep space not orbiting anything, it could be orbiting a stable 4 neutron star system (ooh that would be sweet!- Hubble please get fixed so you can find one!), or it could be just a regular ol’ planet around a regular star.

    Maybe I’m just too much of a layman, but it seems like we’re trying to make this too complicated.

  44. rob

    i propose using the kirk criteria:

    it isn’t a planet unless captain kirk went there and made out with an indigenous female.

  45. Er, Phil, I don’t follow the argument you’re making at all. I don’t see how the size of a Hill Sphere should imply that something is (or is not) a planet. Even if I did see the implication there, I don’t think that an example of how orbital dynamics not being a sensible part of a definition rules out all orbital dynamical considerations. Amid this whole controversy I’ve heard a lot of ire at dynamical astronomers (including a fair bit from our colleagues in planetary science), but I have yet to hear an explanation of why orbits are less important than, say, geological considerations.

    (This isn’t to say that I think that the IAU definition works the way it should. The real consideration shouldn’t be whether the orbit is ‘cleared’, it’s whether the would-be planet is part of a family of similar objects, where similar means dynamics, size, and history.)

  46. jess tauber

    Is it or isn’t it- all depends on what the meaning of is is….

    In multifactor cognitive prototypes there are ‘best’ representatives, then not so good ones as you go in different directions from there. Is a platypus a mammal, a reptile, a mammal-like reptile or a reptile-like mammal? If you like hair, then the former, but if you look at all sorts of different features, then the latter.

    But people tend to like simple and absolute definitions, which is too bad, because the real world is often a tad more complex, and likes it that way. Blame it on the evolution of symbolic language. Some languages, though, have large numbers of what are called ideophones, forms that act as if they were constructed on a kind of semantic algebra, where one can break them down not just to roots or affixes, but all the way down to phonemes and even their underlying acoustic and articulatory features, and still be able to assign meanings to them. These are NOT symbolic, but rather iconic.

    Such a system is capable of generating enormous finely detailed semantic qualification. Every tiny shift from one sound to another leads to a dependable meaning shift. A deliberately created analogue could easily capture all the variability of masses in space, all in a single word.

    Jess Tauber

  47. Doc

    I think Jupiter should be promoted to a new category: failed star.

  48. MDF


    I feel the same. I have felt since grade school that Pluto was an ‘odd duck’ and wondered why it was considered a planet along with the other eight. It had always seemed to me to be a KBO whipped into its odd orbit.

    @IVAN3MAN @Jeeves

    Also agree with these people. it’s the orbital configuration, people!


  49. MarshallDog

    Doc, I think Jupiter would have to be about 80 times more massive than it currently is to become a star. But again, the line between stars and planets is fuzzy too.

  50. John Powell

    My problem with the ‘sweep the orbit’ requirement is that they don’t apply it to Neptune – after all Pluto hasn’t been swept away by Neptune’s gravity. IAU – be consistent!

  51. Hey Phil,

    I have to point out one little error of logic your (and Alan’s) claim that Earth would not be a planet if it were in the outer solar system. That might not actually be true. If an Earth-sized rock formed in the outer solar system, it would likely accumulate lots and lots of material and grow into a gas giant. That’s how Saturn and Jupiter did it. Pluto didn’t because, like other Kuiper belt objects, it was likely flung out of the growing zone while the larger objects were growing (by accumulating gas and some other Pluto wannabes) into Jupiter, Saturn, etc. That’s pretty much what “clearing it’s orbit” refers to, and it’s why I think the IAU decision was a reasonable one; it takes into account the formation process of the object.

    Having said that, I don’t think this debate is all that useful unless it leads to many more people actually coming to understand how our solar system formed. I don’t see much evidence of that, which is a pity. The “planet” label itself is not really important.

    By the way, I learned about all this stuff in my solar system dynamics course back at CU. The course was team-taught (fabulously) by Hal Levison and (of all people) Alan Stern.

  52. David Klassen

    I see nothing wrong with including orbital dynamics in trying to define planet—and, no, a non-stellar object just floating through space would not be a planet in my book. It may be a “planetary body” or “planetoid” but I’d reserved “planet” for something that orbits a star.

