Congratulations! It’s a planet!

By Phil Plait | August 15, 2006 11:09 pm

Here’s the news the whole world has been waiting for…

Pluto is a planet!

And there’s more news:

We have 12 planets now!

Well, sortof. It’s not official yet, and won’t be until at least Thursday. Sit back, folks, because this’ll take some ‘splaining.

Here’s the deal. There is a subgroup in the International Astronomical Union that decides about the naming of names, and the categorizing of, um, categories for astronomical objects. For quite some time, they have been pondering whether to call Pluto a planet or not. There has been a lot of controversy about this, mostly in the media and the public, since most astronomers don’t care all that much.

Why argue this at all? Because Pluto isn’t like other planets. It’s weird. It’s little, far smaller than any other planet. It’s an ice ball, where other planets are rock, or metal, or gas giants. It’s way far out from the Sun, and on a very elliptical orbit, and the orbit is tilted with respect to the other planets. Far worse, it appears that there are other objects out there beyond Neptune that look an awful lot like Pluto – if we call Pluto a planet, we have to call them planets too… and there might be a lot of them. A lot. Like maybe millions, or even billions. Trillions? Possibly. That’s a lot of planets.

So we have do an issue here, but not the one most people think. It’s not whether Pluto is a planet or not, the real problem is what the heck is a planet?

I don’t think you can scientifically decide what’s a planet and what isn’t (as I’ll get to below). But that’s what the IAU Commission had to decide. And it looks like they did decide. Here’s what they say is now a planet:

1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet.”

(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.

(4) All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

These rules have some good things going for them. Most people expected them to come down on the side of planetary status for the little guy. I think the big surprise (well, one of two, but again, patience, I’ll get to it) is calling Ceres a planet.

But overall I’m not happy.

For one thing, if a planet the size of, say, Saturn, gets ejected from a solar system — a scenario that can happen early in the life of a forming system — then under Rule 1 it wouldn’t be a planet. That’s silly. Why should location matter? Also, imagine two planets which are the same size, but one is solid rock and the other is a less solid rock. It’s easy to imagine the first being able to retain a lumpy shape due to its structural strength, while the other one doesn’t, even though it’s the same overall size and mass! Yet under these rules, one is a planet and the other is not.

Rule 3 is arbitrary. What if we find an object bigger than Pluto but with an orbit of 199 years? Actually, there are no planets like this, but I’m making a point: Is that a pluton? What do they mean, exactly, by "highly" inclined? 10°? 20°? I’m not a fan of arbitrary rules, especially in science.

But given these rules, here’s the impact on our solar system:

  1. There are 8 major planets in the solar system. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
  2. There is a newly named kind of planet: "plutons". Pluto is one, of course. So is Charon, it’s moon (the second big surprise I mentioned above), and UB313, an object slightly larger than Pluto that orbits even farther from the Sun.
  3. Ceres, once called the largest asteroid, is also now a planet.
Welcome to our new solar system!

Now, at the moment this is just a proposed set of rules, and has not yet been ratified. If it does pass, then we have 12 planets in our solar system, and the number will certainly grow as more plutons are found. The final vote will be made by astronomers attending the IAU meeting going on right now in Prague, on Thursday August 24th at 14:00 Prague time, which is 5:00 a.m. Pacific time. I’ll post an update on the vote when I find out how it went. You an also check the group IAU blog as well for updates.

So what do I think of all this? The more I think about it, the less I like it. Ceres is the largest asteroid, and big enough that its own gravity can crush it into a sphere. No other asteroid is big enough or massive enough to do that, but several of the larger asteroids are massive enough that their shapes are modified by gravity, and are "nearly round". Why aren’t they on the list? Maybe they’re not at hydrostatic equilibrium, and geological forces dominate over gravity (in other words, the gravity isn’t enough to totally morph the thing into a ball). But this seems like another somewhat arbitrary line in the sand. How far away from a perfect sphere does an object have to be before it’s tossed out of the planet club?

Pluto being a planet is fine, I suppose, though it opens a can of worms in the way they defined things. Remember, statistically speaking, there really could be millions or more objects like Pluto in the deep dark. As time goes on and our instruments get better, we’ll find ’em. Guaranteed. We could have millions of planets in the solar system in less than a century as telescopes and techniques get really good.

However, including Charon, Pluto’s moon, as a planet — even as a pluton – opens up a bigger can of worms. Here is the committee’s reasoning:

For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are “satellites”. Under this definition, Pluto’s companion Charon is a planet, making Pluto-Charon a double planet.

The barycenter is the center-of-mass (CoM) of a system. Since Charon is pretty big compared to Pluto, that CoM is outside Pluto’s surface, so, by their reckoning, Charon is a planet in its own right. But moons’ orbits move over their lifetime, changing the CoM of the system. Right now, the barycenter of the Earth-Moon system is about 1700 kilometers beneath the Earth’s surface. The Moon recedes from the Earth by about 4 centimeters per year, so in 40 million years or so (an eye blink in solar system time) 3 billion years the barycenter will be outside the Earth. Will we call the Moon a planet then (if we’re still around and arguing about this)?

So this rule isn’t really arbitrary, it’s just not very satisfying. But imagine the rule we’d need instead: if you have two objects that orbit each other, and one or both are planets by Rules 1 and 2, then the more massive on is a planet, and the other is just a moon. That’s silly too. What if one is Jupiter-sized and the other is Earth-sized?

Which brings me, finally, to my big point. This is all incredibly silly. We’re not arguing science here. We’re arguing semantics. For years people have tried to make a rigid definition of planet, but it simply won’t work. No matter what parameter you include in the list, I can come up with an example that screws the definition up. I’ve shown that already, and I’m just warming up.

The problem here is simple, really: we’re trying to wrap a scientific definition around a culturally-defined word that has no strict definition. Doing this will only lead to trouble. Why? For one thing, it’s divisive and silly. How does a definition help us at all? And how does it make things less confusing than they already are? Charon is a planet? It’s smaller than our own Moon!

A big step in understanding a new object is being able to categorize it. Is it icy, or rocky? Is the orbit circular, elliptical, far from the Sun, nearby, tilted? This type of information leads to insight on how the object formed, what it’s doing, and how it behaves. This is all important, and so it is a good idea to try to categorize objects. But definitions are like little boxes, containers in which ideas sit. But sometimes they’re more like prison cells. They frame our minds, make us see things too rigidly. Thinking of Pluto as a planet might make us miss some important characteristic because we’re too narrow in our thinking. I’ve seen it happen before, even with me. It’s too easy to be rigid with a definition in your hand.

However, in a sense this doesn’t matter. What’s in a name? Scientists will probably still of Pluto as they always have– an ice ball at the edge of the main solar system. The public will still think of it as a planet, so that won’t change. And, well, there is something cool about this new set of rules. Maybe, just maybe, in a few years we’ll have a solar system with hundreds or even thousands of planets, instead of just the 9 — nuts, I mean 12 — that we have now.


Comments (229)

Links to this Post

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  9. Shannon and Mike ☆ Net » Barron of Blog » I, For One, Welcome Our Pluton Overloads | August 16, 2006
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  25. Congratulations! It sa planet!You an also check the group IAU blog -- Centplus Tech | September 15, 2006
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  1. Horseshoe

    I challenge the readers of this Blog to come up with a new mnemonic to replace the soon-to-be outdated versions (i.e. My Very Excellent Mother Just Served Us Nine Pies) 😀

  2. Christian Burnham

    How does the BA find time to write all this stuff? It takes me half a year to write a couple of scientific sounding paragraphs- and I get paid to do it.

  3. I had plenty of time today while the server was crashed. :-)

    Plus, I’ve been expecting this for a while, and thinking about it for years, so I knew what I wanted to say when the rules became available. I get press releases before the public does, since I am considered (amazingly) to be a journalist. So I’ve had this news since this morning.

  4. Eric Ingram


    Moving vans endanger many cats journeying slowly upwards near pest-controlled universities.

  5. Don’t forget that UB313 has not officially been named yet. The unofficial name is (blech) Xena.

  6. Kyle_Carm

    Just before people go smacking on non-PhD’s, remember we were told Pluto was a planet since it was discovered. We were happy, PhD’s decided we weren’t right and that we were wrong in saying “Pluto is a planet”. It has only been in recent years, since 1992, with newer discoveries that there are other things out there, and that Pluto might not be rightly called a planet. Neil DeGrasse Tyson has even said no matter what the IAU decides they are wrong Pluto will never be a planet to him and he will not say it is (I saw the interview on a Science Channel program about Pluto, yeah he is SMART but dang that is ARROGANT and condecending to us normal folk). Other lesser known astronomers have said the same. So what does that tell me and other non-scientist? “Your wrong I’m right and I have a PhD”? To a certain extent yes. Hey the “leaders” of your field have spoken and will shortly vote, if it doesn’t go your way well tough and eat some humble pie. If things change in another 62 years then you get the last laugh and get to serve a whole heaping dinner of humble.

    The last thing we need at this time is a conflict within science over a simple definition of a planet. Don’t you think the anit-science people will jump all over that and use it to there own ends to discredit ALL of science if they can?

  7. Eric Ingram

    (unoffically) …near pest-controlled xenoliths.

    No wonder I never win games of acro.

  8. Bleh, this looks like it could get messy. Fast.

  9. Added difficulty: sometimes Charon is closer to the Sun. So you might get


    or it might be


    … at least, until UB313 is named.

  10. Gregory

    “I challenge the readers of this Blog to come up with a new mnemonic to replace the soon-to-be outdated versions (i.e. My Very Excellent Mother Just Served Us Nine Pies) :D”

    My Very Enigmatic Mother Can’t Justify Sewing New Pants ?????? Darn, if it only wasn’t for the 12th planet.

  11. Gregory

    Wait, Charon’s a moon too, ok, make that My Very Enigmatic Mother Can’t Justify Sewing New Cargo Pants + something.

  12. Navneeth

    A planet is a celestial body that… is in orbit around a star, and is neither a star nor a satellite of a planet.

    Well, that seems a bit circular.

  13. Max Fagin

    I for one am shocked. I could have sworn that the IAU would come down on the “not a planet” side of things.

    But I guess your right Phil, the sky belongs to everyone, and nothing the IAU says will change how we percive the solar system.

    By the way, I was watching “The Colbert Report” last night, and Stephen Colbert gave me an idea. How about mobilizing your following to get UB313 named after you? After all this astrology bashing and moon hoaxs thrashing, dosen’t the sky owe you a debt of thanks?

    Is anyone with me here?

    I hereby cast my vote for Plaitos.

  14. Rumour Mongerer

    If I may make a few predictions:
    1) There’ll be a large complaint about updating all the text books.
    2) Religious fanatics will once again cry “Look how Science changes! Come trust our religious books!”
    3) Astrologers will be given mroe air time than Astronomers (generally true anyway…).
    4) Some scientists will get really annoyed.
    5) Some won’t give a rat’s…
    6) In five months time, we’ll be like “What you mean, there were once nine planets?”

    (And yes, this is applying for a million dollars 😉 )

  15. Horseshoe

    “Added difficulty: sometimes Charon is closer to the Sun”

    If the order is “PC”, we can use the phrase “Politically Correct” in the mnemonic:

    “My Very Enigmatic Mother Can’t Justify Sewing Unusually New Politically Correct Underpants/X???”

    BA – which, um… celestial body… is “usually” closest to the Sun: Pluto or Charon?

    …and don’t say it’s 50/50 😛

  16. Stephen Eaton

    Cigars for everyone! It’s a buoy! Or at least a floater…

    The only major impact that this um, ruling, can have, is on astronomy education. Does no one else see this? For several decades (as indicated by Horseshoe’s post), students have been taught that there are nine planets. BUT! I see it as an influential impact. Every teacher, no matter the grade (K-12, college and beyond) must make the most out of this information. I’ve TA’d in college physics and astronomy labs and lectures, and let me tell you: many, many, so many kids have no informed, structured ideas about space, inner and outer “planets”, our solar system, the Kuiper Belt, or even the necessary conditions for an eclipse!! Ask 1000 people what a “main sequence star” is and I’ll bet you’ll get 998 blank stares. How horrible! Many kids haven’t had the opportunity to learn this stuff in high school, but any kid in college should be required to take basic physics and astronomy as a prerequisite to graduation. My education rant is now adjourned.
    Re: Horseshoe–in increasing AUs, it would be MVEMCJSU(CP)X (for Xena, harhar). Hmm… tough one. How ’bout “My Very Ecumenical Minister Condones Jaw Socking Until Catholic Papals Xenophize”… the last few words in that meme might need some work.

  17. So Phil, why could you not be there to bang heads together and make them see sense?

    What would your definition of a planet be?

    IIRC, the name “planet” meant wanderer, so that definition could have applied to cmmets and asteroids as well.

  18. Alexander Whiteside

    On the bright side, the next planet we find will be Planet 13, which is quite possibly the best title for a scary-new-planet sci-fi flick since Planet X fell out of favour.

