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Ceres and Charon: A Matter of Gravity

Someone with more cultural insight than Centauri Dreams will have to explain why the designation of Pluto as a planet has captivated so large an audience. The issue is front page on my local newspaper this morning and I’m being asked about it by people who have never shown the slightest interest in space exploration. Perhaps it’s the overturning of things memorized long ago, as if someone had changed the multiplication tables, or decided to modify what makes up the letters of the alphabet.

Whatever the case, the IAU’s draft definition seems to lead to a de facto loss of planetary dignity for Pluto even while maintaining its tenuous identification as one of the tribe. For ‘plutons’ — that category to describe planets whose orbits take more than 200 years to complete, with large orbital inclination and eccentricity — will no doubt soon comprise the vast majority of planets as we discover more and more material in the Kuiper Belt. Which will inevitably lead to an accurate sense that the ‘classical’ planets and these other objects are fundamentally different things.

Three new planets?

Image: The world’s astronomers, under the auspices of the International Astronomical Union (IAU), have concluded two years of work defining the lower end of the planet scale – what defines the difference between “planets” and “solar system bodies”. If the definition is approved by the astronomers gathered 14-25 August 2006 at the IAU General Assembly in Prague, three of the bodies in the Solar System will be assigned new status as planets: Ceres, Charon (Pluto’s companion) and 2003 UB313. There is no change in the planetary status of Pluto. In this artist’s impression the planets are drawn to scale, but without correct relative distances. Credit: The International Astronomical Union/Martin Kornmesser.

And that brings us to Ceres and Charon, and the useful fact that the proposed IAU definition would rest upon twin conditions for planetary status, neither of which accepts Pluto’s diameter as a criterion:

  • The object must orbit a star while not being one itself; neither can it be the satellite of a planet
  • The object must be massive enough for its own gravity to form it into a spherical shape. That implies objects of sufficient mass with a diameter larger than 800 kilometers or so are planets, though borderline cases will have to be determined as they arise
  • Which strikes this observer as emminently sensible because it relies on the way objects behave in a solar system environment. As Richard Binzel, a member of the Planet Definition Committee puts it, “Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet.” More discussions to follow and perhaps touch-ups to the draft resolution, with a vote to be taken on the 24th.

    A later addition for clarity: Pluto and Charon are considered a double planet rather than a planet and its satellite. Why aren’t the Earth and Moon treated the same way? The reason is found in the IAU’s definition of a double planet, which is this: “A pair of objects, which each independently satisfy the definition of ‘planet’ are considered a ‘double planet’ if they orbit each other around a common point in space that is technically known as the ‘barycentre’. In addition, the definition of ‘double planet’ requires that this ‘barycentre’ point must not be located within the interior of either body.” All of which makes Pluto and Charon a double planet but rules out the Earth and Moon as a double.

    Comments on this entry are closed.

    • boomslang August 16, 2006, 16:41

      I think you’re on the right track. There’s a quote, apocryphally-attributed to Max Planck, that “Science advances one funeral at a time.”

      What we’re stuck with here is resistance to change from “learned it that way” types on the one hand, and “fearful of uncertainty” types on the other. The first group is in thrall to nostalgia; the latter prefer simplicity.

      The proposed definition, relying only on gravitational factors, is quite elegant, and the version I saw in the press is impishly self-referential (a virtue to the algorithmically-minded): 1) the object is massive enough to be roughly spherical yet not massive enough to begin atomic fusion, and 2) the object does not orbit a planet.

      Note that the second criterion is not arbitrary; Luna is a moon of Earth (or “Terra” I suppose, to be consistent) because the system’s center of mass is within the sphere of the Earth, but Pluto and Charon are a binary planet system because that system’s center of mass does not reside within the sphere of either one of them.

      Unless I am mis-remembering about the center of mass for Earth-Luna and it actually is outside the Earth’s surface, in which case, let me be the first to welcome our new binary planet companion…

    • Administrator August 16, 2006, 20:18

      Good points all, and your comment reminded me to make an addition to that post re the differences between a planet and its satellite and a double planet. Thanks! As you say, it all has to do with the location of the barycenter.

    • ankur August 17, 2006, 11:38

      I’ve seen at some forums that people are not happy with the possibility of some 100+ planets, pointing out among other things that we’ll have to learn a few more names. Mike Brown has linked a page on his home page with the title “How many planets are there if everything round is a planet?” http://www.gps.caltech.edu/~mbrown/whatsaplanet/howmanplanets.html . I’ve even seen claims that the definition is arbitrary.

      I for one am happy, I think the definition is pretty easy to understand and does not impose any arbitrary size or angle of orbit kind of restrictions that won’t work in another solar system. Even if the definition is accepted not much will actually change. If there can be a thousand asteroids, there can be a hundred planets, for all I care the original 8 planets are important ones.

      In our solar system M-V-E-M-J-S-U-N are still the ‘classical planets’, and I believe will continue to be in school books under that name, with (I hope) the addition of pluto and other plutons as a small topic. Though Ceres, since it is not a pluton, and is simply a planet will very likely be classified as “minor planet” a term not exactly endorsed by IAU.

    • Administrator August 17, 2006, 14:35

      I’m with you. Any planetary definition will be arbitrary — that’s what definitions are — but this one has the advantage of being ingenious and logical. I like it more and more…

    • boomslang August 17, 2006, 22:18

      The prospect of learning new names should be a source of excitment, not dread, to the true scientist. (How about an “astronomy bee” for school kids to go along with spelling bees and math bees?) And I am secretly hoping to see a worldwide petition to make “Xena” the official name of 2003 UB313, if for no other reason than to punish the IAU for inflicting that mouthful “Quaoar”(on deck for planethood in the next batch of candidates) on us for all time… :)

    • Adam Crowl August 18, 2006, 0:02

      Ceres almost qualifies as a ‘pluton’ in a compositional sense, as the latest modelling indicates an extensive ice mantle, just like Triton and Pluto. All have similar density, but Ceres won’t have any volatile gases like nitrogen and methane, except as clathrates. What atmosphere it has is due to the sublimation of ices, just like Triton and Pluto too, albeit just water vapour.

    • Eric James August 18, 2006, 3:15

      Oh no! I have to take my socks off to count that many!

    • Ankur August 18, 2006, 13:30

      @Eric learn to count in binary using fingers :) http://www.intuitor.com/counting/

      When I was a kid I used this mnemonic to memorize the order of planets “My Very Educated Mother Just Showed Us Nine Planets” I wonder what I’d do now.

    • Administrator August 18, 2006, 13:50

      There’s also “My Very Energetic Mother Just Served Us Nine Pizzas…” We need to prepare for some extended mnemonics! On the other hand, there are plenty of grumblings ongoing at the IAU assembly in Prague, so that vote on the 24th could be interesting.

    • Gerard Crotty August 25, 2006, 3:35

      Suggestion for new mnemonic in the aftermath of the vote:

      Most Voted Eight, Meaning Just Slightly Under Nine

    • Administrator August 25, 2006, 19:02

      An inspired mnemonic. Well done!

    • ljk July 5, 2008, 10:06

      New Horizons Team Celebrates 30th Anniversary of Charon’s

      This week the New Horizons mission team celebrates the 30th
      anniversary of the discovery of Pluto’s largest and first moon,
      Charon, by U.S. Naval Observatory astronomers James Christy
      and Robert Harrington.