    Of course, the IAU “sweeping out” definition is silly. As already pointed out, Jupiter certainly did NOT sweep its orbit clean of the Trojan asteroids (hmmm… if we’re going to discuss the meaning of planet, what about asteroid?!). I think the better dynamical consideration is population size statistics. You look at the object and all other objects “nearby” or in the same/similar relative orbit. If the size distribution of those objects is “one object significantly larger then everything else” then its a planet. This works for Jupiter and the Trojans; it works for the Earth and the NEO’s (and even the Moon, although we could get into an argument about binary planets if you’d like). However, if the object you are trying to classify is merely one in a broad, somewhat continuous size distribution of objects, then it’s not a planet, but one of a group of “other things” (and, no, not dwarf planet—what a completely inane classification!). Look at Ceres; there’s LOTS of stuff that sorta share its orbit and they all form a near-continuous distribution. Thus, none of them is a planet. Pluto is in the same boat.

    I didn’t come up with this all on my own, this was at one time the preferred definition offered up by Mike Brown, discoverer of Eris and you can see his argument at (look at the 4th bullet point).

  53. GloomyGus

    “Definitions tend to restrain us, and our minds like to follow the path of least resistance.” I like that sentence a lot and it got me to thinking. Is it possible that the reason we are having problems coming up with a suitable definition of planet is that we are still far from having a good theory of solar systems, their formation and evolution? My sense is that we know far more about an atom than we do our solar system — in terms of a rigorous theory. Obviously we have a lot of data on specific features of our solar system and the two items I am comparing against each other are markedly different in many ways. We do know for example that Jupiter has the bulk of the Solar System’s angular momentum. We also know that many stellar systems in the neighborhood have planets. Thus planets and stellar systems go together. As we learn more (really, we’ve been working for centuries with only a few data points and it only recently has started to change) about these stellar systems, as we start to measure the next decimal points if you will, is it possible that a true scientific definition of “planet” will emerge from this new and far more sophisticated theory than what we have now? I won’t push this any further because the QM of an atom is very different indeed from the CM (celestial mechanics) of a solar system, but it might be suggestive of an approach. In other words, no need to get hung up on a definition now. Let’s just focus on getting more data and better theory.

  54. Ben

    I mostly agree with Invader Xan and Cheyenne.
    Any definition is an arbitrary rule anyway, why not make it something that can be clearly stated and measured.
    How about anything that orbits a star and has a mass grater then 7.36 × 1022 kilograms (Mass of the moon). That way there is no gray area about how cleared the orbit is or if it is round enough.
    Why can’t it be that simple?

  55. Anchor

    This debate about the proper definition of “planet” is long since excruciatingly tiresome.

    Defining what a “planet” is has much less to do with astronomical reality than it has to do with establishing what we mean by a word. It’s NOT an important scientific classification issue. Never has been. It’s a lexicographical issue, not a scientific one.

    It should remain the generic term it’s always been, applicable to any object one chooses to call a “planet”. (Even third-rate poets understand this).

    Nothing but rubbish is generated by all this silliness: Worlds not being a planet if it is a moon (even though planets orbiting stars are moons of those stars);

    worlds not being a planet because it doesn’t orbit within a common plane established by other worlds (even though it is possible to have planets perturbed

    into orbits that can have a high-inclination to such a plane); worlds that aren’t planets because they don’t orbit a star and are isolated in interstellar space (even though they may once have done so and been kicked away); worlds that aren’t planets because they do not “gravitationally clear their orbits” (even though trojan and resonance objects can and do often share them); worlds that aren’t planets because they do not orbit the Sun (even though worlds every bit as substantial orbit other suns); worlds that aren’t planets because they aren’t “round” (even though tiny bits of molten rock or water easily attain such spherical pulchritude with surface tension and blobs hardly more massive than a bus can remain molten long enough to attain voluptuous roundness gravitationally)…and so on, ad nauseam.

    Distractions like this, just because of a lousy word.

    So what is this exercise in nonsense all about? So that kids can count out the “proper number of planets” in the Solar System or in any exoplanetary system? Or so astronomers can? This is an important scientific question? Really?

    Somebody has to say it: if one is doing astronomy, it is NOT important to dertermine a proper definition of ‘planet’. If you want it to be important, take up lexicography.