  19. You an also check the group IAU blog as well for updates.

    This will be the same IAU blog that said: “Seed magazine links here, but predicts that you will be able to find out if Pluto is a planet here. No, you won’t! I think this is an incredibly unimportant topic, it’s not what this meeting is about and I will not mention it at all. Well, not any more, that is”?

  20. idlemind

    Well, the general public wouldn’t accept losing Pluto, but adding new “planets?” How exciting! The media will lap it up. For a brief moment people will look skyward.

    I think the IAU is doing the political thing here, as you imply. Tossing Pluto is the only solution that makes much sense IMHO. But in ain’t gonna happen.

  21. Tim G

    No more ordinal numbers for the planets? Earth is considered the 3rd planet from the sun, Jupiter the 5th, etc. Now with Ceres interjected, Jupiter would now be the 6th. However, Pluto and Charon co-orbit, alternating between the 10th and 11th spots.

    Anyway, with Ceres and UB313 considered planets, funding may be more easily secured for probes.

  22. Liam

    I always knew Pluto was a planet! And now they’ve done scientific experimenty stuff and PROVED I was right all along. Go Pluto! Go Pluto! Go Pluto!

    Or maybe I’m not getting this..

  23. Tim G

    The link to the video tour of the New Solar System keeps prompting for a user name and password. *Ugh*

  24. Horseshoe

    Many planets are named after Roman Gods. Also, Julius Caesar was declared by the Roman Senate to be a God.

    Therefore, the twelth planet should be named “Caesar” 😀

    “My Very Enigmatic Mother Can’t Justify Sewing Unusually New Politically Correct Clothes”

  25. Nachor

    Wouldn’t the Asteroid called Sedna (90377) be a planet under this definition, too?

    1700 km diameter, spherical, high excentricy…

  26. Personally I find this all very exciting and interesting. New planets! Yay!

    So, do these new planets have/get pretty picture-symbols like the other ones do?

    You know, the fanfic writers are probably going to have a field day with this in every series with any sort of planet-affiliated powers…

  27. I Wonder Why

    I really don’t see the point of leaving the largest “moons” out of all this discussion about planets. After all, e.g. Titan is 5150kms in diameter, has an atmosphere and it is round. Ok, so it is the satellite of a BIG planet, so? Well a lot of compromises are being made with Charon and Pluto and Ceres, so why not with this issue? I’m sure if we inhabitted Titan, we would call it our home “planet” not our home “moon”.

  28. DrFlimmer

    I won´t call Pluto a planet for myself as I never really did before. He is more or less a trans-neptunian-object – that´s what I think about it!
    I´ve been sure the IAU would throw Pluto out of the window, but they even go on further.
    Welcome to a solar system with countless bodys called planets.
    I´m somehow shocked!

  29. Didn’t they recently discover two new tiny moons of Pluto? Since the system’s barycentre is ‘outside the primary’ doesn’t that make them planets too?

  30. Michelle Rochon

    I really think they should just leave the dumb titles as it is. It sounds childish, and it bumps in people’s lives.

    Not to mention they are making a fuss about nothing. The three new “planets” won’t hold a party about it. I’m pretty sure they’re what they are.

  31. Nachor

    @ Adam:

    I don’t think so – Hydra and Nix are probably too small for the forming-a-spheroid-under-their-own-gravity part of the definition (40 and 160 km diameter, according to wiki).

    Another problem would be the composition of a “planet” – if a body is mostly ice, it should form into a sphere much easier, shouldn’t it? (Ask your local glacier for details.) So, what says the IAU about what to call that? Cometary body? Ice-planet? Where would be the the boundary for a “planet” and a spherical “icewhateveryouwanttocallthat”? 50 % non-ice materiel? 75 %?


  32. As a science teacher, I have had to help my students with the concepts of planet/moon/planetoid/satellite/human litter. As a biologist by training, I’ve taken the evolutionary path: You understand what something is by knowing where it came from. History teachers would agree with that. So, to help my students understand solar system evolution, we’ve included ‘a non-fusor created in orbit around a fusor’ with the rest of it. So yes, if Jupiter had been a bit bigger it could have evolved beyond planet status. Our Moon, created in orbit around the Earth, would not be a planet. And Pluto, with its weird orbit, would be considered a capture and not one of the 8 current planets. So what happened to Selida? For a while we were doing ‘My Very Excellent Mother Just Sent Us Nine Pizza Slices’

  33. Kullat Nunu

    Pluto’s new satellites are way too small.

    I don’t know how did they come up with 12 planets. There’s already at least another 12 almost-planets, some of which are larger than Charon and Ceres.

    If this proposal is adopted, I’m pretty sure that eventually nobody will call those “dwarf planets” “true” planets anymore. The eight big are the important ones, and I can’t see why children should memorize all the smaller ones (the important point is to know that they’re there and what they are).

    The good point in the proposal (something that BA missed) is that the largest asteroids and Kuiper belt objects are rather different from smaller ones. Pluto (and most certainly 2003 UB313, maybe even 2005 FY) has transient atmosphere; Ceres and especially Vesta have complex geological histories–the latter may have been volcanicall active in its youth–something that small chunks of rocks don’t have. Lumping Pluto and Jupiter into same group as “planets” is about as dumb as grouping Pluto and for example 433 Eros into same group as “asteroids”. It’s only good thing if the “dwarf planet” definition sticks.

    What comes to interstellar objects, we already know objects that are too small to call even brown dwarfs (failed stars). One proposed term is “planemo”. It’s probably impossible to distinguish ejected planets from those sub-brown dwarfs which formed directly from the interstellar cloud and never have orbited a star. Maybe ejected “rogue planets” could be also called “planemos”.

  34. schneider

    [picard]There… are… NINE… Planets![/picard]

  35. nebularain

    [quote]Don’t you think the anit-science people will jump all over that and use it to there own ends to discredit ALL of science if they can? [/quote]
    IMHO, the discredit won’t be over the change (after all, change is a part of science – we’ve established that before). The discredit will be that these scientists based their decision off of naustalgia and politics rather than logic and reason.

    I’m not happy with the decision, either.

  36. I think, besides UB313, there’s Quaoar and Sedna out there. What about them?

  37. It’s Bill Arnett who keeps updated I feel sorry for. He’ll have to shell out for a new domain name!

  38. It does seem a pointless argument. Like Nachor, I immediately wondered, “What about Sedna?”

    I also agree that “planet” is not a scientific term. It has far too much meaning among everyday folk like me. And further, trying to pin a meaning to it (particularly one as fractured as the above) is probably not a good idea. As far as arbitrary cut-offs… I don’t have nearly as much an aversion to that as Phil does. We use a base 10 numbering system because of an arbitrary decision based on our number of fingers. There’s dozens of other ways it could have gone, but didn’t.

    I do like the idea of having subclassifications of planets, such as jovian planets, terrestrial planets, and minor planets.

  39. P. Edward Murray

    I’m amazed that they did not use Pluto as a lower limit but it’s fine with me.

    Professional Astronomers mostly don’t come in contact with the public but we amateurs do all the time. It will be a heck of a lot easier to explain this then to try to explain Pluto as not being a planet.

    As to the thought of cultural or scientific, I refer everyone back to a statement of a few days ago…

    It doesn’t take 76 years of usage for a new word and it’s definition to be included into a new Edition of Websters. If the Astronomical community can’t agree on something this simple then it leaves fellow human beings to question their sanity.

  40. P. Edward Murray

    And as far as nomenclature goes, a better word for ‘Minor Planet” or Asteroids is Planetoid or again Minor Planet.

    But maybe that’s too much to ask.

    Oh, and according to what I have seen thus far, Sedna is considered to be in a group that has yet to be decided on. So, it’s possible it might be called a planet at a later date.

    Of course now this opens up the range of planetary objects that amateurs will have to look at now that they are considered true Planets.
    I see future robotic missions in the works locally in the Solar System to keep Astronomers and the public busy for years and years.

  41. El Guapo

    I don’t understand this.

    Why not set the rules which make sense, and Pluto is not a planet… but, then make a special exemption for historical reasons to name Pluto as a planet? Kind of like Michelle Wie getting into the men’s majors tournaments.

    This would seem to solve all the problems and prevent making up obtuse rules just to shoehorn Pluto into the definition and cause all these other consequences.

  42. There’s one glaring omission in this otherwise excellent post: what impact is this news likely to have on the field of astrology, and by extrapolation, human destiny? I mean, for centuries, gullible people have been categorizing themselves and their personalities (and patterns in their lives) according to the placement of planets in the heavens. How can we possibly understand ourselves and make wise decisions based on planetary motions if scientists can’t even decide on what constitutes a planet? :)

    Seriously, I tend to agree with Phil. The discussion appears to be more about semantics, and the new “plutons” category is just going to complicate matters further. Whatever happened to Occam’s Razor?

  43. P. Edward Murray

    If an object the size of Saturn gets ejected from our Solar System or anyone else’s…who cares?

    How can you care if you can’t see it and if you can’t see it either visually or by any other means, how do you know it’s there in the first place?

  44. Lunatik

    Might this “changed rule” attact more attention from the younger minds out there? Of course it will! I can remember staring up at full moon (after the broadcast on tv) when Apollo landed there for first time. Thinking about how a man was there…

    Sometimes the rules must change.

  45. Bruno Domingues

    New planets, new planet… why all the confusion?

    Like BA said, this will endeed lead to us having 100’s or 1000’s of them in some decades. Phil also pointed about our moon becoming a planet in 40M years or so. Nice.

    IMO, the diference in size magnitude in near objects should be very important, making them not “isolated” planets, but part of a “population” or a belt. Ceres and Pluto have more similarities with near objects of their “population” than with Mars, Jupiter or Neptuno, right?

    So IMO, it would be a far more scientific if the IAU had the guts to “scrap” Pluto from the planet list and call it a inner Kuiper belt body. Like Ceres is the big asteroid, Pluto would be the big Kuiper object, even if closer to Sol then the rest of is tribe.

    I understand that the IAU’s purpose is not to make astronomy more appealing or something but think of it:
    Future Planets like “2011SW1” and “2012OP9” will sure make astronomy classes more interesting… whatever… if they are all named, we will run our of BD characters really fast…
    A kid in 2023 asks is mate: – How many planets exist? 546. No, 551. No, wait… Ahhh!… …who cares?!?… :-(

    … please forgive my typos, I’m not an english speaker

  46. Tom K

    There’s one upside to millions of possible new planets: now there can be a Planet Claire.

    Thank you, B-52s.

  47. Andy Varga

    Was there this much controversy when they came up with the term “minor planet”?

    And since all these new objects (including Pluto apparently) will be categorized as “plutons” or “pluton planets”, doesn’t it seem we are actually down to 8 “planets” ( or rather 8 non-prefixed planets)?

  48. I’m with El Guapo, the solution of coming up with a decent definition that fits everything else planet-y, and then just grandfathering Pluto in on the grounds of history when it turns out it doesn’t fit, seems so *obvious* a way to solve this very silly argument, that I’m frankly rather baffled that isn’t being proposed.

  49. Well, my opinion ran over 1000 words, so maybe I’ll just link it:
    My Very Endearing Mother Certainly Just Shot Up Near Prison Cell 2003

    The main points:
    Pluto has more evidence of active planetary processes than Mercury does, and this is more deserving of planetary status.

    This whole setup looks like it is designed to engineer an anti-Pluto backlash, in order to weaken public resistance to the cryophobic proposals.

    “Pluton” is a term already in use in planetary science. It refers to a nonplanar intrusive igneous body. see

  50. jimBOB

    I second El Guapo – call the “big eight” planets and grandfather in Pluto. Call the rest “subplanets” if you need to shoehorn them into a category. Anything else is just overthinking.

    Personally I like the “Xena” designation, especially with the ultra-cute “Gabrielle” to orbit it (though I have to confess to being something of a fan of the show). In 50 years these names would either sound really dumb, or have been around so long everyone would accept them in spite of the fact that their origin would be forgotten. For astronomers who feel these names pollute their chosen field of study, remember that the old roman-god names are just the names of characters from mythic tales, and Xena/Gabrielle are also mythic characters – albeit modern ones. If the Romans had had teevee, they would likely have had a “Venus” show.

  51. Grand Lunar

    Given Pluto’s diamter of 2274 km, I’d say add a size requirement of an object to be over 2000 km in size. After all, how common a size can that be for the Kuiper Belt?
    It also bumps out Ceres and Charon from the picture.

    For the case mentioned by the BA if Saturn was no longer in orbit of the Sun….they’d also have to add the definition of an object that is not a brown dwarf but not orbiting a parent body.

    Hopefully, this mess will be sorted out.

    I’d hate to think that we would wind up with 12 planets. Oy vey……

  52. Aubri

    > There’s one upside to millions of possible new planets: now there can be
    > a Planet Claire.
    > Thank you, B-52s.

    “You can’t call a planet Bob!”
    “Well, no one is making you live on Bob.”

  53. Bruce

    If Charon is a planet, how do you explain pygmies and dwarves?!?

    dang, maybe that only works on PZ’s site.

