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Eight Planets It Is

So now we know what a planet is. As confirmed by the passage of a revised resolution at the International Astronomical Union’s general assembly today in Prague, a planet meets the following criteria:

  • It must be in orbit around a star
  • It must possess sufficient mass to allow it to assume a round shape; i.e., it assumes hydrostatic equilibrium
  • It is large enough that it has cleared the orbit through which it moves
  • The third item, of course, is the interesting part, for it rules out Ceres, about which there had been some controversy. I mean, it was one thing to consider 2003 UB313 as a planet, but to delve into the middle of the Solar System and define a new planet in medias res seemed a stretch too far for some people (though not for me). Pluto is also ruled out because it moves for part of its orbit inside the orbit of Neptune; Charon likewise is left without planetary designation.

    What does happen to Pluto is that it becomes a ‘dwarf planet,’ a new kind of object that also includes Ceres and 2003 UB313, and of course we will be finding numerous other dwarf planets as we continue to refine our observations of the Kuiper Belt and surrounding space (as many as a dozen candidate dwarves are already on the IAU watchlist — click here for a list of known dwarf planets).

    So there you have it, eight planets that can be considered ‘classical’ and a new category that subsumes all the rest. Which for the traditionalist in me is OK, while the logician says that the third criteria involving clearing out a planetary orbit is likewise sensible, and we can forget about the Pluto/Charon barycenter. Sigh.

    There is precedent for planetary demotion, incidentally. After its discovery in 1801, Ceres was generally thought to be a planet, but by the early 1850s so many of what we now call ‘asteroids’ were being discovered that planetary status for all was ruled out. It shouldn’t be long before the IAU ruling becomes accepted and similarly relegated to a historical footnote, but the controversy was intense while it lasted.

    But let’s move on to other things, which tomorrow includes a new planet around Mu Arae, a system that is getting more crowded (and better characterized) all the time, and the methods used to find it.

    Update: A telling comment from Hal Weaver at Johns Hopkins’ Applied Physics Laboratory. apropos of dynamically cleared orbits:

    “Regarding the resolution itself, I’m with Andy Cheng in concluding that the situation is still somewhat muddled. What exactly is meant by a planet ‘clearing its neighborhood?’ Since many ‘plutinos’ … (including Pluto) …cross Neptune’s orbit, I’d say Neptune’s neighborhood still needs some clearing! … It just seems a bit risky to me to base a definition on a theoretical construct (‘dynamically cleared regions’) that’s only approximate at best and may change significantly as our understanding of planet formation evolves over time.

    “I further note that there have been particularly large swings in the theories of outer solar system dynamical evolution during the past decade. What was ‘conventional wisdom’ five years ago has been replaced with the latest fad, and I don’t expect that situation to change any time soon.”

    Comments on this entry are closed.

    • boomslang August 24, 2006, 15:28

      Actually, it’s not eight, it’s six. Jupiter has an estimated 50000 Trojan asteroids that share its orbit, and therefore it has not “cleared’ it. Likewise, as long as Cruithne is doing its complicated little dance around our orbit, Earth is no longer a planet, either.

      A completely scientifically unsound definition. The dolts at the IAU will be revisiting this issue again, you can be certain…

    • boomslang August 24, 2006, 15:53

      Also, I have yet to see the exact wording of the second criterion. As reported in the press, it would appear that all brown dwarfs and dwarf stars that orbit stars are now also “planets”, since the non-fusion feature seems to have been eliminated and they are large enough to be round yet comparatively small enough to be “in orbit” around their primary “suns”.

    • Administrator August 24, 2006, 19:10

      Not to mention the Neptune problem. See the update above.

    • djlactin August 24, 2006, 22:39

      And then there’s the problem of what to call all those bodies that were ejected from star systems during their chaotic early period and are now wandering interstellar… When we find one, then what? Seems like ‘wanderer’ (planetoi) would better refer to these than to the relictual orbiters

    • Joseph Baneth Allen August 25, 2006, 5:44

      Its rather curious that the history behind the discovery of Pluto has been largely ignored by the IAU. And its also rather amusing to watch as the 450 IAU members who voted on this “new” definition of a planet forget that science is a dynamic field of discourse and in about nine years they might be forced vote again on what a planet is when New Horizon flies by Pluto – the ninth planet.

    • Ralph Buttigieg August 25, 2006, 17:13


      And what about Sedna? Its big enough to be round and so far out its not a KBO. It might be part of the inner Oort Cloud but that has still to be confirmed. At the moment its out there on its lonesome. By their difinition Sedna is a planet.



    • Adam August 26, 2006, 8:45

      Hi All

      Good points. The dynamicists might be trying to base planethood on gravitational dominance – which Jupiter and Earth have over their associated Trojans – and it kind of makes sense BUT… what if we find exoplanets – say a superJovian with Trojan Mars/Earth mass ‘dwarfs’ sharing its orbit? That seems to be a plausible and ridiculous outcome of their new proposal.