    The classification maniacs in the latter who insisted on tackling the “issue” at the IAU didn’t do anybody any favors. It simply isn’t a scientific issue. I hate to say it of my colleagues, but it will go down in the annals of astronomy as an unprecedented act of constipation, followed by a seemingly endless outpouring of contending wills.

    Stupidity beyond belief.

  56. Donald

    Is a Hill sphere actually a sphere? Wouldn’t it be stretched away from the Sun by tidal forces? Or am I being overly pedantic?

  57. The best idea expressed here so far is Lao Tzu’s system of subcategories.
    Having an inclined orbit not on the same plane as the solar system’s other eight planets does not make Pluto or any other object not a planet. If we look at the exoplanet systems discovered so far, we see many stars with planets that have eccentric orbits and/or multiple planets not orbiting on the same plane. Pluto only appeared to be an oddball because we were not yet aware that there is an entire class of planets,that is objects in hydrostatic equilibrium, beyond the orbit of Neptune that have inclined orbits. This does not make them not planets. It simply makes them another type of planet.

    Ceres’ demotion ends up having been an error because it is not like the other objects in the asteroid belt. Being round, it is differentiated with geological processes far more akin to those of the big planets. Of course, this was not known in the 19th century, so one can hardly fault astronomers for demoting it. However, today, we know that it is round and therefore fundamentally different from its neighbors in the asteroid belt, which is why it should not be lumped in the same category as them. The same is true for Pluto and other round Kuiper Belt Objects.

    As we discover more types of objects out there with more and more variations, the smart thing to do is add new subcategories, all of which are encompassed under the broad umbrella of planet. So a rogue planet, which does not orbit any star, is still a planet, as it almost certainly once orbited a star only to somehow have been ejected from that orbit. That ejection does not change the fundamentals of what the object is. Similarly, round moons of planets should be classified as secondary planets–which they were for much of the 19th century–since their composition is the same as those of planets but their primary orbit is around another planet as opposed to around a star.

    Orbital dynamics are important but cannot be the only consideration in defining the concept of planet; otherwise we are left with the absurd situations described above where the same object is a planet in one area and not a planet in another. Keeping the term planet broad and using subcateories to account for both orbital dynamics and geophysical composition rightly takes all these considerations into account.

    To answer another question, Ceres and Pluto were never considered minor planets. The term “minor planet” has been used to describe asteroids, shapeless inert objects clearly distinct from those like Ceres and Pluto that are in hydrostatic equilibrium.

    I also disagree that having 35 plus objects that fit the category of planet makes use of the word “unwieldy” for scientists, educators, and students. The number of planets does not matter, and it does not diminish the pedagogical value of the term. How many categories of objects fall under the broad umbrella of “star?” We have billions of stars and many different types of stars, yet this does not pose a problem for educators, scientists, or students. Why should planet be any different? Why does the number have to be scarce for the word to have meaning? Why not instead explain to students that there are many subtypes of planets and help them learn what the categories of each subtype are? That will serve to bring home the immense variety within each type of astronomical object in the universe.

  58. Quiet_Desperation

    I still side with the IAU on Pluto. Not a planet. 😛 Even as a little kid, looking at that orbit, I pondered if it wasn’t a wayward moon. No one knew about the Kuiper belt back then. It doesn’t take anything away from Tombaugh. In fact, it elevates his discovery as the first Kuiper object instead of just the ninth planet.

  59. Mike

    Going back to the beginning…a “planet” is a light in the sky that wanders among the other lights in the sky. Using this classical definition, there are only five planets. Everything else is just stuff.

  60. I’m still amazed that whenever this discussion comes up, no one mentions the fact that a simple criterion has been proposed (by Steven Soter, click my name for his paper) which clearly distinguishes the eight planets from everything else. Simply put, you look at M/m, where M is the mass of the object in question, and m is the total mass of everything else that crosses its orbit in a non-resonant way, and with an orbital period within an order of magnitude of the candidate’s (to keep comets from skewing things). Turns out there’s a big gap between the planets’ M/m value, and Pluto’s, Eris’, Ceres’, etc. (Mars’ M/m value, 5100, is the lowest of the planets; Ceres’ M/m value, 0.3, is the highest of the non-planets.)