  54. Oh, dear. It has to be in hydrostatic equilibrium? Does that leave Earth out? After all, we have some mountains pushed up by tectonic processes that would sink back down somewhat as soon as those processes ceased. How far from hydrostatic equilibrium can you be? As you say, there are a couple of asteroids that are close. Even Ceres isn’t perfectly round. And a number of the Kuiper Belt objects are in a similar situation.

    Oh, and what if we find something just like Pluto, with an orbital period of several hundred years, but it is in a nearly circular orbit of nearly zero inclination? By this defiition, as I understand it, then such a body would not be a planet. In fact, it would be pretty much undefined.

  55. Don’t forget that sometimes Pluto (and thus Charon too) is closer to the sun than Neptune. So, this gives us two more orders:


    Plus, there are even those brief times when Pluto is closer than Neptune, but Charon isn’t, and vice-versa, giving us:


    As to which of Pluto and Charon is closer to the Sun more often, that would be Pluto by a slim margin, so I think we should stick with MVEMCJSUNPCU as it works the plurality of the time.

  56. Scott

    Choose something more solid to base it on:

    1) mass (larger than X)

    2) diameter (wider than X)

    3) atmosphere (exists, more than X atm)

    i.e. something that has a definite measurable value and is very unlikely to be ambiguous. There are a lot of planets out there in the Universe, making for quite a few exceptions to more complex rules! If we can’t figure out what’s a planet in our own Solar System then how are we going to figure out whether something is a planet elsewhere?

  57. Nate_the_ist

    Most Vans Easily Made Cars Junk; Steadily Until Newest Pt-Cruiser 2003

  58. Katherine

    Interesting comments about this interesting (for better or worse) news. As several have mentioned, and having followed the planet-not planet debate for some time, this is not the course of action I would have expected. I’m guessing there are a lot of other astronomers (not the least of which those in the academic system who have to teach these sorts of things to non-scientist folks) who aren’t happy about this either. To me, one of the most important criteria to determine what something “is” is composition – what are a body’s constituent materials? Does it have a core? Is it an amalgam of various materials or does it have a layered structure? And does it follow the “classic” behavior pattern of planetary objects (i.e., small and rocky towards the sun, and large and gaseous away from it)?

    Given all the various ways one could really compare and group objects in the solar system, it seems odd for this kind of major decision to be made based on things like size and potential for sphericity (is that a word?), especially when the vast numbers of yet-undiscovered KBOs, etc. that could potentially meet these rules is considered. People – students, amateur astro buffs, Joe and Jane Average, whoever – need to learn that there ARE big differences between Ceres (an asteroid), Mars (a rocky planet), and large KBOs, and calling them all planets, even in press releases where for example a new, larger-than-Pluto KBO is found, does not help. It makes things MORE confusing, not less – how many people would actually latch onto definitions like “minor planet” and “pluton” that aren’t exposed to astronomy talk on a normal basis?

    While I think Pluto should never have been classified a planet in the first place, I do agree that grandfathering it in and leaving the nine classic “planets” intact would be a lot less confusing and a lot more satisfying than settling on a definition that makes controversial concessions right off the bat and leaves room for a lot more future loopholes.

  59. Someone

    Ok, I feel I have to react on this.

    1) Definitions are arbitrary. All definitions are arbitrary. Why is a star “a bunch of mass (mostly hydrogen) in space that undergoes nuclear fusion”? Because we defined it that way. We could have defined it to mean something else, but we didn’t. Why is a boson “a particle of wich many can occupy one orbital”? Because we defined it that way. Why is a planet “(dreadfull definition given above)”? Again, because we defined it that way.

    2) A definition is better than no definition. Using non-defined words is fundamentally unscientific. What is the meaning of “We discovered a planet in orbit around (some star)” if you don’t know what a planet is? It’s like saying “snorks are blue”, it’s totally meaningless untill you define “snork”.

    True, “planet” might be hardly ever used by professional astronomers, but it’s used sometimes, isn’t it?

    3) Consistent definitions are better than inconsistent definitionds. Let’s call a bat a bird, “for historical reasons”. Let’s call a banana a tree because everyone thinks so. (bananas are herbs).

  60. Please, please, PLEASE: stop saying this is “only” semantics.

    For crying out loud, just try having a discussion about anything without using semantics.

    Don’t diss semantics: semantics IS meaning.

    If you don’t want to argue about definitions, fine – I’ve got no problem with that. If you want to say “This is all incredibly silly. We’re not arguing science here. We’re arguing labels.” then go for it.

    But semantics is far, far more than labels.

    Oh, yes – the point… Well, I like Pluto as a planet – possibly because it’s always been one. And I don’t mind if we get several dozen planets. Why not?

    And anybody who’s watched Firefly knows we won’t have a problem saying “moon” for something people live on. :-)

  61. Yvette Cendes

    My Very Educated Mother Can’t Justify Someone Using New Planetary Conventions… or is that “Conventional Planets” right now? no matter… oh wait, what’s that last one called right now?!? DAMMIT!

    (Yes, I’m sure everyone can tell how I feel on this one…)

  62. Cindy

    Sigh. And a new edition of the textbook I use just came out and I was hoping there wouldn’t be a new one for at least a couple more years. I’m sure the textbook companies were jumping up and down with joy.

  63. Roger

    I think the Moon should be elevated to the status of planet, making ours a double-planet system. It is large enough that if it were in its own orbit about the Sun, it would be a planet with no question. It is larger than Pluto. It is large enough to significantly move the centre of revolution of the system from the parent body’s axis. (The moons of the gas planets don’t meet this criterion.) That should be enough. We could call it Cynthia, the Greek name for it. I like that better than Luna.

    How ’bout “Men Very Easily Made Ceramic Jugs, Saucers, Nectar Pots, and Cracked 2” (2 for 2003 UBwhatever.)

  64. Xav666

    I have a solution to the problem. Just add into the definition that a planet has to have an atomsphere. I know we would lose Mercury and Pluto as planets. We would only have 7 planets, not 12, not millions, not billions, not trillions but 7. I’m aware of the fact that some moons have atomspheres…the definition should go something like this: A planet is a spherical body mass that orbits a star, has an atomsphere, and is not a satellite. I think a much more simpler definition is in order.

  65. My Very Educated Mother Can’t Justify Someone Using New Planetary Conventions (X-that)


  66. Jennifer: I did take into account astrology, two posts back! Take a look. :-)

    Bruce: I read Pharyngula, so I get it!

    Karen: but that’s the point I’m making. Naming may be important, but it makes no real scientific difference, so all this fuss is silly. And names can guide investigation, which can be both good and bad. I’ll have to post my own experiences about that… anyway, I’m not trying to denigrate semantics. I’m saying that people think they’re arguing science, when they’re not.

  67. Betelgeuze

    You can’t ignore the fact that we need a new definition for ‘planet’, because you also can’t ignore the new objects like ‘Xena’ and Sedna. We found them so we have to classify and name them.
    Calling them asteroids doesn’t make sence because we call Pluto a planet. Saying Pluto is not a planet is not a sollution, because without a definition you also cant say why Pluto and other Pluto-like objects wouldn’t be planets.
    Forgetting the name ‘planet’ is also not an option, what would you say when you want to talk about ‘planets’ if the name planet doesnt exist anymore? You cant say ‘object orbiting a star’ because comets do that too.

    My point is we HAVE to define ‘planet’, but its impossible to make a perfect definition. What the IAU is proposing is the best scientific definition I can think of, altought its defenetly not perfect(and it will never be).

    The main reason why people dont like this new definition is because of the size of some of the new planets and because of the amount of planets that could be found in the future. I dont see why those things should be a problem? Why cant we have 1000 of planets in the Sol system? Why do they all have to be massive? I dont see anyone complaining about the sizes of stars and the amount of stars we know of.
    How can Ceres being called planet being more wrong than calling it an asteroid? Is it wrong because its smaller than the mayor planets and because we will probably find other objects like this? It looks like a planet and it acts like a planet and thats what counts for me.

    With all respect, but complaining is always easy when you don’t give a sollution. Do you have any suggestions? What should we do?

  68. Jim Hammond

    All science is either physics or stamp collecting.
    —Ernest Rutherford.

    This issue is the part of astronomy that is stamp collecting. It’s dirty work but someone has to do it.

  69. CD Reed

    I certainly agree with Phil that we’re only talking about schematics here, and not science.

    However, before we dismiss the importance of this move, think back for a second to when you first got interested in the stars and planets. When you were in first grade, walked into that small school library and immediately reached for those outdated books in the science section that said that someday, we would land a man on the Moon.

    This is where the IAU decision could be the most damaging. The last thing we need to do is confuse the kids on this subject who we will need the most in the future,e when we hopefully get off our low-orbit rear-ends and explore the solar system.

    I can imagine the near-future schoolroom conversations now —
    Ms. Jones: OK, class, there are nine, I mean 10, I mean 12 planets.
    Student: Ms. Jones, if Pluto’s moon is a planet, why isn’t our moon a planet.
    Ms. Jones: Uhhhhhh….
    Student: If an asteroid is a planet, why not all of them?
    Ms. Jones: Uuhhhhh. Because they say so.
    Student: Ms. Jones, my daddy said there were hundreds of worlds as big as Charon circling the Sun.. Why aren’t they planets?

    At this point, Ms. Jones throws her papers in the air and gives up.

    We’re also forgetting the implication this could have on planets we discover around other stars.

    Look… There’s an easy way to define a planet that is being overlooked: Any circular planetary object orbiting a star that is larger than 2,000km is a planet …. Period.

    With this, Pluto stays a planet, Xena the Warrior Planet joins the club and that’s it… 10 planets. Sure there might be room for one or two more as yet undiscovered worlds, but we’re not talking about adding hundreds of planets here.

    Adding Ceres and Charon seemingly makes this confusing subject worse than it was before.

  70. The BA mentions that the barycenter of the Earth/moon system will move outside the Earth in 40 million years. He assumes that the moon continues to recede at a rate of 4cm per year. That’s not true, of course, since the process will slow down, but even still he seems to have made a mistake of a couple orders of magnitude. In 40 million years, at 4cm per, it would move 160 million cm, or 1.6 million meters, or 1600 km–which is less than a quarter of the earth diameter.

    Since the moon is about 1/80 earth mass, and is at about 30 earth diameters, it would have to move out another 10 earth diameters, to 40.

    And, unfortunately, the process is not really linear like that, and (according to Physics of the Earth, by Stacey, p.126 in the 3rd ed.) is self-limiting and the recession of the moon would stop at 39 earth diameters!

    Even then, it would take longer than the remaining lifetime of the sun to get there.

    By the way Xav666, doesn’t Pluto have an atmosphere?

  71. I like Ceres as a planet. It brings more attention to the asteroid belt which certainly deserves it since it will likely play a key role in the permanent future of humans beyond earth (assuming there is such a future.)

    It also means that the planets are now nicely spaced in an Titius-Bode kind of way. No more annoying gap! Check out the more modern treatment of this topic by Bakulev.

  72. Spikeman

    My very earnest mother can’t just serve us nine pickles–count xtras!

    I’d have preferred eight planets. On the other hand my wife’s in the “Pluto is too a planet” camp, and I rather like sticking it to the astrologers. And at least there’s the semblance of a physical definition (HSE) to define the lower end of the planet scale, just as there is at the planet/brown dwarf boundary or brown dwarf/star boundary.

  73. Nigel Depledge

    Bruce said:
    “If Charon is a planet, how do you explain pygmies and dwarves?!? …”

    Bruce, you may be interested to know that the plural of “dwarf” is “dwarfs”. The spelling “dwarves” was invented by Tolkien to make it appear to have a similar etymological history to the plural of “elf” (“elves”).

    Isn’t our language fun?

    Meanwhile, back on-topic, I’m starting to agree with Phil. It doesn’t really matter how a planet is defined, because the word will continue to have a certain (vague) meaning in general usage, but will not suddenly start being used more frequently by astronomers (who, if I understand correctly, hardly ever use the term at all).

  74. Toenail

    So what about Oph1622 ? A double-planet rotating in free space ? I can’t wait to read the final vote. In my opinion, the most difficult changes will be for the game question writers and small kids who will have to learn the new definition by heart.

    link to Oph1622 (in frech):

  75. DavidF


    I don’t agree that we have to have a formal definition of a planet just because new objects are being discovered. As the Bad Astronmer says there is no unique definition of a planet possible. So, since it’s arbitrary, why not go with the definition that has historical precedence – keep Pluto as the 9th planet and call everything beyond plutons? It would keep the public happy and wouldn’t have the slightest impcat on planetary physics.

    If people don’t like historical precedence for an argument just think of how IUPAC rules screwed up much chemical nomenclature as compared to historical names. It’s ok to call new molecules by historical names but renaming “ethylene” as “ethene” is stupid.

  76. DavidF

    sorry, I meant “it’s ok to call new molecules (or elements) by IUPAC names but ….

  77. Marlayna

    I, for one, like the definition. Nine “regular” planets (Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune) and some “pluton” planets (Pluto, Charon, UB313 – I can’t believe they named it after the protagonist of a silly show, and probably others) suits me just fine. If in 40M years the moon is also considered a planet, fine, what’s the problem with that? The only thing that bothers me is this aspect: “For one thing, if a planet the size of, say, Saturn, gets ejected from a solar system — a scenario that can happen early in the life of a forming system — then under Rule 1 it wouldn’t be a planet.” Indeed, what would we call such an object?

    Completely off-topic: “Bananas are herbs”? No, a fruit that grows on a tree can’t be considered a herb… how do you define a herb, anyway?

  78. kit

    I don’t get how you can call these rules pure semantics and not science? What definition of the plane will you call science? “The planet is… don’t know – there is no clear definition”. “No definition”… this is science. yeah, sure…
    I agree that the rules are somewhat vague and have room for clarification. But this is at least something. And these definitions can be as easily applied to any other planetery system without humanity-centric aproaches (like caping the size with Pluto’s just because it is the smallest planet in OUR solar system).

    And as far as “pluton” classification goes, this really is semantics. Plutons are still planets (according to the new definition). The definition of a planet is a bit too fresh to start classifying within it I think.

  79. Chris Ledwith

    I think the public will come around to the notion of hundreds of planets. Everyone who has reached maturity realizes that there is no black and white — no absolute order. This new standard would just help the layman to realize that this applies to our solar system as well – making it all seem just as real as the weather outside. I think that’s a good thing.

    As for schoolchildren, well… there are hundreds of countries, no? Students just end up studying the most important ones. The same thing will happen with astronomy education.

  80. I too am with El Guapo.

    I guess it probably doesnt make much sense to have distance from the star be one of the factors, does it?

  81. Apocalipsys

    This is all a semantic non-sense gone mad. I know that there are serious people on the meeting. But look at this way and i think Phil is right, planet shouldn’t be named because of semantics but rather because of their properties like mass, size, main elements (gas, ice, methane, rock, etc), etc. Also a planet should not be tied to the type of orbit it makes or what it orbits. For example if a asteroid of the size of jupiter would hit us it should be called an asteroid; i doubt it; it would be called in the news like “big planet is going to hit us”.

    Anyways we call an asteroid to those small objects that have a strange orbit, and comets to those objects that has a highly elliptic orbit. But if we see them they are almost the same size and almost as same mass, only to be diferent in its orbit, and how cool they look from the telescopes POV. But they are only small objects.

    I prefer to call something a planet by its mass and size parameters. And should be noted that what they orbit and how they orbit is a property that not excludes them from being a planet. So from now i prefer to know that jupiter is a planet that has multiple planets in its orbit and moons too (moons for being smaller than planets).

    If you like my idea is ok, if you dont like it is also ok, but new ideas are always welcomed.

    PS My english is very bad sorry for my errors.

  82. Enlightened

    I can’t believe all the criticism here. This definition makes so much sense. First of all the main qualifying factor to be a planet (roundness) is based on scientific physical properties rather than an arbitrary size designed to keep certain ones in and other very similar objects out. Most people imagine planets as being round and asteroids as being randomly rock-sized so that works.

    Further more, there is a subset of “classical planets” that are those original eight main ones that don’t exist in a belt of many bodies. Also sensible as these are clearly the major important planets and can be labelled as such without offending the plutocrats.

    Thirdly, the “must orbit a star” works well. Moons orbit planets and planets orbit stars. A planet that leaves a solar system will be an escaped planet just as a moon that leaves a planetary system is an escaped moon. The definition isn’t circular logic as moons orbit stars as well as their planet, so “is not a satellite of a planet” is needed as well as “orbits a star” (to distinguish from free-roaming planemos).

    Finally, the pluton term is excellent as it replaces the horrible abbreviations TNO or KBO and is also a wink to Plutos previous special status without making a nonsensical disorderliness.

    The IAU has done a stormer here. I couldn’t believe how happy I was with the definition today.

  83. Matija

    One technical correction. It is true that the Moon is receding from Earth, but it will certaiinly not become a “planet” (by the new definition) in just 40 million years. It’s orbit is growing by 4 cm a year, but the Earth-Moon center of mass is moving at only 1/81 of that (CoM is 81 times closer to Earth than the Moon). The CoM will on average be outside Earth’s surface only when the Moon reaches average distance of 81 Earth radii. This will probably not happen before the Sun becomes a red giant (I get 10 billion years from now, with many assumptions). What might happen before that is that the CoM would start being outside Earth when the Moon is at apogee and inside when the Moon is at perigee. I think this illustrates well how messy the dividision between ‘planets with satellites’ and ‘double planets’ is.

    As for the IAU planet definition, I think it makes things too confusing. The word ‘pluton’ is annoying, as this is exactly how Pluto is called in many European languages. Also, now one can say that “only plutons among plutinos are Pluto and Charon” . Makes my head spin! Being a professional Solar System astronomer, I usually distinguish major and minor planets in a somewhat selfish way. Do you need to include Pluto in oder to model the orbit of Neptune with a reasonable precision? No? Then it’s a minor planet. How about Ceres? The same, doesn’t do much to Mars or anything else. Mercury’s influence, however, is important for modelling Venus’s orbit, so it must be included. The other bits and pieces orbiting the Sun are planets for sure, but minor planets, which is how all small bodies (asteroids, NEAs, Trojans, TNOs, Centaurs etc. ) were always officially known.

    When Pluto was discovered, it was thought to be much bigger, so it was designated a major planet. Through the 80s and 90s astronomers almost always ignored Pluto when modelling the larger Solar System, but before “Xena” there was no reason to open that can of worms. The only reason why Pluto wasn’t demoted sooner (like Ceres) was that it took them much longer to sort out how big it really is. So I think that it’s better and more honest to just demote proor old Pluto from a “major” to a “minor planet” than come up with new, confusing categories.

  84. kit

    Will the planets orbiting other stars be Major or Minor planets? More and more of them are being discovered. And I’d be greatly surprised if any astronomer uses any of them for modeling Solar System. so… this makes them Minor planets, right? Even if the planet is the size of Jupiter (or larger)?

    The new definition is good enough to be aplied to any object orbiting any star.

    Also, the whole “pluton” thing is not really a part of a definition of a planet. Please don’t bring it up to argue how bad the definition is.

  85. WTF?!? Pluto is not a planet! I could run around the perimeter in just 15 minuets!! What kind of crap is this?! @*$&@!!!!!!!

    They also named that astroid a planet?! What the heck is wrong with these people? Pluto and Ceres SHOULD NOT BE CALLED PLANETS FOR $*#&@ CRYING OUT LOUD!

    It’s so small to be a planet! It’s just a big orbiting chunk of ice! >.

  86. Chris Hecker

    “Planet” was coined as a word by the Greeks centuries ago. It meant “wanderer”, because some planets reverse their course through the sky at times, due to their orbital motions relative to an observer on Earth.

    “Planet” is a cultural word that has become passe scientifically. It’s still a useful word to describe round objects that aren’t stars, but I think there are better ways to describe astronomical objects. For example, Mercury has more in common with the moons of Jupiter than it has with Jupiter itself.

    For starters, why not try this nomenclature:

    Atmos: A gaseous round non-star that is more than 50% atmosphere by volume:
    Jupiter, Saturn, Uranus, and Neptune are Atmoses.
    (From the Greek word for “Air”, as in “atmos-sphere”)
    Petros: A rocky round non-star that is less than 50% atmoshpere by volume:
    Mercury, Venus, Earth, and Mars are Petroses. So is Ceres, for that matter.
    (From the Greek word for “Rock”.)
    Cryos: A cold round non-star that is less than 50% atmosphere by volume. This is reserved
    for Petros objects in the cold outer reaches of a solar system, like Pluto or “Xena”.
    (From the Greek word for “Cold”.)
    Moon: A round non-star that orbits an Atmos, Petros, or Cryos. It can also be an
    Atmos, Petros, or Cryos in itself — it’s just got an orbital relationship with a
    bigger Atmos/Petros/Cryos. You could have a Neptune-sized Moon orbiting
    a Jupiter-sized Atmos, for example. This nicely removes the need for the “double
    planet” nomenclature except in cases where the objects are (almost) the same size.

    The Atmos classification would apply to bodies up to the size of a brown dwarf star: i.e. it isn’t an Atmos if it has undergone significant Deuterium fusion at some point in its life.

    As for what is a “planet”? Scientists wouldn’t really care. Maybe we could have a reality TV show where the public voted on which ones to keep and which ones to throw out. 😉

  87. Matija: Yes, I suddenly realized that this morning, and haven’t had time to correct the entry. That was a dumb mistake on my part, spurred partially by trying to write under a deadline coupled with all the server issues I had yesterday. No excuse, of course. But I’ll correct it very soon.

  88. Hurricane

    Hurray! If Pluto’s a planet then “Xena” is too – let’s give her a real name, already!

    Surprise – Ceres is a planet too. Okay, most *regular* people would agree that if it’s big, spherical and orbits the Sun it’s a planet.

    Charon? The major surprise, IMO! Kinda looks like a moon to me! LOL

    And planets should orbit a star to have planethood. Can moons exist that aren’t satellites? Of course not.

    So much for *ending* the debate on What’s a Planet – Goodbye Pluto, Xena and Charon… Hello, Sedna, Quaoar, Ixion, Easterbunny, Santa, Varuna, AW197, TX300, etc., etc. Good – gives us something to fuss about! LOL

  89. I applaud this definition of planet and I propose that we move on to much thornier definitions, like , “What is a Continent?”

  90. Thad Ritchards

    The problem with the 2,000km criteria is that…why 2,000? What if there’s something 1,999km? (Which is likely) It’s just as arbitary a line in the sand as most of what else is being discussed here.

  91. James Fox

    I personally think that the best compromise would be to emphasize that ‘dwarf planets’ should not be regared as important as ‘classical planets’. That way, they can be ignored by those who only care about memorizing lists of planet names and imparting basic information. The 8 planets are individualists, the Plutons can be described collectivly, and that can probably hold even if new, icy Plutons the size of Mercury are discovered.

    The problem with excluding Pluto and other large TNO’s is that they differ from the large planets only by degree. They are colder, more inclined, smaller, more eccentric : but intermediate cases can be envisioned. Thus any attempt to make a definition that excludes Pluto has the stink of, well, making a definition just to exclude Pluto and others like it. That even goes for the ‘population’ defintions: the width of the Kuiper belt is larger than the inner solar syatem.

    The comittee wanted to avoid arbitrary terms in the basic definition. but made a compromise with the division into ‘dwarf planets’ and ‘classical planets’, in order to make things easier for those hating the idea of numerous mini-planets: just ignore the lesser ones! However, pluto-haters gripe, unable to accept anything but complete demotion, while ignoring the physical differences between the new dwarf planets and lumpy asteroids. Meanwhile, other overlook that the special status of the ‘big eight’ has been preserved, and you don’t have to force information about every single dwarf planet down everyone’s throat.

  92. Chris Hecker


    If I interpret what you’re saying correctly, you’re approaching this the same way I am: by classifying planetary systems according to their development and structure, not by the attributes of individual planets.

    So our solar system can be divided into three zones: inner planets, Gas Giant planets, and TNOs / Kuiper Objects plus whatever is further out. Then base a definition of “planet” on the analysis of the whole system.

    That makes sense to me.

  93. Bart O'Brien

    I find the four-point definition – at least as presented – a rather messy piece of analysis.
    Point 1 says “a planet is any object in the universe which meets the following criteria ..” That’s it: proposed complete definition of the concept planet.
    Points 2-3 are quite different. They propose a way to sub-classify all the planets (by the above definition) which happen to exist in just one particular solar system out of all those there are in the universe: namely our solar system.
    Thus Point 1 has an entirely different logical status from points 2-3.
    If the authors of the proposal made that much clearer, the whole debate could be more focussed.
    Moreover it would then become evident that Point 1 is the important one. We need reliable nomenclature to help discuss and understand the bodies in other solar systems as we discover them.

  94. The three zones are not very well-separated in space or in formation-time. What about TNOs that got tossed into the inner solar system? Asteroids that got tossed into the Kuiper belt? What about the comets in the Main Belt? Or comets in the Kuiper Belt that pose as asteroids? It’s a strange mixed-up world out there.. :-)

  95. philip

    Its amazing that a heap of scientists can not, by analytical (scientific) methods, determin what would be a generally (in other solar systems than ours) valid taxonomy of celestial objects.

    It is, for scientific usage, completely irrelevant wether a body circling an exergetic object is called a planet as long as this object is describable in mass, size, orbit a.s.o.

    Planet is a historical term ending with Pluto. Its a word of language and literature, not science. Ceres and Charon, not being called planets or being planets or whatever will not change their physical status. Could the planetary scientists call the things they must define for certain reasons something like “Gragtet” or “Murpf”? (inclined eliptical orbit or member of a group of similar objects).

    Since Planet can mot be made fit to a selection of objects ,as this debate clearly points out the only real answer left is that all things circling a star are planets, and those may be devided into subgroups. Or that the word planet must be abandoned and replaced in science.

  96. Why don’t we look at the original meaning of the word “Planet” and stick to that? It means “Wanderer”. To me, that means the classical line-up that could be seen with the naked eye. For anything else, how about calling them “worlds”? Definition of a “world” – spherical (non-stellar) body in space. Keep it simple!

  97. Rumour Mongerer

    I think the most controversy here will be in the world of Sailor Moon! (There’s no Sailor Charon, but there is a Sailor Ceres…)

  98. Mike


    Well out of all the possibilities, I’d have to say that this is one of the most dissapointing.

    Although the definition is simple, it certainly lacks depth. By this loose definition, we can call an enormously large amount of bodies planets. Some of which would have never been considered prior to this event.

    The definition is too simple and not rigid enough, allowing for a wealth of weird “planets”, some of which were considered asteroids.

    It would have been alot more elegant to define a minimum mass for a planet and call any body under that mass a dwarf planet. Hell, we do it with stars, why not planets? The equivalent of this defintion for stars would allow all deuterium burning brawfs to be considered in the same class of stars as the the larger ones.

    Really, they should have put constaints on orbit, terrain, and mass. And it doesn’t need to compromise the elegance of the defintion, but in this way we can call anything under the threshold as drawf planet and thus not have a variety of asteroids planets.

  99. Brant D.

    In my opinion, as long as the eight “major” or “classic” planets are set aside from the rest of the lot by the language, I don’t have any problems with this. That would be the best choice based on nature itself, I think. And that’s all that matters, right?

  100. John Guerin

    Here’s my mnemonic (or Friday’s headline; take your pick):

    Many Varied, Extra, Moon-like Chunks Jostle Scientists Union’s Named Planets, Coming UneXpectedly

    (UneXpectedly works for either UB313 or Xena, depending on whether it gets that name or not)

  101. Aerimus


    “It is, for scientific usage, completely irrelevant wether a body circling an exergetic object is called a planet as long as this object is describable in mass, size, orbit a.s.o.”

    True, but the problem with just leaving that at that is that scienist have this need to want to explain things to common folk. For common folk, a solid definition of planet, or any other definition, is much easier to visualize and graps then just the facts and figures involved.

    For example, you could say Ceres is a 9.5×1020 kg, thus massive enough to make it nearly spherical, has a radius of 466(nine planets doesn’t give units, but I’d assume km), and has an eliptical orbit around the sun; it’s perihelion 2.544 AU and Aphelion 2.987 AU. Or you could say Ceres is a large Asteroid in the Main Asteroid Belt that is so large that it is so massive, it is almost spherical. Which one do you think most people will be able to visualize and understand?

    So while scientist may not really need the definition, having one is helpful since it allows them to convey their work to the people. And considering that (a) scientific funding can be help through greater understand by the populace and (b) most scientist WANT to educate people, having a good definition is not a bad thing.

  102. I see that there has been a lot of activity on the Ceres entry at Wikipedia. It’s looking pretty good now:

  103. justawriter

    Actually, an honest species would say that this system had only four planets, Jupiter, Saturn, Uranus and Neptune, and there are a bunch of little turds floating around that are considered important by the fleas living on the third turd.

  104. A bunch of mnemonics (hey, who needs more planets when we can have so much fun with words?):

    MVEMCJSUNPC2003 (The 2003UB313 Version)
    Most vehicles emit malignant carbon; journeys should usually not proceed, circa 2003.

    MVEMCJSUNPCU (The UB313 Version)
    More velocity equals more carbon; just standing uses none. Please consider unmovingness.

    MVEMCJSUNPCU (The Meta-Mnemonic)
    Mississippi? Very easy. Massachusetts? Just spell using nous. Philippines? Can’t understand.

    MVEMCJSUNPCX (The Xena Version)
    Musical venues emit magical, jewel-like sounds, unless negligent players consider xylophones.

    MVEMJSUNP (The Backlash Version)
    My very earnest man, just show us nine planets.

    MVEMJSUN (The Purist Version)
    Making vigorous emendations means jettisoning some useful notions.

  105. Whoops! Forgot Ceres in two of those.

    Mississippi? Very easy. Massachusetts? Can just spell using nous. Philippines? Can’t understand.

    Musical venues emit magical, cintillating, jewel-like sounds, unless negligent players consider xylophones.

  106. Magnus

    So, does this mean we’ll get three entirely new Disney characters. I propose Charon to be Donald Ducks new pet mouse. Ceres could be Scrooge’s lost, evil twin brother. And UB131 could be the hight tech version of Donalds car, with an A.I. personality (as i recall the licence plate number on DD’s car is 131)

  107. Brant D.

    On the topic of astrology, I like this naming scheme. Including multiple new “planets” into the family every other year or so is sure to make the astrologers pull their hair out.

  108. Keith Thompson

    Why did they choose 800 kilometers for the cutoff? 1000 is a rounder number, and it would have excluded Ceres while keeping Pluto (and Charon — oh, well).

  109. Dead Boy

    This may be a bit off topic, but it is my most sincerest of hopes that the International Astronomical Union, when making the final call in declaring that we have 12 planets (so far), that they officially accept Dr. Mike Brown’s name of 2003UB and it’s moon; planet Xena and Gabriel. Sure, it may be the geek in me that finds that really cool, but the man discovered it and as such it’s only fair that he get to name them.

  110. Like I said earlier, if only Phil had been there to talk some sense into this committee.

    But this is just a proposal, it has to be voted on. Is there any chance that the IAU will through this proposal out?

  111. Ryan S

    I only read about half the posts (sorry, I was at work :) ), and I feel like we got shafted. It’s not necessarily bad that Pluto is a planet, but now we are going to have gajillions of them. What if 3 asteroids and a comet collide and sphere it up? Zoinks, another planet! Then we’ll have to change the books, and the websites, and on and on, and the whole debate will continue. Most people don’t care about the sky as it is – if it keeps changing, they’ll say “Screw it, I just can’t keep up remembering new planet names!” and they’ll just abort.

    I don’t know what the solution should be. Maybe we just leave well enough alone, and put a * beside the ninth planet with a disclaimer – “Pluto was found back in the day when telescopes were crappy, etc, etc… and so it’s not really a “planet” [ya ya, semantics], but it’s in here for historical reasons”.

    I’ve finally made a decision. I’m going to sit on the fence. :)

  112. tom

    If the newly proposed set of rules that define planets are accepted, then there are actually at least 15 planets discovered so far. Along with the 9 recognized until yesterday, Ceres, Charon and UB2003, there are:

    50000 Quaoar (1260km diameter),
    90377 Sedna (1180-1800km diameter),
    90482 Orcus (840-1880km diameter),
    84522 2002 TC302 (1200km diameter),
    2005 FY9 (1600-2000km diameter),
    20000 Varuna (1000km diameter),
    possibly 28978 Ixion (just about 800km diameter),
    55637 2002UX (910km diameter),
    55636 2002TX (900km diameter)…

    So it’s 20 or 21 planets now, isn’t it? At least at the moment.

  113. tom

    Actually there were 15 when I first thought of Quaoar, Sedna and Orcus, but I decided to look up any others that might qualify and voila!

  114. Does it bother anyone else that, in the long image of our new solar system, the lighting of Neptune and Uranus does not match the other planets (nor the direction the sun is)?

  115. Aaron F.

    Marlayna — An herb is basically a plant without a woody stem. Banana “trees” don’t have woody stems, so they’re not trees… they’re herbs. If I recall correctly, the part of the banana plant that looks like a woody trunk is actually a bundle of leaves.

  116. Ditch the term planet and have:

    Terrestrial Worlds (Earth etc.)
    Jovian Worlds (Jupiter etc.)
    Asteroids (Ceres etc)
    Kuiper Belt Objects (Pluto, 2003UB313 etc)
    Other stuff (comets, centaurs etc)

  117. (Man, I can’t believe I misspelled “scintillating”. Wishful thinking, I guess. Make that “coruscating”. These mnemomnimcs are diffimcult.)

  118. Cate Mato

    So does this mean that the Hayden Planetarium will get Pluto ‘back’, or will Neil DeGrasse Tyson continue to annoy and disappoint visitors:)?

  119. tom

    Imagine just how much more detailed horoscopes there’ll be now:

    “Expect phone call around 2pm. It’ll be your mother…”

  120. tom

    The more I think about the rules for planet definition, the more it looks as if the rules were written in such a way that would make Pluto a planet (by sufficiently low diameter treshold), yet keep Moon, Titan and similarly big satellites out (by applying system barycentre rule – although Charon is not affected by this rule since Pluto is too small). As for arbitrary 800km diameter rule – this puzzles me a bit. Had it been 2000km diameter rule, we would have 10 planets, Pluto included, plus UB313, no Charon, no Ceres. Maybe this will be changed yet.

  121. Whet Smith

    Let’s be honest here. This debate is all about Pluto, and the choices are A) drop Pluto as a planet or B) if we’re going to be intellectually honest, include Pluto and by necessity include some other stuff. Pluto’s discovery and inclusion as a planet is really a historical accident, and it seems like in all the semantics we know that if it were discovered today, it would never be considered a planet. It also seems to me that having probably millions of planets stretches whatever usefulness that the word has.

    I suggest another criteria. A planet must be a reasonably unique body in it’s area/neighborhood/rough distance from the Sun/star. It should not be one of a group of other objects. Mercury would qualify because no other rock near that big exists that close to the Sun. But, it wouldn’t be a planet if it were located in the KB. The next seven planets are pretty unique regardless of distance. Pluto would not be a planet, it’s a KBO. However, using this definition would explain for history why Pluto was considered a planet for 75 years. Before we knew of the KB, it made sense that Pluto was a planet. Ceres is a closer call, and I’ll be honest in that I’m not sure how similar Ceres is to other asteroids, but I’d lean to saying it’s not unique enough.

  122. i think it’s sorta funny, planets 1 -11 (with the exception of Earth) are named after Gods, planet 12 is named after a TV character

  123. PsyberDave

    My wife suggests that Pluto and Charon be considered planetless moons. Brilliant!

    I’d also like to add that I think UB313 should be called Phil. And while you may smile and snigger, it is still consistent with with the current method of nomenclature. Phil is a Greek name (isn’t it?), and who wouldn’t argue that Phil Plait is some sort of god among men?

    Xena? Please! Why don’t you just call it Seven of Nine or Princess Leia for crying out loud!

  124. robert jason

    What ever happened to calling them minor planets??

  125. Betelgeuze

    ‘minor planets’ is already used for asteroids.

  126. evilekin

    What about Triton?
    Under the new rules, Triton is still a moon, but used to be a planet before it was captured out of the Kuiper belt by Neptune?

    I’m for dropping Pluto, let’s not add more confusion to science please…

  127. Regarding mnemonics, someone on Wikipedia came up with a pretty good, and a-propos, mnemonic:

    “My Very Educated Mother Considers Just Slightly Understood Nine Planet Concept Uneducated”

  128. devodave

    To further add to the confusion, “pluton” is already a geological term. A pluton is a large mass of intrusive rock.

  129. JoseC

    Why don’t we call one of the new planets Vulcan or maybe Vulcanus in honor of the greatest science fiction telivision series ever made?

  130. Jesse and Jonathan Holmes


  131. Keith Thompson

    Here’s something I posted to rec.humor.funny back in February of 1999.

    Reprieve for Pluto

    The International Astronomical Union recently rejected a proposal that would have revoked Pluto’s status as a planet, reclassifying it as a minor planet.

    In a compromise vote, however, the IAU has approved a censure resolution.

  132. Keith Thompson

    devodave writes:
    To further add to the confusion, “pluton” is already a geological term. A pluton is a large mass of intrusive rock.

    I was going to suggest using the word “plutino”, but apparently that already has a more speciific meaning. According to Wikipedia, a “plutino” is “a trans-Neptunian object that has a 2:3 orbital resonance with Neptune.”

    How about Plutoid? Plutesque? Pluthiness?

    Or just “Goofy”.

  133. erekose

    I propose Eris (greek goddess of chaos and discord) as the new name for UB313. Only fitting, considering the fuss its discovery has caused…I dont think its been taken yet either, which is suprising, given the proliferation of asteroid names

  134. Triangleman

    – – “Let’s be honest here. This debate is all about Pluto, and the choices are A) drop Pluto as a planet or B) if we’re going to be intellectually honest, include Pluto and by necessity include some other stuff. ”

    My thoughts exactly! I feel that this whole Planet – Pluton thing is just shoehorning. I’d have rather seen Pluto dropped to TNO without creating a new nomicker that I doubt anyone is going to use. I’m guessing science classes in elementary schools will just list the 8 planets and briefly note that Pluto once was but isn’t anymore. I doubt they’ll get into Ceres, Xena and all of the others.

    Could you imagine a sci-fi movie:
    Captain: “There’s a planet on the sensors!”
    Officer: “Actually, I think it’s a pluton Captain”
    Captain: “Shut up and head for it doofus!” :)

  135. MCS

    I’m in the “Pluto is not a planet” camp, and haven’t been since I found out about KBOs and plutinos. The term “planet” should be reserved for the most significant bodies, I think, and Pluto, as one of the larger members of a very large class of objects, doesn’t seem distinct enough.

    The proposed IAU definition has a certain amount of subjectivity and arbitrariness built into it, since there will be a lot of argument about whether marginal bodies are in hydrostatic equilibrium, plus getting sufficient information about a body to make that determination would be difficult. Also, while hydrostatic equilibrium might be important in some cases, it’s not necessarily a critical distinction for all purposes – distinguishing spherical and non-spherical KBOs isn’t necessarily important for all purposes, their KBO status could be the most important thing. When teaching astronomy, saying that Pluto is a KBO would seem more useful than to say it’s a planet. They aren’t mutually exclusive, but calling Pluto a planet doesn’t seem to add anything, nor does calling Ceres a planet seem to help much either. For that matter, making the distinction between terrestrial planets and gas giants also seems like a good idea.

    If there’s going to be a certain amount of arbitrainess, then an arbitrary diameter cutoff seems as good as anything. Since Mercury is somehwat marginal, I’d use a 2,000 km radius (not diameter) as a cutoff. Mercury’s large iron core and relative uniqueness help save it.

    Perhaps it would be better to wait until we get a better idea of what the size range of TNOs are. If we discover Mercury-sized or larger objects out there, we’ll have a better idea of where we stand. Whatever the case, the IAU proposal doesn’t work for me, because I think it trivializes the meaning of “planet” by including too many minor bodies. Since it seems like demoting Pluto isn’t going to happen, I hope this proposal gets defeated, and the whole issue put on hold until we get a better idea of what’s out there.

  136. JonUk

    How about defining a planet as having enough mass to retain some semblance of atmosphere, anything wrong with that?

  137. Pluto-Charon is a double planet…uh… why? Charon is way, way, way more tightly bound to Pluto than it is to the Sun. In fact, it’s 578 times more tightly bound to Pluto than it is to the Sun! In my programming language/calculating tool “Frink”, ( ) this is easily calculated by:

    (plutomass / charondist^2) / (sunmass/plutodist^2)

    (The first half of this equation is the acceleration of Charon due to Pluto, the second half is the acceleration of Charon due to the sun.)

    On the other hand, Earth’s moon is more tightly bound to the sun than it is to the Earth,

    (sunmass/sundist^2) / (earthmass/moondist^2)

    by a factor of about 2.2. So Earth’s moon is more a planet in co-orbit with the earth around the sun, (its orbit is at all points concave with respect to the sun) while Charon is without a doubt a moon of Pluto. Unless you’re the IAU.

    They also have some criterion that the body has to be big enough that it’s squished into about a sphere, but with absolutely no rigorous definition of how to judge this.

    Sigh. Thanks for bringing rigor to the process, IAU.

  138. RAD

    What I want to know is how soon before I can have my horoscope changed so its right? I mean these new definitions will really wack out astrology.

  139. Sylas

    Repeating a comment I left at the Pandas Thumb:

    The more I think about this proposed definition, the more I like it.

    What is really exciting about planets these days is that we keep finding them around other stars. This means we need a nice clear simple definition, that can be applied in many contexts. This is what the IAU is proposing to give us.

    So what if Pluto is an ice-ball. Why should that rule it out? As for calling it a Kuiper belt object… so it is; but the Kuiper belt is a local solar system structure. Other stars have similar belts; but the structure of belts in other stars is going to vary a lot, and will depend on what other planets there are to push and pull on smaller planets. Kuiper belt is not a quality that can be used to distiguish planets and other bodies, because we can’t apply it consistently for other systems.

    The definition they have given is beautifully elegant. It’s not a case of some arbitrary number chosen on permissible size, location, eccentricity or whatever. They don’t give a number, but a quality. It it is big enough for the surface to be defined by gravitational equilibrium, then it is a planet. Now this is not perfect; it allows for a grey area as the shape is more and more constrained. But it is way better than just taking a number out of a hat; it is trying to identify a quality that transposes easily to other contexts.

    And the asteroid Ceres is to be a planet! Great! I see this as an injustice rectified at last. I like it that we recognize this small world in the inner solar system. It’s not devaluing the big guys… it’s acknowledging one of the little guys, which even so all on its own accounts for about a third of the mass of the asteroid belt. I’d love to visit there one day — and I hope with it will get increased recognition for that rather interesting part of our solar system.

    Charon is a planet! How cool is that!? We have a binary planet in the solar system. It’s been spoken of as such before this, but now this can become official, and we know what it means to say it is a binary planet. And if anyone else wants to find another binary planet, they know just what to look for.

    It also opens the way for new discoveries. It’s a good thing that we don’t know how many planets there are in our solar system. It leaves open the way to search and find more worlds, and honour those who find them as discoverers of planets.

    This is an excellent proposal.

    Cheers — Sylas (Chris)

  140. Bart O'Brien

    This discussion is so parochial!
    To read these messages anyone would think that this solar system was unique in the universe, and none of the other stars had anything that might be regarded as planets.
    The interesting and useful issue is: Which bodies IN THE UNIVERSE should we regard as belonging to a special category named planet?
    Only when you have a sensible answer to that question should you begin working out which particular bodies in our own particular solar system conform to the definition, which don’t, whether there should be solar-system-specific subcategories etc.

  141. Enlightened

    You guys are incorrigible! The 800km isn’t “arbitrary” its the *rough* size diameter that usually equates to an object being round. If a 799km round object is found its a planet. If an 801km irregular object is found its not. (It will change depending how far away from the Sun the object is, and also its composition) Nature’s forces determine a planet in this sense and that’s the way it should be. Not a truly arbitrary line or 100km in the sand designed to fit our solar system and based on one particular measurement system created by man. Besides the “big eight” will now have special importance as classical planets. This should be a great solution that keeps everyone happy. But, as usual, people will always find reasons to moan.

  142. but if Charon is a satellite of Pluto then why is it being included as one of the new planets? Doesnt that go against the first of the new rules?

  143. I am SO MAD about astronomers addin new planets. Now, I don’t know much about astronomy, but WHY can’t they just leave the solar system alone? Admit they made a mistake with Pluto but stop adding planets! Make a new classification for these “giant ice balls” but leave Pluto. Make sense? This is INSANE.

  144. zpt


    There is an exclusion to rule 1(b): “For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are “satellites”. Under this definition, Pluto’s companion Charon is a planet, making Pluto-Charon a double planet.

    BTW, has anyone noticed that the perihelion distance of 2003 UB313 (37.7 A.U) is closer than the aphelion distance of Pluto (49.3 A.U.) ?

    I’m not even going to try coming up with a new mnemonic taking this into account.

  145. Hey Phil, I wrote about this on my blog this morning (tipped a hat to you of course). When I first read this yesterday I didn’t like it (per my comment somewhere up there in that pile thereof) but having time to read the full definition, think about it, sleep on it, etc. I’ve decided I like it, and the more I think about it, the more I like it.

    It’s a good definition, and I think we should support it. AFA kids’ textbooks and all that, how old were you when Charon was discovered? I was 11, and I was *thrilled*. I didn’t care that I had to memorize a new celestial body. Any kid who actually cares about the solar system is going to be stoked that there are 12 planets, and the idea that many more may be named is similarly cool.

    Coming up with the class “pluton” is a good idea… it differentiates Pluto (and worlds like it) from the other planets, which ought to be enough to satisfy the PINAP (Pluto is not a planet) crowd.

    I like the binary planet distinction… you can have binary stars, why not binary planets? The definition seems good as any. If Saturn and Uranus were in orbit around each other, I don’t think anyone would seriously argue that Uranus is not a planet.

    So I guess you can file me under one of the converted. I’m happy to see these three newly classified planets… especially Ceres, I bet Piazzi would be thrilled.

    As you note, it’s kind of a silly argument anyway. If you could actually go there and land on it or dive into it, would it really matter whether you called it “moon”, “planet”, or something else?

  146. icemith

    When I started reading this post, there were 156 comments. Just checking before this, it had grown to 164. That was some read!

    I have a few very short points to make:

    1. Pluto/Charon spends years inside the orbit of Neptune, and as it appears that from zpt’s comment just above, 2003 UB 313 also skips inside Pluto’s orbit. Whether it is co-committent with it’s (Pluto et al) excursion inside Neptune territory, I don’t know.

    2. Though Xena is a problem for some to reconcile, because of the pronunciation, I always imagine it spelled “Zena”. Surely that will have the nmenomic crowd happier with an easier choice of words! Use “Z”. But honestly, it is far easier to learn the actual names of the planets in the first place; why learn baby-talk, only to have to learn what it means as well?

    3. Nobody has mentioned the obvious name for a Minor Planet. Simply call it a “Planette”. By the usual English definition, a small planet. This satisfies the senses as it is pronounced the same, or it could have an extra stress on that last syllable. So anything that was questionably not quite large enough to pass as a planet under the new criteria, could be in that class until it grew up, or wandered off somewhere else. At least it would not cause the confusion as “Pluton” does. Some have asserted that it is already used in other disciplines, and it is too similar to its namesake, Pluto.

    Basically, I feel the IAU have to rule on the whole question of what constitutes a planet, moon etc., and I hope they don’t surprise or disappoint us, but given the many comments above, I’m afraid we won’t all be satisfied. So what’s new?

    3+a bit. The Moon can never be a planet as it appears from previous comments, except if “somebody” removes the Earth. Now I don’t want to be around if that happens! The previous questions would be purely academic.


  147. mircear

    Although the text was written in a haste, and by an (very) amateur, have a look at this:

  148. James

    Not sure if this has been mentioned, but your calculation of the time it will take for the barycenter to move beyond Earth’s surface is incorrect:

    Distance to barycenter: M_Moon / (M_Earth + M_moon) * new_distance

    Set this equal to the Earth’s radius, we have:
    .0122 * new_distance = 6378km =>
    new_distance = 524000km

    distance_traveled = new_distance – current_distance = 524000km – 385000km = 139000km.

    Time to travel distance = (139000km) / (4 cm/yr) = 3.48 BILLION years.

    What you did was divide the distance of the barycenter from the surface by the rate of travel, which is NOT correct. has the figure as being “in the few billions” as well.

  149. Luke

    The definition of planet should also include having an atmosphere.

  150. Hawaiian_Physicist

    I think part of the reason for the complex scientific definition in terms of roundness etc. is that they want the definition to be able to apply to an arbitrary solar system, not just ours. If you try to pick something in terms of some physical quantity (mass, diameter, distance from the sun, inclination etc.) then you will invariably run into problems in the event that you hit something unusual or very close to the limit. Just as an example, if you take it that a planet must in the ecliptic, say, what do you do if there are only two “planets”, both with different inclination angles, which otherwise meet the criteria. The definition falls apart because the system doesn’t have a real ecliptic plane. I personally like the idea of the classification system suggested above of essentially “icy planets” “gas planets” and “rocky planets” or some equivalent system.

    For the people who don’t believe Charon should be a planet because it orbits Pluto, here’s a question… what if Charon was 1/2 the mass of Jupiter and Pluto was the mass of Jupiter. Would you still feel that Charon should be a moon?

  151. Doodler

    Honestly, I don’t see what the fuss about thousands of possible planets is. We deal with billions of trillions of stars without tearing our hair out, we have the possibility of tens of thousands of asteroids (with only minor heartburn about Earthcrossers), and QUADRILLIONS of KBOs and Oort objects.

    So long as whatever definition evolves out of this discussion is internally consistent (Phil’s belief that what it is is more important than where it is pretty much reflects my own opinion on the matter), I’m not going to be intimidated by large numbers.

  152. Keith Thompson

    MCS writes:

    I’m in the “Pluto is not a planet” camp, and haven’t been since I found out about KBOs and plutinos. The term “planet” should be reserved for the most significant bodies, I think, and Pluto, as one of the larger members of a very large class of objects, doesn’t seem distinct enough.

    (Let’s see if the “blockquote” tag works. Pity there’s no “Preview” button.”

    What if we suddenly found a few dozen bodies the size of Neptune in similar orbits? Would Neptune then no longer be a planet?

    Of course that’s not likely to happen in our Solar system, but it could easily happen elsewhere. Any definition of “planet” should cover such cases.

    Personally, I think that a planet-like body not orbiting a star should still be considered a planet. I wouldn’t be surprised if there are a whole bunch of such things out there. (The word is particularly apt since “planet” is derived from the Greek word for “wanderer”.)

  153. Keith Thompson

    The mnemonic I learned for the old nine-planet system (ah, nostalgia) was:

    Mother Visits Every Monday, Just Stays Until Noon, Period.

    (Of course, I always have to use the order of the planets to remember the mnemonic!)

    Here’s a revised version:

    Mother Vists Every Monday, Comma, Just Stays Until Noon, Period, Comma, UB313.

    Or, if the tentative name for UB313 actually catches on:

    Mother Vists Every Monday, Comma, Just Stays Until Noon, Period, Comma, Xylophone.