      It’s a fair concern that if we let every sphericalised rock into the planet list that we’ll soon have hundreds of planets swarming in the Edgeworth-Kuiper Belt and the Inner Oort, but so what? There are over 200 exoplanets now known and almost all have no names – should educators be concerned that children can’t reel off a list of names? No. Some good examples of each class are sufficient, I think, for children to get the idea – that the Universe is a big, amazing place, and planets come in basic types. Thus Pluto and its kin are good examples of ‘ice dwarfs’, Uranus and Neptune are ‘ice giants’ and Saturn and Jupiter are gas giants, while super-Jupiters and brown ‘dwarfs’ are next up the ladder and so on.

      Until we have some exo-terrestrials, then we’re restricted to the five/six known terrestrial planemos – counting the Moon and Io as terrestrial Planetary Mass Objects (planemos.) Europa is borderline, while Ganymede and Callisto are ice-planemos, and Ceres/Triton/Pluto are ‘ice dwarf planemos’…

      Oh for the pre-telescope days of the perfectly reflecting celestial spheres – “Seven planets is enough, surely!”

      Nah. A hundred planets or more is fine by me.


    • pfdietz August 26, 2006, 22:26

      I am concerned about the ‘clearing the orbit’ bit, since this would occur more slowly far from the Sun. In a sufficiently large orbit, even Jupiter would not clear its orbit in the lifetime of the solar system. Does this mean Jupiter would cease to be a planet if it were in a much larger orbit?

    • Eric James August 27, 2006, 21:07

      Oh good. I can put my socks back on!

    • Will August 30, 2006, 19:31

      I think you are correct

    • ljk February 1, 2007, 13:28

      If this theory turns out to be fact, what does this make
      Neptune, since one of the requirements for being a planet
      by the IAU was the clearing of debris in its orbital path
      around Sol.

      Neptune May Have Thousands of Escorts

      Neptune may be escorted in its orbit by thousands of asteroid-like objects,
      perhaps more than exist in the entire asteroid belt.


    • Administrator February 1, 2007, 13:29

      Right on target, Larry. Yet another problem for the planet-definers! That swarm of Trojans doesn’t help things one bit.

    • ljk February 14, 2007, 12:08

      Online article in the January, 2007 issue of Scientific American

      What Is a Planet?

      The controversial new official definition of “planet,” which banished Pluto, has its flaws but by and large captures essential scientific principles

      By Steven Soter


    • ljk August 11, 2008, 22:49

      A planet-sized debate

      Later this week the Applied Physics Lab will host a conference on how
      a planet should be defined. Jeff Foust reports that, two years after
      the IAU approved an official definition for the term, some scientists
      are still clamoring for a change.


    • ljk August 13, 2008, 7:52

      PSI Director Promotes 13-Planet Solar System During Great Planet Debate


      “Mark Sykes says that if a non-stellar object is massive enough
      to be round and orbits a star, it ought to be a planet. The key
      here is that once an object gets that big, important geophysical
      processes begin.”

    • ljk August 14, 2009, 13:28

      Friday, August 14, 2009

      Pluto fizzled at IAU’s Rio meeting

      The IAU’s Rio meeting never discussed the sensitive issue of Pluto’s status.

      “Pluto still not a planet after astronomy meeting”

      by Dan Vergano

      August 13, 2009

      USA Today

      An international astronomy meeting ended Thursday in Rio de Janeiro, and Pluto is still not a planet. The closing ceremonies of the 2009 International Astronomical Union (IAU) concluded with nary a peep about the planetary brouhaha that saw walk-outs and table-banging three years ago at the venerable astronomer’s assembly, in Prague.

      “None of that stuff at all came up,” said IAU spokesman Lars Lindberg Christensen, speaking from a taxi speeding away from the meeting for one of Rio’s restaurants. “I believe people still disagree, but the focus was really on science at the meeting.”

      In 2006, the IAU meeting assembly demoted Pluto into a “dwarf planet” (sometimes called a “plutoid”) because it has failed to have “cleared the neighborhood around its orbit,” in the words of the assembly’s resolution. Pluto only weighs 0.07 as much as the rest of the objects in its orbit, which includes “Kuiper Belt” comets and Neptune, an ice giant planet. Earth, in contrast, weighs 1.7 million times more than the remaining objects in its orbit, asteroids and the moon, making it a planet, according to the just-released Pluto Confidential: An Insider Account of the Ongoing Battles Over the Status of Pluto, by astronomers Steve Maran and Laurence Marschall.

      The two authors split on whether Pluto is a planet, with Maran arguing for the demoted world. “Pluto is still the people’s planet and, if anything, more popular as an underdog than ever before,” Maran writes.

      Full article (and cartoon) here: