Simon’s Backup Weblog

Twelve planets?

Posted in Uncategorized by Simon Bisson on August 16, 2006

It’s looking more and more likely that we’ll soon have to change those mnemonics for the planets. No more Matilda Visits Every Monday And Just Stays Until Noon Period. We’ll have to add in Ceres, Charon and 2003 UB313…

Professor Owen Gingerich, who chairs the IAU planet definition committee, said: “In a sense we’re demoting Pluto by taking it off the list of classical planets. But we’re promoting it by making it the prototype of this new category of plutons.”


The basis for this re-evaluation is a new scientific definition of a planet which uses gravity as the determining factor.

According to this definition, two conditions must be satisfied for an object to qualify as a planet:

* The object must be in orbit around a star, but must not itself be a star
* It must have enough mass for the body’s own gravity to pull it into a nearly spherical shape

On whether he was confident the resolution would be passed, Professor Gingerich told the BBC News website: “It will be a very awkward situation if they don’t.

“On Sunday afternoon, we proposed it out of the blue for the division chairmen and they voted unanimously that they would be prepared to back it. That’s a good cross-section of astronomers.

“I’m sure it will be controversial to those with a stake in some other solution, but I hope we will get an overwhelming endorsement.”

Ah, the politics of the IAU. Long may they debate.


15 Responses to 'Twelve planets?'

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  1. andrewducker said,

    Well, we were going to have either 12(+) planets or 8, so either way round people were going to feel cross.

  2. rfmcdpei said,

    I’m pleased that Ceres is about to become a planet, myself. I’ve a soft spot in my heart for that world.

  3. rowanf said,

    Hmm, my Ceres is in Libra. But Charon doesn’t seem to be amongst the minor planets on my chart. I guess it was still considered Pluto’s moon and not something that would have a lot of influence on an earther’s chart. (The minor planets that are covered – Chiron, Ceres, Pallas Athena, Juno and Vesta. I’m guessing more for mythological reasons than astronomical.)

  4. ivory_goddess said,

    OK, I want clarification: why isn’t our Moon now a pluton?

    It’s bigger (3,476 km diameter) than Charon (1,250 km) or Pluto (2,360km) (duno about Ceres or UB313). Yes, it is in orbit around us rather than the Sun, but as I understand it Charon is in obit around Pluto, not the Sun, yet that still counts as a pluton!

    NB: Anyone else think ‘pluton’ is a bloody stupid name? What’s wrong with ‘minor planet’?

  5. the_gardener said,

    “Pluton” sounds like one of the systems of measurement used in the original Battlestar Galactica!

  6. childeric said,

    Oh dear, I do realise what you’re saying here, but the first time I read that I was really wondering why it would make the round people in particular cross. Fellow feeling with similarly-shaped objects, perhaps?

  7. surliminal said,

    I quite like pluton, though it does sound more like an atomic particle.
    Yeh and isn’t Pluto round? How round do you have to be??

  8. rowanf said,

    In Heinlein it was a unit of currency based on plutonium.

    And of course it is an epithet of Dis Pater Pluton (the giver of wealth).

  9. cobrabay said,

    Also a French nuclear-armed short-range ballistic missile

  10. grey_lady said,

    I am rather hoping that poor 2003 UB313 ends up with a more memorable name at some point….

  11. del_c said,

    The minor planet category already exists, it was created to strip the Italians of all their planets they found in the nineteenth century (the solar system has had eleven planets in it before). This whole controversy is about the Americans squealing at their planet suffering the same fate, so that wouldn’t be a compromise solution.

  12. del_c said,

    It will, it just has to be established who gets to name it. If it turns out to be a minor planet, then its discoverers (Brown and Trujillo IIRC) can call it whatever they want within reason. But if it’s a planet, then the IAU has to decide on a name. I believe Brown has said he has a name all ready, and is just keeping quiet about it until he finds out whether he has the right to it.

    This compromise of calling Pluto and UB313 “intermediate planetoids” of some kind also means that the IAU now have to decide who gets to name one.

  13. del_c said,

    PS as dumb category names go, they would have to go a long way to beat “cubewano”. Without looking it up, or giving the game away if you already know, would anyone care to guess how that name was coined?

  14. davesslave said,

    This is very fascinating! I confess I’m one of those geeks who likes to check the internet for scientific discoveries when I have the time. Pity I don’t.

  15. khavrinen said,

    One of the articles I read said that they are counting Pluto and Charon as a sort of double planet pair, because rather than Charon actually orbiting Pluto they are co-orbiting around a center of gravity which is in space between the two of them. Because of the much greater differences in size, however, the centers of gravity for groupings like Earth & Luna, or Jupiter and its fleet of moons, are well below the surface of the dominant central planet — a distinction which I suspect will cause endless headaches for elementary school teachers trying to cover low level astronomy from now on.

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