    Presumably, objects in the solar system got their individual M/m values due to the processes by which they formed and ended up in their current orbits. If so, then the gap between Pluto, etc. and the planets is an indication of their different origins. Isn’t that why we’re trying to come up with a definition of “planet” in the first place: To create a physically meaningful way to categorize things that orbit a star?

  61. rkolter

    How about determining if a rocky body is a planet by it’s estimated average gravitational pull at its surface? Anything over 1/10th a G is a planet.

    If multiple rocky bodies are in orbit around each other, all of them that meet the above definition are planets, and all of them are also considered moons of the most massive planet.

    If you meet such an object in the orbital plane of a star, outside that orbital plane, around a gas giant, or floating free in space, it wouldn’t matter. It’d be a planet.

    As we find other solar systems, our definition will really need to be radically altered. Why not get rid of all the traditional ideas and just pick a template we can reliably use anywhere?

  62. gopher65

    Clearly instead of definitions of the “drawing lines in the sand” type we need to create a “continuum” type definition. Those *always* make more sense.

  63. @ Laurel Kornfeld

    While there are many objects that fit in the category of “star,” they all share fundamental characteristics – thermonuclear fusion in the core, a main sequence of some sort, existence mostly as a ball of superheated plasma. That there are many different sizes of stars with different chemical components doesn’t mean that the word “star” doesn’t have a pretty specific meaning.

    Planets, on the other hand, seem to come in many different flavors. There are your rocky terrestrials, your gas giants – but the fact that they are all orbiting stars – and the fact that they are all on the same orbital plane – gives me a much more concise feeling about their definition.

    If you took Pluto and put it in a much more wildly eccentric orbit that made it come significantly closer to the sun, you’d call it a comet, because it would be one. That wouldn’t be the case for any of the other objects we consider planets.

    The fact that Pluto is NOT a planet makes it more interesting, not less. The idea of trying to nail down what planets are – and how they relate to the solar system at large – is a good thing.

  64. IVAN3MAN

    @ Laurel Kornfeld

    There also are many categories of music genres: Classical, Folk, Jazz, Country & Western, Rhythm & Blues, Rock & Roll, Pop, Heavy Metal, Punk Rock, Reggae, Electronic, Acid, Techno, Rap, Skiffle (kitchen sink music), etc.

    As to the question, “Is Pluto a planet?” Is Rap ‘music’ Classical?

    The term classic means something that has stood the test of time and has the “highest rank or excellence”; therefore, the distinction of “planet” should be reserved for the classical planets: The Rocky Planets — Mercury, Venus, Earth, Mars; The Gas Giants — Jupiter, Saturn, Uranus, Neptune.

    Everything else can be be given sub-categories. That is it. Period!

  65. As I blogged last month, the orbital clearing definition has problems with the recent discovery that planets (especially around other stars) do not have static orbits. It is hard to clear your neighborhood if you are constantly moving.

    For Example, using the IAU definition, our solar system had 8 planets 4 billion years ago. But shortly after that, resonance between Jupiter and Saturn disrupted the orbits of the outer solar system. So 100 million years later, at 3.9 million years, we only had at most two planets- Mercury, and Venus. Earth, Mars, Jupiter, Saturn, Uranus, and Neptune didn’t cease to exist, but they stopped adhering to the definition of ‘planet’. This is because gas giants tried to clear each other out of their orbits, and in the process they pushed the asteroid belt and/or the inner kupier belt inward, into Mars and Earth’s neighborhood. The big impact basins on the moon are a result of the Earth-Moon system “reclearing” their orbit at this time.

  66. KC

    Thank you Chris A. When discussing the definition of a planet people get all tangled up in the IAU language instead of looking at the science behind it. Go look at the article Chris linked to. If you want to get right to heart of the matter skip to Figure 1 on page 21. For a body a that mu>100 its a planet (has it cleared its orbit of every speck of dust? *No* But does it *dominate* its orbit: Yes!). If not then no. Simple as that. I rather like this rather that the planetary scientist’s definition because of the reasons the BA cite above plus the fact tha tan oblect can be in equilibrium and yet not be round (take Haumea which is in equilibrium but is shaped like a cigar!)

    To satisfy Lab Lemming – note mu applies only to mature systems that have stopped planetary accretion (a birthday cake looks a lot different when you start than when you finish, obviously there’s going to be a bunch of steps along the way before reaching the final product!)