    Ok, it needs some work.

  154. CR

    I’m with… who was it so many entries ago? Bob Allee?
    I’m normally so jazzed about astronomy news, especially planetary astronomy news, that people think I’m FROM another planet. But this whole thing just has me a little bored and, I hate to admit it, turned off.

    I’m still interested in the planets and other objects that orbit our star, and I really can’t wait to see details about Pluto itself some year (along with Charon, of course).

    But this whole debate has me somewhat numb. I can only imagine what people not as interested in astronomy think of it all… then again, I’m guessing in America at least, many people are probably more interested in the latest “Rock Star” results and don’t care.

  155. HawaiiArmo

    The fact that Pluto’s considered a planet has less to do with semantics, then it has with stagnation towards progress. So many people are lulled to sleep every night by the fact that there were 9 planets, that the thought, that perhaps the 9th shouldn’t be considered a planet at all will give them years of insomnia. Justbecause you’ve read about something being one way for years, doesnt’ mean it should remain classified as such. The same debate rages in Paleontological circles about whether T-rex was a carrion eater, or an active hunter. Those of us in the Scientific community tend to laugh at the ignorance and stagnation of those New Earth Creationists who take the bible literally, yet, some of us cannot break the indoctrinated beliefs we’re raised with.

    I know it’s comforting to retain the status of Pluto as a planet, which by extention, results in the future possibility of hundreds of planets, but let’s keep in mind that change is not necessarily a bad thing. If actual thought went into the nomenclature, we would have 8 planets, and the rest would be classified as asteroids, kuiper objects, etc.

  156. Sylas

    Thought did go into this… and there is one excellent reason to call Pluto a planet.

    Simply put: other stars have planets. So we need a definition that works beyond the narrow confines of our solar system.

    If our solar system was the universe, then we might single out the four major planets, and call the rest debris. Or we might single out the planets that are roughly in the plane of the elliptic, and have eight. Both these ways of defining planets are parochial — they only make sense in the context of one particular solar system where we have certain kinds of distinctions we can apply between known bodies. It would be really hard to come up with meaningful definitions that would single out the four, or the eight, or the nine, in such a way that it could be expected to transfer naturally to the wider view of other stars and their attendent bodies, now opening up to our instruments.

    The proposed definition neatly captures a quality that does not rely on comparisons with other bodies, but is intrinsic to the body being classified.

    Cheers — Sylas

  157. MacNasty

    Lets call the last one TURD anything further out can be a Dingleberry!! Simplify this ‘hunk of rock’ stuff..!!

  158. Lee

    Interesting thought.

    Given the “Orbits around a star” part of the definition added with Charon becoming a planet; doesn’t that promote Jupiter to being a Sub Brown Dwarf star instead of a planet.

    Jupiter’s barycenter lies outside of the sun.

  159. patrick

    i wouldnt call jupiter a sub brown dwarf. just because it really is a gas giant. and from what i know it has never, and probably never will have nuclear fusion.

    with that said

    with pluto and charon. i personally think that they are both just esacaped moons of neptune. yadda yadda. but they should be classified as a planet. well, double planet. and if that means ceres should be a planet so be it. as long as its circular its fine. just set a given distance that is just out of plutos or that ub313 orbit and be done with it…

    or we could just do the turd thing

  160. John Neff

    I also thought it was silly but after think about the defenition I realized that they may have done some good.

    What they have done is to set a lower limit to the bodies self gravitational force by requiring that the body be nearly spherical. The problem with this approach is that the strength of materials of ice rich bodies is much lower that that of iron rich bodies so they have set two lower limits instead of one. As a consequence there will be a bunch of new ice ball planets and no new iron rich planets other than Ceres.

    However I think they goofed in proposing that the Pluto-Charon system is a double planet.

  161. I get so confused so easily. I liked the idea/proposal. I don’t really care how many planets there are. If this is ONLY a semantics debate then why have astronomers and other scientists decided to take it up.

    It seems to me that the fact that BA has been thinking about it for years shows that it is also a scientific debate.

    People just want to be able to categorize things I think. I agree that planet is largely a culturally defined term, but I also note that scientists have used the term for years to describe their discoveries and categorize the solar system, you can’t have it both ways.

  162. I don’t doubt that there is a scientific debate. What I doubt is the need or even the possibility of a scientific definition.

    Hal Levinson and Gibor Basri are both planetary scientists, and both published white papers about a definition for “planet” a couple of years back (Hal cowrote his with planetary scientist superstar Alan Stern– their paper is here (in PS format) and Gibor’s is here). And both, IMO, don’t work. Gibor, Hal, and Alan are incredibly good scientists, and very smart guys. They wanted to put out ideas to get people to chew over the “debate”. I think that’s a fine idea, but it showed to me that you simply cannot come up with a good definition of planet. We’re arguing over how many angels can dance on the head of a pin– the entire first premise is wrong.

    I talked with Gibor about this a couple of years ago, and got him to agree with me (after I browbeat him) that it is always going to possible to come up with an object that is obviously a planet but would not be included in a definition, and to come up with an object that is obviously not a planet but would be included in a definition.

    To me, this shows that a scientific definition of any kind is impossible. And that’s my argument:

    There is no scientific merit to defining what a planet is.

  163. Hey Phil,

    With great respect I have to at least partly disagree with you. Nomenclature in science is important,
    it constrains the questions we can ask. But any definitional boundary has grey-area cases.
    The test is whether the typical member of class A is really a different kind of animal from the typical member of class B. If the distribution is bimodal so that there are few grey area cases, that’s
    much better, but not essential.

    Clearly the distinction between worlds that are gravity dominated and worlds where structural strength dominates gravity is a physically significant one and I think it’s a very useful definition. The Soter proposal for dynamical-dominance-in-its-orbit-regime provides a reasonable physical argument for distiguishing major planets from dwarf planets, but I’m not super-convinced by it.
    The IAU double-planet-barycenter rule I’m also not convinced by. Your argument that
    something that’s a planet now may not be a gigayear from now is irrelevant. Something
    that’s a star now may be a supernova remnant a megayear from now and was part of
    a giant molecular cloud some time back, and noone objects
    to that.

    The real problem is that the word ‘planet’ has a common meaning that carries both a sense of size (large compared to Phobos, small compared to Sol) and of dynamics (goes round the Sun not round Jupiter). As Velikovsky presciently pointed out (KIDDING!!), *Dynamical states are not stable
    over a Hubble time*. I assert that several real scientific questions are being mixed here:

    – Is Pluto more like Mars than it is like Eros?
    – Is Titan more like Epimetheus than it is like Pluto?
    – Is Ceres different from Mercury and Venus in any *important* way?
    Sure, it’s smaller, but apart from (recent) tradition is there any reason to set the boundary
    between them – ignoring their original and orbital situation? I claim it’s the same kind of
    object as Mercury, and not the same kind of object as Itokawa.
    – Is the dynamical state (orbital dominance, barycenter position, etc) an interesting thing
    to use as classification boundary?
    – Is the formation history (accretion history; formation of pulsar planets, etc) an important
    thing to use in classification
    – Do I need to change my website to be www.

    There is also a cultural question: do any of the above distinctions merit having the word
    ‘planet’ associated with them?

    I tend to think that IAU Res 5 Item 1a is a great definition of the concept ‘world’ (as opposed to
    ‘bit of debris’). I think that ‘world that currently goes round the Sun (or another star)’
    (as per Res 5 Item 1b) is a reasonable definition of ‘planet’. I agree that the double-planet issue
    needs to be addressed even though I am not certain that the IAU barycenter rule is the
    correct solution. I think that a distinction between ‘classical’ (I prefer ‘major’) planets
    and dwarf planets is useful from a cultural point of view, although scientifically perhaps
    not needed. I see no need to distinguish at this level between dwarf planets and “Plutons”,
    although at a deeper level of classification I think a distinction between terrestrial
    worlds, gas giant worlds and ice worlds is useful (but not between terrestrial *planets*
    and ice *planets*, since the dynamical state is not relevant here).

    These distinctions help form our mental pictures, and I claim there is scientific merit
    to codifying them.
    all best wishes, Jonathan

  164. John Neff

    I reviewed all of the planets and satellites that are known to be nearly spherical and found the the sphericity condition is effectively the same as saying the radius must be 500 km or larger.

    A body orbiting the sun that is made of condensed matter and has a radius greater than or equal to 500 km is a planet. Such objects are nearly spherical and have masses larger than 1.0e20 kg.

    In decending order by mass Jupiter, Saturn, Neptune, Uranus, Earth, Venus, Mars, Mercury, followed by seven satellites, Pluto, three more satellites and Charon, and five more satellites.

  165. Alexander

    “I challenge the readers of this Blog to come up with a new mnemonic to replace the soon-to-be outdated versions (i.e. My Very Excellent Mother Just Served Us Nine Pies)”

    That’s easy: My Very Educated Mother Can’t Just Show Us Nine Planets. Can U?

  166. Nick

    Sorry, You can tell us in the “lay public” what the “new” things are, Pluto is a planet — that’s because we get to decide how laymen interpret things. You can change the technical definition, or even insist on what it is — once the public has ahold of something, you can forget about YOUR definition of it.

    Just look at the computer term “hacker” — it got mis-appropriated years ago by journalists, entered the minds of the public, and we’ve never been able to fix up their(journalist’s) stupidity. To them, there is no difference between “hacker” and “cracker”.

    “Planet” as used by Laymen will include Pluto for all eternity.

  167. John Neff

    It is a science education issue it does not add anything to sum of scientific knowledge. Establishing a definition of a planet based on the objects physical properties has the consequence that more objects than the classical nine planets are included. So what is so bad about that.

  168. Vega Altair

    I don’t know if this was mentioned earlier, and my apologies if it was.

    I remember reading a few years ago, that the Moon didn’t actually go around the Earth. That is, when viewed from ‘above’, the Moon never moves ‘backwards’ in its orbit relative to the Sun. If correct, wouldn’t that make the Moon a planet by the above definitions?

  169. Algoviano

    Hi Phil, you said “Charon is a planet ? It’s smaller than our own moon !”

    May I remind you, that the supposed volume of 2003 UB313 plus that of Pluto plus Charon plus Ceres plus most of the planet candidates we have now ALL TOGETHER is smaller than that of our own moon ! And our moon’s volume fits easily 2.5 times into Mercury ! Which makes me think that we should stay with our good old eight classical planets and call the rest “small planets”, “minor planets” or something. In Chinese the word “asteroid” anyhow is nothing else than “small planet”. For what scientific or whatever reason should we now distinguish between “normal small planets” and “IAU-planets” (roundish small planets with IAU approval) ?? Has the IAU become mad ?

    Actually I think that the real reason behind all of this is the fact that the IAU has got TWO COMMITTEES to decide about names, and they fight a battle about which committee is entitled to get work, and public attention – and funds. If this crazy planet definition gets accepted, the IAU will have to set up even more committees – one to decide about which of the other committees gets the work, i.e. which celestial body is round enough to get the planet-approval, maybe also another committee to decide whether a celestial body needs renaming after it was christened a planet (e.g. Quaoar) ? A good chance to get every now and then into the media, to proclaim another member of the solar system to have been elected to become an “official IAU-planet” (why not send a golden medal to everyone ?), or to announce another object-naming or renaming. Enough committee work for decades from now on. Is this how SCIENCE works ?

    The difference between a planet and an asteroid basically has always been only a matter of size ! Why can we not simply define a number like e.g. 4000 km and – leave moons aside – call everything above a “planet” and everything below a “small planet” (if it is not too small) ? It is arbitrary, yes, why not ? It is always arbitrary ! But that is how it would easily work – and would save us from scientifically absolutely useless committees and debates.

    The real decision we need here is the decision to form ONE committee for naming celestial bodies instead of TWO ! And the naming conventions have to be unified too ! Then the planet/non-planet difference would automatically shrink to its real scientific size (= meaninglessness), and newly found celestial bodies could get their final names in a decent time – independent of their physical properties, including their “roundliness” (sphericity) that could eventually take centuries to determine.

  170. Algoviano

    Sorry, I meant “independent of their physical properties EXCEPT SIZE” …

    And I also want to remember that the word “planet” has been around for thousands of years, and means nothing but “wandering star”. Also Saturn was once believed to look highly irregular (until the ring structure was discovered) and nevertheless was accepted as planet from the beginning.
    PLANET is not a scientific term, it is a normal language word, and only the people, not a committee, may decide what to call a planet. If the IAU wants to define something, they have to call it differently, how about “ROUNDINO” or “ACCRETINO” or something ?

  171. I am very happy to see that other people are equally unhappy about the “satellite rules”.
    There has to be a definition of a planet. The fast pace of new objects discovery in and out the solar system leaves no choice to the scientific community.

    The good thing with a lower mass/size limit is that it is easy to determine empirically. Yes, it is arbitrary but any other rules would be. We all agree with that.