  67. Right on KC. My opinion was swayed in favor of the IAU definition when I had the whole “sweeping its orbit” thing explained to me by Stern and Levison, through the paper found by clicking my name. It’s an older paper; you can see it referenced in the article Chris A. linked to.

    In this paper, they wrote about how the “sweeping of the orbit,” or dynamical dominance can be calculated. It clearly excluded Pluto, Ceres and their types from the list of planets… ok, ok, so he called them überplanets and unterplanets- all planets of some type, I suppose… but I think if you look at the math behind the funny words (here I mean “sweeping up its orbit” not “überplanet” and “unterplanet”) it makes sense.

    So what if Neptune hasn’t swept up Pluto? The little guy is in a stable, resonant orbit thanks, at least in part, to Neptune’s superior gravity. Same with the trojan asteroids in Jupiter’s orbit; they’re there, but they’re being dominated by Jupiter’s gravity and have been swept into a couple neat little, Lagrangian piles.

  68. Craig

    Forgive this simple layperson who has an avid interest in all things astronomical for a simple and perhaps naive question or two or three:

    Why do we need a precise definition of a planet? “A rose by any other name smells as sweet…” or words to that effect.

    Shouldn’t we spend time LOOKING for new stuff instead of arguing about this? Don’t we have other more important things that would benefit from studied debate?

  69. @ Justin: Stern and Levison make it very clear in their article that they consider both “uber-planets” and “unter-planets” as still being planets, just of two different types. They do not exclude Ceres and Pluto from being planets; they simply classify them and other objects that do not dominate their orbits as “unter-planets.” If you’re going to use their research, you should accurately portray the conclusions they reach.

    @IVAN3Man: All the types of music you list still fall under the broad category of music. They illustrate the tremendous variety of styles that music encompasses. No one is suggesting calling the objects that don’t dominate their orbits “classical planets.” That term can be reserved for what Stern and Levison label the “uber-planets.” You said it best in the statement: “everything else can be given subcategories.” So just like all the styles you listed fall under the broad umbrella of music, all objects in hydrostatic equilibrium fall under the umbrella of planets. The “unter-planets” can be distinguished from the 8 larger ones without taking away their status as one subtype of planet.

    @ Greg: Like stars, planets may come in many varieties, but they do share a fundamental characteristic, namely being in hydrostatic equilibrium; being differentiated internally with a core, mantle, and crust; and having meteorological and geological processes. The issue is not their being in hydrostatic equilibrium in and of itself but the fact that when an object gets large enough to attain this state, it starts developing the same geological processes and differentiation as planets (processes that inert asteroids do not have). Haumea may not be round, but it is large enough to have this differentiation. And any planet placed close enough to its parent star would grow a comet-like tail due to atmospheric outgassing. You are incorrect in saying this would not apply to any of the other planets. Place Earth in Mercury’s orbit, and it too would start looking and acting like a comet. Therefore, this argument is nothing more than a “red herring.” Also, Pluto is significantly larger than any known comets.

    Pluto, Haumea, Makemake, and Eris can be considered to have dual status as both Kuiper Belt Objects and as planets (specifically as “unter-planets” or what we call dwarf planets).

    If we make the #1 criterion for planethood be that an object orbits in the same plane as its entire planetary system, how do we classify exoplanet systems with planets orbiting in several different planes? Are none of them planets? Do we pick one and use its orbital plane, call that a planet, and the others not planets? By this criterion, we should demote Mercury, which has an orbit inclined to the plane of most of the solar system’s planets by about seven degrees.

  70. Buzz Parsec

    Hey, if a planet is anything we decide to call a planet, I’m starting the Interplanetary Planet Registry. I’ll be selling names and publishing them on my web site, coming soon to a URL near you!


  71. Ray

    If the definition for planet is to include the ability to sweep the local area clear of debris then the (aledged) planets that are known to have rings of debris orbiting around them should be excluded. Goodbye Saturn, and others out there.

  72. Ken G

    I understand the argument over whether Pluto is a planet, but I think it’s useful to forget that whole discussion or at least reserve it for recreational purposes.

    When I talk to school kids about the solar system (on rare occasions), I like to emphasize that the solar system looks like this:

    * four terrestrial planets
    * four jovian planets
    * asteroids
    * Kuiper belt objects (of which Pluto is one of the largest but not the largest now).

    and also

    * Comets and the Oort cloud
    * Dust

  73. Ken, in talking with kids about the solar system, why do you not distinguish between Ceres and the other asteroids and between Pluto, Haumea, Makemake, and Eris and the other KBOs. There are very real differences between those objects in hydrostatic equilibrium and those that are not, and it is a disservice to ignore or gloss over those differences.

  74. BA,

    Confusion in a different direction. If our Moon “orbits Earth” because of the Hill Sphere argument – as you point out in your first post, then why doesn’t Charon “orbit Pluto” for the same reason – especially if Pluto’s Hill Sphere is so much bigger?

    Does the barycenter argument trump the Hill sphere argument?


  75. “If you found a Mars-sized object floating randomly in interstellar space, would you consider it a planet? No, of course not, it isn’t orbiting a star.”

    If you found a perfect cube exactly one meter on each side composed of pure titanium halfway between the sun and Alpha Centauri, would you call it a cube of titanium or something else? It isn’t orbiting a star. Why does your answer change when we shift from titanium to silicon/oxygen/etc. and increase the size?

    Ask any class of kindergarten kids. If we find a purple cow on Gliese 581c it’s still a purple cow.

  76. IVAN3MAN

    Laurel Kornfeld:

    If we make the #1 criterion for planethood be that an object orbits in the same plane as its entire planetary system, how do we classify exoplanet systems with planets orbiting in several different planes? Are none of them planets? Do we pick one and use its orbital plane, call that a planet, and the others not planets? By this criterion, we should demote Mercury, which has an orbit inclined to the plane of most of the solar system’s planets by about seven degrees.

    Au contraire, mon ami. The planetary plane is determined by the Sun’s equator, not the ecliptic (which you’re thinking of), and the orbital inclination to the Sun’s equator of the 8 major planets are: Mercury — 3.38°; Venus — 3.86°; Earth — 7.25°; Mars — 5.65°; Jupiter — 6.09°; Saturn — 5.51°; Uranus — 6.48°; Neptune — 6.43°. All well within 8°.

    Whereas the orbital inclination of Pluto to the Sun’s equator is 11.88° (O.K., not much, but a benchmark has to be set somewhere), and its orbital eccentricity makes it an oddball, as I have already stated above.

    As for your hypothesis of extra-solar planets in eccentric orbits, I don’t think it is a coincidence that all the major 8 planets of the Solar System orbit more or less within the planetary plane — Protoplanetary disc.

  77. IVAN3MAN

    @ Tom Marking

    In the Kingdom of the Blind, the man with one good eye is King; however, where depth perception is required, he is handicapped.

  78. “Au contraire, mon ami. The planetary plane is determined by the Sun’s equator”

    Why? Shouldn’t it be the plane that defines the solar system’s net angular momentum? That would be (approximately) the orbit of Jupiter- with a dash of Saturn thrown in, and Mercury still fail.

    KC: The LHB happened half a billion years after accretion finished. My point is that the assumption that planetary orbits must be stable is probably not valid.

    @ Greg: Mercury does have a tail. It isn’t very bright, but there are plenty of pictures of it floating around the web. Pluto is an icy planet just like Triton, Ganymede, Enceladus, etc. It is big, round, and has a geologic history. Comets meet none of those criteria.

  79. IVAN3MAN

    @ Lab Lemming

    You’re referring to the invariable plane. All planetary orbital planes wobble around the invariable plane, and the inclination of the Earth’s orbit has a 100,000-year cycle relative to the invariable plane, which varies from 0.1° to 3°. This cycle closely matches the 100,000-year pattern of ice ages.

    It has been hypothesized that a disk of dust and other debris is in the invariable plane, and that this has influence on the Earth’s climate. The Earth currently traverses this plane around January 9th and July 9th, when there is an increase in radar-detected meteors and meteor-related noctilucent clouds.*

    *Source: Wikipedia — Milankovitch cycles.