    Now, why can’t we call a satellite a planet? What if Saturn did get ejected from the solar system? What would it be?
    The full logic of defining a planet through its size and mass characteristics is to revolutionize once more the astronomy semantics by defining Ganymede, Titan, Callisto, the Moon, Io, Europa and many others as… planets!

    What matters is to show the non-scientific communauty how diversified the solar system is.

    Have you ever tried to convince someone that Titan is an important place to explore? Isn’t it frustrating that Titan is only defined as a satellite of Saturn and not as a “planet” on nearly equal footing with Mars?

    How can we explain to the wider public than the Jupiter system is like a “miniature” solar system? Wouldn’t it be easier to say that this system includes 4 more (smaller) planets: Europa, Io, Ganymede and Callisto?

  172. Algoviano

    If the definition for a planet is supposed to be “physical”, then the trajectory should nothing have to do with it, because the object doesn’t care much whether it is on a nearly straight line (outside of the solar system), on an ellipse around a star, or on a trajectory around another object, and together with that one around a star. And together with the star around the barycenter of the galaxy. And what about horseshoe-type orbiting objects ?

    It is the same with cars, I call it a car no matter whether it is going straight, in a circle, or through many curves …
    The orbit type should be classified separately from the object.

    And I call it also a car, no matter whether it is round or bulky, or whether the sunroof hangs down for gravity … we do not need a committee that is deciding on a case by case basis, whether a vehicle is shapy enough to be a car, or whether a storm is turbulent enough to be a hurricane, or whether a celestial body is round enough to be an IAU-planet. Wouldn’t it be interesting if we found odd-shaped planets too ?

    Yes, such a trajectory-free and shape-free definition of a planet would make sense and can be determined empirically (no committee involved) ! I insist that an arbitrary lower mass/size limit shall be defined – THIS IS the task the IAU is entitled to do. If this is going to be 2000 km, then Pluto could stay in the planet club, and by dropping the moon/planet-distinction the new planets would be Ganymede, Titan, [Mercury], Callisto, Io, Our Moon (yes !), Europa, Triton, 2003 UB313, and [Pluto] !
    (Me personally would prefer a somewhat higher limit like e.g. 3000 km that would cut the list after Europa).

  173. Soppy sentimentality is cleraly going to get in the way here and every effort will no doubt be made to keep Pluto as a planet instead of the obvious re-classification it needs. If it’s a Kuiper belt object, then surely thats what it should be called. Of course, in the minds of the sentimental public (and many IAU members) losing Pluto as a planet is like losing a member of the family and who wants that? Lets ignore the science and and really mess things up by adding lots more rocky bits and pieces to our planetary number.

  174. John Neff

    Most solar system bodies made of condensed matter are not spherical and there is no apparent connection between body shape and type of orbit. All the requirement of near spherical shape does is to set a lower mass limit of 5.0e20 kg and a lower radius limit of 400 km.

    This raises the level of the discussion without adding any new knowledge.

    What troubles me is the mass of Ceres is 8.7e20 kg and that of the water in the Pacific Ocean is 6.7e20 kg. I think Ceres is too small to be anything other than a minor planet.

  175. Algoviano

    Actually the near spherical shape condition sets different lower mass limits to icy bodies and rocky bodies. And the lower mass limit for rocky bodies is much higher than that for icy bodies – so, iceballs go go !?

    Under this IAU proposition we will end up with a lot of iceball “planets” – and with a lot of rocky “Minor Solar System Bodies” that are heavier and very often even larger than the iceballs …

  176. It’s official…Pluto is not a planet…it is a capture.

    Our Solar System now has 8 planets.

  177. Hurricane

    Science won over popular opinion. T’was Xena that killed Pluto. Go back to sleep Ceres. How many planets? Eight.

  178. Bruno Domingues

    Well, it is a planet, but a dwarf planet, right?
    Or is it a dwarf planet, not a planet at all?

    I’m a bit sad, I hoped we could have a lot of planets, but looks like we will only get a lot of dwarf planets…

    At least they (IAU) decided something…

  179. Jenn

    My Very Excellent Mother Can Just Serve Us Nine Pies Cause U B Crazy 😛

    This is frikkin rediculous

  180. Mikhail Bragoria

    My Very Excellent Mother Just Served Us Noodles 😛

  181. Juan

    Hmmmmm maybe this isn’t the only thing we are wrong about? Confusion about the planets makes me wonder if there is confusion about the age of the Earth as well.

  182. Rodrigo

    Might not be important. But this mess is very much cultural stuff. After Ceres was discovered and considered a Planet, it changed its status as it was so extremelly small compared to the others known then. Pluto then was discovered, and it is also diminute. What’s more, they discovered that the centre of gravity between Pluto and Caronte it’s not inside Pluto, but it was still considered a Planet.

    This seems very strange. With yesterday’s definition Ceres reaches Pluto status; but we call them plutons for Pluto, instead of calling them cerens for the first that was discovered.

    You might like it or not, guys, but Pluto is the only -niknamed- Planet discovered by someone of the USA, in our Solar System. And as your lobby is so powerfull… this might not be science, but it’s history, for good, bad or for who cares.

    I’ll be very interested to know what it’s discovered of Pluto in 2015, and I won’t care the fact that Pluto should never have been called a Planet.

  183. cheruvaaka

    Indian astrologers today said the announcement about dropping Pluto as a planet had endorsed the mathematical and astrological treatises written by Aryabhatta and Varahamihira centuries ago.'Aryabhatta,~Varahamihira's~mathematical~science~vindicated

  184. Science takes a turn for the worse

    Science has taken a turn for the worse here. What constitutes a planet is now no more than a definition or opinion.

    After all if you change a definition of what a planet is to prevent more planets from entering into the numbers, then aren’t you quilty of fanaticism. The old definition of what a Planet is, will be and always should remain permanent. Otherwise you can change it whenever you please, just on a whim based on how many planets you want in our Solar system or Universe. In which case it is not Scientific NOR is it opinion, but rather an agenda of your own personal accord.

    Thus not only should the Science community stay away from this debate, they should accept wholeheartedly that objects of spherical shape orbiting the sun is and always should be called a planet. That’s the old definition, dont redefine it because your frightened of how many planets in the Universe there will be.

    There are billions of planets, you cant continually redefine the Planet definition so many times too keep your precious Nine as only planets in the Universe. This is not only foolish to do so, it shames Science altogether.

    Anyone attempting to redefine or exclude planets from what is widely excepted should not only be expelled from the Science community, they should have their degrees stripped by the Universities for going against Science and making it a mockery.

    For along time now, Scientists have been making it known there are billions of planets in the Universe. Is it so strange to find more near us? Is it so hard for you to accept change that you want to force OTHERS into your own personal beliefs? If so you are as quilty as the guy from Rome who said because there are only 4 elements from the four corners of the Earth then there must only be 4 Gospels.

    Hard to see if your on the other side of the debate I know, but from my side you look just as guilty as a religious fanatic.

    If the object in question is or is not a Planet, you must define rather it has the same characteristics or not as the other planets. Since all the planets are different, you can’t exclude another from being different from the rest so long as it has the same similar characteristics as what you define the other planets. Thus PLUTO is a planet based on the similarities Earth has with Jupiter. Just because Jupiter doesn’t have a cozy atmosphere doesn’t mean we should exclude, just because it if BIG doesn’t mean we should exclude it. The same LOGIC, the business Scientists should be in, is the reason Pluto shouldn’t be excluded as a Planet or sub-categorized as anything other than a Planet, nor any Planets that are similar TOO Pluto should be excluded either just because there are more of them. What if you find another Mars in another distant system? Should we exclude Mars now because it has a sister some billions of light years away?

    As of now we have more Planets in the Universe than you realized. Thats what Astronomy is all about, its about that Discovery. Dont know why its in the business of excluding all Planets from existence other than the Nine. Get used too it and find yourself a new career. You knew this would be the case that there are more than Nine planets in the Universe, obviously you have not realized this and joined the wrong career in Astronomy.

    If your in the business of finding Planets, find some, then declare they aren’t good enough to be a Planet because there is already a Planet called Pluto that has too many of the same qualities, why the hell should you be funded? I think everyone should pull their Funding from Astronomy until you get a clue. If you find an important discovery you better stand by your discovery and accept the discovery of your nearest friendly Astronomer too. How many more so called Non-Planets will you find when the funding goes dry? You aren’t going to continue to get Funding and claim everything that isn’t the Nine is not a planet.

    You have a business too run, you need to make sure your Great Sciencific Discoveries aren’t washed away like some piece of trash. If you can’t get with the program and back up your fellow Scientists, you need to find a new career because you are not a team player.

    Would you listen to yourselves…

    ‘There are only 9 Planets, there are only 9 Planets! The Earth is flat, the Earth is flat.

    No lets call everything else Kuiper Belt Objects. No lets call everything else floating balls of material out in space. No only our 8 must be in, No only Nine. No more Planets.

    Those aren’t planets, Hoth from Star Wars is a Kuiper Belt Object, its not a planet. Can’t be its not one of the Nine or Eight! Its just a big ball of ice!

    There can’t be more than Nine because that’s all I learned in school when I was a baby, so the Tenth Planet isn’t a tenth planet its a a dwarf circular ball of nothing’

  185. John Neff

    The definition a planet is primarily a science education issue. Has the IAU helped science teachers explain to their students what a planet is? They tried to avoid setting an arbitrary lower limit to the mass of spherical body orbiting the sun and in my opinion they failed.

    In effect they have said a panet is a a spherical body made of condensed matter that is more massive than the moon and is in orbit about the sun. Where the mass of the moon was chosen as the arbitrary lower mass limit because it is a familiar object.

  186. Specusci

    The issue is hardly over.

    By the new definition, not only is Pluto not a planet, but neither is Jupiter.

    And Earth might not even be considered a planet.

    Neither planets have cleared their orbit.

  187. SpiderQuark

    More Violence Each Month Comes Jumping Stairs Up Nearby Painted Ceilings Everyday.

    No one said it had to make sense, right?

  188. Oblivion

    All in all, I think the Dark Lord of Death and the Underworld appreciates His representative body being designated as a Binary Ice Dwarf Planet. It’s so Black Metal!

    And now…Planet Eris. The Discordians are so jubilant they will include a bun with their hot dogs tonight. (No that is not a sexual joke.)(Yer such a perv!)

    My Very Excellent Mother Just Served Us Nine Prepared Exquisite Crumb Cakes

    My Very Excellent Mother Just Served Us Nine Prepared Elegant Crumb Cakes

    My Very Excellent Mother Just Served Us Nine Pretty Exquisite Crumb Cakes

    My Very Excellent Mother Just Served Us Nine Pretty Elegant Crumb Cakes

  190. Angus GF

    What a whole load of absolute piffle these decision(s) have been !

    In the next few hundreds of years there is small doubt that a virtually complete range of masses, sizes and species of exoplanets will be discovered and there is a call for some ‘sensible’ physical properties to be considered when discriminating between all of the possible variations of planet that likely exist.

    First and foremost, I think that the actual ‘planet’ term should itself be dumped in favour of some as yet uncoined alternative… it is just too entrenched in the public imagination to be fiddled around with, adding prefixes/suffixes or whatever !

    Objects in the celestial realms would be far better described perhaps, with (maybe) mass/diameter as the primary concept in terms of naming non-stellar & non-brown-dwarf objects.

    If mass were to be so used, then degrees in masses could be discriminators, yotta-objects, zetta-objects, peta-objects and so on for the SI units describing their masses in (Kg./tonnes).

    Further discriminators which could co-describe objects might be their ‘state’ (of matter) – solid/fluid; their state of hydrostatic equilibrium and so forth.

    Nice and simple, easy and the names themselves would contain plenty of (user-redundant) information.

    Its hardly rocket science (although; I suppose; its pretty darned close !)

  191. Adam2

    im doing a school project on the dedate between removing pluto off the list of planets. Can some1 please explain to me, Who are the 2 sides debating weither Pluto is a planet or not? and also what was the final decision? Because all i can see is this debate has got very messy, ( and to me, very interesting ) Thanks to anyone in advance who replies to this.


  192. I think your right it is a planet

  193. it is good that is a planet and i new it so now we have 12 planets thats good and bad but what can we do menkind are distroying our planet we mite aswel go live on pluto lol !!!!!!!!!!!!!!! any way we should protect out planet because its not just good for some people this goes to everyone that is waisting because it is for our won good ou planet would burn and stop cutting the trees we need them to breath we will die and it is all menskind tat are doin it and if your the one that is doing it please stop it is for your own good you mite not lisen but watch when you go jail and you get caught because we are not destroying them we are hurting ourselfs xxxxxxxxxxxxxxxx love everyone

  194. YAY!!!!!Ice World Planets!!!Eris,Sedna, Orcus, Make Make Ceres Dysnomia
    GO Kuipier BELT

  195. Hooray for Pluto and Charon!!Hooray for Eris and Dysnomia! Horray For Sedna and MakeMake!!!

  196. okay My Very Entertaining Mother Just Said Uranus Needs Pants, a Coat and Earrings.
    That’s My verse


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