  80. Leon A Davis

    The proper definition of a planet is as follows: a body of sufficient mass to form a sphere which orbits a star. However, if the orbital mass has been discovered by an American, then this definiton in non-operational. (please pass the chablis and brie…)

  81. I am a little goofy I know in saying this but perhaps “Planet” should be like an honorary title instead of a scientific term. I would propose we go to the ancient Greek meaning of the word and use it for any object that you can see in the sky with the un-added eye that moves against the “apparent” fixed stars. We should just come up with a different Scientific term for everything else that doesn’t fit that criteria and doesn’t bother all the little kids and parents. This of course would make the Sun and the Moon planets and perhaps not just demote Pluto, but Neptune as well. You can’t win them all.

  82. Oli

    The word Planet comes from Greek πλανήτης, meaning ‘wanderer’. So basically, anything that seems to randomly ‘wander’ in our sky, whether it is a planet, moon, asteroid rogue star or whatever, would be a planet.

    However, I personally think that this isn’t a real ‘definition’. We now say that the first criterion for naming something a planet, is that it must orbit a star. Otherwise, it is either a star itself or a rogue planet (‘planetar’). I think most people would agree with this criterion.

    It would, however, demote Pluto – it isn’t just orbiting the Sun, it’s orbiting a barycenter (its and Charon’s) that is not within Pluto itself. So I’d consider Pluto-Charon a double planet if I look at this criterion only.

    The second criterion is that it must have reached ‘Hydrostatic Equilibrium’ – its shape must resemble a nice roundish ball. Most objects that are considered planets, like Earth, Ceres, Pluto or Charon, have reached this balance. This makes ‘big rocks’ like comets, small asteroids, etc. ‘other objects’ – they aren’t round.

    The third would be ‘clearing the neighbourhood’. I disagree with this one, per your reasoning – even Jupiter would have a hard time doing so at 200 AU.

    I suggest we change the third criterion to this: The object must orbit the Sun near the Invariable Plane or the plane of the Sun’s equator. All current planets could therefore be considered planets, but Pluto or Eris couldn’t, as their inclinations are huge.
    However, Ceres’s is only ~10°. Would it be a planet? I don’t know. It’s round, it orbits the Sun, but it’s tiny – its surface is only just greater than the island of Madagascar’s.

    A further criterion, to differ dwarf stars (like Sirius B) from planets, is that they can only produce so much light by themselves – our current planets don’t produce light, they only reflect it. Dwarf stars do produce light, making them stars and not planets.

    I suggest:
    Planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
    Dwarf planets – Ceres, Eris, Haumea, Makemake, Sedna, Orcus and all the others that we have yet to discover
    Double dwarf planets – Pluto-Charon
    Minor planets – asteroids, comets, trojans, etc.

    ps: I think we also need a definition of ‘moon’ (Jupiter has over 60 moons atm – 60!), so that only big chunks like Ganymede, the Moon, Titan, etc. are moons, but Phobos, Deimos, Nix, etc. are dwarf moons. But that’s beside the point.

  83. John McDonnell

    In defining what a planet is, the simplest and most general approach is best. Moreover a planet should be defined only by intrinsic characters, that is things that can be determined of the object itself without regard to its orbial parameters or relation to other objects.

    I would simply say a planet is a natural spheroidal object in space, not a star, one of insufficient mass to sustain nuclear fusion in the core but of sufficient mass to overcome bulk machanical rigidity and thus attain “hydostatic equilibrium”. Objects like the Moon, and the larger moons of Jupiter and Saturn are planets by this definition in their own right. In is primarily by habit, and convenience, that they are excluded. By excluding orbital or positional constraints, we allow objects orbiting other stars and even those such as free-fleating Jupiters, less massive than methane dwarfs to be planets.

    Pluto may be a dwarf planet, but it’s still a planet, just as the Sun or Alpha Centauri may be dwarf stars, but still stars. We don’t say Alpha Centauri, or the Sun isn’t a star, its a dwarf star and its equally rediculous to say Pluto isn’t a planet, its a dwarf planet.

  84. webtrekker

    Pluto is NOT a planet., as even a child will inform you.

    Pluto is, in fact, the small canine friend of a very famous Disney rodent! :-)

    Goofy idea? Not really. As it’s so very small, why not call it a Minnie-Planet instead?

    (Think I’d better Duck out of this discussion while the going’s good!).


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